Inhibition of cuneate neurones: its afferent source and influence on dynamically sensitive "tactile" neurones

Reduced Cerebellar Brain Inhibition and Vibrotactile Perception in Response to Mechanical Hand Stimulation at Flutter Frequency

The cerebellum is traditionally considered a movement control structure because of its established afferent and efferent anatomical and functional connections with the motor cortex. In the last decade, studies also proposed its involvement in perception, particularly somatosensory acquisition and prediction of the sensory consequences of movement. However, compared to its role in motor control, the cerebellum's specific role or modulatory influence on other brain areas involved in sensory perception, specifically the primary sensorimotor cortex, is less clear. In the present study, we explored whether peripherally applied vibrotactile stimuli at flutter frequency affect functional cerebello-cortical connections. In 17 healthy volunteers, changes in cerebellar brain inhibition (CBI) and vibration perception threshold (VPT) were measured before and after a 20-min right hand mechanical stimulation at 25 Hz. 5 Hz mechanical stimulation of the right foot served as an active control condition. Performance in a Grooved Pegboard test (GPT) was also measured to assess stimulation's impact on motor performance. Hand stimulation caused a reduction in CBI (13.16%) and increased VPT but had no specific effect on GPT performance, while foot stimulation had no significant effect on all measures. The result added evidence to the functional connections between the cerebellum and primary motor cortex, as shown by CBI reduction. Meanwhile, the parallel increase in VPT indirectly suggests that the cerebellum influences the processing of vibrotactile stimulus through motor-sensory interactions.

Dexamphetamine widens temporal and spatial binding windows in healthy participants

BACKGROUND

The pathophysiology of psychosis is complex, but a better understanding of stimulus binding windows (BWs) could help to improve our knowledge base. Previous studies have shown that dopamine release is associated with psychosis and widened BWs. We can probe BW mechanisms using drugs of specific interest to psychosis. Therefore, we were interested in understanding how manipulation of the dopamine or catecholamine systems affect psychosis and BWs. We aimed to investigate the effect of dexamphetamine, as a dopamine-releasing stimulant, on the BWs in a unimodal illusion: the tactile funneling illusion (TFI).

METHODS

We conducted a randomized, double-blind, counterbalanced placebo-controlled crossover study to investigate funnelling and errors of localization. We administered dexamphetamine (0.45 mg/kg) to 46 participants. We manipulated 5 spatial (5-1 cm) and 3 temporal (0, 500 and 750 ms) conditions in the TFI.

RESULTS

We found that dexamphetamine increased funnelling illusion (p = 0.009) and increased the error of localization in a delay-dependent manner (p = 0.03). We also found that dexamphetamine significantly increased the error of localization at 500 ms temporal separation and 4 cm spatial separation (p _interaction = 0.009; p _{500ms|4cm v. baseline} = 0.01).

LIMITATIONS

Although amphetamine-induced models of psychosis are a useful approach to understanding the physiology of psychosis related to dopamine hyperactivity, dexamphetamine is equally effective at releasing noradrenaline and dopamine, and, therefore, we were unable to tease apart the effects of the 2 systems on BWs in our study.

CONCLUSION

We found that dexamphetamine increases illusory perception on the unimodal TFI in healthy participants, which suggests that dopamine or other catecholamines have a role in increasing tactile spatial and temporal BWs.

Sensory computations in the cuneate nucleus of macaques

Tactile nerve fibers fall into a few classes that can be readily distinguished based on their spatiotemporal response properties. Because nerve fibers reflect local skin deformations, they individually carry ambiguous signals about object features. In contrast, cortical neurons exhibit heterogeneous response properties that reflect computations applied to convergent input from multiple classes of afferents, which confer to them a selectivity for behaviorally relevant features of objects. The conventional view is that these complex response properties arise within the cortex itself, implying that sensory signals are not processed to any significant extent in the two intervening structures-the cuneate nucleus (CN) and the thalamus. To test this hypothesis, we recorded the responses evoked in the CN to a battery of stimuli that have been extensively used to characterize tactile coding in both the periphery and cortex, including skin indentations, vibrations, random dot patterns, and scanned edges. We found that CN responses are more similar to their cortical counterparts than they are to their inputs: CN neurons receive input from multiple classes of nerve fibers, they have spatially complex receptive fields, and they exhibit selectivity for object features. Contrary to consensus, then, the CN plays a key role in processing tactile information.

Effect of Remote Masking on Tactile Perception of Electrovibration

Masking has been used to study human perception of tactile stimuli, including those created by electrovibration on touch screens. Earlier studies have investigated the effect of on-site masking on tactile perception of electrovibration. In this article, we investigated whether it is possible to change the absolute detection threshold and intensity difference threshold of electrovibration at the fingertip of index finger via remote masking, i.e., by applying a (mechanical) vibrotactile stimulus on the proximal phalanx of the same finger. The masking stimuli were generated by a voice coil (the Haptuator). For 16 participants, we first measured the detection thresholds for electrovibration at the fingertip and for vibrotactile stimuli at the proximal phalanx. Then, the vibrations on the skin were measured at four different locations on the index finger of subjects to investigate how the mechanical masking stimulus propagated as the masking level was varied. Later, masked absolute thresholds of eight participants were measured. Finally, for another group of eight participants, intensity difference thresholds were measured in the presence/absence of vibrotactile masking stimuli. Our results show that vibrotactile masking stimuli generated sub-threshold vibrations around the fingertip, and hence, probably did not mechanically interfere with the electrovibration stimulus. However, there was a clear psychophysical masking effect due to central neural processes. We measured the effect of masking stimuli, up to 40 dB SL, on the difference threshold at four different intensity standards of electrovibration. We proposed two models based on hypothetical neural signals for prediction of the masking effect on intensity difference thresholds for electrovibration: amplitude and energy models. The energy model was able to predict the effect of masking more accurately, especially at high intensity masking levels.

Emergence of an Invariant Representation of Texture in Primate Somatosensory Cortex

A major function of sensory processing is to achieve neural representations of objects that are stable across changes in context and perspective. Small changes in exploratory behavior can lead to large changes in signals at the sensory periphery, thus resulting in ambiguous neural representations of objects. Overcoming this ambiguity is a hallmark of human object recognition across sensory modalities. Here, we investigate how the perception of tactile texture remains stable across exploratory movements of the hand, including changes in scanning speed, despite the concomitant changes in afferent responses. To this end, we scanned a wide range of everyday textures across the fingertips of rhesus macaques at multiple speeds and recorded the responses evoked in tactile nerve fibers and somatosensory cortical neurons (from Brodmann areas 3b, 1, and 2). We found that individual cortical neurons exhibit a wider range of speed-sensitivities than do nerve fibers. The resulting representations of speed and texture in cortex are more independent than are their counterparts in the nerve and account for speed-invariant perception of texture. We demonstrate that this separation of speed and texture information is a natural consequence of previously described cortical computations.

Sub-Second Temporal Integration of Vibro-Tactile Stimuli: Intervals between Adjacent, Weak, and Within-Channel Stimuli Are Underestimated

Tactile estimation of sub-second time is essential for correct recognition of sensory inputs and dexterous manipulation of objects. Despite our intuitive understanding that time is robustly estimated in any situation, tactile sub-second time is altered by, for example, body movement, similar to how visual time is modulated by eye movement. The effects of simpler factors, such as stimulus location, intensity, and frequency, have also been reported in temporal tasks in other modalities, but their effects on tactile sub-second interval estimation remain obscure. Here, we were interested in whether a perceived short interval presented by tactile stimuli is altered only by changing stimulus features. The perceived interval between a pair of stimuli presented on the same finger apparently became short relative to that on different fingers; that of a weak-intensity pair relative to that of a pair with stronger intensity was decreased; and that of a pair with the same frequency relative to one with different frequencies was underestimated. These findings can be ascribed to errors in encoding temporal relationships: nearby-space/weak-intensity/similar-frequency stimuli presented within a short time difference are likely to be integrated into a single event and lead to relative time compression.

An Adaptation-Induced Repulsion Illusion in Tactile Spatial Perception

Following focal sensory adaptation, the perceived separation between visual stimuli that straddle the adapted region is often exaggerated. For instance, in the tilt aftereffect illusion, adaptation to tilted lines causes subsequently viewed lines with nearby orientations to be perceptually repelled from the adapted orientation. Repulsion illusions in the nonvisual senses have been less studied. Here, we investigated whether adaptation induces a repulsion illusion in tactile spatial perception. In a two-interval forced-choice task, participants compared the perceived separation between two point-stimuli applied on the forearms successively. Separation distance was constant on one arm (the reference) and varied on the other arm (the comparison). In Experiment 1, we took three consecutive baseline measurements, verifying that in the absence of manipulation, participants’ distance perception was unbiased across arms and stable across experimental blocks. In Experiment 2, we vibrated a region of skin on the reference arm, verifying that this focally reduced tactile sensitivity, as indicated by elevated monofilament detection thresholds. In Experiment 3, we applied vibration between the two reference points in our distance perception protocol and discovered that this caused an illusory increase in the separation between the points. We conclude that focal adaptation induces a repulsion aftereffect illusion in tactile spatial perception. The illusion provides clues as to how the tactile system represents spatial information. The analogous repulsion aftereffects caused by adaptation in different stimulus domains and sensory systems may point to fundamentally similar strategies for dynamic sensory coding.

Vibrotactile stimulation of fast-adapting cutaneous afferents from the foot modulates proprioception at the ankle joint

It has previously been shown that cutaneous sensory input from across a broad region of skin can influence proprioception at joints of the hand. The present experiment tested whether cutaneous input from different skin regions across the foot can influence proprioception at the ankle joint. The ability to passively match ankle joint position (17° and 7° plantar flexion and 7° dorsiflexion) was measured while cutaneous vibration was applied to the sole (heel, distal metatarsals) or dorsum of the target foot. Vibration was applied at two different frequencies to preferentially activate Meissner's corpuscles (45 Hz, 80 μm) or Pacinian corpuscles (255 Hz, 10 μm) at amplitudes ∼3 dB above mean perceptual thresholds. Results indicated that cutaneous input from all skin regions across the foot could influence joint-matching error and variability, although the strongest effects were observed with heel vibration. Furthermore, the influence of cutaneous input from each region was modulated by joint angle; in general, vibration had a limited effect on matching in dorsiflexion compared with matching in plantar flexion. Unlike previous results in the upper limb, we found no evidence that Pacinian input exerted a stronger influence on proprioception compared with Meissner input. Findings from this study suggest that fast-adapting cutaneous input from the foot modulates proprioception at the ankle joint in a passive joint-matching task. These results indicate that there is interplay between tactile and proprioceptive signals originating from the foot and ankle.

Processing afferent proprioceptive information at the main cuneate nucleus of anesthetized cats

Medial lemniscal activity decreases before and during movement, suggesting prethalamic modulation, but the underlying mechanisms are largely unknown. Here we studied the mechanisms underlying proprioceptive transmission at the midventral cuneate nucleus (mvCN) of anesthetized cats using standard extracellular recordings combined with electrical stimulation and microiontophoresis. Dual simultaneous recordings from mvCN and rostroventral cuneate (rvCN) proprioceptive neurons demonstrated that microstimulation through the rvCN recording electrode induced dual effects on mvCN projection cells: potentiation when both neurons had excitatory receptive fields in muscles acting at the same joint, and inhibition when rvCN and mvCN cells had receptive fields located in different joints. GABA and/or glycine consistently abolished mvCN spontaneous and sensory-evoked activity, an effect reversed by bicuculline and strychnine, respectively; and immunohistochemistry data revealed that cells possessing strychnine-sensitive glycine receptors were uniformly distributed throughout the cuneate nucleus. It was also found that proprioceptive mvCN projection cells sent ipsilateral collaterals to the nucleus reticularis gigantocellularis and the mesencephalic locomotor region, and had slower antidromic conduction speeds than cutaneous fibers from the more dorsally located cluster region. The data suggest that (1) the rvCN-mvCM network is functionally related to joints rather than to single muscles producing an overall potentiation of proprioceptive feedback from a moving forelimb joint while inhibiting, through GABAergic and glycinergic interneurons, deep muscular feedback from other forelimb joints; and (2) mvCN projection cells collateralizing to or through the ipsilateral reticular formation allow for bilateral spreading of ascending proprioceptive feedback information.

Sensory transmission in cerebellar granule cells relies on similarly coded mossy fiber inputs

The computational principles underlying the processing of sensory-evoked synaptic inputs are understood only rudimentarily. A critical missing factor is knowledge of the activation patterns of the synaptic inputs to the processing neurons. Here we use well-defined, reproducible skin stimulation to describe the specific signal transformations that occur in different parallel mossy fiber pathways and analyze their representation in the synaptic inputs to cerebellar granule cells. We find that mossy fiber input codes are preserved in the synaptic responses of granule cells, suggesting a coding-specific innervation. The computational consequences of this are that it becomes possible for granule cells to also transmit weak sensory inputs in a graded fashion and to preserve the specific activity patterns of the mossy fibers.

Impairment of human proprioception by high-frequency cutaneous vibration

These experiments assessed whether the impairment in proprioceptive acuity in the hand during 'interfering' cutaneous stimulation could be caused by inputs from Pacinian corpuscles. The ability to detect passive movements at the proximal interphalangeal joint of the index finger was measured when vibrotactile stimuli were applied to the adjacent middle finger and thenar eminence at frequencies and amplitudes that favour activation of rapidly adapting cutaneous afferents. Inputs from Pacinian corpuscles are favoured with high-frequency vibration (300 Hz), while those from Meissner corpuscles are favoured by lower frequencies (30 Hz). Detection of movement was significantly impaired when 300 Hz (20 microm peak-to-peak amplitude) complex vibration or 300 Hz (50 microm) sinusoidal vibration was applied to the middle finger and thenar eminence. In contrast, detection of movements was not altered by low-frequency sinusoidal vibration at 30 Hz with an amplitude of 50 microm or with a larger amplitude matched in subjective intensity to the 300 Hz sinusoidal stimulus. Thus it is unlikely that the impairment in detection was due to attention being diverted by vibration of an adjacent digit. In addition, an increase in amplitude of 300 Hz vibration led to a greater impairment of movement detection, so that the impairment was graded with the input. The time taken to nominate the direction of applied movement also increased during 300 Hz but not during 30 Hz sinusoidal vibration. These findings suggest that stimuli which preferentially activate Pacinian, but not Meissner corpuscles, impair proprioceptive acuity in a movement detection task.

Cortical influences on rapid brainstem plasticity

Cortical contributions to brainstem plasticity in the somatosensory system are poorly understood. Tactile receptive fields (RFs) of brainstem dorsal column nuclei (DCN) neurons rapidly enlarge when peripheral inputs are disrupted by local anesthetic blocks with lidocaine (LID). Cortical inputs appear to influence this plasticity because enlargements have been shown to be greater when cortical inputs are disrupted. Like disruptions of peripheral inputs, disruptions of DCN inhibition by DCN administration of the GABAA receptor antagonist bicuculline methiodide (BMI) also cause rapid enlargements of DCN RFs when cortical inputs are intact. These findings leave questions about interactions between cortical inputs, DCN inhibition, and DCN RF plasticity. To study potential interactions, the present experiments evaluated RF sizes of DCN tactilely responsive neurons in anesthetized rats following DCN microinjection of BMI when cortical inputs were acutely disrupted or intact. These tests were also supplemented by subsequent LID tests to directly compare post-BMI and post-LID effects on the same RF. BMI caused DCN RF enlargements when cortical inputs were disrupted or intact; however, enlargements after cortical input disruption were greater than when cortical inputs were intact. Following RF enlargement and retraction after BMI, LID often caused a second enlargement of the same RF, across skin that partially matched skin involved in the enlargement after BMI. This occurred when cortical inputs were disrupted or intact. We hypothesize that cortical inputs are not required for BMI and LID to initiate partially matching enlargements in individual DCN tactile RFs, however, cortical inputs constrain magnitudes of these enlargements.

Processing of Vibrotactile Inputs From Hairy Skin by Neurons of the Dorsal Column Nuclei in the Cat

The capacity of single neurons of the dorsal column nuclei (DCN) for coding vibrotactile information from the hairy skin has been investigated in anesthetized cats to permit quantitative comparison first with the capacities of DCN neurons responding to glabrous skin vibrotactile inputs and second with those of spinocervical tract neurons responding to vibrotactile inputs from hairy skin. Dynamically sensitive tactile neurons of the DCN the input of which came from hairy skin could be divided into two classes, one associated with hair follicle afferent (HFA) input, the other with Pacinian corpuscle (PC) input. The HFA-related class was most sensitive to low-frequency (<50 Hz) vibration and had a graded response output as a function of vibrotactile intensity changes. PC-related neurons had a broader vibrotactile sensitivity, extending to ≥300 Hz and appeared to derive their input from the margins of hairy skin, near the footpads, or from deeper PC sources such as the interosseous membranes or joints. HFA-related neurons had phaselocked responses to vibration frequencies up to ∼75 Hz, whereas PC neurons retained this capacity up to frequencies of ∼300 Hz with tightest phaselocking between 50 and 200 Hz. Quantitative measures of phaselocking revealed that the HFA-related neurons provide the better signal of vibrotactile frequency up to ∼50 Hz with a switch-over to the PC-related neurons above that value. In conclusion, the functional capacities of these two classes of cuneate neuron appear to account for behavioral vibrotactile frequency discriminative performance in hairy skin, in contrast to the limited capacities of vibrotactile-sensitive neurons within the spinocervical tract system.

Vibrotactile coding capacities of spinocervical tract neurons in the cat

The response characteristics and tactile coding capacities of individual dorsal horn neurons, in particular, those of the spinocervical tract (SCT), have been examined in the anesthetized cat. Twenty one of 38 neurons studied were confirmed SCT neurons based on antidromic activation procedures. All had tactile receptive fields on the hairy skin of the hindlimb. Most (29/38) could also be activated transynaptically by electrical stimulation of the cervical dorsal columns, suggesting that a common set of tactile primary afferent fibers may provide the input for both the dorsal column-lemniscal pathway and for parallel ascending pathways, such as the SCT. All but 3 of the 38 neurons studied displayed a pure dynamic sensitivity to controlled tactile stimuli but were unable to sustain their responsiveness throughout 1s trains of vibration at vibration frequencies exceeding 5-10 Hz. Stimulus-response relations revealed a very limited capacity of individual SCT neurons to signal, in a graded way, the intensity parameter of the vibrotactile stimulus. Furthermore, because of their inability to respond on a cycle-by-cycle pattern at vibration frequencies >5-10 Hz, these neurons were unable to provide any useful signal of vibration frequency beyond the very narrow bandwidth of approximately 5-10 Hz. Similar limitations were observed in the responsiveness of these neurons to repetitive forms of antidromic and transynaptic inputs generated by electrical stimulation of the spinal cord. In summary, the observed limitations on the vibrotactile bandwidth of SCT neurons and on the precision and fidelity of their temporal signaling, suggest that SCT neurons could serve as little more than coarse event detectors in tactile sensibility, in contrast to DCN neurons the bandwidth of vibrotactile responsiveness of which may extend beyond 400 Hz and is therefore broader by approximately 40-50 times than that of SCT neurons.

Vibrotactile frequency discrimination in human hairy skin

The human capacity for vibrotactile frequency discrimination has been compared directly for glabrous and hairy skin regions by means of a two-alternative, forced-choice psychophysical procedure in five subjects. Sinusoidal vibratory stimuli, delivered by means of a 4-mm-diam probe, were first used to obtain detection threshold values for the two skin sites, the finger tip and the dorsal forearm, at four standard frequencies, 20, 50, 100, and 200 Hz. Values confirmed previous results showing detection thresholds were markedly higher on hairy skin than on glabrous skin. For the discrimination task, each standard frequency, at an amplitude four times detection threshold, was paired with a series of comparison frequencies, and discrimination capacity then was quantified by deriving from psychometric function curves, measures of the discriminable frequency increment (Deltaf) and the Weber Fraction (Deltaf/f), which, when plotted as a function of the four standard frequencies, revealed similar capacities for frequency discrimination at the two skin sites at the standard frequencies of 20, 100, and 200 Hz but an equivocal difference at 50 Hz. Cutaneous local anesthesia produced a marked impairment in vibrotactile detection and discrimination at the low standard frequencies of 20 and 50 Hz but little effect at higher frequencies. In summary, the results reveal, first, a striking similarity in vibrotactile discriminative performance in hairy and glabrous skin despite marked differences in detection thresholds for the two sites, and, second, the results confirm that vibrotactile detection and discrimination in hairy skin depend on superficial receptors at low frequencies but depend on deep, probably Pacinian corpuscle, receptors for high frequencies.

Transmission security for single kinesthetic afferent fibers of joint origin and their target cuneate neurons in the cat

Transmission between single identified, kinesthetic afferent fibers of joint origin and their central target neurons of the cuneate nucleus was examined in anesthetized cats by means of paired electrophysiological recording. Fifty-three wrist joint afferent-cuneate neuron pairs were isolated in which the single joint afferent fiber exerted suprathreshold excitatory actions on the target cuneate neuron. For each pair, the minimum kinesthetic input, a single spike, was sufficient to generate cuneate spike output, often amplified as a pair or burst of spikes, particularly at input rates up to 50-100 impulses per second. The high security was confirmed quantitatively by construction of stimulus-response relationships and calculation of transmission security measures in response to both static and dynamic vibrokinesthetic disturbances applied to the joint capsule. Graded stimulus-response relationships demonstrated that the output for this synaptic connection between single joint afferents and cuneate neurons could provide a sensitive indicator of the strength of joint capsule stimuli. The transmission security measures, calculated as the proportion of joint afferent spikes that generated cuneate spike output, were high (>85-90%) even at afferent fiber discharge rates up to 100-200 impulses per second. Furthermore, tight phase locking in the cuneate responses to vibratory stimulation of the joint capsule demonstrated that the synaptic linkage preserved, with a high level of fidelity, the temporal information about dynamic kinesthetic perturbations that affected the joint. The present study establishes that single kinesthetic afferents of joint origin display a capacity similar to that of tactile afferent fibers for exerting potent synaptic actions on central target neurons of the major ascending kinesthetic sensory pathway.

Transmission security for single kinesthetic afferent fibers of joint origin and their target cuneate neurons in the cat

Transmission between single identified, kinesthetic afferent fibers of joint origin and their central target neurons of the cuneate nucleus was examined in anesthetized cats by means of paired electrophysiological recording. Fifty-three wrist joint afferent-cuneate neuron pairs were isolated in which the single joint afferent fiber exerted suprathreshold excitatory actions on the target cuneate neuron. For each pair, the minimum kinesthetic input, a single spike, was sufficient to generate cuneate spike output, often amplified as a pair or burst of spikes, particularly at input rates up to 50-100 impulses per second. The high security was confirmed quantitatively by construction of stimulus-response relationships and calculation of transmission security measures in response to both static and dynamic vibrokinesthetic disturbances applied to the joint capsule. Graded stimulus-response relationships demonstrated that the output for this synaptic connection between single joint afferents and cuneate neurons could provide a sensitive indicator of the strength of joint capsule stimuli. The transmission security measures, calculated as the proportion of joint afferent spikes that generated cuneate spike output, were high (>85-90%) even at afferent fiber discharge rates up to 100-200 impulses per second. Furthermore, tight phase locking in the cuneate responses to vibratory stimulation of the joint capsule demonstrated that the synaptic linkage preserved, with a high level of fidelity, the temporal information about dynamic kinesthetic perturbations that affected the joint. The present study establishes that single kinesthetic afferents of joint origin display a capacity similar to that of tactile afferent fibers for exerting potent synaptic actions on central target neurons of the major ascending kinesthetic sensory pathway.

Modeling population responses of rapidly-adapting mechanoreceptive fibers

The population response of rapidly-adapting (RA) fibers is one component of the physiological substrate of the sense of touch. Herein, we describe a computational scheme based on the population-response model by K.O. Johnson (J. Neurophysiol. 37: 48-72, 1974) which we extended by permitting the capability to include the spatial distributions of receptors in the glabrous skin linked to RA fibers. The hypothetical cases simulated were rectangular, uniformly random and proximo-distally Gaussian distributions. Each spatial organization produced qualitatively distinct population-response profiles that also varied due to stimulus parameters. The effects of stimulus amplitude, average innervation density and contactor-probe location were studied by considering various response measures: number of active fibers, summated firing rate and the average firing rate of a subset of the modeled population. The outcome of the measures were statistically compared among simulated anatomical distributions. The response is the same for rectangular and uniformly random distributions, both of which have a homogeneous innervation density. However, the Gaussian distribution produced statistically different responses when the measure was not averaged over the subset population which represented the receptive field of a higher-order neuron. These results indicate that, as well as stimulus parameters, the anatomical organization is a significant determinant of the population response. Therefore, reconstructing population activity for testing psychophysical hypotheses must presently be done with care until the organization of the receptors within the skin has been clarified.

Interaction of tactile input in the human primary and secondary somatosensory cortex--a magnetoencephalographic study

Interaction of simultaneous tactile input at two finger sites in primary (SI) and secondary somatosensory cortex (SII) was studied by whole-head magnetoencephalography. Short pressure pulses were delivered to fingers of the right and left hand at an interstimulus interval of 1.6 s. The first phalanx of the left digit 1 and four other sites were stimulated either separately or simultaneously. We compared four sites with increasing distance: the second phalanx of left digit 1, left digit 5, and digits 1 and 5 of the right hand. The temporal evolution of source activity in the contralateral SI and bilateral SII was calculated using spatiotemporal source analysis. Interaction was assessed by comparing the source activity during simultaneous stimulation with the sum of the source activities elicited by separate stimulation. Significant suppressive interaction was observed in contralateral SI only for stimuli at the same hand, decreasing with distance. In SII, all digits of the same and the opposite hand interacted significantly with left digit 1. When stimulating bilaterally, SII source waveforms closely resembled the time course of the response to separate stimulation of the opposite hand. Thus, in bilateral simultaneous stimulation, the contralateral input arriving first in SII appeared to inhibit the later ipsilateral input. Similarly, the separate response to input at two unilateral finger sites which arrived slightly earlier in SII dominated the simultaneous response. Our results confirm previous findings of considerable overlap in the cortical hand representation in SII and illustrate hemispheric specialization to contralateral input when simultaneous stimuli occur bilaterally.

Characterization of tactile afferent fibers in the hand of the marmoset monkey

The marmoset monkey, Callithrix jacchus, has increasingly been the subject of experiments for the analysis of somatosensory system function in simian primates. However, as response properties of the mechanoreceptive afferent fibers supplying the skin have not been characterized for this primate, the present study was undertaken to classify fibers innervating the glabrous skin of the marmoset hand and determine whether they resembled those described for other mammalian species, including cat, macaque monkey, and human subjects. Forty-seven tactile afferent fibers with receptive fields (RFs) on the glabrous skin of the hand were isolated in fine median and ulnar nerve strands. Controlled tactile stimuli, including static indentation and skin vibration, were used to classify fibers. Twenty-six (55%) responded to static indentation in a sustained manner and were designated slowly adapting (SA) fibers, while 21 (45%) were selectively sensitive to the dynamic components of the stimulus. The SA fibers had well-defined boundaries to their RFs, lacked spontaneous activity in most cases (23/26 fibers), had an irregular pattern of discharge to static skin indentation, and displayed graded response levels as a function of indentation amplitude, attributes that were consistent with the properties of slowly adapting type I (SAI) fibers described in other species. The dynamically sensitive afferent fibers could be subdivided into two distinct functional classes, based on their responses to vibrotactile stimulation. The majority (15/21) responded best to lower frequency vibration (~10-50 Hz) and had small RFs, whereas the second class responded preferentially to higher frequency vibration (50-700 Hz) with maximal sensitivity at ~200-300 Hz. These two classes resembled, respectively, the rapidly adapting (RA) and Pacinian corpuscle-related (PC) fiber classes found in other species, and like them, responded to vibration with tightly phase-locked patterns of response over a wide range of frequencies. The results demonstrate that the functional classes of tactile afferent fibers that supply the glabrous skin in the marmoset monkey appear to correspond with those described previously for the cat and macaque monkey, and are similar to those supplying the human hand and fingers, although the SA fibers in the human hand appear to fall into two classes, the SAI and SAII fibers. With the increasing use of the marmoset monkey as a primate model for somatosensory system studies, these data now allow tactile neurons identified at central locations, such as the cerebral cortex and thalamus, to be classified in relation to inputs from the peripheral classes identified in the present study.

Spatial and cortical influences exerted on cuneothalamic and thalamocortical neurons of the cat

This work aimed to study the responses of cuneothalamic and thalamocortical cells to electrical stimulation of the body surface in alpha-chloralose-anaesthetized cats. It was found that both classes of cells had a central excitatory receptive field, an edge overlapping the field centre whose stimulation elicited inhibitory-excitatory (cuneothalamic cells) and excitatory-inhibitory (thalamocortical cells) sequences, and a surrounding or peripheral area usually being inhibitory. Manipulating the descending corticofugal activity by removing the fronto-parietal cortex, electrical stimulation, or by placing picrotoxin or muscimol over the sensorimotor cortex demonstrated that the cortical feedback potentiated effects driven from the field centre and the surround. In particular this potentiated centre-driven excitation and surround-driven inhibition, but some of the data points to more complex patterns. The inhibition elicited in cuneothalamic cells from the edge and the surround of the field was faster than the excitation induced from the field centre. Effects at the edge of the field centre included late excitatory responses relayed via the cerebral cortex. There were also direct corticofugal excitatory inputs to the field centre. Excitatory surrounds were occasionally observed, the assumption being that in most cases these were suppressed by the enhanced inhibition driven from the cortex. The data indicate that the cortico-subcortical feedback contributes not only to enhance the surround antagonism of a centre response but also to increase the time resolution of thalamic and cuneate relay somesthetic neurons.

The effects of neonatal median nerve injury on the responsiveness of tactile neurones within the cuneate nucleus of the cat

1. The capacity of cuneate neurones to attain normal functional properties following neonatal median nerve injury was investigated with single neurone recording in anaesthetized cats, 12-24 months subsequent to a controlled crush injury. Effectiveness of the peripheral nerve injury was confirmed by the abolition of the median nerve compound action potential following the crush. 2. Cuneate recording was carried out after denervation of the forearm, apart from the median nerve, to ensure that neurones studied had receptive fields within the distribution zone of the regenerated median nerve. Controlled and reproducible tactile stimuli were used to evaluate the functional capacities of neurones to determine whether they were consistent with those reported earlier for cuneate neurones in cats that had normal peripheral nerve development. 3. Twenty-two cuneate neurones with well-defined tactile receptive fields within the distribution zone of the regenerated median nerve were classified according to their adaptation characteristics and functional properties. Slowly adapting neurones responded throughout static skin indentations and had graded and approximately linear stimulus-response relations over indentation ranges up to 1.5 mm. Rapidly adapting neurones responded to the dynamic phases of skin indentations and could be divided into two broad classes, one most sensitive to vibrotactile stimuli at 200-400 Hz which appeared to receive a predominant input from Pacinian corpuscle receptors, and a non-Pacinian group that included neurones most sensitive to skin vibration at 5-50 Hz which appeared to receive glabrous skin input from the rapidly adapting class of afferent fibres. 4. Based on the stimulus-response relations and on measures of phase locking in the responses to vibrotactile stimuli, it appears that the functional properties of cuneate neurones activated from the field of a regenerated median nerve subsequent to a neonatal nerve crush injury were consistent with those reported previously for 'control' cuneate neurones. The results indicate that cuneate neurones can acquire normal tactile coding capacities despite the disruption caused by prior crush injury to their peripheral nerve source.

Effects of a dorsal column lesion on temporal processing within the somatosensory system of primates

A dorsal column (DC) lesion has lasting effects on behavioral tasks that require temporal processing of tactile information (e.g., frequency and duration discrimination). The present experiments describe physiological correlates of these deficits in temporal discrimination. Compound action potentials evoked by electrocutaneous stimulation were recorded from the major white matter subdivisions of the spinal cord in anesthetized monkeys, and relationships between stimulation frequency and evoked potential (EP) amplitude were determined for the ascending pathways. At 10 pulses per second (Hz) EPs recorded in the lateral spinal columns were attenuated slightly (by 15% or less, relative to 1.5 Hz), whereas potentials recorded from the DCs were not attenuated. The attenuation increased with stimulation frequencies up to 50 Hz, reaching 80% for the anterolateral column and 38% for the dorsolateral column, but only 15% for the DC. Epidural EPs were recorded, before and after interruption of the contralateral DC, from awake animals with electrodes chronically implanted over primary somatosensory cortex (SI). Following the lesion. EP responses to 1.5-Hz stimulation were 46% of preoperative responses to the same stimulus. At 10 Hz, EP amplitudes were attenuated even more, to 27% of the preoperative amplitude at 1.5 Hz. Principal components analysis was employed to quantify alterations in EP conformation and stimulus frequency was varied from 1.5 to 10 Hz, before and after a DC lesion. Interruption of the DC resulted in a significant decrease in the information provided by the EP about changes in stimulus frequency. EPs were also recorded from different locations along the anterior-posterior dimension of the hindlimb region of SI in lightly anesthetized animals. Principal components analysis revealed that there was less information present in the EP about changes in stimulus frequency (1.5-10 Hz) at all recording locations in animals with a DC lesion, compared with the cortex of normal animals. The DC lesion significantly decreased the amplitude of cortical EPs evoked by repetitive stimulation. At 10 Hz the EP was nearly buried in noise, consistent with behavioral deficits in discrimination of the duration of 10 Hz stimulation following interruption of the DC. Also, significantly less information was present in the cortical EPs about changes in stimulus frequency in the absence of intact DCs, which is consistent with deficits in frequency discrimination. This reduction could be explained in part by a lesser capacity of spinal pathways in the lateral column to follow repetitive stimulation above 10 Hz. However, more rostral manifestations of a DC lesion, at either the thalamus or the cortex, are likely to contribute to the reduced capacity of animals with DC lesions to make temporal discriminations.

The immediate effects of peripheral deafferentation on neurons of the cuneate nucleus in raccoons

Single-unit recordings were obtained from 42 neurons in the cuneate nucleus of 12 anesthetized raccoons. All neurons had receptive fields on the glabrous skin of a forepaw digit. Temporary removal of the dominant excitatory input to a neuron, by injection of lidocaine into the base of the digit, did not result in any expansion of the excitatory receptive field onto adjacent, "off-focus" digits. Similarly, the responses evoked from the off-focus digits by electrical stimulation, which had a longer latency and a higher threshold, were not improved during the lidocaine block. Inhibition was produced in the majority of neurons by high-intensity mechanical stimulation of the off-focus digits, but this was also unchanged when the dominant excitatory input to the neurons was blocked. Since this from of inhibition is not apparent in the somatosensory thalamus before denervation, the spontaneous activity of thalamic neurons must be controlled by inputs other than the cuneate nucleus. These results also indicate that the long-term reorganization seen in the thalamus and cortex is not attributable to a simple unmasking of connections from the adjacent digits within the cuneate nucleus, but may involve strengthening of the connections responsible for longer-latency responses. The only significant change induced in cuneate neurons by temporary denervation was a decrease in the firing rates of 69% of the neurons that had spontaneous activity. Since it is unlikely that any of the large-diameter afferents from touch receptors can account for this finding, mechanically insensitive afferent fibers from the digit may contribute to the spontaneous activity of cuneate neurons, either directly or via a relay in the spinal cord.

Perceived intensity of vibrotactile stimuli: the role of mechanoreceptive channels

The perceived intensity of vibrotactile stimuli was studied by means of free magnitude estimation. Eighty different sinusoidal stimuli ranging in frequency from 10 to 200 Hz, and in amplitude from 2.4 to 154 microns, were presented to the left index fingerpad of psychophysical observers through a 5-mm-diameter contactor. Estimates at a given frequency increased with amplitude in all four subjects, and estimates at a given amplitude increased with frequency in three. For the fourth subject, however, intermediate frequencies (25-75 Hz) produced the most intense sensations; the relative sensory effectiveness of different frequencies suggested that in her case, perceived vibrotactile intensity was determined largely by signals in Meissner afferents. From the data of this unusual subject, and from high-frequency (200-Hz) measurements on the normal subjects, quantitative descriptions were derived of the signals in Meissner and Pacinian channels, respectively, that could contribute to subjective intensity. Candidate algorithms by which the signals from the two channels might interact were then evaluated by comparison of modeled and empirically determined subjective intensity values. It was found that subjective intensity is given by the sum of (1) the stronger of the two channels' signals, and (2) half the weaker signal, the latter apparently being reduced by cross-channel suppression that occurs only at suprathreshold levels. Adapting to 25-Hz vibration selectively reduces the perceived intensity of low frequencies, whereas adapting to 200-Hz vibration has a corresponding effect at high frequencies. It is concluded that an understanding of perceived vibrotactile intensity requires knowledge of the signals in vibrotactile channels, and of the interactions between those channels.

Responses of cat ventroposterolateral thalamic neurons to vibrotactile stimulation of forelimb footpads

Responses of neurons in the ventroposterolateral nucleus of the thalamus to vibration applied to the forelimb footpads were analyzed in anesthetized cats in order to describe the signalling properties of thalamic neurons that received input from the different classes of tactile afferents innervating the glabrous skin of the distal forelimb. Seventy-six thalamic neurons, the majority of which (60 of 76) were positively identified as thalamocortical projection neurons, were classified into two broad groups according to their responses to 1-s step indentations of the skin. A minority (24%) comprised neurons that had slowly adapting (SA) responses, whereas the remainder (76%), the dynamically sensitive neurons, had transient responses to the onset and offset phase of the step and were further classified according to their sensitivity to cutaneous vibrotactile stimuli into those activated by low-frequency vibration (rapidly adapting, RA, neurons) and those activated by high frequencies (Pacinian afferent, PC, neurons). Thalamic RA neurons displayed phaselocked responses to vibration at frequencies up to approximately 100 Hz, while PC neurons displayed phaselocked responses to vibration up to 400-500 Hz. Thalamic SA neurons varied in their responses to vibrotactile stimuli; half were most sensitive to vibration frequencies of 50 Hz or less, while the others responded over a broader range of frequencies. Although three major classes of footpad-related thalamic neurons were identified, there was evidence of convergent input to a small proportion of them. The study demonstrates that thalamic neurons have the capacity for responding to cutaneous vibration with phaselocked, patterned impulse trains, which would enable them to encode information about vibrotactile frequencies up to approximately 300 Hz.

Input-output relation of the somatosensory system for mechanical air-puff stimulation of the index finger in man

This study examined input-output relation of the somatosensory system in response to mechanical air-puff stimuli applied to the volar aspect of the tip of the index finger. Compound sensory nerve action potentials (SNAPs) from the median nerve at the wrist and cerebral somatosensory evoked potentials (SEPs) were simultaneously recorded at six levels of stimulus intensity above threshold. Using the time-integral of the SNAPs and SEPs as measures of peripheral and central neural activity, a strongly accelerating power function with an exponent of 1.35 was found to describe peripheral neural function, while central neural function was described by a negatively accelerating function with a power exponent of 0.50, suggesting suppression of SEPs by recruitment of units with increasing stimulus intensity. It was concluded that input-output relation of the somatosensory system can be described by a decelerating power function with the exponent of 0.37.

Physiological characteristics of responses of wide dynamic range spinal neurones to cutaneously applied vibration in the cat

Extracellular single-unit recordings were made from wide dynamic range neurones in the lumbar dorsal horn of anaesthetized or decerebrated cats. Vibration applied to the skin at a frequency of 80 Hz could evoke 3 distinct types of response--excitation, depression or a biphasic response consisting of excitation followed by depression. By applying vibration at different sites, a given neurone was found to show more than one type of response. Parametric studies of the depressant and biphasic responses were made because previous studies indicated that adenosine mediates the depression in these types of response. Thus, amplitude- and frequency-response relationships were determined at individual stimulation sites: amplitude was varied from 0.001 to 1.0 mm (frequency, 80 Hz) and the frequencies studied were 10, 20, 40, 80, 120 and 240 Hz (amplitude, 0.15 mm). Vibration at amplitudes greater than 0.15 mm caused a decrease in the rate of discharge during the period of stimulation, the magnitude of this decrease varying directly with amplitude; at amplitudes of 0.15 mm and less vibration had no statistically significant effect. With regard to the frequency-response relationship, a decrease in discharge rate occurred at frequencies of 120 and 240 Hz, with the more pronounced effect at 240 Hz; excitation occurred at 40 Hz and there was no statistically significant effect at other frequencies. Amplitude- and frequency-response relationships for the depressant and the biphasic responses were analyzed separately. In the case of depressant responses, the magnitude was monotonically related to the amplitude of stimulation and depression occurred only at frequencies of 80 Hz or greater, with higher frequencies being more effective. The biphasic responses appeared to consist of 2 subtypes termed biphasic-1 and biphasic-2 responses. For biphasic-1 responses, the amplitude- and frequency-response curves were similar to those of depressant responses. Biphasic-2 responses differed in that the response was biphasic when the stimulation frequency was 80 Hz or greater and the amplitude was 0.3 mm or more, yet, at lower frequencies and/or amplitudes vibration evoked excitation. The similarities in the amplitude- and frequency-response relationships of depressant and biphasic-1 responses raise the possibility that these responses might be mediated by a single class of primary afferent. Both depressant and biphasic responses were evoked when stimulation parameters (2 microns, 240 Hz) were used which selectively activate Pacinian corpuscle afferents. Depression with 240-Hz stimulation was attenuated by administration of caffeine (60 mg/kg i.v.) suggesting that the depressant and biphasic-1 responses may be mediated by afferents from Pacinian corpuscles.

Gating of tactile input from the hand. I. Effects of finger movement

Intraneural microstimulation within the median nerve of alert healthy subjects was used to evoke tactile sensations at threshold for conscious detection. The effect of movement on these sensations was studied by asking the subjects to estimate their magnitude before, during and after movement of the appropriate finger at different speeds. It was found that sensations of flutter and pressure were both attenuated by movement, as was the magnitude of spontaneous paraesthesiae. The degree of sensory inhibition correlated positively with speed of movement and was comparable to the previously reported reduction in cortical somatosensory evoked potentials by movement, using suprathreshold stimuli. These results indicate that (i) movement inhibits tactile sensations of different qualities, (ii) such inhibition is velocity-dependent, and (iii) threshold sensations are amenable to central modulation short of their abolition. It is likely that the mechanisms of inhibition of exteroceptive inputs during movement are contingent upon the character of the sensory stimulus and the nature of the motor task.

Spatial distribution of pre- and postsynaptic sites of axon terminals in the dorsal horn of the frog spinal cord

Axon terminals which could be interpreted as dorsal root boutons, were photographed from a series of 98 ultrathin sections with a Jeol 100B electron microscope. A total of 13 boutons were recovered for computer reconstruction. Two of them were terminal boutons, eight en passant boutons and three boutons were only partially recovered. All boutons contained multiple synaptic sites (maximum 33 and minimum seven) at which axodendritic and axoaxonic synapses were established. Axodendritic synapses were of the asymmetric type and they were directed toward adjacent dendrites. In axoaxonic synapses, which were of the symmetric type, the boutons were invariably on the postsynaptic side. Among the presynaptic profiles axons with spherical and pleomorphic vesicles and dendrites with flattened vesicles could be discerned. On average, each 2.67-microns2 bouton surface area contained one presynaptic site at which an axodendritic synapse was established, and each 7-microns2 surface area contained one postsynaptic site for an axoaxonic (or dendroaxonic) contact. A tendency of grouping of synaptic sites was observed. Distance measurements between the closest neighbours of all synaptic sites were made in four combinations in boutons with the original and with a random distribution of synaptic sites. The arithmetic mean of distances measured between the presynaptic and the closest postsynaptic sites was almost twice as big as that measured in the reverse direction. The difference between these values became greatly reduced in the case of random distribution. The arithmetic mean of distances between the closest neighbours of presynaptic sites was about the same as that between the closest neighbours of postsynaptic sites. This latter value was considerably increased with randomly distributed synaptic sites. The results suggest a non-random distribution of synaptic sites on the surface of boutons. The analysis of cluster formation of synaptic sites performed with a numerical taxonomy technique revealed that the majority of the 153 synaptic sites were comprised in 27 clusters containing both pre- and postsynaptic sites within the 1-micron similarity level. All postsynaptic sites were within 1 micron of one or more presynaptic sites. On the basis of the assumption that the postsynaptic sites are occupied by inhibitory axoaxonic synapses, it is suggested that the transmitter release from the presynaptic sites can be individually controlled in this structural arrangement. A probable mechanism of this function may be the passive invasion of the bouton by the impulse propagating actively along the dorsal root fibre.

Cutaneous excitatory and inhibitory input to neurones of the postsynaptic dorsal column system in the cat

1. In chloralose-anaesthetized cats single-unit microelectrode recordings were made from axons in the dorsal columns, at the lumbar level, identified as belonging to the postsynaptic dorsal column (PSDC) system. 2. Excitatory and inhibitory receptive field arrangements of a sample of seventy-five PSDC neurones were examined in detail using natural cutaneous stimuli. 3. The sample was characterized by a high degree of convergent input: 80% of units were activated by both light tactile and noxious mechanical stimuli and more than half of those examined were excited by noxious radiant heat. In addition, three-quarters of the units had inhibitory receptive fields on the ipsilateral limb. 4. Twenty-three units (27%) were influenced by input from areas of both hairy and glabrous skin covering the foot and distal limb. Neurones in this group had complex receptive fields, many of which occupied several discontinuous areas of skin. Background and evoked activity of these units could frequently be inhibited by light tactile and/or noxious stimuli. Their inhibitory receptive fields occupied small areas of skin overlapping or adjacent to excitatory fields. 5. Fifty-two units (73%) had receptive fields restricted to areas of hairy skin on the thigh and upper hindlimb. Half the units in this group had coextensive low- and high-threshold excitatory areas but about one-third had a concentric receptive field organization; a high-threshold excitatory component extending beyond, or around, a central low-threshold area. The discharge of these units could be inhibited only by light tactile stimuli. Their inhibitory receptive fields covered extensive areas of skin, sometimes completely surrounding the excitatory field. 6. The complex receptive field arrangements observed for neurones of the postsynaptic dorsal column system are discussed in relation to previous observations on dorsal horn neurones of other ascending tracts.

Quantitative analysis of laryngeal mechanosensitivity in the cat and rabbit

1. Single afferent fibres in the internal branch of the superior laryngeal nerve which responded to light touch or gentle probing of discrete areas of the exposed epithelium of the opened larynx were identified in anaesthetized, paralysed cats (148 fibres) and rabbits (58 fibres). 2. A quantitative examination of the sensitivity of these laryngeal mechanoreceptors to both static (step indentations) and dynamic (vibratory) forms of mechanical stimulation was undertaken using a servo-controlled mechanical stimulator. 3. In both species two predominant classes of mechanoreceptors were observed (Boushey, Richardson, Widdicombe & Wise, 1974). One class was distinguished by a regular and continuous pattern of activity at a frequency of 10-70 Hz (tonic fibres, sixty-six in cat, thirty-five in rabbit). The other class was silent or (more rarely) irregularly active at a very low frequency (silent fibres, eighty-two in cat, twenty-three in rabbit). 4. The location of the receptive fields was determined by manual probing. Inter-species and regional variations in receptive field location were observed for the two fibre groups. 5. Conduction velocity was measured for twenty-one tonic and seven silent fibres in the rabbit by a pre-triggered averaging technique. The results obtained (tonic: range 10.8-30.0, mean +/- S.E. of mean 21.4 +/- 1.2 m/s; silent: 14.8-28.6, 20.4 +/- 1.8 m/s) were characteristic of group III afferent fibres but were not significantly different for the two classes. 6. Both classes of receptor showed a response at the onset of a step indentation of the region of the mucosa that corresponded to their receptive field. Subsequent to this brief initial response the behaviour of the two classes diverged markedly. Tonic fibres were invariably slowly adapting whereas most (forty-four out of fifty-five in cat; twenty-two out of twenty-three in rabbit) silent fibres were rapidly adapting, at least for smaller indentation amplitudes. 7. Receptors of both classes were readily entrained to discharge at the same frequency as the probe stimulator (1:1 entrainment) when this was made to vibrate upon the receptive area for test periods of 0.5 or 1.0 s. Tuning curves were constructed of the minimum amplitudes required to elicit 1:1 entrainment throughout an entire test period at various frequencies. 8. Individual fibres in the two classes could be entrained at frequencies up to 400 Hz or more at sensitive (e.g. less than 100 microns) vibratory amplitudes. However, all fibres were less sensitive at these higher frequencies than at some lower point on the frequency scale.(ABSTRACT TRUNCATED AT 400 WORDS)

Temporal patterning in the responses of gracile and cuneate neurones in the cat to cutaneous vibration

1. Recordings were made in decerebrate cats from gracile and cuneate neurones responding to vibration-induced inputs from Pacinian corpuscle (P.c.) receptors of the hind-limb and forelimb footpads. The two groups of neurones were compared, in particular for their capacities for responding to cutaneous vibration with phase-locked impulse patterns. 2. In both nuclei the P.c. neurones were most sensitive to vibration in the range 80 to greater than 600 Hz. Stimulus-response relations were similar for the two groups, as were measures derived from these relations such as response levels, absolute thresholds and the dynamic range (defined as the vibration amplitude range over which responses were graded). 3. At frequencies up to 300-400 Hz, responses for some neurones in both nuclei remained well phase locked to the vibration; however, quantitative analysis using a factorial analysis of variance indicated that the phase locking was poorer in gracile than cuneate neurones. 4. In both nuclei there was marked variability from neurone to neurone in measures of phase locking which may reflect variations in the extent of convergence of P.c. fibres upon different target neurones. For neurones in either nucleus that had comparatively tight phase locking of responses to vibration it is proposed that their output is functionally dominated by one or a few of their convergent P.c. input fibres.

Integrative processing of vibratory information in cat dorsal column nuclei neurones driven by identified sensory fibres

1. In decerebrate or anaesthetized cats, the vibration-induced responses of dorsal column nuclei neurones were examined, first, when their input came from simultaneously recorded pairs or other combinations of identified Pacinian corpuscle (P.c.) afferent fibres of the interosseous nerve, and secondly, when different convergent sets of P.c. fibres were engaged by footpad vibration. 2. Suprathreshold actions were observed on individual dorsal column nuclei neurones from two or more identified P.c. fibres. Recruitment of these convergent fibres usually led to summation in the dorsal column nuclei neurone as reflected in higher response levels compared with those evoked by single-fibre inputs. 3. When the input was increased from one to two or more identified P.c. fibres the dorsal column nuclei neurones could retain a single, dominant phase of response to high-frequency (greater than 100 Hz) vibration even though these fibres, in isolation, evoked responses in the target neurone at substantially different latencies. However, on average, phase locking was significantly tighter in response to single-fibre input than to multiple P.c.-fibre input. 4. Dorsal column nuclei neurones were also able to retain phase-locked responses to high-frequency vibration when phase differences between different convergent inputs were systematically introduced to alter the degree of synchrony in the activity arriving over convergent, identified P.c. fibres. 5. When the input to dorsal column nuclei neurones came from the skin it was found that with the recruitment of two converging sets of P.c. fibres the dorsal column nuclei neurones were able to retain phase-locked responses to high-frequency vibration even when phase shifts were introduced between the two sets of P.c. inputs. 6. In conclusion, the observed integrative processing by dorsal column nuclei neurones of vibration-induced inputs arriving over identified, convergent P.c. fibres, or sets of P.c. fibres, is consistent with our hypothesis that the retention of phase-locked responses to vibration at frequencies greater than or equal to 100 Hz may reflect the functional domination of the target neurone by just one or a few of its convergent input fibres.

Actions of single sensory fibres on cat dorsal column nuclei neurones: vibratory signalling in a one-to-one linkage

1. The synaptic linkage between single, identified sensory fibres associated with Pacinian corpuscle (P.c.) receptors and central neurones of the dorsal column nuclei was examined in decerebrate or anaesthetized cats. Paired recordings were made from individual neurones in the gracile division of the dorsal column nuclei and from the hind-limb interosseous nerve in which it is possible to identify and monitor the activity of each P.c. fibre activated when recording from the intact nerve with a platinum hook electrode. Individual P.c. fibres were activated by vibration delivered with an 0.2 mm diameter probe to the interosseous P.c. receptors. 2. Thirty-five P.c. fibre-gracile neurone pairs were isolated in which activity in the single, identified P.c. fibre evoked suprathreshold responses (mean latency +/- S.D., 10.3 +/- 1.5 ms) in the gracile neurone. A single impulse arriving over one P.c. fibre could generate pairs or triplets of output spikes from several target neurones thus revealing a potent synaptic organization within the dorsal column nuclei for the transmission and amplification of weak sensory signals. 3. The potency of the linkage for some pairs resulted in post-synaptic response levels of up to 400 impulses s-1 when a single input fibre was discharging one impulse on each vibration cycle at 200-400 Hz. 4. Gracile neurones driven by single P.c. fibres had phase-locked responses to vibration at frequencies of up to 400-500 Hz. However, the responses displayed much greater phase dispersion than those of P.c. fibres, indicating that a major component of phase dispersion in the vibration-induced responses of dorsal column nuclei neurones is attributable to the properties of the synaptic linkage between an individual fibre and the target neurone. 5. The potent actions of single, identified P.c. fibres on their target neurones are consistent with the hypothesis that phase-locked responses in dorsal column nuclei neurones to vibration at 100-400 Hz may reflect the functional domination of the target neurone's output by one or a few of its converging fibres.

Effect of mechanical stimulus spread across glabrous skin of raccoon and squirrel monkey hand on tactile primary afferent fiber discharge

The role of spread of skin deformation in activating cutaneous mechanoreceptors at a distance from their threshold receptive fields (RFs) was examined in glabrous skin of the North American raccoon and the squirrel monkey. One feedback-controlled mechanical stimulus probe was used to indent the skin to a controlled depth at a constant velocity, at varying distances from a second probe, which was used to monitor vertical displacement depth and velocity at this distant site. In many instances, the monitor probe was positioned over the RF of a cutaneous mechanoreceptor, and single-unit action potentials were simultaneously recorded from individual fibers of the median or ulnar nerve. With distance from the site of stimulation, there was a systematic, monotonic decline in indentation depth and velocity; velocity fell off with distance more rapidly than depth. The degree of diminution with distance varied with the size, shape, and curvature of the digital or palm pad stimulated. Spread of indentation was more restricted on digital than on palm pads, and was more restricted across monkey skin than across raccoon skin. Spread was less with higher-velocity than with lower-velocity indentations, but was seemingly unaffected by indentation depth. As expected from the findings noted above, the number of spikes discharged by slowly adapting mechanoreceptive afferent fibers declined more rapidly with distance between stimulus site and RF for digital than for palmar RFs, in squirrel monkey than in raccoon skin, and with higher-velocity than with lower-velocity stimuli. Furthermore, the number of spikes occurring during either ramp or early static indentation phases of stimulation dropped to zero more rapidly with distance than did either vertical indentation depth or velocity. Decreases with distance in both indentation depth and velocity acted to restrict the size of suprathreshold RFs. For most units, horizontal components of mechanical stimulation subtracted from the effects of vertical components. It is suggested, on the basis of this and other studies, that many neural and perceptual phenomena usually attributed to central mechanisms of afferent inhibition may be attributable, at least in part, to mechanical properties of the skin. In addition, the present data suggest that regional variations in the two-point limen may be associated with variations in spread of mechanical deformation. The conclusion that glabrous skin and subjacent soft tissues act as a low-pass filter system provides a mechanical basis for the relative efficacy of high-frequency vibratory stimuli in tactile pattern perception.(ABSTRACT TRUNCATED AT 400 WORDS)

Vibrotactile masking effects on airpuff-elicited sensations vary with skin region in the human hand

Inhibitory interactions between two tactile signals take place predominantly within mechanoreceptive submodality channels. This finding was utilized in the present study to determine the mechanoreceptive channels contributing to tactile sensations elicited by brief airpuff stimuli applied to the hairy and glabrous skin of the human hand. A reaction time paradigm was used to estimate the sensitivity of four subjects to airpuffs without and during continuous vibration (masker) of low (30 Hz) or high (240 Hz) frequency. The sensitivity to airpuffs (test stimuli) was decreased by a low-frequency masker in the hairy skin and by low- and especially by high-frequency maskers in the glabrous skin. The masking effect was enhanced in both skin areas by increasing the intensity of the masker and by decreasing the intensity of the test stimulus. The results suggest that the mechanisms underlying airpuff-elicited sensations consist of the low-frequency channel in the hairy skin, and of both the low- and high-frequency channels in the glabrous skin.

The pattern of spinal and medullary projections from a cutaneous nerve and a muscle nerve of the forelimb of the cat: a study using the transganglionic transport of HRP

The transport of HRP into the spinal cord and medulla in the cat has been examined from a forelimb cutaneous nerve, the lateral superficial radial nerve (LSR), and from the muscle nerves supplying both heads of the forelimb muscle, extensor carpi radialis (ECR). HRP transported by the LSR was widely distributed in the spinal cord throughout laminae I-IV in the vicinity of the root entry zone and from spinal segments T1 to C5. HRP was also transported from the LSR to the medulla where there was intense patchy, discontinuous labelling in the main cuneate nucleus. The pattern of labelling in the cuneate nucleus did not follow any simple somatotopic plan. Exposure of the muscle nerve to HRP led to labelling in the spinal dorsal horn in lamina I, in the deep dorsal horn on the lamina V/VI border, and in lateral and medial lamina VI at sites that contain cells of origin of spinocerebellar tracts. The medial lamina VI label was contiguous with a deposit that extended medially to the central canal. The label in lateral lamina VI was patchy and formed a discontinuous column from T1 to C5. HRP transported by the muscle nerve also produced label in the more ventral regions of the cuneate nucleus where it had a lacy appearance, in part due to its extensive distribution around dendrites. A relatively dense, patchy, and discontinuous deposit of reaction product was also present in the external cuneate nucleus after muscle nerve exposure. This deposit was most intense on the dorsomedial surface of this nucleus, but another, less intense, deposit was also present ventrally.

Mechanoreceptive submodality channel interactions: single unit analysis of afferent inhibition in the primary somatosensory cortex of the cat

This study examined whether afferent inhibition generated by activation of one mechanoreceptive submodality influences the response of neurons of the other submodality tested. Response properties of quickly adapting hair and Pacinian neurons in primary somatosensory cortex of the cat were evaluated to assess afferent inhibition generated by single-cycle sinusoidal mechanical stimuli. Animals were lightly anesthetized with sodium thiopental. Stimulation at 20 Hz (low-velocity stimulus) was used to activate hair cells, the receptive fields of which were located in the skin; stimulation at 200 Hz (high-velocity stimulus) was used to activate Pacinian cells, the receptive fields of which were located in the deeper tissues. The skin was partially dissected from the deeper tissue in order to uncouple mechanically the effective receptive surfaces and to achieve greater selectivity. Hair and Pacinian cell test responses were paired with single-cycle 20 and 200 Hz conditioning stimuli. A 20 Hz stimulus, more effective in activating hair cells than Pacinian cells, strongly inhibited the test response of hair cells only and a 200 Hz stimulus, which is more effective in activating Pacinian cells than hair cells, markedly inhibited the test response of Pacinian cells only. Our data indicate that afferent inhibition generated by activation of one submodality channel is largely confined to that submodality channel and is not distributed to the other.

Effects of spinal lesions on temporal resolution of cutaneous sensations

In order to test the possibility that the dorsal columns (DCs) contribute to temporal resolution of tactile stimuli, Macaca speciosa monkeys were trained to discriminate different frequencies of stimulation delivered to the glabrous surface of the left foot. Brief (11-msec) pulses of 550-micron indentation from the skin surface were presented for 1 sec at a standard rate of 10 pulses per second (pps), and subsequent trains of the same duration either were replications of the standard or consisted of a higher (comparison) frequency of up to 35 pps. Correct performance consisted of a lever press in response to the comparison stimulus when it occurred as the second or third train in a trial. Signal detection analysis of response tendencies in the second stimulus interval revealed difference threshold values of 2.8 pps, on the average, for 75% correct responding. Transections of the ipsilateral dorsolateral column or the contralateral anterolateral column or both did not significantly affect discrimination of the frequency of cutaneous stimulation. However, following interruption of the ipsilateral DC, two monkeys could not discriminate 10 from 35 pps throughout testing for more than 1 year. These results contrast with a preservation of spatiotactile resolution that has been demonstrated repeatedly for animals following lesions of the DCs or the lateral columns, indicating that a unique function of the DC pathway relates to temporal coding.

Response properties of raccoon cuneothalamic neurons

Microelectrodes were used to record the extracellular activity of 80 single neurons of the main cuneate nucleus (MCN) of raccoons anesthetized with either methoxyflurane or pentobarbital sodium. All 80 MCN neurons had peripheral receptive fields (RFs) that lay entirely on the glabrous surfaces of the forepaw and were responsive to light mechanical stimulation. Neurons were characterized according to the nature of their response to mechanical stimulation of their RFs, as well as to their response to electrical stimulation of the contralateral thalamic ventrobasal complex (VB). All antidromically activated neurons (64% of sample) were histologically verified as falling within the clusters region of the MCN, while synaptically activated neurons (19% of sample), as well as neurons not responsive to VB stimulation (17% of sample), were located in both the clusters and the polymorphic regions. Antidromically activated neurons typically responded with a single fixed-latency spike, although a few responded with a burst of 3 or more spikes. Others responded with a single antidromic spike followed by a train of synaptically activated spikes. In these latter neurons, it was often possible to block the synaptic spikes selectively. MCN neurons were classed according to their response to controlled mechanical stimuli as rapidly adapting (RA), slowly adapting (SA), or Pacinian (Pc). The proportions of neurons falling into these categories did not vary significantly with the type of response to thalamic stimulation, and the overall percentages were 56% RA, 24% SA, and 20% Pc. These figures are very similar to those previously obtained in a sample of primary afferent fibers of the raccoon cervical cuneate fasciculus (L. M. Pubols and Pubols, 1973). Absolute displacement, displacement velocity, and force thresholds, which ranged between 4 and 326 micron, 0.01 and 16.3 micron/msec, and 120 and 3600 mg, respectively, are comparable to those previously found for primary afferents supplying mechanoreceptors of the glabrous surfaces of the raccoon's forepaw. Neither displacement nor force thresholds differed for RA versus SA neurons; however, displacement velocity thresholds were significantly lower for SA than for RA neurons.

Convergence in the somatosensory pathway between cutaneous afferents from the index and middle fingers in man

Average short-latency cerebral potentials were recorded from the parietal scalp to mechanical stimulation of the index and middle fingers and to electrical stimulation of the digital nerves in normal subjects. The early components of the cerebral potential, representing the arrival of the afferent volley at the sensorimotor cortex, were studied during stimulation of the fingers separately and together. When strong or moderate stimuli were used there was a suppressive interaction between the afferent input from the two fingers with either electrical or mechanical stimulation. During simultaneous stimulation of both fingers the size of the early component of the cerebral potential was less than predicted by simple addition of the potentials produced by stimulation of the fingers individually. When very weak stimuli, close to the level necessary for detection by the subject, the input from the two fingers produced additive or facilitatory interactions in the early components of the cerebral potential. These results suggest that there is convergence between the afferent inputs from the index and middle fingers along the somatosensory pathway. At levels of stimulation comparable to those which produced facilitation in the electrophysiological studies, simultaneous stimulation to both fingers was detected significantly more frequently than would be expected from the detection of stimulation to individual fingers.

Interactions between cutaneous and muscle afferent projections to cerebral cortex in man

In order to demonstrate interactions between cutaneous and muscle afferent volleys in the ascending somatosensory pathways, different nerves of the lower limb were stimulated together in a conditioning-test paradigm, the changes in the earliest component of the cerebral potential evoked by the test stimulus being taken to indicate such an interaction. It was first confirmed that the cerebral potential evoked by stimulation of the posterior tibial nerve at the ankle is derived from muscle afferents in the mixed nerve and has shorter latencies than the cerebral potential evoked by purely cutaneous volleys in the sural nerve (see Burke et al. 1981). Complete suppression of the cerebral potential evoked by stimulation of muscle or cutaneous afferents was produced by conditioning volleys in a different nerve or in a different fascicle of the same nerve. The major factors determining the degree of suppression were found to be the relative sizes of the conditioning and test volleys and their timing, rather than whether the volleys were of cutaneous or muscular origin. It is concluded that the transmission of cutaneous or muscle afferent volleys to cortex can be profoundly altered in normal subjects by conditioning activity. The possibility that normal background afferent activity can similarly modify afferent transmission has implications for diagnostic studies, particularly when they are performed under non-standard conditions, such as in the operating theatre or intensive care unit. It is also concluded that, although a subject may perceive cutaneous paraesthesiae when the posterior tibial nerve is stimulated at the ankle, there may be no cutaneous component to the evoked cerebral potential.

Afferent inhibition on various types of cats cuneate neurons induced by dynamic and steady tactile stimuli

Afferent inhibition in several distinct types of cuneate neurons was studied using controlled natural stimuli in 35 lightly anesthetized cats. Mechanoreceptive cuneate neurons were recorded extracellularly with microelectrodes from the middle and caudal divisions of the main nucleus. They were classified into several modality subtypes based on their response to adequate mechanical stimuli. Emphasis was laid on the neurons which had their receptive fields (RFs) in the forepaw. Afferent inhibition was induced by conditioning tactile stimuli in 31 out of 168 neurons (18%) tested. There were particular combinations between the neuron types inhibited and conditioning stimulus modalities. Dynamic stimuli such as high frequency vibration and hair movement by air-jet stimuli applied to areas beyond the excitatory RFs induced inhibition on touch (T), hair (H) and slowly adapting pad (SA) units predominantly in the paw region. In contrast, steady pressure stimulation on the skin adjacent to the excitatory RFs induced inhibition in exclusively slowly adapting neurons receiving afferent inputs from hairy skin such as touch (T), joint (J) and subcutaneous (Deep) units in the paw, elbow and shoulder regions. Most of the inhibitory RFs were organized laterally or eccentrically rather than concentrically around the excitatory RF. Two J units were found to be inhibited by steady pressure applied to the shoulder region of the contralateral forelimb. Functional significance of the intramodality and cross modality inhibition of cuneate neurons is discussed.

Specificity of tactile connections in neonatal cuneate nucleus

In the neonatal kitten, cuneate neurones activated by tactile stimulation of the forelimb foot pads are divisible into 3 functional classes each of which appears to receive its glabrous skin input selectively from a single class of tactile receptors. Their properties suggest that the functionally specific patterns of tactile convergence characteristic of the adult cuneate nucleus are established by the time of birth.

Functional capacities of tactile afferent fibres in neonatal kittens

1. Responses were recorded from individual tactile afferent fibres isolated by microdissection from the median nerve of pentobarbitone-anaesthetized neonatal kittens (1-5 days post-natal age). Experiments were also conducted on adult cats to permit precise comparisons between neonatal and adult fibres.2. Neonatal fibres with receptive fields on the glabrous skin of the foot pads were classified into two broad groups, a slowly adapting class (40%) which responded throughout a 1 sec period of steady indentation and a rapidly adapting or dynamically sensitive class comprising 60% of units. Fibres in these two groups had overlapping conduction velocities in the range 4.3 to 7.5 m/sec and were believed to be the developing Group II afferents of the adult.3. Neonatal slowly adapting fibres qualitatively resembled their adult counter-parts. They displayed graded stimulus-response relations which, over the steepest segment of the curves, had mean slopes of 15.7 impulses/100 mum of indentation. Plateau levels of response were often reached at amplitudes of skin indentation of < 0.5-0.7 mm.4. Dynamically sensitive fibres with receptive fields on the glabrous skin were studied using sinusoidal cutaneous vibration which in the adult enables them to be divided into two distinct classes. However, in the neonate, they formed a continuum whether criteria of sensitivity or responsiveness were used.5. In response to vibration neonatal fibres differed from adult ones according to the following quantitative indices: (i) sensitivity as measured by both absolute thresholds and thresholds for a 1: 1 pattern of response, both of which were higher in the neonate than in the adult at all frequencies > 50 Hz and differed by an order of magnitude at frequencies >/= 200 Hz; (ii) responsiveness based on the mean impulse rate evoked at a fixed amplitude of cutaneous vibration; (iii) band width of vibratory sensitivity which in the neonate was confined to approximately 5-300 Hz whereas in the two classes of adult units it covered the range 5-800 Hz; (iv) capacity for coding information about vibration frequency. Impulse activity of neonatal fibres was less tightly phase-locked to the vibratory stimulus and showed a poorer reflection of the periodic nature of the vibratory stimulus than impulse patterns of adult units.6. The results reveal that tactile receptors and afferent fibres in the neonate are functionally immature. Their restricted coding capacities suggest that peripheral tactile sensory mechanisms impose limits on the ability of the new-born animal to derive information about its tactile environment.