Responses of areas 3b and 1 in anesthetized squirrel monkeys to single- and dual-site stimulation of the digits

Theta-burst stimulation over primary somatosensory cortex modulates tactile acuity of tongue

Background

Emerging studies in humans have established the modulatory effects of repetitive transcranial magnetic stimulation (rTMS) over primary somatosensory cortex (S1) on somatosensory cortex activity and perception. However, to date, research in this area has primarily focused on the hand and fingers, leaving a gap in our understanding of the modulatory effects of rTMS on somatosensory perception of the orofacial system and speech articulators.

Objective

The present study aimed to examine the effects of different types of theta-burst stimulation-continuous TBS (cTBS), intermittent TBS (iTBS), or sham-over the tongue representation of left S1 on tactile acuity of the tongue.

Methods

In a repeated-measures design, fifteen volunteers participated in four separate sessions, where cTBS, iTBS, sham, or no stimulation was applied over the tongue representation of left S1. Effects of TBS were measured on both temporal and spatial perceptual acuity of tongue using a custom vibrotactile stimulator.

Results

CTBS significantly impaired spatial amplitude threshold at the time window of 16-30 minutes after stimulation, while iTBS improved it at the same time window. The effect of iTBS, however, was smaller than cTBS. In contrast, neither cTBS nor iTBS had any effect on the temporal discrimination threshold.

Conclusions

The current study establishes the validity of using TBS to modulate somatosensory perception of the orofacial system. Directly modifying somatosensation in the orofacial system has the potential to benefit clinical populations with abnormal tactile acuity, improve our understanding of the role of sensory systems in speech production, and enhance speech motor learning and rehabilitation.

Central sensitization mechanisms in chronic migraine with medication overuse headache: a study of thalamocortical activation and lateral cortical inhibition

BACKGROUND

It is unclear whether cortical hyperexcitability in chronic migraine with medication overuse headache (CM-MOH) is due to increased thalamocortical drive or aberrant cortical inhibitory mechanisms.

METHODS

Somatosensory evoked potentials (SSEP) were performed by electrical stimulation of the median nerve (M), ulnar nerve (U) and simultaneous stimulation of both nerves (MU) in 27 patients with CM-MOH and, for comparison, in 23 healthy volunteers (HVs) of a comparable age distribution. We calculated the degree of cortical lateral inhibition using the formula: 100 - [MU/(M + U) × 100] and the level of thalamocortical activation by analyzing the high frequency oscillations (HFOs) embedded in parietal N20 median SSEPs.

RESULTS

Compared to HV, CM-MOH patients showed higher lateral inhibition (CM-MOH 52.2% ± 15.4 vs. HV 40.4% ± 13.3; p = 0.005), which positively correlated with monthly headache days, and greater amplitude of pre-synaptic HFOs (p = 0.010) but normal post-synaptic HFOs (p = 0.122).

CONCLUSION

Our findings suggest that central neuronal circuits are highly sensitized in CM-MOH patients, at both thalamocortical and cortical levels. The observed changes could be due to the combination of dysfunctional central pain control mechanisms, hypersensitivity and hyperresponsiveness directly linked to the chronic intake of acute migraine drugs.

Multi-finger receptive field properties in primary somatosensory cortex: A revised account of the spatiotemporal integration functions of area 3b

The leading view in the somatosensory system indicates that area 3b serves as a cortical relay site that primarily encodes (cutaneous) tactile features limited to individual digits. Our recent work argues against this model by showing that area 3b cells can integrate both cutaneous and proprioceptive information from the hand. Here, we further test the validity of this model by studying multi-digit (MD) integration properties in area 3b. In contrast to the prevailing view, we show that most cells in area 3b have a receptive field (RF) that extends to multiple digits, with the size of the RF (i.e., the number of responsive digits) increasing across time. Further, we show that MD cells' orientation angle preference is highly correlated across digits. Taken together, these data show that area 3b plays a larger role in generating neural representations of tactile objects, as opposed to just being a "feature detector" relay site.

Modular Organization of Signal Transmission in Primate Somatosensory Cortex

Axonal patches are known as the major sites of synaptic connections in the cerebral cortex of higher order mammals. However, the functional role of these patches is highly debated. Patches are formed by populations of nearby neurons in a topographic manner and are recognized as the termination fields of long-distance lateral connections within and between cortical areas. In addition, axons form numerous boutons that lie outside the patches, whose function is also unknown. To better understand the functional roles of these two distinct populations of boutons, we compared individual and collective morphological features of axons within and outside the patches of intra-areal, feedforward, and feedback pathways by way of tract tracing in the somatosensory cortex of New World monkeys. We found that, with the exception of tortuosity, which is an invariant property, bouton spacing and axonal convergence properties differ significantly between axons within patch and no-patch domains. Principal component analyses corroborated the clustering of axons according to patch formation without any additional effect by the type of pathway or laminar distribution. Stepwise logistic regression identified convergence and bouton density as the best predictors of patch formation. These findings support that patches are specific sites of axonal convergence that promote the synchronous activity of neuronal populations. On the other hand, no-patch domains could form a neuroanatomical substrate to diversify the responses of cortical neurons.

Mapping the Integration of Sensory Information across Fingers in Human Sensorimotor Cortex

The integration of somatosensory signals across fingers is essential for dexterous object manipulation. Previous experiments suggest that this integration occurs in neural populations in the primary somatosensory cortex (S1). However, the integration process has not been fully characterized, as previous studies have mainly used 2-finger stimulation paradigms. Here, we addressed this gap by stimulating all 31 single- and multifinger combinations. We measured population-wide activity patterns evoked during finger stimulation in human S1 and primary motor cortex (M1) using 7T fMRI in female and male participants. Using multivariate fMRI analyses, we found clear evidence of unique nonlinear interactions between fingers. In Brodmann area (BA) 3b, interactions predominantly occurred between pairs of neighboring fingers. In BA 2, however, we found equally strong interactions between spatially distant fingers, as well as interactions between finger triplets and quadruplets. We additionally observed strong interactions in the hand area of M1. In both M1 and S1, these nonlinear interactions did not reflect a general suppression of overall activity, suggesting instead that the interactions we observed reflect rich, nonlinear integration of sensory inputs from the fingers. We suggest that this nonlinear finger integration allows for a highly flexible mapping from finger sensory inputs to motor responses that facilitates dexterous object manipulation.SIGNIFICANCE STATEMENT Processing of somatosensory information in primary somatosensory cortex (S1) is essential for dexterous object manipulation. To successfully handle an object, the sensorimotor system needs to detect complex patterns of haptic information, which requires the nonlinear integration of sensory inputs across multiple fingers. Using multivariate fMRI analyses, we characterized brain activity patterns evoked by stimulating all single- and multifinger combinations. We report that progressively stronger multifinger interactions emerge in posterior S1 and in the primary motor cortex (M1), with interactions arising between inputs from neighboring and spatially distant fingers. Our results suggest that S1 and M1 provide the neural substrate necessary to support a flexible mapping from sensory inputs to motor responses of the hand.

Optical imaging reveals functional domains in primate sensorimotor cortex

Motor cortex (M1) and somatosensory cortex (S1) are central to arm and hand control. Efforts to understand encoding in M1 and S1 have focused on temporal relationships between neural activity and movement features. However, it remains unclear how the neural activity is spatially organized within M1 and S1. Optical imaging methods are well-suited for revealing the spatio-temporal organization of cortical activity, but their application is sparse in monkey sensorimotor cortex. Here, we investigate the effectiveness of intrinsic signal optical imaging (ISOI) for measuring cortical activity that supports arm and hand control in a macaque monkey. ISOI revealed spatial domains that were active in M1 and S1 in response to instructed reaching and grasping. The lateral M1 domains overlapped the hand representation and contained a population of neurons with peak firing during grasping. In contrast, the medial M1 domain overlapped the arm representation and a population of neurons with peak firing during reaching. The S1 domain overlapped the hand representations of areas 1 and 2 and a population of neurons with peak firing upon hand contact with the target. Our single unit recordings indicate that ISOI domains report the locations of spatial clusters of functionally related neurons. ISOI is therefore an effective tool for surveilling the neocortex for “hot zones” of activity that supports movement. Combining the strengths of ISOI with other imaging modalities (e.g., fMRI, 2-photon) and with electrophysiological methods can open new frontiers in understanding the spatio-temporal organization of cortical signals involved in movement control.

Mapping mesoscale cortical connectivity in monkey sensorimotor cortex with optical imaging and microstimulation

To map in vivo cortical circuitry at the mesoscale, we applied a novel approach to map interareal functional connectivity. Electrical intracortical microstimulation (ICMS) in conjunction with optical imaging of intrinsic signals (OIS) was used map functional connections in somatosensory cortical areas in anesthetized squirrel monkeys. ICMS produced activations that were focal and that displayed responses which were stimulation intensity dependent. ICMS in supragranular layers of Brodmann Areas 3b, 1, 2, 3a, and M1 evoked interareal activation patterns that were topographically appropriate and appeared consistent with known anatomical connectivity. Specifically, ICMS revealed Area 3b connections with Area 1; Area 1 connections with Areas 2 and 3a; Area 2 connections with Areas 1, 3a, and M1; Area 3a connections with Areas M1, 1, and 2; and M1 connections with Areas 3a, 1, and 2. These somatosensory connectivity patterns were reminiscent of feedforward patterns observed anatomically, although feedback contributions are also likely present. Further consistent with anatomical connectivity, intra-areal and intra-areal patterns of activation were patchy with patch sizes of 200-300 μm. In summary, ICMS with OIS is a novel approach for mapping interareal and intra-areal connections in vivo. Comparisons with feedforward and feedback anatomical connectivity are discussed.

Characterization of multi-channel intraneural stimulation in transradial amputees

Although peripheral nerve stimulation using intraneural electrodes has been shown to be an effective and reliable solution to restore sensory feedback after hand loss, there have been no reports on the characterization of multi-channel stimulation. A deeper understanding of how the simultaneous stimulation of multiple electrode channels affects the evoked sensations should help in improving the definition of encoding strategies for bidirectional prostheses. We characterized the sensations evoked by simultaneous stimulation of median and ulnar nerves (multi-channel configuration) in four transradial amputees who had been implanted with four TIMEs (Transverse Intrafascicular Multichannel Electrodes). The results were compared with the characterization of single-channel stimulation. The sensations were characterized in terms of location, extent, type, and intensity. Combining two or more single-channel configurations caused a linear combination of the sensation locations and types perceived with such single-channel stimulations. Interestingly, this was also true when two active sites from the same nerve were stimulated. When stimulating in multi-channel configuration, the charge needed from each electrode channel to evoke a sensation was significantly lower than the one needed in single-channel configuration (sensory facilitation). This result was also supported by electroencephalography (EEG) recordings during nerve stimulation. Somatosensory potentials evoked by multi-channel stimulation confirmed that sensations in the amputated hand were perceived by the subjects and that a perceptual sensory facilitation occurred. Our results should help the future development of more efficient bidirectional prostheses by providing guidelines for the development of more complex stimulation approaches to effectively restore multiple sensations at the same time.

Neural Basis of Touch and Proprioception in Primate Cortex

The sense of proprioception allows us to keep track of our limb posture and movements and the sense of touch provides us with information about objects with which we come into contact. In both senses, mechanoreceptors convert the deformation of tissues-skin, muscles, tendons, ligaments, or joints-into neural signals. Tactile and proprioceptive signals are then relayed by the peripheral nerves to the central nervous system, where they are processed to give rise to percepts of objects and of the state of our body. In this review, we first examine briefly the receptors that mediate touch and proprioception, their associated nerve fibers, and pathways they follow to the cerebral cortex. We then provide an overview of the different cortical areas that process tactile and proprioceptive information. Next, we discuss how various features of objects-their shape, motion, and texture, for example-are encoded in the various cortical fields, and the susceptibility of these neural codes to attention and other forms of higher-order modulation. Finally, we summarize recent efforts to restore the senses of touch and proprioception by electrically stimulating somatosensory cortex. © 2018 American Physiological Society. Compr Physiol 8:1575-1602, 2018.

Connectivity of neuronal populations within and between areas of primate somatosensory cortex

Functions of the cerebral cortex emerge via interactions of horizontally distributed neuronal populations within and across areas. However, the connectional underpinning of these interactions is not well understood. The present study explores the circuitry of column-size cortical domains within the hierarchically organized somatosensory cortical areas 3b and 1 using tract tracing and optical intrinsic signal imaging (OIS). The anatomical findings reveal that feedforward connections exhibit high topographic specificity, while intrinsic and feedback connections have a more widespread distribution. Both intrinsic and inter-areal connections are topographically oriented across the finger representations. Compared to area 3b, the low clustering of connections and small cortical magnification factor supports that the circuitry of area 1 scaffolds a sparse functional representation that integrates peripheral information from a large area that is fed back to area 3b. Fast information exchange between areas is ensured by thick axons forming a topographically organized, reciprocal pathway. Moreover, the highest density of projecting neurons and groups of axon arborization patches corresponds well with the size and locations of the functional population response reported by OIS. The findings establish connectional motifs at the mesoscopic level that underpin the functional organization of the cerebral cortex.

Development of an optogenetic toolkit for neural circuit dissection in squirrel monkeys

Optogenetic tools have opened a rich experimental landscape for understanding neural function and disease. Here, we present the first validation of eight optogenetic constructs driven by recombinant adeno-associated virus (AAV) vectors and a WGA-Cre based dual injection strategy for projection targeting in a widely-used New World primate model, the common squirrel monkey Saimiri sciureus. We observed opsin expression around the local injection site and in axonal projections to downstream regions, as well as transduction to thalamic neurons, resembling expression patterns observed in macaques. Optical stimulation drove strong, reliable excitatory responses in local neural populations for two depolarizing opsins in anesthetized monkeys. Finally, we observed continued, healthy opsin expression for at least one year. These data suggest that optogenetic tools can be readily applied in squirrel monkeys, an important first step in enabling precise, targeted manipulation of neural circuits in these highly trainable, cognitively sophisticated animals. In conjunction with similar approaches in macaques and marmosets, optogenetic manipulation of neural circuits in squirrel monkeys will provide functional, comparative insights into neural circuits which subserve dextrous motor control as well as other adaptive behaviors across the primate lineage. Additionally, development of these tools in squirrel monkeys, a well-established model system for several human neurological diseases, can aid in identifying novel treatment strategies.

Cortical Merging in S1 as a Substrate for Tactile Input Grouping

Perception is a reconstruction process guided by rules based on knowledge about the world. Little is known about the neural implementation of the rules of object formation in the tactile sensory system. When two close tactile stimuli are delivered simultaneously on the skin, subjects feel a unique sensation, spatially centered between the two stimuli. Voltage-sensitive dye imaging (VSDi) and electrophysiological recordings [local field potentials (LFPs) and single units] were used to extract the cortical representation of two-point tactile stimuli in the primary somatosensory cortex of anesthetized Long-Evans rats. Although layer 4 LFP responses to brief costimulation of the distal region of two digits resembled the sum of individual responses, approximately one-third of single units demonstrated merging-compatible changes. In contrast to previous intrinsic optical imaging studies, VSD activations reflecting layer 2/3 activity were centered between the representations of the digits stimulated alone. This merging was found for every tested distance between the stimulated digits. We discuss this laminar difference as evidence that merging occurs through a buildup stream and depends on the superposition of inputs, which increases with successive stages of sensory processing. These findings show that layers 2/3 are involved in the grouping of sensory inputs. This process that could be inscribed in the cortical computing routine and network organization is likely to promote object formation and implement perception rules.

Somatosensory maps

Somatosensory areas containing topographic maps of the body surface are a major feature of parietal cortex. In primates, parietal cortex contains four somatosensory areas, each with its own map, with the primary cutaneous map in area 3b. Rodents have at least three parietal somatosensory areas. Maps are not isomorphic to the body surface, but magnify behaviorally important skin regions, which include the hands and face in primates, and the whiskers in rodents. Within each map, intracortical circuits process tactile information, mediate spatial integration, and support active sensation. Maps may also contain fine-scale representations of touch submodalities, or direction of tactile motion. Functional representations are more overlapping than suggested by textbook depictions of map topography. The whisker map in rodent somatosensory cortex is a canonic system for studying cortical microcircuits, sensory coding, and map plasticity. Somatosensory maps are plastic throughout life in response to altered use or injury. This chapter reviews basic principles and recent findings in primate, human, and rodent somatosensory maps.

Study of single and multidigit activation in monkey somatosensory cortex using voltage-sensitive dye imaging

Toward the goal of understanding cutaneous sensory integration during manual behavior, we used voltage-sensitive dye (VSD) imaging to study the organization and dynamics of anesthetized monkey primary somatosensory cortex (SI) in response to single and multidigit tactile stimulation. We find that in both macaque and squirrel monkey SI, VSD reveals clear focal digit topography consistent with previous electrophysiological and intrinsic signal imaging studies. VSD also reveals interactions in SI in response to multidigit stimulation. With a tactile funneling paradigm in areas 3b and 1 in squirrel monkeys, VSD reveals two-digit induction of subthreshhold influences, consistent with lateral intracortical inhibition. In response to tactile apparent motion stimuli, VSD reveals preferential response to motion stimuli over static tactile stimuli in both areas 1 and 3b. Comparison of the response at different digit locations to "toward digit" stimuli suggests the presence of direction-selective response in area 1; however, further study is needed. These exciting results indicate that VSD constitutes a powerful tool for studying somatosensory cortical processing in nonhuman primates and should be further developed for future somatosensory studies in awake behaving monkeys.

High spatial correspondence at a columnar level between activation and resting state fMRI signals and local field potentials

Although blood oxygenation level-dependent (BOLD) fMRI has been widely used to map brain responses to external stimuli and to delineate functional circuits at rest, the extent to which BOLD signals correlate spatially with underlying neuronal activity, the spatial relationships between stimulus-evoked BOLD activations and local correlations of BOLD signals in a resting state, and whether these spatial relationships vary across functionally distinct cortical areas are not known. To address these critical questions, we directly compared the spatial extents of stimulated activations and the local profiles of intervoxel resting state correlations for both high-resolution BOLD at 9.4 T and local field potentials (LFPs), using 98-channel microelectrode arrays, in functionally distinct primary somatosensory areas 3b and 1 in nonhuman primates. Anatomic images of LFP and BOLD were coregistered within 0.10 mm accuracy. We found that the point spread functions (PSFs) of BOLD and LFP responses were comparable in the stimulus condition, and both estimates of activations were slightly more spatially constrained than local correlations at rest. The magnitudes of stimulus responses in area 3b were stronger than those in area 1 and extended in a medial to lateral direction. In addition, the reproducibility and stability of stimulus-evoked activation locations within and across both modalities were robust. Our work suggests that the intrinsic resolution of BOLD is not a limiting feature in practice and approaches the intrinsic precision achievable by multielectrode electrophysiology.

Altered Spatiotemporal Dynamics of Cortical Activation to Tactile Stimuli in Somatosensory Area 3b and Area 1 of Monkeys after Spinal Cord Injury

Reactivation of deafferented cortex plays a key role in mediating the recovery of lost functions, although the precise mechanism is not fully understood. This study simultaneously characterized the dynamic spatiotemporal features of tactile responses in areas 3b and 1 before and 6–8 weeks after partial dorsal column lesion (DCL), and examined how the reactivation relates to the recovery of simple hand use in squirrel monkeys. A combination of high spatiotemporal resolution functional intrinsic optical imaging, microelectrode mapping, behavioral assessment, and tracer histology methods were used. Compared with the normal cortex, we found that the responses of deafferented areas 3b and 1 to 3 s of continuous 8 Hz tactile stimulation of a single digit were significantly weaker and more transient. This finding indicates a loss of response to sustained tactile stimuli. The activation area enlarged for areas 3b and 1 in both directions along digit representation (medial–lateral) and across areas (anterior–posterior). All subjects showed behavioral deficits in a food reaching-grasping-retrieving task within the first 5 weeks after DCL, but recovered at the time when optical images were acquired. Summarily, we showed that these populations of cortical neurons responded to peripheral tactile inputs, albeit in significantly altered manners in each area, several weeks after deafferentation. We propose that compromised ascending driven inputs, impaired lateral inhibition, and local integration of input signals may account for the altered spatiotemporal dynamics of the reactivated areas 3b and 1 cortices. Further investigation with large sample sizes is needed to fully characterize the effects of deafferentation on area 1 activation size.

Functional consequences of experience-dependent plasticity on tactile perception following perceptual learning

Continuous training enhances perceptual discrimination and promotes neural changes in areas encoding the experienced stimuli. This type of experience-dependent plasticity has been demonstrated in several sensory and motor systems. Particularly, non-human primates trained to detect consecutive tactile bar indentations across multiple digits showed expanded excitatory receptive fields (RFs) in somatosensory cortex. However, the perceptual implications of these anatomical changes remain undetermined. Here, we trained human participants for 9 days on a tactile task that promoted expansion of multi-digit RFs. Participants were required to detect consecutive indentations of bar stimuli spanning multiple digits. Throughout the training regime we tracked participants' discrimination thresholds on spatial (grating orientation) and temporal tasks on the trained and untrained hands in separate sessions. We hypothesized that training on the multi-digit task would decrease perceptual thresholds on tasks that require stimulus processing across multiple digits, while also increasing thresholds on tasks requiring discrimination on single digits. We observed an increase in orientation thresholds on a single digit. Importantly, this effect was selective for the stimulus orientation and hand used during multi-digit training. We also found that temporal acuity between digits improved across trained digits, suggesting that discriminating the temporal order of multi-digit stimuli can transfer to temporal discrimination of other tactile stimuli. These results suggest that experience-dependent plasticity following perceptual learning improves and interferes with tactile abilities in manners predictive of the task and stimulus features used during training.

Chronic recordings reveal tactile stimuli can suppress spontaneous activity of neurons in somatosensory cortex of awake and anesthetized primates

In somatosensory cortex, tactile stimulation within the neuronal receptive field (RF) typically evokes a transient excitatory response with or without postexcitatory inhibition. Here, we describe neuronal responses in which stimulation on the hand is followed by suppression of the ongoing discharge. With the use of 16-channel microelectrode arrays implanted in the hand representation of primary somatosensory cortex of New World monkeys and prosimian galagos, we recorded neuronal responses from single units and neuron clusters. In 66% of our sample, neuron activity tended to display suppression of firing when regions of skin outside of the excitatory RF were stimulated. In a small proportion of neurons, single-site indentations suppressed firing without initial increases in response to any of the tested sites on the hand. Latencies of suppressive responses to skin indentation (usually 12-34 ms) were similar to excitatory response latencies. The duration of inhibition varied across neurons. Although most observations were from anesthetized animals, we also found similar neuron response properties in one awake galago. Notably, suppression of ongoing neuronal activity did not require conditioning stimuli or multi-site stimulation. The suppressive effects were generally seen following single-site skin indentations outside of the neuron's minimal RF and typically on different digits and palm pads, which have not often been studied in this context. Overall, the characteristics of widespread suppressive or inhibitory response properties with and without initial facilitative or excitatory responses add to the growing evidence that neurons in primary somatosensory cortex provide essential processing for integrating sensory stimulation from across the hand.

Lateral inhibition in the somatosensory cortex during and between migraine without aura attacks: Correlations with thalamocortical activity and clinical features

Background

We studied lateral inhibition in the somatosensory cortex of migraineurs during and between attacks, and searched for correlations with thalamocortical activity and clinical features.

Participants and methods

Somatosensory evoked potentials (SSEP) were obtained by electrical stimulation of the right median (M) or ulnar (U) nerves at the wrist or by simultaneous stimulation of both nerves (MU) in 41 migraine without aura patients, 24 between (MO), 17 during attacks, and in 17 healthy volunteers (HVs). We determined the percentage of lateral inhibition of the N20–P25 component by using the formula [(100)–MU/(M + U)*100]. We also studied high-frequency oscillations (HFOs) reflecting thalamocortical activation.

Results

In migraine, both lateral inhibition (MO 27.9% vs HVs 40.2%; p = 0.009) and thalamocortical activity (MO 0.5 vs HVs 0.7; p = 0.02) were reduced between attacks, but not during. In MO patients, the percentage of lateral inhibition negatively correlated with days elapsed since the last migraine attack ( r = −0.510, p = 0.01), monthly attack duration ( r = −0.469, p = 0.02) and severity ( r = −0.443, p = 0.03), but positively with thalamocortical activity ( r = −0.463, p = 0.02).

Conclusions

We hypothesize that abnormal migraine cycle-dependent dynamics of connectivity between subcortical and cortical excitation/inhibition networks may contribute to clinical features of MO and recurrence of attacks.

Rapid-rate paired associative stimulation over the primary somatosensory cortex

Rapid-rate paired associative stimulation (rPAS) involves repeat pairing of peripheral nerve stimulation and Transcranial magnetic stimulation (TMS) pulses at a 5 Hz frequency. RPAS over primary motor cortex (M1) operates with spike-timing dependent plasticity such that increases in corticospinal excitability occur when the nerve and TMS pulse temporally coincide in cortex. The present study investigates the effects of rPAS over primary somatosensory cortex (SI) which has not been performed to date. In a series of experiments, rPAS was delivered over SI and M1 at varying timing intervals between the nerve and TMS pulse based on the latency of the N20 somatosensory evoked potential (SEP) component within each participant (intervals for SI-rPAS: N20, N20-2.5 ms, N20 + 2.5 ms, intervals for M1-rPAS: N20, N20+5 ms). Changes in SI physiology were measured via SEPs (N20, P25, N20-P25) and SEP paired-pulse inhibition, and changes in M1 physiology were measured with motor evoked potentials and short-latency afferent inhibition. Measures were obtained before rPAS and at 5, 25 and 45 minutes following stimulation. Results indicate that paired-pulse inhibition and short-latency afferent inhibition were reduced only when the SI-rPAS nerve-TMS timing interval was set to N20-2.5 ms. SI-rPAS over SI also led to remote effects on motor physiology over a wider range of nerve-TMS intervals (N20-2.5 ms – N20+2.5 ms) during which motor evoked potentials were increased. M1-rPAS increased motor evoked potentials and reduced short-latency afferent inhibition as previously reported. These data provide evidence that, similar to M1, rPAS over SI is spike-timing dependent and is capable of exerting changes in SI and M1 physiology.

Connectivity of somatosensory cortical area 1 forms an anatomical substrate for the emergence of multifinger receptive fields and complex feature selectivity in the squirrel monkey (Saimiri sciureus)

Converging evidence shows that interaction of digit-specific input, which is required to form global tactile percepts, begins as early as area 3b in the primary somatosensory cortex with the involvement of intrinsic lateral connections. How tactile processing is further elaborated in area 1, the next stage of the somatosensory cortical hierarchy, is less understood. This question was investigated by studying the tangential distribution of intrinsic and interareal connections of finger representations of area 1. Retrogradely labeled cell densities and anterogradely labeled fibers and terminal patches were plotted and quantified with respect to the hand representation by combining tract tracing with electrophysiological mapping and intrinsic signal optical imaging in somatosensory areas. Intrinsic connections of distal finger pad representations of area 1 spanned the representation of multiple digits indicating strong cross-digit connectivity. Area 1 distal finger pad regions also established high-density connections with homotopic regions of areas 3b and 2. Although similar to area 3b, connections of area 1 distributed more widely and covered a larger somatotopic representation including more proximal parts of the finger representations. The lateral connectivity pattern of area 1 is a suitable anatomical substrate of the emergence of multifinger receptive fields, complex feature selectivity, and invariant stimulus properties of the neurons.

Cortical connections to single digit representations in area 3b of somatosensory cortex in squirrel monkeys and prosimian galagos

The ventral posterior nucleus of thalamus sends highly segregated inputs into each digit representation in area 3b of primary somatosensory cortex. However, the spatial organization of the connections that link digit representations of areas 3b with other somatosensory areas is less understood. Here we examined the cortical inputs to individual digit representations of area 3b in four squirrel monkeys and one prosimian galago. Retrograde tracers were injected into neurophysiologically defined representations of individual digits of area 3b. Cortical tissues were cut parallel to the surface in some cases and showed that feedback projections to individual digits overlapped extensively in the hand representations of areas 3b, 1, and parietal ventral (PV) and second somatosensory (S2) areas. Other regions with overlapping populations of labeled cells included area 3a and primary motor cortex (M1). The results were confirmed in other cases in which the cortical tissues were cut in the coronal plane. The same cases also showed that cells were primarily labeled in the infragranular and supragranular layers. Thus, feedback projections to individual digit representations in area 3b mainly originate from multiple digits and other portions of hand representations of areas 3b, 1, PV, and S2. This organization is in stark contrast to the segregated thalamocortical inputs, which originate in single digit representations and terminate in the matching digit representation in the cortex. The organization of feedback connections could provide a substrate for the integration of information across the representations of adjacent digits in area 3b.

Cortical neuron response properties are related to lesion extent and behavioral recovery after sensory loss from spinal cord injury in monkeys

Lesions of the dorsal columns at a mid-cervical level render the hand representation of the contralateral primary somatosensory cortex (area 3b) unresponsive. Over weeks of recovery, most of this cortex becomes responsive to touch on the hand. Determining functional properties of neurons within the hand representation is critical to understanding the neural basis of this adaptive plasticity. Here, we recorded neural activity across the hand representation of area 3b with a 100-electrode array and compared results from owl monkeys and squirrel monkeys 5-10 weeks after lesions with controls. Even after extensive lesions, performance on reach-to-grasp tasks returned to prelesion levels, and hand touches activated territories mainly within expected cortical locations. However, some digit representations were abnormal, such that receptive fields of presumably reactivated neurons were larger and more often involved discontinuous parts of the hand compared with controls. Hand stimulation evoked similar neuronal firing rates in lesion and control monkeys. By assessing the same monkeys with multiple measures, we determined that properties of neurons in area 3b were highly correlated with both the lesion severity and the impairment of hand use. We propose that the reactivation of neurons with near-normal response properties and the recovery of near-normal somatotopy likely supported the recovery of hand use. Given the near-completeness of the more extensive dorsal column lesions we studied, we suggest that alternate spinal afferents, in addition to the few spared primary axon afferents in the dorsal columns, likely have a major role in the reactivation pattern and return of function.

Distinct fine-scale fMRI activation patterns of contra- and ipsilateral somatosensory areas 3b and 1 in humans

Inter-areal and ipsilateral cortical responses to tactile stimulation have not been well described in human S1 cortex. By taking advantage of the high signal-to-noise ratio at 7 T, we quantified blood oxygenation level dependent (BOLD) response patterns and time courses to tactile stimuli on individual distal finger pads at a fine spatial scale, and examined whether there are inter-areal (area 3b versus area 1) and interhemispheric response differences to unilateral tactile stimulation in healthy human subjects. We found that 2-Hz tactile stimulation of individual fingertips evoked detectable BOLD signal changes in both contralateral and ipsilateral area 3b and area 1. Contralateral digit activations were organized in an orderly somatotopic manner, and BOLD responses in area 3b were more digit selective than those in area 1. However, the area of cortex that was responsive to stimulation of a single digit (stimulus-response field) was similar across areas. In the ipsilateral hemisphere, response magnitudes in both areas 3b and 1 were significantly weaker than those of the contralateral hemisphere. Digit activations exhibited no clear somatotopic organizational pattern in either area 3b or area 1, yet digit selectivity was retained in area 1 but not in area 3b. The observation of distinct digit-selective responses of contralateral area 3b versus area 1 supports a higher order function of contralateral area 1 in spatial integration. In contrast, ipsilateral cortices may play a less discriminative role in the perception of unilateral tactile sensation in humans.

Intrinsic horizontal connections process global tactile features in the primary somatosensory cortex: neuroanatomical evidence

To understand manual tactile functions in primates, it is essential to explore the interactions between the finger pad representations in somatosensory cortex. To this end, we used optical imaging and electrophysiological mapping to guide neuroanatomical tracer injections into distal digit tip representations of Brodmann area 3b in the squirrel monkey. Retrogradely labeled cell densities and anterogradely labeled fibers and terminal patches in somatosensory areas were plotted and quantified with respect to tangential distribution. Within area 3b, reciprocal patchy distribution of anterograde and retrograde labeling spanned the representation of the distal pad of multiple digits, indicating strong cross-digit connectivity. Inter-areal connections revealed bundles of long-range fibers projecting anteroposteriorly, connecting area 3b with clusters of labeled neurons and terminal axon arborizations in area 1. Inter-areal linkage appeared to be largely confined to the representation of the injected finger. These findings provide the neuroanatomical basis for the interaction between distal finger pad representations observed by recent electrophysiological studies. We propose that intra-areal connectivity may be heavily involved in interdigit integration such as shape discrimination, whereas long-range inter-areal connections may subserve active touch in a digit-specific manner.

Optical imaging of cortical networks via intracortical microstimulation

Understanding cortical organization is key to understanding brain function. Distinct neural networks underlie the functional organization of the cerebral cortex; however, little is known about how different nodes in the cortical network interact during perceptual processing and motor behavior. To study cortical network function we examined whether the optical imaging of intrinsic signals (OIS) reveals the functional patterns of activity evoked by electrical cortical microstimulation. We examined the effects of current amplitude, train duration, and depth of cortical stimulation on the hemodynamic response to electrical microstimulation (250-Hz train, 0.4-ms pulse duration) in anesthetized New World monkey somatosensory cortex. Electrical stimulation elicited a restricted cortical response that varied according to stimulation parameters and electrode depth. Higher currents of stimulation recruited more areas of cortex than smaller currents. The largest cortical responses were seen when stimulation was delivered around cortical layer 4. Distinct local patches of activation, highly suggestive of local projections, around the site of stimulation were observed at different depths of stimulation. Thus we find that specific electrical stimulation parameters can elicit activation of single cortical columns and their associated columnar networks, reminiscent of anatomically labeled networks. This novel functional tract tracing method will open new avenues for investigating relationships of local cortical organization.

The relationship of anatomical and functional connectivity to resting-state connectivity in primate somatosensory cortex

Studies of resting-state activity in the brain have provoked critical questions about the brain's functional organization, but the biological basis of this activity is not clear. Specifically, the relationships between interregional correlations in resting-state measures of activity, neuronal functional connectivity and anatomical connectivity are much debated. To investigate these relationships, we have examined both anatomical and steady-state functional connectivity within the hand representation of primary somatosensory cortex (areas 3b and 1) in anesthetized squirrel monkeys. The comparison of three data sets (fMRI, electrophysiological, and anatomical) indicate two primary axes of information flow within the SI: prominent interdigit interactions within area 3b and predominantly homotopic interactions between area 3b and area 1. These data support a strikingly close relationship between baseline functional connectivity and anatomical connections. This study extends findings derived from large-scale cortical networks to the realm of local millimeter-scale networks.

Interaction of finger representations in the cortex of individuals with autism: a functional window into cortical inhibition

An established neural biomarker of autism spectrum disorder (ASD) has the potential to provide novel biological and pharmacological targets for treatment. Lower level of inhibition in brain circuits is a leading biomarker candidate. A physiological investigation of the functional levels of inhibition in the cortex of individuals with autism can provide a strong test of the hypothesis. The amplitude of cortical response to the stimulation of adjacent fingers is controlled by the level of cortical inhibition and provides just such a test. Using magnetoencephalography, we recorded the response of the somatosensory cortex to the passive tactile stimulation of the thumb (D1), and index finger (D2), and to the simultaneous stimulation of both fingers combined (D1,D2) of the dominant (right) hand of young subjects with and without autism. For each participant, we measured the response to the stimulation of both fingers combined (D1,D2) relative to the post hoc sum of the responses to the stimulation of each finger alone (D1+D2) in multiple different ways and linearly regressed the ASD and neurotypical (NT) groups' responses. The resulting slopes were then compared: Smaller slope values imply attenuated response to paired finger stimulation, and enhanced levels of inhibition. The short-latency M40 and mid-latency M80 response slopes of the group with autism obtained in different ways were either significantly smaller, or statistically indistinguishable from NT. The result does not support reduced inhibition in the somatosensory cortex of individuals with autism, contrary to the seminal hypothesis of reduced inhibition. Implications are discussed including refinements of current theory.

Continuous theta-burst stimulation modulates tactile synchronization

Background

Temporal order judgement (TOJ) is the ability to detect the order of occurrence of two sequentially delivered stimuli. Previous research has shown that TOJ in the presence of synchronized periodic conditioning stimuli impairs TOJ performance, and this phenomenon is suggested to be mediated by GABAergic interneurons that cause perceptual binding across the two skin sites. Application of continuous theta-burst repetitive TMS (cTBS) over primary somatosensory cortex (SI) alters temporal and spatial tactile perception. The purpose of this study was to examine TOJ perception in the presence and absence of synchronized periodic conditioning stimuli before and after cTBS applied over left-hemisphere SI. A TOJ task was administered on the right index and middle finger (D2 and D3) in two separate sessions in the presence and absence of conditioning stimuli (a background low amplitude sinusoidal vibration).

Results

CTBS reduced the impact of the conditioning stimuli on TOJ performance for up to 18 minutes following stimulation while sham cTBS did not affect TOJ performance. In contrast, the TOJ task performed in the absence of synchronized conditioning stimulation was unaltered following cTBS.

Conclusion

We conclude that cTBS suppresses inhibitory networks in SI that mediate perceptual binding during TOJ synchronization. CTBS offers one method to suppress cortical excitability in the cortex and potentially benefit clinical populations with altered inhibitory cortical circuits. Additionally, TOJ measures with conditioning stimuli may provide an avenue to assess sensory processing in neurologically impaired patient populations.

The functional architecture of S1 during touch observation described with 7 T fMRI

Recent studies indicate that the primary somatosensory cortex (S1) is active not only when touch is physically perceived but also when it is merely observed to be experienced by another person. This social responsivity of S1 has important implications for our understanding of S1 functioning. However, S1 activity during touch observation has not been characterized in great detail to date. We focused on two features of the S1 functional architecture during touch observation, namely the topographical arrangement of index and middle finger receptive fields (RFs), and their dynamic shrinkage during concurrent activation. Both features have important implications for human behavior. We conducted two fMRI studies at 7 T, one where touch was physically perceived, and one where touch was observed. In the two experiments, participants either had their index finger and/or middle finger stimulated using paintbrushes, or just observed similar touch events on video. Our data show that observing and physically experiencing touch elicits overlapping activity changes in S1. In addition, observing touch to the index finger or the middle finger alone evoked topographically arranged activation foci in S1. Importantly, when co-activated, the index and middle finger RFs not only shrank during physical touch perception, but also during touch observation. Our data, therefore, indicate a similarity between the functional architecture of S1 during touch observation and physical touch perception with respect to single-digit topography and RF shrinkage. These results may allow the tentative conclusion that even primary somatosensory experiences, such as physical touch perception, can be shared amongst individuals.

Dynamic reorganization of digit representations in somatosensory cortex of nonhuman primates after spinal cord injury

Somatosensory cortices of adult primates reactivate over time after sensory loss. The time course and the neural mechanisms underlying the cortical reactivation are not well understood. Here we report that longitudinal high-resolution functional magnetic resonance imaging (fMRI) studies on anesthetized squirrel monkeys revealed dynamic reorganizations of digit activations in area 3b, within 2 months after severely disrupting afferent inputs by dorsal column section. We found that digit regions in which inputs were severely disrupted exhibited fMRI tactile responses. Reorganization was characterized by an early moving away phase and a late returning phase, as indicated by spatial shifts of individual digit activation centers in relation to the pre-lesion activation sites. Subsequent optical imaging studies confirmed fMRI activations, and dense microelectrode penetrations identified weak neuronal activity at the reactivated sites. Activation zones detected by fMRI and optical imaging were significantly larger in input-deprived than normal input single-digit regions and were larger than regions defined by neuronal spiking activity. This study captures the dynamic reorganization of digit representations after dorsal column lesions and reveals differences between functional imaging and microelectrode recording maps. Our observations suggest that subthreshold activity plays an important role in the reactivation of deafferented cortex and could promote behavioral recovery.

Columnar distribution of activity dependent gabaergic depolarization in sensorimotor cortical neurons

Background

GABA, the major inhibitory neurotransmitter in CNS, has been demonstrated to paradoxically produce excitation even in mature brain. However activity-dependent form of GABA excitation in cortical neurons has not been observed. Here we report that after an intense electrical stimulation adult cortical neurons displayed a transient GABA excitation that lasted for about 30s.

Results

Whole-cell patch recordings were performed to evaluate the effects of briefly applied GABA on pyramidal neurons in adult rodent sensorimotor cortical slice before and after 1 s, 20 Hz suprathreshold electrical stimulation of the junction between layer 6 and the underlying white matter (L6/WM stimulation). Immediately after L6/WM stimulation, GABA puffs produced neuronal depolarization in the center of the column-shaped region. However, both prior to or 30s after stimulation GABA puffs produced hyperpolarization of neurons. 2-photon imaging in neurons infected with adenovirus carrying a chloride sensor Clomeleon revealed that GABA induced depolarization is due to an increase in [Cl^-]_i after stimulation. To reveal the spatial extent of excitatory action of GABA, isoguvacine, a GABA_A receptors agonist, was applied right after stimulation while monitoring the intracellular Ca²⁺ concentration in pyramidal neurons. Isoguvacine induced an increase in [Ca²⁺]_i in pyramidal neurons especially in the center of the column but not in the peripheral regions of the column. The global pattern of the Ca²⁺ signal showed a column-shaped distribution along the stimulation site.

Conclusion

These results demonstrate that the well-known inhibitory transmitter GABA rapidly switches from hyperpolarization to depolarization upon synaptic activity in adult somatosensory cortical neurons.

Perceptual grouping over time within and across auditory and tactile modalities

In auditory scene analysis, population separation and temporal coherence have been proposed to explain how auditory features are grouped together and streamed over time. The present study investigated whether these two theories can be applied to tactile streaming and whether temporal coherence theory can be applied to crossmodal streaming. The results show that synchrony detection between two tones/taps at different frequencies/locations became difficult when one of the tones/taps was embedded in a perceptual stream. While the taps applied to the same location were streamed over time, the taps applied to different locations were not. This observation suggests that tactile stream formation can be explained by population-separation theory. On the other hand, temporally coherent auditory stimuli at different frequencies were streamed over time, but temporally coherent tactile stimuli applied to different locations were not. When there was within-modality streaming, temporally coherent auditory stimuli and tactile stimuli were not streamed over time, either. This observation suggests the limitation of temporal coherence theory when it is applied to perceptual grouping over time.

Somatosensory information processing in the aging population

While it is well known that skin physiology – and consequently sensitivity to peripheral stimuli – degrades with age, what is less appreciated is that centrally mediated mechanisms allow for maintenance of the same degree of functionality in processing these peripheral inputs and interacting with the external environment. In order to demonstrate this concept, we obtained observations of processing speed, sensitivity (thresholds), discriminative capacity, and adaptation metrics on subjects ranging in age from 18 to 70. The results indicate that although reaction speed and sensory thresholds change with age, discriminative capacity, and adaptation metrics do not. The significance of these findings is that similar metrics of adaptation have been demonstrated to change significantly when the central nervous system (CNS) is compromised. Such compromise has been demonstrated in subject populations with autism, chronic pain, acute NMDA receptor block, concussion, and with tactile–thermal interactions. If the metric of adaptation parallels cortical plasticity, the results of the current study suggest that the CNS in the aging population is still capable of plastic changes, and this cortical plasticity could be the mechanism that compensates for the degradations that are known to naturally occur with age. Thus, these quantitative measures – since they can be obtained efficiently and objectively, and appear to deviate from normative values significantly with systemic cortical alterations – could be useful indicators of cerebral cortical health.

A novel device for the study of somatosensory information processing

Current methods for applying multi-site vibratory stimuli to the skin typically involve the use of multiple, individual vibrotactile stimulators. Limitations of such an arrangement include difficulty with both positioning the stimuli as well as ensuring that stimuli are delivered in a synchronized and deliberate manner. Previously, we reported a two-site tactile stimulator that was developed in order to solve these problems (Tannan et al., 2007a). Due to both the success of that novel stimulator and the limitations that were inherent in that device, we designed and fabricated a four-site stimulator that provides a number of advantages over the previous version. First, the device can stimulate four independent skin sites and is primarily designed for stimulating the digit tips. Second, the positioning of the probe tips has been re-designed to provide better ergonomic hand placement. Third, the device is much more portable than the previously reported stimulator. Fourth, the stimulator head has a much smaller footprint on the table or surface where it resides. To demonstrate the capacity of the device for delivering tactile stimulation at four independent sites, a finger agnosia protocol, in the presence and absence of conditioning stimuli, was conducted on seventeen healthy control subjects. The study demonstrated that with increasing amplitudes of vibrotactile conditioning stimuli concurrent with the agnosia test, inaccuracies of digit identification increased, particularly at digits D3 and D4. The results are consistent with prior studies that implicated synchronization of adjacent and near-adjacent cortical ensembles with conditioning stimuli in impacting TOJ performance (Tommerdahl et al., 2007a,b).

Continuous theta-burst rTMS over primary somatosensory cortex modulates tactile perception on the hand

OBJECTIVE

Theta-burst stimulation (TBS) over the primary somatosensory cortex (SI) alters cortical excitability, and in its intermittent form (iTBS) improves tactile spatial acuity. The effects of continuous TBS (cTBS) on tactile acuity remain unknown. The present study examined the influence of cTBS over SI on temporal and spatial tactile acuity on the contralateral hand.

METHODS

In separate experiments, temporal discrimination threshold (TDT) and spatial amplitude discrimination threshold (SDT) were obtained from the right hand before and for up to 34 min following real and sham cTBS (600 pulses) over left-hemisphere SI.

RESULTS

CTBS reduced temporal and spatial tactile acuity for up to 18 min following real cTBS. Tactile acuity was unaltered in the groups receiving sham cTBS.

CONCLUSIONS

CTBS over SI impairs both temporal and spatial domains of tactile acuity for a similar duration.

SIGNIFICANCE

CTBS over SI appears to decrease neural activity within targeted cortex and has potential utility in reducing excessive sensory processing.

Interactions within the hand representation in primary somatosensory cortex of primates

Previous studies indicate that primary somatosensory cortical area 3b in macaques contains a somatotopic map of the hand, encompassing representations of each digit. However, numerous observations including recent findings in anesthetized New World monkeys indicate that that the digit representations within the map are not discrete. We assessed the generality and spatial extent of these effects in awake macaques. We show that, within a given digit representation, (1) there is response to stimulation of all other digits tested, extending across most or all of the digit map, and (2) response to stimulation of the locally preferred digit is modulated by concurrent stimulation of each of the other digits. Control experiments rule out effects of attention and mechanical spread of stimulation. We thus confirm that, even at the first level of somatosensory cortical processing, inputs from potentially all of the digits frame the context within which the input to a single digit is represented.

Mislocalization of near-threshold tactile stimuli in humans: a central or peripheral phenomenon?

Principles of brain function can be disclosed by studying their limits during performance. Tactile stimuli with near-threshold intensities have been used to assess features of somatosensory processing. When stimulating fingers of one hand using near-threshold intensities, localization errors are observed that deviate significantly from responses obtained by guessing - incorrectly located stimuli are attributed more often to fingers neighbouring the stimulated one than to more distant fingers. Two hypotheses to explain the findings are proposed. The 'central hypothesis' posits that the degree of overlap of cortical tactile representations depends on stimulus intensity, with representations less separated for near-threshold stimuli than for suprathreshold stimuli. The 'peripheral hypothesis' assumes that systematic mislocalizations are due to activation of different sets of skin receptors with specific thresholds. The present experiments were designed to decide between the two hypotheses. Taking advantage of the frequency tuning of somatosensory receptors, their contribution to systematic misclocalizations was studied. In the first experiment, mislocalization profiles were investigated using vibratory stimuli with frequencies of 10, 20 and 100 Hz. Unambiguous mislocalization effects were only obtained for the 10-Hz stimulation, precluding the involvement of Pacinian corpuscles in systematic mislocalization. In the second experiment, Pacinian corpuscles were functionally eliminated by applying a constant 100-Hz vibratory masking stimulus together with near-threshold pulses. Despite masking, systematic mislocation patterns were observed rendering the involvement of Pacinian corpuscles unlikely. The results of both experiments are in favor of the 'central hypothesis' assuming that the extent of overlap in somatosensory representations is modulated by stimulus intensity.

High-resolution functional magnetic resonance imaging mapping of noxious heat and tactile activations along the central sulcus in New World monkeys

This study mapped the fine-scale functional representation of tactile and noxious heat stimuli in cortical areas around the central sulcus of anesthetized squirrel monkeys by using high-resolution blood oxygen level-dependent (BOLD) fMRI at 9.4T. Noxious heat (47.5°C) stimulation of digits evoked multiple spatially distinct and focal BOLD activations. Consistent activations were observed in areas 3a, 3b, 1, and 2, whereas less frequent activation was present in M1. Compared with tactile activations, thermal nociceptive activations covered more area and formed multiple foci within each functional area. In general, noxious heat activations in area 3b did not colocalize with tactile responses. The spatial relationships of heat and tactile activations in areas 3a and 1/2 varied across animals. Subsequent electrophysiological mapping confirmed that the evoked heat and tactile BOLD signals were somatotopically appropriate. The magnitude and temporal profiles of the BOLD signals to noxious heat stimuli differed across cortical areas. Comparatively late-peaking but stronger signals were observed in areas 3b and 2, whereas earlier-peaking but weaker signals were observed in areas 3a, 1, and M1. In sum, this study not only confirmed the involvement of somatosensory areas of 3a, 3b, and 1, but also identified the engagements of area 2 and M1 in the processing of heat nociceptive inputs. Differential BOLD response profiles of the individual cortical areas along the central sulcus suggest that these areas play different roles in the encoding of nociceptive inputs. Thermal nociceptive and tactile inputs may be processed by different clusters of neurons in different areas. To critically bridge animal and human pain studies, human fMRI was related to primate fMRI and electrophysiology of nociceptive processing, examining the functional role of the primary somatosensory cortex in heat nociception and demonstrating that subregion areas 3a, 3b, 1, 2, and M1 are responsive to noxious heat stimuli.

Modular processing in the hand representation of primate primary somatosensory cortex coexists with widespread activation

Neurons in the hand representation of primary somatosensory cortex (area 3b) are known to have discretely localized receptive fields; and these neurons form modules that can be visualized histologically as distinct digit and palm representations. Despite these indicators of the importance of local processing in area 3b, widespread interactions between stimuli presented to locations across the hand have been reported. We investigated the relationship of neuron firing rate with distance from the site of maximum activation in cortex by recording from a 100-electrode array with electrodes spaced 400 μm apart, implanted into the area 3b hand representation in anesthetized owl monkeys. For each stimulated location on the hand, the electrode site where neurons had the highest peak firing rate was defined as the peak activation site. The lesser firing rates of neurons at all other electrode sites in the grid were compared with the firing rates of neurons at the peak activation site. On average, peak firing rates of neurons decreased rapidly with distance away from the peak activation site. The effect of distance on the variance of firing rates was highly significant (P < 0.0001). However, individual neurons retained high firing rates for distances over 3 mm. The clear decline in firing rate with distance from the most activated location indicates that local processing is emphasized in area 3b, while the distance of neurons with reduced but maintained firing rates ≤3-4 mm from the site of best activation demonstrated widespread activation in primary somatosensory cortex.

Differentiation of somatosensory cortices by high-resolution fMRI at 7 T

This study aimed to evaluate the ability of BOLD signals at high MRI field (7 T) to map fine-scale single-digit activations in subdivisions (areas 3b and 1) of the human primary somatosensory cortex (SI) in individual subjects. We acquired BOLD fMRI data from cortical areas around the central suclus in six healthy human subjects while stimulating individual finger pads with 2-Hz air puffs. Discrete, single-digit responses were identified in an area along the posterior bank of the central sulcus corresponding to area 3b and in an area along the crest of the postcentral gyrus corresponding to area 1. In single subjects, activations of digits 1 to 4 in both areas 3b and 1 were organized in a somatotopic manner. The separation of digit representations was measured for adjacent digits and was approximately 1.6 times greater in area 3b than in area 1. Within individual subjects, the cortical responses to single-digit stimulations and the magnitude of the BOLD signals were reproducible across imaging runs and were comparable across subjects. Our findings demonstrate that BOLD fMRI at 7 T is capable of revealing the somatotopic organization of single-digit activations with good within-subject reliability and reproducibility, and activation maps can be acquired within a reasonably short time window, which are essential characteristics for several neurological applications within patient populations.

Cutaneous stimulation of the digits and lips evokes responses with different adaptation patterns in primary somatosensory cortex

Neuromagnetic evoked fields were recorded to compare the adaptation of the primary somatosensory cortex (SI) response to tactile stimuli delivered to the glabrous skin at the fingertips of the first three digits (condition 1) and between midline upper and lower lips (condition 2). The stimulation paradigm allowed to characterize the response adaptation in the presence of functional integration of tactile stimuli from adjacent skin areas in each condition. At each stimulation site, cutaneous stimuli (50 ms duration) were delivered in three runs, using trains of 6 pulses with regular stimulus onset asynchrony (SOA). The pulses were separated by SOAs of 500 ms, 250 ms or 125 ms in each run, respectively, while the inter-train interval was fixed (5s) across runs. The evoked activity in SI (contralateral to the stimulated hand, and bilaterally for lips stimulation) was characterized from the best-fit dipoles of the response component peaking around 70 ms for the hand stimulation, and 8 ms earlier (on average) for the lips stimulation. The SOA-dependent long-term adaptation effects were assessed from the change in the amplitude of the responses to the first stimulus in each train. The short-term adaptation was characterized by the lifetime of an exponentially saturating model function fitted to the set of suppression ratios of the second relative to the first SI response in each train. Our results indicate: 1) the presence of a rate-dependent long-term adaptation effect induced only by the tactile stimulation of the digits; and 2) shorter recovery lifetimes for the digits compared with the lips stimulation.

Response properties of neurons in primary somatosensory cortex of owl monkeys reflect widespread spatiotemporal integration

Receptive fields of neurons in somatosensory area 3b of monkeys are typically described as restricted to part of a single digit or palm pad. However, such neurons are likely involved in integrating stimulus information from across the hand. To evaluate this possibility, we recorded from area 3b neurons in anesthetized owl monkeys with 100-electrode arrays, stimulating two hand locations with electromechanical probes simultaneously or asynchronously. Response magnitudes and latencies of single- and multiunits varied with stimulus conditions, and multiunit responses were similar to single-unit responses. The mean peak firing rate for single neurons stimulated within the preferred location was estimated to be ∼26 spike/s. Simultaneous stimulation with a second probe outside the preferred location slightly decreased peak firing rates to ∼22 spike/s. When the nonpreferred stimulus preceded the preferred stimulus by 10-500 ms, peak firing rates were suppressed with greatest suppression when the nonpreferred stimulus preceded by 30 ms (∼7 spike/s). The mean latency for single neurons stimulated within the preferred location was ∼23 ms, and latency was little affected by simultaneous paired stimulation. However, when the nonpreferred stimulus preceded the preferred stimulus by 10 ms, latencies shortened to ∼16 ms. Response suppression occurred even when stimuli were separated by long distances (nonadjacent digits) or long times (500 ms onset asynchrony). Facilitation, though rare, occurred most often when the stimulus onsets were within 0-30 ms of each other. These findings quantify spatiotemporal interactions and support the hypothesis that area 3b is involved in widespread stimulus integration.

Intra- and inter-subject variability of high field fMRI digit maps in somatosensory area 3b of new world monkeys

This study evaluates the intra- and inter-subject variability of digit maps in area 3b of anesthetized squirrel monkeys. Maps were collected using high field blood oxygenation level dependent (BOLD) functional magnetic resonance imaging (fMRI). BOLD responses to individual digit stimulations were mapped and their response properties (location, area of activation, % signal change, time to peak response) were compared within and across imaging sessions separated by up to 20 months. During single digit stimulation using a block design, the spatiotemporal response of the BOLD signal for individual runs within and across sessions and animals was well conserved, with a time to peak BOLD response of 20+/-4 s. The variability in the center of BOLD activation in area 3b was 0.41+/-0.24 mm (mean+/-SD) across individual 5-7 min runs within a scanning session and 0.55+/-0.15 mm across sessions. The average signal change across all animals, runs and sessions was 0.62+/-0.38%, and varied 32% within and 40% across sessions. In a comparison of the stability and reproducibility of the area of single digit activation obtained using three approaches, use of a fixed statistical threshold (P<10(-5)) yielded an average area of 4.8+/-3.5 mm(2) (mean+/-SD), adaptive statistical thresholding 1.32+/-1.259 mm(2) (mean+/-SD), and combined fixed statistical and adaptive BOLD signal amplitude 4.4+/-2.5 mm(2) (mean+/-SD) across image runs and sessions. The somatotopic organization was stable within animals across sessions, while across animals, there was some variation in overall activation pattern and inter-digit distances. These results confirm that BOLD activation maps of single digits in area 3b as characterized by activation center, signal amplitudes, and temporal profile are very stable. The activation sizes determined by various criteria are the most variable measure in this preparation, but adaptive statistical thresholding appears to yield the most stable and reproducible maps. This study serves as a baseline assessment of the limits imposed on the detection of plastic changes by experimental variations of the digit BOLD fMRI activation maps in normal animals, and as an indicator of the likely performance limits in human studies.

Optical imaging of digit topography in individual awake and anesthetized squirrel monkeys

Topographic maps and columnar structures are fundamental to cortical sensory information processing. Most of the knowledge about detailed topographic maps and columnar structure comes mainly from experiments conducted on anesthetized animals. Towards the goal of evaluating whether topographic maps change with respect to behavioral demands, we used intrinsic signal optical imaging in alert monkeys to examine the spatial specificity of cortical topographic representation. Specifically, the somatotopies of neighboring distal finger pad representation in areas 3b and 1 were examined in the same awake and anesthetized squirrel monkey. In comparison to the anesthetized animal, we found larger cortical activation sizes in the alert animal in area 3b, where activation widths were found to overlap with even non-adjacent digits. This may suggest that in the alert animal, there is less inhibition across the somatotopic map within area 3b.

Area-specific representation of mechanical nociceptive stimuli within SI cortex of squirrel monkeys

While functional imaging studies in humans have consistently reported activation of primary somatosensory cortex (SI) with painful stimuli, the specific roles of subdivisions of areas 3a, 3b, and 1 within SI during pain perception are largely unknown, particularly in the representation of mechanical evoked pain. In this study, we investigated how modality, location, and intensity of nociceptive stimuli are represented within SI by using high-spatial resolution optical imaging of intrinsic signals in Pentothal-anesthetized squirrel monkeys. Perceptually comparable mechanical nociceptive and innocuous tactile stimuli were delivered by indenting the glabrous skin of the distal finger pads with 0.2 and 2mm diameter probes, respectively. Within each of areas 3a, 3b, and 1, activations to mechanical nociceptive stimulation of individual distal finger pads were spatially distinct and somatotopically organized. We observed differential cortical activation patterns. Areas 3a, 3b, and 1 were all activated during mechanical nociceptive stimulation and were modulated by nociceptive stimulus intensity. However, with innocuous tactile stimulation, mainly areas 3b and 1 exhibited response modulation with different levels of stimulation. In summary, mechanical nociceptive inputs are area-specific and topographically represented within SI. We propose that all areas of SI are implicated in encoding the features of mechanical nociception, where areas 3a and 3b are distinctively involved in coding nociceptive and pressure sensation components of stimulation.