Cortical areas within the lateral sulcus connected to cutaneous representations in areas 3b and 1: a revised interpretation of the second somatosensory area in macaque monkeys

Cytoarchitecture and cortical connections of the posterior cingulate and adjacent somatosensory fields in the rhesus monkey

The cytoarchitecture and connections of the caudal cingulate and medial somatosensory areas were investigated in the rhesus monkey. There is a stepwise laminar differentiation starting from retrosplenial area 30 towards the isocortical regions of the medial parietal cortex. This includes a gradational emphasis on supragranular laminar organization and general reduction of the infragranular neurons as one proceeds from area 30 toward the medial parietal regions, including areas 3, 1, 2, 5, 31, and the supplementary sensory area (SSA). This trend includes a progressive increase in layer IV neurons. Area 23c in the lower bank and transitional somatosensory area (TSA) in the upper bank of the cingulate sulcus appear as nodal points. From area 23c and TSA the architectonic progression can be traced in three directions: one culminates in areas 3a and 3b (core line), the second in areas 1, 2, and 5 (belt line), and the third in areas 31 and SSA (root line). These architectonic gradients are reflected in the connections of these regions. Thus, cingulate areas (30, 23a, and 23b) are connected with area 23c and TSA on the one hand and have widespread connections with parieto-temporal, frontal, and parahippocampal (limbic) regions on the other. Area 23c has connections with areas 30, 23a and b, and TSA as well as with medial somatosensory areas 3, 1, 2, 5, and SSA. Area 23c also has connections with parietotemporal, frontal, and limbic areas similar to areas 30, 23a, and 23b. Area TSA, like area 23c, has connections with areas 3, 1, 2, 5, and SSA. However, it has only limited connections with the parietotemporal and frontal regions and none with the parahippocampal gyrus. Medial area 3 is mainly connected to medial and dorsal sensory areas 3, 1, 2, 5, and SSA and to areas 4 and 6 as well as to supplementary (M2 or area 6m), rostral cingulate (M3 or areas 24c and d), and caudal cingulate (M4 or areas 23c and d) motor cortices. Thus, in parallel with the architectonic gradient of laminar differentiation, there is also a progressive shift in the pattern of corticocortical connections. Cingulate areas have widespread connections with limbic, parietotemporal, and frontal association areas, whereas parietal area 3 has more restricted connections with adjacent somatosensory and motor cortices. TSA is primarily related to the somatosensory-motor areas and has limited connections with the parietotemporal and frontal association cortices.

Ipsilateral representation of oral structures in human anterior parietal somatosensory cortex and integration of inputs across the midline

Anterior parietal somatosensory areas 3a, 3b, 1, and 2 generally contain cells with receptive fields that are on the contralateral body. However, inputs from midline structures such as the mouth must be uniquely integrated across the midline. This hypothesis is supported by studies of these fields from nonhuman primates that demonstrate ipsilateral representations of oral structures. We used magnetoencephalography (MEG) to examine the cortical representations of the lips and tongue in humans and to examine the time course of interaction of bilateral inputs from these structures. Ipsilateral activation was observed in response to tactile stimulation of the upper lip in 69% of cases, the lower lip in 85% of cases, and the tongue in 88% of cases. In the contralateral hemisphere, the map of oral structures tended to be in agreement with that from nonhuman primates, although variation was large and source locations were not statistically significantly different from each other. There were no differences in latency of activation for ipsi-vs. contralateral responses (about 30 msec), and cortical sources from ipsi-and contralateral stimulation tended to be located together. Differential activation for bilateral vs. unilateral stimulation occurred later than activation in S1, around 110 msec, and was localized to the upper bank of the Sylvian sulcus. Our findings indicate that, unlike nonhuman primates, humans have an ipsilateral representation of the lips in 3b/1, possibly related to the precise manipulation necessary for the articulation of speech. The distinct pattern of differential activation for uni-vs. bilateral stimulation suggests a unique neural mechanism of integration across the midline for inputs from the mouth.

Anatomical and functional organization of somatosensory areas of the lateral fissure of the New World titi monkey (Callicebus moloch)

The organization of anterior and lateral somatosensory cortex was investigated in titi monkeys (Callicebus moloch). Multiunit microelectrode recordings were used to identify multiple representations of the body, and anatomical tracer injections were used to reveal connections. (1) Representations of the face were identified in areas 3a, 3b, 1, S2, and the parietal ventral area (PV). In area 3b, the face was represented from chin/lower lip to upper lip and neck/upper face in a rostrocaudal sequence. The representation of the face in area 1 mirrored that of area 3b. Another face representation was located in area 3a. Adjoining face representations in S2 and PV exhibited mirror-image patterns to those of areas 3b and 1. (2) Two representations of the body, the rostral and caudal ventral somatosensory areas (VSr and VSc), were found in the dorsal part of the insula. VSc was roughly a reversal image of the S2 body representation, and VSr was roughly a reversal of PV. (3) Neurons in the insula next to VSr and VSc responded to auditory stimuli or to both auditory and somatosensory stimuli. (4) Injections of tracers within the hand representations in areas 3b, 1, and S2 revealed reciprocal connections between these three areas. Injections in areas 3b and 1 labeled the ventroposterior nucleus, whereas injections in S2 labeled the inferior ventroposterior nucleus. The present study demonstrates features of somatosensory cortex of other monkeys in titi monkeys, while revealing additional features that likely apply to other primates.

The human primary somatosensory cortex response contains components related to stimulus frequency and perception in a frequency discrimination task

Somatosensory stimulation of primary somatosensory cortex (SI) using frequency discrimination offers a direct, well-defined and accessible way of studying cortical decisions at the locus of early input processing. Animal studies have identified and classified the neuronal responses in SI but they have not yet resolved whether during prolonged stimulation the collective SI response just passively reflects the input or actively participates in the comparison and decision processes. This question was investigated using tomographic analysis of single trial magnetoencephalographic data. Four right-handed males participated in a frequency discrimination task to detect changes in the frequency of an electrical stimulus applied to the right-hand digits 2+3+4. The subjects received approximately 600 pairs of stimuli with Stim1 always at 21 Hz, while Stim2 was either 21 Hz (50%) or varied from 22 to 29 Hz in steps of 1 Hz. Both stimuli were 1 s duration, separated by a 1 s interval of no stimulation. The left-SI was the most consistently activated area and showed the first activation peak at 35-48 ms after Stim1 onset and sustained activity during both stimulus periods. During the Stim2 period, we found that the left-SI activation started to differ significantly between two groups of trials (21 versus 26-29 Hz) within the first 100 ms and this difference was sustained and enhanced thereafter (approximately 600 ms). When only correct responses from the above two groups were used, the difference was even higher at later latencies (approximately 650 ms). For one subject who had enough trials of same perception to different input frequencies, e.g. responded 21 Hz to Stim2 at 21 Hz (correct) and 26-29 Hz (error), we found the sustained difference only before 650 ms. Our results suggest that SI is involved with the analysis of an input frequency and related to perception and decision at different latencies.

Cortical connections of the second somatosensory area and the parietal ventral area in macaque monkeys

To gain insight into how cortical fields process somatic inputs and ultimately contribute to complex abilities such as tactile object perception, we examined the pattern of connections of two areas in the lateral sulcus of macaque monkeys: the second somatosensory area (S2), and the parietal ventral area (PV). Neuroanatomical tracers were injected into electrophysiologically and/or architectonically defined locations, and labeled cell bodies were identified in cortex ipsilateral and contralateral to the injection site. Transported tracer was related to architectonically defined boundaries so that the full complement of connections of S2 and PV could be appreciated. Our results indicate that S2 is densely interconnected with the primary somatosensory area (3b), PV, and area 7b of the ipsilateral hemisphere, and with S2, 7b, and 3b in the opposite hemisphere. PV is interconnected with areas 3b and 7b, with the parietal rostroventral area, premotor cortex, posterior parietal cortex, and with the medial auditory belt areas. Contralateral connections were restricted to PV in the opposite hemisphere. These data indicate that S2 and PV have unique and overlapping patterns of connections, and that they comprise part of a network that processes both cutaneous and proprioceptive inputs necessary for tactile discrimination and recognition. Although more data are needed, these patterns of interconnections of cortical fields and thalamic nuclei suggest that the somatosensory system may not be segregated into two separate streams of information processing, as has been hypothesized for the visual system. Rather, some fields may be involved in a variety of functions that require motor and sensory integration.

Regional cerebral blood flow correlations of somatosensory areas 3a, 3b, 1, and 2 in humans during rest: a PET and cytoarchitectural study

The concept of functional connectivity relies on the assumption that cortical areas that are directly anatomically connected will show correlations in regional blood flow (rCBF) or regional metabolism. We studied correlations of rCBF of cytoarchitectural areas 3a, 3b, 1, and 2 in the brains of 37 subjects scanned with PET during a rest condition. The cytoarchitectural areas, delineated from 10 postmortem brains with statistical methods, were transformed into the same standard anatomical format as the resting PET images. In areas 3a, 3b, and 1, somatotopically corresponding regions were intercorrelated. Area 2 was correlated with the dorsal pre-motor area. These results were in accordance with the somatosensory connectivity in macaque monkeys. In contrast, we also found correlations between areas 3b and 1 with area 4a, and SMA, and among the left and right hand sector of areas 3a, 3b, and 1. Furthermore, there were no correlations between areas 3b, 1, and 2 with SII or other areas in the parietal operculum, nor of other areas known to be directly connected with areas 3a, 3b, 1, and 2 in macaques. This indicates that rCBF correlations between cortical areas during the rest state only partly reflect their connectivity and that this approach lacks sensitivity and is prone to reveal spurious or indirect connectivity.

Correlated neuronal discharges that increase coding efficiency during perceptual discrimination

During a sensory discrimination task, the responses of multiple sensory neurons must be combined to generate a choice. The optimal combination of responses is determined both by their dependence on the sensory stimulus and by their cofluctuations across trials-that is, the noise correlations. Positively correlated noise is considered deleterious, because it limits the coding accuracy of populations of similarly tuned neurons. However, positively correlated fluctuations between differently tuned neurons actually increase coding accuracy, because they allow the common noise to be subtracted without signal loss. This is demonstrated with data recorded from the secondary somatosensory cortex of monkeys performing a vibrotactile discrimination task. The results indicate that positive correlations are not always harmful and may be exploited by cortical networks to enhance the neural representation of features to be discriminated.

Somatosensory cortex of prosimian Galagos: physiological recording, cytoarchitecture, and corticocortical connections of anterior parietal cortex and cortex of the lateral sulcus

Compared with our growing understanding of the organization of somatosensory cortex in monkeys, little is known about prosimian primates, a major branch of primate evolution that diverged from anthropoid primates some 60 million years ago. Here we describe extensive results obtained from an African prosimian, Galago garnetti. Microelectrodes were used to record from large numbers of cortical sites in order to reveal regions of responsiveness to cutaneous stimuli and patterns of somatotopic organization. Injections of one to several distinguishable tracers were placed at physiologically identified sites in four different cortical areas to label corticortical connections. Both types of results were related to cortical architecture. Three systematic representations of cutaneous receptors were revealed by the microelectrode recordings, S1 proper or area 3b, S2, and the parietal ventral area (PV), as described in monkeys. Strips of cortex rostral (presumptive area 3a) and caudal (presumptive area 1-2) to area 3b responded poorly to tactile stimuli in anesthetized galagos, but connection patterns with area 3b indicated that parallel somatosensory representations exist in both of these regions. Area 3b also interconnected somatotopically with areas S2 and PV. Areas S2 and PV had connections with areas 3a, 3b, 1-2, each other, other regions of the lateral sulcus, motor cortex (M1), cingulate cortex, frontal cortex, orbital cortex, and inferior parietal cortex. Connection patterns and recordings provided evidence for several additional fields in the lateral sulcus, including a retroinsular area (Ri), a parietal rostral area (PR), and a ventral somatosensory area (VS). Galagos appear to have retained an ancestral preprimate arrangement of five basic areas (S1 proper, 3a, 1-2, S2, and PV). Some of the additional areas suggested for lateral parietal cortex may be primate specializations.

Single trial fMRI reveals significant contralateral bias in responses to laser pain within thalamus and somatosensory cortices

Pain is processed in multiple brain areas, indicating the complexity of pain perception. The ability to locate pain plays a pivotal role in immediate defense and withdrawal behavior. However, how the brain localizes nociceptive information without additional information from somatotopically organized mechano-receptive pathways is not well understood. We used single-trial functional magnetic resonance imaging (fMRI) to assess hemodynamic responses to right and left painful stimulation. Thulium-YAG-(yttrium-aluminium-granate)-laser-evoked pain stimuli, without concomitant tactile component, were applied to either hand in a randomized order. A contralateral bias of the BOLD response was investigated to determine areas involved in the coding of the side of stimulation, which we observed in primary (SI) and secondary (SII) somatosensory cortex, insula, and the thalamus. This suggests that these structures provide spatial information of selective nociceptive stimuli. More importantly, this contralateral bias of activation allowed functionally segregated activations within the SII complex, the insula, and the thalamus. Only distinct subregions of the SII complex, the posterior insula and the lateral thalamus, but not the remaining SII complex, the anterior insula and the medial thalamus, showed a contralaterally biased representation of painful stimuli. This result supports the hypothesis that sensory-discriminative attributes of painful stimuli, such as those related to body side, are topospecifically represented within the forebrain projections of the nociceptive system and highlights the concept of functional segregation and specialization within these structures.

Flutter discrimination: neural codes, perception, memory and decision making

Recent studies combining psychophysical and neurophysiological experiments in behaving monkeys have provided new insights into how several cortical areas integrate efforts to solve a vibrotactile discrimination task. In particular, these studies have addressed how neural codes are related to perception, working memory and decision making in this model. The primary somatosensory cortex drives higher cortical areas where past and current sensory information are combined, such that a comparison of the two evolves into a behavioural decision. These and other observations in visual tasks indicate that decisions emerge from highly-distributed processes in which the details of a scheduled motor plan are gradually specified by sensory information.

From sensation to action

Key to understanding somatosensation is the form of how the mechanical stimuli are represented in the evoked neuronal activity of the brain. Here, we focus on studies that address the question of which components of the evoked neuronal activity in the somatosensory system represent the stimulus features. We review experiments that probe whether these neuronal representations are essential to somatosensation. We also discuss recent results that suggest how the somatosensory stimuli are represented in the brain during short-term memory. Finally, we review data that show the neuronal correlates of a decision during somatosensory perception.

Processing of tactile and kinesthetic signals from bilateral sides of the body in the postcentral gyrus of awake monkeys

Earlier studies pointed out that in the primary somatosensory cortex (SI) the receptive fields (RF) of bilateral neurons were related exclusively to the body midline. We recently found a substantial number of neurons with bilateral RFs on hand digits, shoulders/arms or legs/feet in the caudalmost part (areas 2 and 5) of the postcentral gyrus in awake macaque monkeys. The RFs of these neurons were generally of the most complex types found in this region of the cortex, and thus they were considered to be at the highest level along the hierarchical chain of information processing. We conclude that there are two types of bilateral RFs in the postcentral gyrus, one representing the midline structures such as the intraoral cavity, chin or trunk and the other related to limb structures such as fingers, hands, arms, shoulders, legs and girdles. Functional significance of the bilateral activity could be understood in behavioral context as it is seen more extensively in the body parts where bilateral coordination is essential.

Exploring the cortical evidence of a sensory-discrimination process

Humans and monkeys have similar abilities to discriminate the difference in frequency between two consecutive mechanical vibrations applied to their fingertips. This task can be conceived as a chain of neural operations: encoding the two consecutive stimuli, maintaining the first stimulus in working memory, comparing the second stimulus with the memory trace left by the first stimulus and communicating the result of the comparison to the motor apparatus. We studied this chain of neural operations by recording and manipulating neurons from different areas of the cerebral cortex while monkeys performed the task. The results indicate that neurons of the primary somatosensory cortex (S1) generate a neural representation of vibrotactile stimuli which correlates closely with psychophysical performance. Discrimination based on microstimulation patterns injected into clusters of S1 neurons is indistinguishable from that produced by natural stimuli. Neurons from the secondary somatosensory cortex (S2), prefrontal cortex and medial premotor cortex (MPC) display at different times the trace of the first stimulus during the working-memory component of the task. Neurons from S2 and MPC appear to show the comparison between the two stimuli and correlate with the behavioural decisions. These neural operations may contribute to the sensory-discrimination process studied here.

Parabrachio-cortical connections with the lateral hemisphere in the madagascan hedgehog tenrec: prominent projections to layer 1, weak projections from layer 6

The present study was undertaken to further characterize and subdivide the rhinal cortex (insular and perirhinal areas) in the hedgehog tenrec (Echinops telfairi), a placental mammal with a rather low encephalisation index. Injections of wheat germ agglutinin-horseradish peroxidase into the dorsolateral pontine tegmentum revealed a prominent layer 1 projection to several rhinal target areas, while the rhinal cortex only stained weakly for the calcitonin gene-related peptide. Among the regions retrogradely labeled following tracer injections into the rhinal cortex, the parabrachial nucleus was considered the main origin of the tegmento-cortical projection. This conclusion was based on the circumscribed pattern of termination, as well as the differences noted between the pattern of anterograde labeling and the pattern obtained by thyrosine hydroxylase immunohistochemistry. The tracer injections into the dorsolateral tegmentum also revealed numerous retrogradely labeled cells in the layer 5 of the dorsomedial frontal cortex. In contrast, the rhinal cortex only showed few labeled cells and most of these cells were located in the layer 6/7. A comparison with other species indicates that the tenrec's parabrachial nucleus gives rise to the most extensive cortical projections but receives the least prominent input from the lateral cerebral hemisphere. The layer 6/7 projection may be a common mammalian feature but it is overshadowed by the layer 5 projection in higher mammals.

Temporal evolution of a decision-making process in medial premotor cortex

The events linking sensory discrimination to motor action remain unclear. It is not known, for example, whether the motor areas of the frontal lobe receive the result of the discrimination process from other areas or whether they actively participate in it. To investigate this, we trained monkeys to discriminate between two mechanical vibrations applied sequentially to the fingertips; here subjects had to recall the first vibration, compare it to the second one, and indicate with a hand/arm movement which of the two vibrations had the higher frequency. We recorded the activity of single neurons in medial premotor cortex (MPC) and found that their responses correlate with the diverse stages of the discrimination process. Thus, activity in MPC reflects the temporal evolution of the decision-making process leading to action selection during this perceptual task.

Cortical and thalamic connections of the parietal ventral somatosensory area in marmoset monkeys (Callithrix jacchus)

Microelectrode mapping methods were used to define the parietal ventral somatosensory area (PV) on the upper bank of the lateral sulcus in five marmosets (Callithrix jacchus). In the same animals, neuroanatomical tracers were placed into electrophysiologically identified sites in PV and/or the second somatosensory area (S2). Foci of anterograde and retrograde label were related to electrophysiological maps of cortical areas and cortical and thalamic architecture. The results lead to the following conclusions: (1) Multiunit recordings from cortex on the upper bank of the lateral sulcus demonstrate that PV is somatotopically organized, with the face representation adjoining area 3b and the hindlimb and tail representations away from this border in cortex deep on the upper bank of the lateral sulcus. The forelimb representation is caudal in PV adjacent to the S2 forelimb representation. The body surface representation in PV approximates a mirror image of that in S2; (2) Areas PV and S2 are less myelinated and have less cytochrome oxidase enzyme activity than area 3b; (3) The ventroposterior inferior nucleus (VPI) of the thalamus provides the major somatosensory projections to PV. PV is reciprocally connected with VPI and anterior pulvinar; (4) PV has ipsilateral cortical connections with areas 3a, 3b, 1, and M1 and higher order somatosensory fields, and at least most of these connections are somatotopically matched; and (5) Callosal connections of PV are with S2 and PV of the other cerebral hemisphere. These results further establish PV as one of at least four somatosensory areas of the lateral sulcus of primates.

Timing and connectivity in the human somatosensory cortex from single trial mass electrical activity

Parallel-distributed processing is ubiquitous in the brain but often ignored by experimental designs and methods of analysis, which presuppose sequential and stereotypical brain activations. We introduce here a methodology that can effectively deal with sequential and distributed activity. Regional brain activations elicited by electrical median nerve stimulation are identified in tomographic estimates extracted from single trial magnetoencephalographic signals. Habituation is identified in both primary somatosensory cortex (SI) and secondary somatosensory cortex (SII), often interrupted by resurgence of strong activations. Pattern analysis is used to identify single trials with homogeneous regional brain activations. Common activity patterns with well-defined connectivity are identified within each homogeneous group of single trials across the subjects studied. On the contralateral side one encounters distinct sets of single trials following identical stimuli. We observe in one set of trials sequential activation from SI to SII and insula with onset of SII at 60 msec, whereas in the other set simultaneous early co-activations of the same two areas.

Brain images of pain

Combined magneto- and encephalography proves the sequential involvement of multiple cortical structures in pain processing. Bilateral activity in secondary somatosensory cortices reflects the sensory-discriminative component and is reduced in states of unconsciousness. Later activity in the posterior cingulum reflects the emotional-aversive component, which is blocked by narcoanalgesics.

Touch and go: decision-making mechanisms in somatosensation

A complex sequence of neural events unfolds between sensory receptor activation and motor activity. To understand the underlying decision-making mechanisms linking somatic sensation and action, we ask what components of the neural activity evoked by a stimulus are directly related to psychophysical performance, and how are they related. We find that single-neuron responses in primary and secondary somatosensory cortices account for the observed performance of monkeys in vibrotactile discrimination tasks, and that neuronal and behavioral responses covary in single trials. This sensory activity, which provides input to memory and decision-making mechanisms, is modulated by attention and behavioral context, and microstimulation experiments indicate that it may trigger normal perceptual experiences. Responses recorded in motor areas seem to reflect the output of decision-making operations, which suggests that the ability to make decisions occurs at the sensory-motor interface.

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.

Evidence for interhemispheric processing of inputs from the hands in human S2 and PV

In the present investigation, we identified cortical areas involved in the integration of bimanual inputs in human somatosensory cortex. Using functional magnetic resonance imaging (fMRI) and magnetoencephalography (MEG), we compared the responses to unilateral versus bilateral stimulation in anterior parietal cortex and areas in the Sylvian fissure of the contralateral hemisphere. The extent of fMRI activation on the upper bank of the Sylvian fissure, in the second somatosensory (S2) and the parietal ventral (PV) areas, was significantly larger for bilateral stimulation than for unilateral stimulation. Using MEG, we were able to describe the latency of response in S1 and S2/PV to unilateral and bilateral stimulation. The MEG response had three components under both stimulus conditions. An early peak in S1 at 40 ms, a middle peak in S2/PV at 80-160 ms, and three late peaks in S2/PV at 250-420 ms. There was an increase in magnetic field strength in S2/PV to bilateral stimulation at 300-400 ms post stimulus. The fMRI results indicate that, as in monkeys, S2/PV receives inputs from both the contralateral and ipsilateral hand. The MEG data suggest that information is processed serially from S1 to S2. The very late response in S2/PV indicates that extensive intrahemispheric processing occurs before information is transferred to the opposite hemisphere. The neural substrate for the increased activation and field strength at long latencies during bilateral stimulation can be accounted for in three ways. Under bilateral stimulus conditions, more neurons may be active, neuronal firing rate may increase, and/or neural activity may be more synchronous.

Dopamine transporter immunoreactivity in monkey cerebral cortex: regional, laminar, and ultrastructural localization

Dopamine (DA) influences a number of cognitive and motor functions that are mediated by the primate cerebral cortex, and the DA membrane transporter (DAT) is known to be a critical regulator of DA neurotransmission in subcortical structures in rodents. To gain insight into the possible functional role of cortical DAT, we compared the regional, laminar, and ultrastructural distribution of DAT immunoreactivity to that of tyrosine hydroxylase (TH), the rate-limiting enzyme in DA synthesis, in the cerebral cortex of macaque monkeys. DAT-immunoreactive (DAT-IR) axons were present throughout the cortical mantle, with substantial differences in density and laminar distribution across cytoarchitectonic areas. In particular, high densities of DAT-IR axons were present in certain regions (e.g., posterior parietal cortex, dentate gyrus) not previously thought to receive a substantial DA input. The laminar distribution of DAT-IR axons ranged from a restricted localization of labeled axons to layer 1 in lightly innervated regions to the presence of axons in all six cortical layers, with a particularly dense plexus in deep layer 3, in highly innervated regions. These regional and laminar patterns paralleled those of TH-IR axons, but several differences in fiber morphology and ultrastructural localization of DAT were observed. For example, in contrast to TH, DAT immunoreactivity in the cortex was localized predominantly to small-diameter profiles, whereas, in the dorsolateral caudate nucleus, DAT and TH immunoreactivities were present in both large-diameter and small-diameter profiles, which may represent varicose and intervaricose axon segments, respectively. Overall, the distribution of DAT-IR axons confirms and extends the results of previous reports, using other markers of DA axons, that the DA innervation of the primate cerebral cortex is global but specialized on both a regional basis and a laminar basis. In particular, these observations reveal an anatomical substrate for a direct and potent influence of DA over neuronal activity in posterior parietal cortex and in certain regions of the temporal lobe. However, due to its predominant distribution to small-diameter profiles, immunoreactivity for DAT may not be an appropriate ultrastructural marker for larger DA varicosities in the primate cortex. Moreover, this distribution of DAT suggests that cortical DA fibers may permit greater neurotransmitter diffusion than subcortical DA axons.

Referred phantom sensations and cortical reorganization after spinal cord injury in humans

To test the hypothesis that cortical remapping supports phantom sensations, we examined referred phantom sensations and cortical activation in humans after spinal-cord injury (SCI) at the thoracic level (T3-T12). Of 12 SCI subjects, 9 reported phantom sensations, and 2 reported referred phantom sensations. In both of these subjects, referred phantom sensations were evoked by contact in reference zones (RZ) that were not adjacent in the periphery and were not predicted to be adjacent in the postcentral gyrus (PoCG), suggesting that representations separated by centimeters of cortical space were simultaneously engaged. This finding was supported by functional MRI (fMRI). In a subject with a T6-level complete SCI, contact in RZ on the left or right forearm projected referred phantom sensations to the ipsilateral chest. During fMRI, contact in either forearm RZ evoked activity in the central PoCG (the position of the forearm representation) and the medial PoCG (the position of the chest representation) with >/=1.6 cm of nonresponsive cortex intervening. In contrast, stimulation in non-RZ forearm and palm regions in this subject and in lesion-matched SCI subjects evoked central but not medial PoCG activation. Our findings support a relation between PoCG activation and the percept of referred phantom sensations. These results, however, present an alternative to somatotopic cortical reorganization, namely, cortical plasticity expressed in coactivation of nonadjacent representations. The observed pattern suggests that somatotopic subcortical remapping, projected to the cortex, can support perceptual and cortical reorganization after deafferentation in humans.

Corticocortical connections of visual, sensorimotor, and multimodal processing areas in the parietal lobe of the macaque monkey

We studied the corticocortical connections of architectonically defined areas of parietal and temporoparietal cortex, with emphasis on areas in the intraparietal sulcus (IPS) that are implicated in visual and somatosensory integration. Retrograde tracers were injected into selected areas of the IPS, superior temporal sulcus, and parietal lobule. The distribution of labeled cells was charted in relation to architectonically defined borders throughout the hemisphere and displayed on computer-generated three-dimensional reconstructions and on cortical flat maps. Injections centered in the ventral intraparietal area (VIP) revealed a complex pattern of inputs from numerous visual, somatosensory, motor, and polysensory areas, and from presumed vestibular- and auditory-related areas. Sensorimotor projections were predominantly from the upper body representations of at least six somatotopically organized areas. In contrast, injections centered in the neighboring ventral lateral intraparietal area (LIPv) revealed inputs mainly from extrastriate visual areas, consistent with previous studies. The pattern of inputs to LIPv largely overlapped those to zone MSTdp, a newly described subdivision of the medial superior temporal area. These results, in conjunction with those from injections into other parietal areas (7a, 7b, and anterior intraparietal area), support the fine-grained architectonic partitioning of cortical areas described in the preceding study. They also support and extend previous evidence for multiple distributed networks that are implicated in multimodal integration, especially with regard to area VIP.

Mapping of architectonic subdivisions in the macaque monkey, with emphasis on parieto-occipital cortex

The intraparietal sulcus (IPS) of the macaque monkey contains numerous areas associated with different aspects of cortical function, including motor control as well as visual, somatosensory, vestibular, and possibly auditory processing. This study focuses largely on the architectonic organization of areas within and near the IPS, but also examines remaining portions of the hemisphere with which the IPS is interconnected. We charted the location of up to 72 architectonically distinct areas plus numerous architectonic zones in individuals over a region covering most of the cortical hemisphere. Identified cortical subdivisions (areas plus zones) were represented on computationally generated flat maps in relation to gyral and sulcal geography, thereby facilitating the analysis of consistent as well as variable aspects of the sizes and relative positions of subdivisions across animals. Using myelin and Nissl stains, plus immunohistochemical staining with the SMI-32 antibody, 17 architectonic subdivisions were identified that are largely or entirely contained in the intraparietal and parieto-occipital sulci. This includes four newly identified zones: a heavily myelinated lateral occipitoparietal zone, termed LOP; a strongly SMI-32 immunoreactive zone termed 7t (near the tip of the IPS); plus medial and lateral subdivisions (VIPm and VIPl) of ventral intraparietal area (VIP), which was previously regarded as an anatomically homogeneous area. Within the superior temporal sulcus, we identified a densely myelinated zone termed the dorso-posterior subdivision of the medial superior temporal area (MSTdp) that bordered middle temporal area (MT). We charted the extent of numerous other architectonically defined subdivisions throughout the cortical hemisphere by using criteria largely based on previous studies, but in some instances involving revised or expanded identification criteria.

Attending to and remembering tactile stimuli: a review of brain imaging data and single-neuron responses

Clinical and neuroimaging observations of the cortical network implicated in tactile attention have identified foci in parietal somatosensory, posterior parietal, and superior frontal locations. Tasks involving intentional hand-arm movements activate similar or nearby parietal and frontal foci. Visual spatial attention tasks and deliberate visuomotor behavior also activate overlapping posterior parietal and frontal foci. Studies in the visual and somatosensory systems thus support a proposal that attention to the spatial location of an object engages cortical regions responsible for the same coordinate referents used for guiding purposeful motor behavior. Tactile attention also biases processing in the somatosensory cortex through amplification of responses to relevant features of selected stimuli. Psychophysical studies demonstrate retention gradients for tactile stimuli like those reported for visual and auditory stimuli, and suggest analogous neural mechanisms for working memory across modalities. Neuroimaging studies in humans using memory tasks, and anatomic studies in monkeys support the idea that tactile information relayed from the somatosensory cortex is directed ventrally through the insula to the frontal cortex for short-term retention and to structures of the medial temporal lobe for long-term encoding. At the level of single neurons, tactile (such as visual and auditory) short-term memory appears as a persistent response during delay intervals between sampled stimuli.

Cortical representation of pain: functional characterization of nociceptive areas near the lateral sulcus

Many lines of evidence implicate the somatosensory areas near the lateral sulcus (Sylvian fissure) in the cortical representation of pain. Anatomical tracing studies in the monkey show nociceptive projection pathways to the vicinity of the secondary somatosensory cortex in the parietal operculum, and to anterior parts of insular cortex deep inside the Sylvian fissure. Clinical observations demonstrate alterations in pain sensation following lesions in these two areas in human parasylvian cortex. Imaging studies in humans reveal increased blood flow in parasylvian cortex, both contralaterally and ipsilaterally, in response to painful stimuli. Painful stimuli (such as laser radiant heat) evoke potentials with a scalp maximum at anterior temporal positions (T3 and T4). Several dipole source analyses as well as subdural recordings have confirmed that the earliest evoked potential following painful laser stimulation of the skin derives from sources in the parietal operculum. Thus, imaging and electrophysiological studies in humans suggest that parasylvian cortex is activated by painful stimuli, and is one of the first cortical relay stations in the central processing of these stimuli. There is mounting evidence for closely located but separate representations of pain (deep parietal operculum and anterior insula) and touch (secondary somatosensory cortex and posterior insula) in parasylvian cortex. This anatomical separation may be one of the reasons why single unit recordings of nociceptive neurons are scarce within regions comprising low-threshold mechanoreceptive neurons. The functional significance (sensory-discriminative, affective-motivational, cognitive-evaluative) of the closely spaced parasylvian cortical areas in acute and chronic pain is only poorly understood. It is likely that some of these areas are involved in sensory-limbic projection pathways that may subserve the recognition of potentially tissue damaging stimuli as well as pain memory.

Neuronal correlates of sensory discrimination in the somatosensory cortex

Monkeys are able to discriminate the difference in frequency between two periodic mechanical vibrations applied sequentially to the fingertips. It has been proposed that this ability is mediated by the periodicity of the responses in the quickly adapting (QA) neurons of the primary somatosensory cortex (S1), instead of the average firing rates. We recorded from QA neurons of S1 while monkeys performed the vibrotactile discrimination task. We found that the periodic mechanical vibrations can be represented both in the periodicity and in the firing rate responses to varying degrees across the QA neuronal population. We then computed neurometric functions by using both the periodicity and the firing rate and sought to determine which of these two measures is associated with the psychophysical performance. We found that neurometric thresholds based on the firing rate are very similar to the animal's psychometric thresholds whereas neurometric thresholds based on periodicity are far lower than those thresholds. These results indicate that an observer could solve this task with a precision similar to that of the monkey, based only on the firing rate produced during the stimulus periods.

Differential organization of touch and pain in human primary somatosensory cortex

Processing of tactile stimuli within somatosensory cortices has been shown to be complex and hierarchically organized. However, the precise organization of nociceptive processing within these cortices has remained largely unknown. We used whole-head magnetoencephalography to directly compare cortical responses to stimulation of tactile and nociceptive afferents of the dorsum of the hand in humans. Within the primary somatosensory cortex (SI), nociceptive stimuli activated a single source whereas tactile stimuli activated two sequentially peaking sources. Along the postcentral gyrus, the nociceptive SI source was located 10 mm more medially than the early tactile SI response arising from cytoarchitectonical area 3b and corresponded spatially to the later tactile SI response. Considering a mediolateral location difference between the hand representations of cytoarchitectonical areas 3b and 1, the present results suggest generation of the single nociceptive response in area 1, whereas tactile stimuli activate sequentially peaking sources in areas 3b and 1. Thus nociceptive processing apparently does not share the complex and hierarchical organization of tactile processing subserving elaborated sensory capacities. This difference in the organization of both modalities may reflect that pain perception rather requires reactions to and avoidance of harmful stimuli than sophisticated sensory capacities.

Somatotopic organization of cortical fields in the lateral sulcus of Homo sapiens: evidence for SII and PV

The human somatosensory cortex in the Sylvian fissure was examined using functional magnetic resonance imaging to describe the number and internal organization of cortical fields present. Somatic stimuli were applied to the lips, face, hand, trunk, and foot of 18 human subjects. Activity patterns were transposed onto three-dimensional magnetic resonance images of the brain so that the location of activity associated with the different stimuli could be related to specific regions of the cortex. There were several consistent findings. First, there were three regions of activity in the lateral sulcus associated with stimulation of the contralateral body. The most consistent locus of activation was on the upper bank of the lateral sulcus, continuing onto the operculum. The other two areas, one rostral and one caudal to this large central area, were smaller and were activated less consistently. Second, when activity patterns in the large central area resulting from stimulation of all body parts were considered, this region appeared to contain two fields that corresponded in location and somatotopic organization to the second somatosensory area (SII) and the parietal ventral area (PV). Finally, patterns of activation within SII and PV were somewhat variable across subjects. Repeated within-subject stimulus presentation indicated that differences across subjects were not due to inconsistent stimulus presentation. Comparisons with other mammals suggest that some features of organization are found only in primates. It is hypothesized that these features may be associated with manual dexterity and coordination of the hands, a characteristic generally restricted to the primate lineage.

Short communication: mapping of somatosensory cortices with functional magnetic resonance imaging in anaesthetized macaque monkeys

Functional magnetic resonance imaging (fMRI) in macaque monkeys is emerging as a potent candidate to bridge the gap between data from human fMRI studies and data from anatomy, electrophysiology and lesion studies in monkeys. The primary (SI) and secondary (SII) somatosensory cortices are the principal regions for somatosensory information processing and contain systematic representations of the body surface map (somatotopy). To examine the functional organization of the somatosensory cortices in anaesthetized macaque monkeys with fMRI, we asked whether focal and differential activation could be observed in SI and SII in response to tactile stimulation with two parameters: body sides (right and left) and body regions (hand and face). We found that changes in stimulus parameters elicited differential focal activation in both SI and SII in two ways. First, the hand and face stimulation activated SI and SII in the contralateral, but not in the ipsilateral, hemisphere. Second, the hand and face stimulation differentially activated two adjacent regions in both SI and SII. These fMRI results appear to correlate with previous mapping studies by other methods in the macaque somatosensory cortices. This study shows the feasibility of fMRI studies in mapping multiple sensory areas in monkeys by which we can distinguish between adjacent functionally distinct regions.

Role of the corpus callosum in the somatosensory activation of the ipsilateral cerebral cortex: an fMRI study of callosotomized patients

To verify whether the activation of the posterior parietal and parietal opercular cortices to tactile stimulation of the ipsilateral hand is mediated by the corpus callosum, a functional magnetic resonance imaging (fMRI, 1.0 tesla) study was performed in 12 control and 12 callosotomized subjects (three with total and nine with partial resection). Eleven patients were also submitted to the tactile naming test. In all subjects, unilateral tactile stimulation provoked a signal increase temporally correlated with the stimulus in three cortical regions of the contralateral hemisphere. One corresponded to the first somatosensory area, the second was in the posterior parietal cortex, and the third in the parietal opercular cortex. In controls, activation was also observed in the ipsilateral posterior parietal and parietal opercular cortices, in regions anatomically corresponding to those activated contralaterally. In callosotomized subjects, activation in the ipsilateral hemisphere was observed only in two patients with splenium and posterior body intact. These two patients and another four with the entire splenium and variable portions of the posterior body unsectioned named objects explored with the right and left hand without errors. This ability was impaired in the other patients. The present physiological and anatomical data indicate that in humans activation of the posterior parietal and parietal opercular cortices in the hemisphere ipsilateral to the stimulated hand is mediated by the corpus callosum, and that the commissural fibres involved probably cross the midline in the posterior third of its body.

Brain structures related to active and passive finger movements in man

A PET study was performed in six normal volunteers to elucidate the functional localization of the sensory afferent component during finger movement. Brain activation during the passive movement driven by a servo-motor was compared with that during an auditory-cued active movement which was controlled kinematically in the same way as the passive one. A newly developed device was used for selectively activating proprioception with a minimal contribution from tactile senses. Active movement was associated with activation of multiple areas, including the contralateral primary sensorimotor cortex, premotor cortex, supplementary motor area (SMA), bilateral secondary somatosensory areas and basal ganglia and ipsilateral cerebellum. In contrast, only the contralateral primary and secondary somatosensory areas were activated by the passive movement. It is likely that the contribution of proprioceptive input to the activation of the premotor cortex, SMA, cerebellum and basal ganglia, if any, is small. However, the present results do not rule out the possibility that the cutaneous afferent input or the combination of cutaneous and proprioceptive input participates in the activation of those areas during the active movement.

Responses of contralateral SI and SII in cat to same-site cutaneous flutter versus vibration

The methods of (14)C-2-deoxyglucose ((14)C-2DG) metabolic mapping and optical intrinsic signal (OIS) imaging were used to evaluate the response evoked in the contralateral primary somatosensory receiving areas (SI and SII) of anesthetized cats by either 25 Hz ("flutter") or 200 Hz ("vibration") sinusoidal vertical skin displacement stimulation of the central pad on the distal forepaw. Unilateral 25-Hz stimulation consistently evoked a localized region of elevated (14)C-2DG uptake in both SI and SII in the contralateral hemisphere. In contrast, 200-Hz stimulation did not evoke elevated (14)C-2DG uptake in the contralateral SI but evoked a prominent, localized region of increased (14)C-2DG uptake in the contralateral SII. Experiments in which the OIS was recorded yielded results that complemented and extended the findings obtained with the 2DG method. First, 25-Hz central-pad stimulation evoked an increase in absorbance in a region in the contralateral SI and SII that corresponded closely to the region in which a similar stimulus evoked increased (14)C-2DG uptake. Second, 200-Hz stimulation of the central pad consistently evoked a substantial increase in absorbance in the contralateral SII but very little or no increase in absorbance in the contralateral SI. And third, 200-Hz central-pad stimulation usually evoked a decrease in absorbance in the same contralateral SI region that underwent an increase in absorbance during same-site 25-Hz stimulation. Experiments in which the OIS responses of both SI and SII were recorded simultaneously demonstrated that continuous (>1 s) 25-Hz central-pad stimulation evokes a prominent increase in absorbance in both SI and SII in the contralateral hemisphere, whereas only SII undergoes a sustained prominent increase in absorbance in response to 200-Hz stimulation to the same central-pad site. SI exhibits an initial, transient increase in absorbance in response to 200-Hz stimulation and at durations of stimulation >1 s, undergoes a decrease in absorbance. It was found that the stimulus-evoked absorbance changes in the contralateral SI and SII are correlated significantly during vibrotactile stimulation of the central pad-positively with 25-Hz stimulation and negatively with 200-Hz stimulation. The findings are interpreted to indicate that 25-Hz central-pad stimulation of the central pad evokes spatially localized and vigorous neuronal activation within both SI and SII in the contralateral hemisphere and that although 200-Hz stimulation evokes vigorous and well maintained neuronal activation within the contralateral SII, the principal effect on the contralateral SI of a 200-Hz stimulus lasting >1 s is inhibitory.

Sensing and deciding in the somatosensory system

Combined psychophysical and neurophysiological experiments have revealed some of the neural codes associated with perception and processing of tactile information. Recently, intracortical microstimulation was used to demonstrate a causal link between primary cortical activity and perception. Evidence for a subsequent link, between a sensory decision process and its expression as a movement, has been found in motor areas.

Simultaneous early processing of sensory input in human primary (SI) and secondary (SII) somatosensory cortices

Simultaneous early processing of sensory input in human primary (SI) and secondary (SII) somatosensory cortices. The anatomic connectivity of the somatosensory system supports the simultaneous participation of widely separated cortical areas in the early processing of sensory input. We recorded evoked neuromagnetic responses noninvasively from human primary (SI) and secondary (SII) somatosensory cortices to unilateral median nerve stimulation. Brief current pulses were applied repetitively to the median nerve at the wrist at 2 Hz for 800-1,500 trials. A single pulse was omitted from the train at random intervals (15% of omissions). We observed synchronized neuronal population activity in contralateral SII area 20-30 ms after stimulation, coincident in time with the first responses generated in SI. Both contra- and ipsilateral SII areas showed prominent activity at 50-60 ms with an average delay of 13 ms for ipsilateral compared with contralateral responses. The refractory behavior of the early SII responses to the omissions differed from those observed at approximately 100 ms, indicative of distinct neuronal assemblies responding at each latency. These results indicate that SII and/or associated cortices in parietal operculum, often viewed as higher-order processing areas for somatosensory perception, are coactivated with SI during the early processing of intermittent somatosensory input.

Patterns of lateral sensory cortical activation determined using functional magnetic resonance imaging

OBJECT

Functional magnetic resonance (fMR) imaging was performed in human volunteers to determine the lateral perisylvian cortical areas activated by innocuous cutaneous stimulation.

METHODS

Eight volunteers who underwent 53 separate experiments form the basis of this report. Eight contiguous coronal slices were obtained using echoplanar fMR imaging techniques while participants were at rest and while somatosensory activation stimuli consisting of vibration or air puffs were delivered to various body areas. The data were analyzed using Student's t-test and cluster analysis to determine significant differences between the resting and activated states. The findings were as follows: the areas in the lateral cortex activated by the stimuli were the primary sensory cortex (SI), the second somatosensory area (SII), the insula, the superior parietal lobule, and the retroinsular parietal operculum (RIPO). Somatotopy was demonstrable in SI but not in the other areas identified. There was a surprisingly low correlation between the amount of cortex activated in the various areas, which could mean separate inputs and functions for the areas identified. The highest correlation was found between activity in SII and RIPO (0.69).

CONCLUSIONS

The authors maintain that fMR imaging can be used to identify multiple lateral somatosensory areas in humans. Somatotopy is demonstrated in SI but not in the other lateral cortical sensory areas. The correlations between the amounts of cortex activated in the different lateral sensory areas are low. Recognition of the multiple lateral sensory areas is important both for understanding sensory cortical function and for safe interpretation of studies designed to identify the central sulcus by activating SI.

Attention modulates both primary and second somatosensory cortical activities in humans: a magnetoencephalographic study

To clarify the role of primary and second somatosensory cortex (SI and SII) in somatosensory discrimination, we recorded somatosensory evoked magnetic fields during a stimulus strength discrimination task. The temporal pattern of cortical activation was analyzed by dipole source model coregistered with magnetic resonance image. Stimulus intensity was represented in SI as early as 20 ms after the stimulus presentation. The later components of SI response (latency 37.7 and 67.9 ms) were enhanced by rarely presented stimuli (stimulus deviancy) during passive and active attention. This supports an early haptic memory mechanism in human primary sensory cortex. Contra- and ipsilateral SII responses followed the SI responses (latency 124.6 and 138.3 ms, respectively) and were enhanced by attention more prominently than the SI responses. Active attention increased SII but not SI activity. These results are consistent with the concept of ventral somatosensory pathway that SI and SII are hierarchically organized for passive and active detection of discrete stimuli.

Hierarchical somatosensory processing

Recent studies of the postcentral and additional somatosensory cortices support a hierarchical scheme for information processing. In the postcentral gyrus, the complexity of receptive field properties increases with caudal progression from area 1. It has been reported that the anterior bank of the intraparietal sulcus, the caudalmost part of the postcentral gyrus, is responsible for the systematic integration of bilateral body parts, as well as of somatic and visual information.

Shape and roughness activate different somatosensory areas in the human brain

Somatosensory stimuli are known to activate the postcentral gyrus, and neurons there fire when the skin is in contact with objects. Also neurons in the lateral fissure, the parietal operculum, fire when rough surfaces are felt. However the localization of somatosensory association areas in humans is largely unknown and differences in functional contributions between somatosensory association areas has not been previously demonstrated. For these reasons the regional cerebral blood flow was measured with 15O-butanol and positron-emission tomography in two groups of young volunteers discriminating the lengths, shapes, and roughness of objects with their right hand. Roughness discrimination activated the lateral parietal opercular cortex significantly more than did length or shape discrimination. A Boolean intersection of the cluster images showing the statistical significant increases of length and shape discrimination demonstrated that shape and length discrimination activated the same cortical field lining the anterior part of the intraparietal sulcus (IPA). Shape and length discrimination activated IPA significantly more than did roughness discriminaton. These findings demonstrate a separation in functional contributions of lateral parietal opercular cortex and IPA. The results indicate different cortical processing streams for the somatosensory submodalities microgeometry and macrogeometry.

Activation of a distributed somatosensory cortical network in the human brain. A dipole modelling study of magnetic fields evoked by median nerve stimulation. Part I: Location and activation timing of SEF sources

Cortical areas responsive to somatosensory inputs were assessed by recording somatosensory evoked magnetic fields (SEF) to electrical stimulation of the left median nerve at wrist, using a 122-SQUID neuromagnetometer in various conditions of stimulus rate, attentional demand and detection task. Source modelling combined with magnetic resonance imaging (MRI) allowed localisation of six SEF sources on the outer aspect of the hemispheres located respectively: (1) in the posterior bank of the rolandic fissure (area SI), the upper bank of the sylvian fissure (parietal opercular area SII) and the banks of the intraparietal fissure contralateral to stimulation, (2) in the SII area ipsilateral to stimulation and (3) in the mid-frontal or inferior frontal gyri on both sides. All source areas were found to be simultaneously active at 70-140 ms after the stimulus, the SI source was the only one active already at 20-60 ms. The observed activation timing suggests that somatosensory input from SI is processed to higher-order areas through serial feedforward projections. However the long-lasting activations of all sources and their overlap in time is also compatible with a top-down control mediated via backward projections.

Changes in glutamate immunoreactivity in the somatic sensory cortex of adult monkeys induced by nerve cuts

Antibodies to glutamate (Glu) were used to study the effects of reduced afferent input on excitatory neurons in the somatic sensory cortex of adult monkeys. In each monkey, immunocytochemical staining was compared to thionin and cytochrome oxidase (CO) staining in adjacent sections. In the cervical spinal cord, dorsal column nuclei, ventroposterior thalamus, and primary somatic sensory cortex (SI), Glu immunoreactivity (Glu-ir) was analogous to that described in normal animals; regions with reduced or absent Glu-ir were never observed and no appreciable differences were noted between the experimental and normal side. There were also no differences in CO or thionin-stained sections from the affected hemisphere. In the insuloparietal operculum, sections in the hemisphere contralateral to the nerve cut showed that most cortical fields had a normal pattern of Glu-ir (pattern a), some exhibited a reduction of Glu-ir (pattern b), and that in the central portion of the upper bank of the central sulcus, which corresponds to the general location of the hand representation of the second somatic sensory cortex (SII), Glu-ir had virtually disappeared (pattern c). Adjacent sections processed for CO or stained with thionin showed that in the regions corresponding to those characterized by pattern c, CO was slightly decreased and that glial cells had increased in number. In the regions of SII characterized by pattern c, small intensely stained glial cells displayed Glu-ir. These findings indicate that Glu-ir is regulated by afferent activity and suggest that changes in Glu levels in neurons as well as in glial cells may trigger the biochemical processes underlying the functional and structural changes occurring during a slow phase of reorganizational plasticity in the cerebral cortex of adult monkeys.

Ipsilateral intracortical connections of physiologically defined cutaneous representations in areas 3b and 1 of macaque monkeys: projections in the vicinity of the central sulcus

This study examined cortical connections of areas 3b and 1 in 17 macaque monkeys in reference to regional somatotopography. The fluorescent retrograde tracers Fast Blue and Diamidino Yellow and the anterograde tracer Rhodamine Dextran were injected into closely related cutaneously responsive sites in primary somatosensory cortex, e.g., adjacent digits. Supra- and infragranular layers in nearly all studied areas contained labeled pyramidal cells. Labeled infragranular cells predominated at the fringes of a distribution where cells labeled from different tracer injections in the same brain intermixed more. All topographical regions across area 1 have reciprocal connections with areas 4, 3a, 3b, 1, 2, and 5. Intrinsic connections within area 1 and between it and area 2 are greatest; those with area 3b are less. Intrinsic connections within area 3b exceed all other nearby projections from this area which reciprocally connects with areas 3a, 1 and 2. Connections appear topographically organized, including those with poorly mapped regions, like area 5. These connections link representations of neighboring skin and skip map regions that include disjoint dermatomal areas. Connections from adjacent digit representations overlap; however, double-labeled cells were not found. Distal and proximal digit zones mostly interconnect within an area. Intrinsic connections spread further in area 1 than in area 3b, thereby joining more disparate topographical zones than interareal connections, which project more homotopically. The domain over which the map in somatosensory cortical area I (SI) dynamically changes following intracortical microstimulation (Recanzone, Merzenich and Dinse, Cerebral Cortex 2:181-196, 1992) may depend on the range of intrinsic connections observed in this study. The extent of connections between cortical areas was less than expected and this challenges the hypothesis that these connections directly create receptive field enlargements.