The somatosensory thalamus of monkeys: cortical connections and a redefinition of nuclei in marmosets.

Thalamic connections of three subdivisions of somatosensory cortex in marmosets were determined by placing wheatgerm agglutinin conjugated with horseradish peroxidase and fluorescent dyes as tracers into electrophysiologically identified sites in S-I (area 3b), S-II, and the parietal ventral area, PV. The relation of the resulting patterns of transported label to the cytoarchitecture and cytochrome oxidase architecture of the thalamus lead to three major conclusions. 1) The region traditionally described as the ventroposterior nucleus (VP) is a composite of VP proper and parts of the ventroposterior inferior nucleus (VPi). Much of the VP region consists of groups of densely stained, closely packed neurons that project to S-I. VPi includes a ventral oval of pale, less densely packed neurons and finger-like protrusions that extend into VP proper and separate clusters of VP neurons related to different body parts. Neurons in both parts of VPi project to S-II rather than S-I. Connection patterns indicate that the proper and the embedded parts of VPi combine to form a body representation paralleling that in VP. 2) VPi also provides the major thalamic input into PV. 3) In architecture, location, and cortical connections, the region traditionally described as the anterior pulvinar (AP) of monkeys resembles the medial posterior nucleus, Pom, of other mammals and we propose that all or most of AP is homologous to Pom. AP caps VP dorsomedially, has neurons that are moderately dense in Nissl staining, and reacts moderately in CO preparations. AP neurons project to S-I, S-II, and PV in somatotopic patterns.     

Connections between area 3b of the somatosensory cortex and subdivisions of the ventroposterior nuclear complex and the anterior pulvinar nucleus in squirrel monkeys.

The goal of this study was to determine whether somatosensory thalamic nuclei other than the ventroposterior nucleus proper (VP) have connections with area 3b of the postcentral cortex in squirrel monkeys. Small injections of the anatomical tracers wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) or 3H-proline were placed in electrophysiologically identified representations of body parts. The results indicate that, besides the well-established somatotopically organized connections with VP, area 3b has connections with three other nuclei of the somatosensory thalamus: the ventroposterior superior nucleus (VPS ["shell" of VP]), the ventroposterior inferior nucleus (VPI), and the anterior pulvinar nucleus (Pa). Injections confined to area 3b or involving adjacent parts of area 3a or area 1 indicate that connections between VPS, VPI, and Pa and the postcentral cortex are somatotopically organized. In VPS, connections related to the hand were found medially, and connections related to the foot were lateral. In VPI, connections with the cortical representations of the mouth, hand, and foot were successively more lateral. In Pa, connections related to the mouth, hand, and foot were successively more ventral, lateral, and caudal, and the trunk region was caudomedial. The findings suggest that VPI contains a representation of all parts of the body, including the face. The connections of Pa with the primary somatosensory cortex, area 3b, the location of Pa relative to the ventroposterior nucleus, and the high degree of topographic order in the connections of Pa with the postcentral cortex suggest that Pa is an integral part of the somatosensory thalamus in monkeys and is homologous to the medial nucleus of the posterior group (Pom) in other mammals. Overall, the results contribute to the growing evidence that individual somatosensory cortical areas in monkeys receive inputs from multiple thalamic sources, and that a single thalamic nucleus has several cortical targets.     

Thalamic Input to Representations of the Teeth,Tongue, and Face in Somatosensory Area 3b of Macaque Monkeys

Representations of the parts of the oral cavity and face in somatosensory area 3b of macaque monkeys were identified with microelectrode recordings and injected with different neuroanatomical tracers to reveal patterns of thalamic projections to tongue, teeth, and other representations in primary somatosensory cortex. The locations of injection sites and resulting labeled neurons were further determined by relating sections processed to reveal tracers to those processed for myeloarchitecture in the cortex and multiple architectural stains in the thalamus. The ventroposterior medial subnucleus (VPM) for touch was identified as separate from the ventroposterior medial parvicellular nucleus (VPMpc) for taste by differential expression of several types of proteins. Our results revealed somatotopically matched projections from VPM to the part of 3b representing intra‐oral structures and the face. Retrogradely labeled cells resulting from injections in area 3b were also found in other thalamic nuclei including: anterior pulvinar (Pa), ventroposterior inferior (VPI), ventroposterior superior (VPS), ventroposterior lateral (VPL), ventral lateral (VL), center median (CM), central lateral (CL), and medial dorsal (MD). None of our injections, including those into the representation of the tongue, labeled neurons in VPMpc, the thalamic taste nucleus. Thus, area 3b does not appear to be involved in processing taste information from the thalamus. This result stands in contrast to those reported for New World monkeys. J. Comp. Neurol. 521:3954–3971, 2013. © 2013 Wiley Periodicals, Inc.     

Thalamic connectivity of the second somatosensory area and neighboring somatosensory fields of the lateral sulcus of the macaque

The thalamocortical relations of the somatic fields in and around the lateral sulcus of the macaque were studied following cortical injections of tritated amino acids and horseradish peroxidase (HRP). Special attention was paid to the second somatosensory area (S2), the connections of which were also studied by means of thalamic isotope injections and retrograde degeneration. S2 was shown to receive its major thalamic input from the ventroposterior inferior thalamic nucleus (VPI) and not, as previously reported, from the caudal division of the ventroposterior lateral nucleus (VPLc). Following small injections of isotope or HRP into the hand representation of S2, only VPI was labeled. Larger injections, which included the representations of more body parts, led to heavy label in VPI, with scattered label in VPLc, the central lateral nucleus (CL), and the posterior nucleus (Po). In addition, small isotope injections into VPLc did not result in label in S2 unless VPI was also involved in the injection site, and ablations of S2 led to cell loss in VPI. Comparison of injections involving different body parts in S2 suggested a somatotopic arrangement within VPI such that the trunk and lower limb representations are located posterolaterally and the hand and arm representations anteromedially. The location of the thalamic representations of the head, face, and intraoral structures that project to S2 may be in the ventroposterior medial nucleus (VPM). The granular (Ig) and dysgranular (Id) fields of the insula and the retroinsular field (Ri) each receive inputs from a variety of nuclei located at the posteroventral border of the thalamus. Ig receives its heaviest input from the suprageniculate-limitans complex (SG-Li), with additional inputs from Po, the magnocellular division of the medial geniculate n. (MGmc), VPI, and the medial pulvinar (Pulm). Id receives its heaviest input from the basal ventromedial n. (VMb), with additional inputs from VPI, Po, SG-Li, MGmc, and Pulm. Ri receives its heaviest input from Po, with additional input from SG-Li, MGmc, Pulm, and perhaps VPI. Area 7b receives its input from Pulm, the oral division of the pulvinar, the lateral posterior n., the medial dorsal n., and the caudal division of the ventrolateral n. These results indicate that the somatic cortical fields, except for those comprising the first somatosensory area, each receive inputs from an array of thalamic nuclei, rather than just one, and that individual thalamic somatosensory relay nuclei each project to more than one cortical field.     

Bilateral effects of spinal overhemisections on the development of the somatosensory system in rats

Connections of the forepaw regions of somatosensory cortex (S1) were determined in rats reared to maturity after spinal cord overhemisections at cervical level C3 on postnatal day 3. Overhemisections cut all ascending and descending pathways and intervening gray on one side of the spinal cord and the pathways of the dorsal funiculus contralaterally. Bilateral lesions of the dorsal columns reduced the size of the brainstem nuclei by 41%, and the ventroposterior lateral subnucleus (VPL) of the thalamus by 20%. Bilateral lesions also prevented the emergence of the normal cytochrome oxidase barrel pattern in forepaw and hindpaw regions of S1. Injections of wheat germ agglutinin conjugated to horseradish peroxidase were placed in the forepaw region of granular S1 and surrounding dysgranular S1 contralateral to the hemisection. The VPL nucleus was densely labeled, whereas the adjoining ventroposterior medial subnucleus, VPM, representing the head, was unlabeled. Thus, there was no evidence of abnormal connections of VPM to forepaw cortex. Foci of transported label in the ipsilateral hemisphere appeared to be in normal locations and of normal extents, but connections in the opposite hemisphere were broadly and nearly uniformly distributed in sensorimotor cortex in a pattern similar to that in postnatal rats. Rats with incomplete lesions that spared the dorsal column pathway on the left side but not the right demonstrated surprisingly normal distributions of callosal connections in the nondeprived right hemisphere, even though the injected left hemisphere was deprived. Thus, the development of the normal pattern of callosal connections depends on dorsal column input and not on normal interhemsipheric interactions.     

The organization and connections of somatosensory cortex in marmosets

Microelectrode mapping methods were used to define and describe 3 representations of the body surface in somatosensory cortex of marmosets: S-I proper or area 3b of anterior parietal cortex, S-II, and the parietal ventral area (PV) of the upper bank of the lateral sulcus. In the same animals, injections of anatomical tracers were placed into electrophysiologically determined sites in area 3b or S-II. Mapping results and patterns of connections were later related to architectonic fields that were delimited in sections cut parallel to the surface of manually flattened cortex and stained for myelin. There were several major results. (1) Recordings from area 3b revealed a characteristic somatotopic organization of foot to face in a mediolateral sequence as previously reported in other members of the marmoset family (Carlson et al., 1986). (2) Multiple injections of WGA-HRP in area 3b demonstrated dense, patchy interconnections with ipsilateral S-II, PV, area 3a, and area 1, less dense interconnections with primary motor cortex (M-I), the supplementary motor area (SMA), limbic cortex of the medial wall (L), and rostrolateral parietal cortex of the lateral sulcus (PR), and callosal connections with areas 3b, S-II, and PV. Injections of 3 different tracers into the representation of 3 body regions in area 3b indicated that the connections with areas 3a, 3b, 1, S-II, and PV are topographically organized. (3) Recordings from cortex on the upper bank of the lateral sulcus demonstrated a somatotopic representation of the body surface that matches that of S-II of other mammals. S-II immediately adjoined areas 3b along the dorsal lip of the lateral sulcus. The face representation in S-II was adjacent to the face representation in 3b while the trunk, hindlimb, and forelimb were represented in a caudorostral sequence deeper in the sulcus. (4) Injections in S-II revealed ipsilateral connections with areas 3a, 3b, 1, a presumptive area 2, PV, PR, M-I, SMA, limbic cortex, the frontal eye fields, and the frontal ventral visual area. Dense callosal connections were with S-II and PV. (5) The recordings also revealed a systematic representation just rostral to S-II that has not been previously described in primates.(ABSTRACT TRUNCATED AT 400 WORDS)     

Cortical and thalamic connections of the representations of the teeth and tongue in somatosensory cortex of new world monkeys

Connections of representations of the teeth and tongue in primary somatosensory cortex (area 3b) and adjoining cortex were revealed in owl, squirrel, and marmoset monkeys with injections of fluorescent tracers. Injection sites were identified by microelectrode recordings from neurons responsive to touch on the teeth or tongue. Patterns of cortical label were related to myeloarchitecture in sections cut parallel to the surface of flattened cortex, and to coronal sections of the thalamus processed for cytochrome oxidase (CO). Cortical sections revealed a caudorostral series of myelin dense ovals (O1-O4) in area 3b that represent the periodontal receptors of the contralateral teeth, the contralateral tongue, the ipsilateral teeth, and the ipsilateral tongue. The ventroposterior medial subnucleus, VPM, and the ventroposterior medial parvicellular nucleus for taste, VPMpc, were identified in the thalamic sections. Injections placed in the O1 oval representing teeth labeled neurons in VPM, while injections in O2 representing the tongue labeled neurons in both VPMpc and VPM. These injections also labeled adjacent part of areas 3a and 1, and locations in the lateral sulcus and frontal lobe. Callosally, connections of the ovals were most dense with corresponding ovals. Injections in the area 1 representation of the tongue labeled neurons in VPMpc and VPM, and ipsilateral area 3b ovals, area 3a, opercular cortex, and cortex in the lateral sulcus. Contralaterally, labeled neurons were mostly in area 1. The results implicate portions of areas 3b, 3a, and 1 in the processing of tactile information from the teeth and tongue, and possibly taste information from the tongue.     

Cell-poor septa separate representations of digits in the ventroposterior nucleus of the thalamus in monkeys and prosimian galagos

The architectonic features of the ventroposterior nucleus (VP) were visualized in coronal brain sections from two macaque monkeys, two owl monkeys, two squirrel monkeys, and three galagos that were processed for cytochrome oxidase, Nissl bodies, or the vesicular glutamate transporter 2 (vGluT2). The traditional ventroposterior medial (VPM) and ventroposterior lateral (VPL) subnuclei were easily identified, as well as the forelimb and hindlimb compartments of VPL, as they were separated by poorly staining, cell-poor septa. Septa also separated other cell groups within VPM and VPL, specifically in the medial compartment of VPL representing the hand (hand VPL). In one squirrel monkey and one galago we demonstrated that these five groups of cells represent digits 1-5 in a mediolateral sequence by injecting tracers into the cortical representation of single digits, defined by microelectrode recordings, and relating concentrations of labeled neurons to specific cell groups in hand VPL. The results establish the existence of septa that isolate the representation of the five digits in VPL of primates and demonstrate that the isolated cell groups represent digits 1-5 in a mediolateral sequence. The present results show that the septa are especially prominent in brain sections processed for vGluT2, which is expressed in the synaptic terminals of excitatory neurons in most nuclei of the brainstem and thalamus. As vGluT2 is expressed in the synaptic terminations from dorsal columns and trigeminal brainstem nuclei, the effectiveness of vGluT2 preparations in revealing septa in VP likely reflects a lack of synapses using glutamate in the septa.     

Intratrigeminal and thalamic projections of nucleus caudalis in the squirrel monkey (Saimiri sciureus): a degeneration and autoradiographic study.

In order to test the hypothesis that thalamic efferents of trigeminal nucleus caudalis (NC) are the cranial analogue of the spinothalamic system, lesion and autoradiographic studies were carried out in the squirrel monkey, and the terminal projection fields in thalamus were noted. Results showed that NC, including lateral reticular formation (LRF), projects to contralateral VPM, the VPM-VPL border and medial VPL, and a region dorsal to ventroposterior nucleus (VP) proper which contains cells larger than those in VPM yet which stain as darkly as VPL neurons; this latter zone of termination may be homologous with VPL_o (Vim) in other species, which is that area receiving lemniscal and cerebellar afferents (Mehler, '71; Walsh and Ebner, '73; Boivie, '74). In addition, a small projection is noted in an area intercalated between dorsomedial MG, limitans nucleus and posterior VP which closely agrees with the medial division of Posterior nucleus (Po) described in rhesus and squirrel monkey (Burton and Jones, '76). No terminations were observed in the gustatory nucleus medial to VPM. Bilateral, terminal projection fields were observed in posterior mediodorsal nucleus (MD), and a paralaminar area (PL) which lies in the ventrolateral strip of MD and is particularly prominent in primates; other bilateral fields were noted in CL, particularly the more medial segment of the nucleus. A sparse projection was noted in contralateral CM. Ipsilateral, intratrigeminal connections between NC and main sensory nucleus (MSV) also were observed. We conclude that, in the squirrel monkey, NC efferents, probably including LRF, may be considered analagous to the spinothalamic system by virtue of terminations in older medial and newer ventroposterior thalamus. Terminations in posterior MD may be specific to Primates. Moreover, projections to an area just dorsal to VP proper in squirrel monkey may be included within the broader definition of a neo-spinothalamic area as reflected in spinothalamic tract projections to the ventrolateral complex in cat (Boivie, '71b; Jones and Burton, '74). The small NC projection to a part of Po is consistent with spinothalamic terminations to a “posterior” thalamic area in other primates (Mehler, '69), and with the suggestion that medial Po transmits pain information (Burton and Jones, '76).     

Recombinant basic fibroblast growth factor spares thalamic neurons from retrograde degeneration after ablation of the somatosensory cortex in rats

Retrograde degeneration of thalamic neurons after cortical ablation has long been recognized. Neuronal loss following axotomy eliminates the possibility of regeneration and might prevent the recovery from axonal injury in patients with brain trauma. We investigated whether CS23, a stable recombinant variant of human basic fibroblast growth factor (bFGF), could protect neurons from retrograde degeneration. Four weeks after ablation of the somatosensory cortex in young female rats, there was extensive neuronal degeneration and loss in the lateral ventro-posterior nucleus (VPL) of the ipsilateral thalamus. When Gelfoam soaked in bFGF(CS23) (1 μg/0.l ml) was applied topically at the time of surgery, this neuronal degeneration in the VPL was markedly reduced and macroscopic atrophy of the lateral and medial ventroposterior nucleus (VPL + VPM) was significantly reduced. In contrast, application of bFGF at three days after surgery failed to prevent retrograde degeneration. These resuts indicate that bFGF can prevent thalamic atrophy after ablation of the somatosensory cortex and that administration of bFGF is only effective in the very early period after brain injury.     

Corticothalamic connections from the second somatosensory area and neighboring regions in the lateral sulcus of macaque monkeys

Corticothalamic connections were shown between the second somatosensory area in primates and the ventroposterior nuclei of the thalamus. These projections were topographically arranged with those from the hindlimb portions of SII traced to the most lateral and posterior parts of the ventroposterior lateral nucleus (VPLc) and those from the forelimb located medially within VPLc. The densest labeling was found ventrally in VPLc and dorsally within ventroposterior inferior n. (VPI) only after injections of the forelimb. A more scattered, dorsal distribution of labeling was seen in the rest of VPLc from injections involving more proximal parts of the body representation in SII.     

Parallel thalamic activation of the first and second somatosensory areas in prosimian primates and tree shrews.

In Tupaia belangeri and Galago senegalensis, microelectrode recordings immediately after ablation of the representation of the forelimb in the midportion of the first somatosensory area, S-I, revealed that all parts of the second somatosensory area, S-II, remained highly responsive to cutaneous stimuli. In this way, prosimian primates, close relatives of simian primates, and tree shrews differ markedly from monkeys in which S-II is deactivated by comparable ablations, and resemble such mammals as cats and rabbits in which S-II also remains highly responsive following ablations in S-I. Thus, it appears that the generalized mammalian condition is that S-I and S-II are independently activated via parallel thalamocortical pathways. A dependence of S-II on serial connections from the thalamus to the S-I region and then to S-II apparently evolved with the advent of anthropoid primates, and may be present only in monkeys and perhaps other higher primates.     

Connections of area 2 of somatosensory cortex with the anterior pulvinar and subdivisions of the ventroposterior complex in macaque monkeys

The principal goal of the present study was to determine the thalamic connections of area 2 of postcentral somatosensory cortex of monkeys. The placement of injections of anatomical tracers (horseradish peroxidase, wheat germ agglutinin, or ³H-proline) was guided by extensive microelectrode maps of cortex in the region of the injection site. These maps identified the body parts represented in the cortex included in the injection site, and provided information about the physiological boundaries of area 2, which was related later to the cortical architecture. Most injections were placed in the representation of the hand in area 2, which was highly responsive to cutaneous stimuli and could be mapped in detail. Injections were also placed in other parts of area 2, area 1, or area 5, and some injections involved more than one area. As other investigators have determined, regions of retrograde and anterograde thalamic label overlapped, demonstrating that connections with cortex are reciprocal. Injections completely confined to area 2 consistently produced label in two locations: the anterior pulvinar (Pa) and a dorsal capping zone of the ventroposterior complex that we term the ventroposterior superior nucleus (VPS). Single restricted injection sites resulted in one region of label in VPS, and multiple foci of label in Pa. In some cases where the injection was confined to the representation of the hand in area 2, label was also found more ventrally in the ventroposterior complex in ventroposterior nucleus proper (VP). Thus, area 2 receives input from Pa, VPS, and, at least in some locations and individuals, VP. Injections of tracers into area 1 confirmed previous findings that area 1 is densely interconnected with VP. In addition, there appear to be sparse connections with VPS. There was no evidence of connections with Pa. Evidence from injection sites that extended from area 2 into areas 5 and 7, and from injection sites in area 5, indicates that the lateral posterior nucleus (LP) projects to rostral areas 5 and 7. The results support the conclusion that area 2 is a functionally distinct subdivision of somatosensory cortex, and indicate that area 2 has thalamic connections that are characteristic of both “sensory” (VP and VPS) and “association” (Pa) cortical fields.     

Immunoreactivity for GAD and Three Peptides in Somatosensory Cortex and Thalamus of the Raccoon

Immuno-cytochemical methods were used to determine the distributions of glutamic acid decarboxylase (GAD), vasoactive intestinal polypeptide (VIP), cholecystokinin (CCK), and somatostatin (SOM) in the primary somatosensory cortex and somatosensory thalamus of adult raccoons. The cortex showed extensive immunoreactivity for GAD, revealing a large population of GABAergic neurons. GAD-labeled cells were numerous in all cortical layers, but were most concentrated in laminae II–IV. The cells were nonpyramidal and of varying morphology, typically with somata of small or medium size. GAD-immunoreactive puncta, presumably synaptic terminals, were widespread and often appeared to end on both GAD-negative and GAD-positive neurons. Immunoreactivity for the peptides was much less extensive than that for GAD, with the number of labeled neurons for VIP > CCK > SOM. Peptidergic cells were preferentially located in the upper and middle cortical layers, especially laminae II and III. The cells were nonpyramidal, often bitufted or bipolar in morphology, and small to medium in size. Their processes formed diffuse plexuses of fibers with terminal-like varicosities that occasionally surrounded nonpeptidergic neurons. The thalamus showed a clearly differentiated pattern of immunoreactivity for GAD, but little or no labeling for the three peptides. Nuclei adjoining the ventral posterior lateral (VPL)/ventral posterior medial (VPM) complex—including the reticular nucleus—contained many GAD-positive neurons and fibers. In contrast, the VPL and VPM nuclei displayed considerably less GAD immunoreactivity, somewhat surprising given the raccoon's highly developed somatosensory system. However, the ventral posterior inferior (VPI) nucleus revealed rather dense GAD labeling, perhaps related to a specialized role in sensory information processing. Thus, the primary somatosensory cortex of the raccoon showed patterns of immunoreactivity for GAD and peptides that were similar to those of other species; the somatosensory thalamus revealed a distinctive profile of GAD immunoreactivity, with labeling that was light to moderate in the VPL/VPM complex and relatively extensive in VPI.     

Physiological and anatomical evidence for a discontinuous representation of the trunk in S-I of tree shrews

Microelectrode mapping methods revealed that the representation of the body surface in the first somatosensory area of cortex, S-I, of the tree shrew is unique in that only the ventral trunk was found in the usual location of the trunk representation in cortex of the dorsolateral surface of the cerebral hemisphere. Instead, the dorsal trunk was found as an extension of the representation of the posterior leg in cortex on the medial wall. The separation of the representation of the trunk occurs along a line that is counter to the orientation of the dorsal root dermatomes, so that S-I of the tree shrew clearly cannot be characterized as a serial representation of dermatomes. Anatomical studies of connections support the conclusion that the representation of the trunk is split in S-I. Both the representation of the dorsal trunk on the medial wall of the cerebral hemisphere and S-I of the dorsolateral surface were found to project to S-II when horseradish peroxidase was injected into S-II.     

Thalamic projections to S-I in macaque monkey.

The organization of thalamic input to functionally characterized zones in primary somatosensory cerebral cortex (S-I) of macaque monkeys (Macaca mulatta) was investigated using the method of labelling by retrograde transport of horseradish peroxidase (HRP). It was found that the cell columns positioned at the posterior margin of the band of cortex representing a given body region receive thalamic input from a posterior level of the ventroposterior thalamic nucleus (VP), and that cell columns at successively more anterior positions within that band receive input from successively more anterior levels of VP. The extreme posterior and anterior margins of the S-I hand, foot and face areas receive input from neuron populations which are not as widely separated in the anteroposterior dimension of VP as the neurons projecting to the extreme anterior and posterior margins of the proximal limb and trunk representations in S-I. These characteristics of the organization of the projections from VP to S-I are consistent with the view that the body representations in VP and S-I have the same connectivity and differential submodality distribution; and with the idea that thalamocortical conncetions only exist between functionally equivalent neuron populations in VP and S-I.     

Spinothalamocortical projections to the secondary somatosensory cortex (SII) in squirrel monkey

Anterograde labeling of the cervical spinothalamic tract was combined with retrograde labeling of thalamocortical cells projecting to the hand region of the second somatosensory cortex (hSII) to identify likely sites in the thalamus for processing and transmitting nociceptive information to hSII. Anterograde labeling of terminals was done with 2% WGA-HRP injections in the cervical enlargement; thalamocortical cells were retrogradely labeled with fluorescent tracers. In one experiment, the contralateral primary somatosensory cortex hand region (hSI) was injected to provide a direct comparison with hSII thalamic label. Both labeled cells and terminal-like structures were visualized in single thalamic sections and their numbers and positions quantitatively analyzed. The number of labeled cells within 100 microns from the STT terminals were counted as overlapping cells. Four thalamic nuclei, ventroposterior inferior (VPI), ventroposterior lateral (VPL), posterior nucleus (PO) and centrolateral nucleus (CL) combined to contain 86.5% of all hSII-projecting overlapping cells. Of all hSII-projecting thalamic overlapping cells, VPI contained the largest number (36.4% of the total) followed by the anterior portion of the posterior nuclear complex (POa; 20.4%), VPL (18.3%) and CL (11.4%). Results of the hSI injection show a different pattern of overlap in agreement with our earlier study. The relative distribution of overlapping cells was dependent on the antero-posterior position of the SII injections. The most anterior injections resulted in small numbers of labeled cells, with the majority of overlapping cells located in PO and CL. The more posterior injections resulted in overlapping cells mainly in VPI and VPL. The results indicate that, in the squirrel monkey, VPI, VPL, POa and CL relay nociceptive information from the spinal cord to the second somatosensory cortex.     

Functional reorganization in adult monkey thalamus after peripheral nerve injury.

Large changes in somatotopic organization can be induced in adult primate somatosensory cortex by cutting peripheral afferents. The role, if any, of the thalamus in these changes has not been investigated previously. In the present experiments, electrophysiological recording in the ventroposterior lateral nucleus (VPL) has revealed that not only can reorganization occur in the thalamus, but it may be as extensive as that revealed in the cortex of the same monkeys. Thus, for at least some types of deafferentation, the reorganization revealed in the cortex may depend largely on subcortical changes.     

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.