Ventroposterior Inferior Research Articles

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.

Evolution of somatosensory and motor cortex in primates.

Inferences about how the complex somatosensory systems of anthropoid primates evolved are based on comparative studies of such systems in extant mammals. Experimental studies of members of the major clades of extant mammals suggest that somatosensory cortex of early mammals consisted of only a few areas, including a primary area, S1, bordered by strip-like rostral and caudal somatosensory fields, SR and SC. In addition, the second somatosensory area, S2, and the parietal ventral area, PV, were probably present. S1, S2, and PV were activated independently via parallel projections from the ventroposterior nucleus, VP. Little posterior parietal cortex existed, and it was unlikely that a separate primary motor area, M1, existed until placental mammals evolved. Early primates retained this basic organization and also had a larger posterior parietal region that mediated sensorimotor functions via connections with motor and premotor areas. The frontal cortex included M1, dorsal and ventral premotor areas, supplementary motor area, and cingulate motor fields. Ventroposterior superior and ventroposterior inferior nuclei were distinct from the ventroposterior nucleus in the thalamus. In early anthropoid primates, areas S1, SR, and SC had differentiated into the fields now recognized as areas 3b, 3a, and 1. Areas 3b and 1 contained parallel mirror-image representations of cutaneous receptors and a parallel representation in area 2 was probable. Serial processing became dominant, so that neurons in areas 1, S2, and PV became dependent on area 3b for activation. Posterior parietal cortex expanded into more areas that related to frontal cortex. Less is known about changes that might have occurred with the emergence of apes and humans, but their brains were larger and posed scaling problems most likely solved by increasing the number of cortical areas and reducing the proportion of long connections.

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.

Distribution of neurons immunoreactive for parvalbumin and calbindin in the somatosensory thalamus of the raccoon

The aim of this study was to assess the distribution of neurons immunoreactive for parvalbumin (PV), calbindin (CaBP), glutamic acid decarboxylase (GAD), and gamma-aminobutyric acid (GABA) in the somatosensory thalamus of the raccoon and to compare these features to those of other species, especially primates. Immunohistochemistry was used to study the location of these neurons in the ventroposterior nucleus (VP), ventroposterior inferior nucleus (VPI), posterior group of nuclei (Po), and reticular nucleus (Rt). A consistent differential pattern of PV-positive (PV+) and CaBP-positive (CaBP+) cells was found in the somatosensory thalamus. Many PV+ neurons were observed in VP and Rt, but very few were found in VPI or Po. In contrast, CaBP+ neurons were distributed throughout VP, VPI, and Po but were very sparse or absent in Rt. In the VP nucleus, PV+ cells were distributed in clusters separated by interclusteral regions with a sparse distribution of PV+ cell bodies. The distributions of PV+ and CaBP+ cells tended to be complementary to each other in VP; regions with a high density of PV+ neurons had a low density of CaBP+ cell bodies. Double-labeling experiments revealed very few neurons in which PV and CaBP immunoreactivities were colocalized. Cells immunoreactive for GAD or GABA were found in PV-dense clusters of VP; fewer GABAergic neurons were present in the CaBP-dense interclusteral regions of VP and in VPI and Po. GAD+ and GABA+ neurons were most prominently distributed in Rt. We conclude that the distributions of PV+ and CaBP+ cell bodies in the raccoon somatosensory thalamus are very similar to those in primates. The density of GABAergic neurons in the somatosensory thalamus of the raccoon is less than that in the cat and monkey, but the relative distribution of GABAergic neurons in the different somatosensory nuclei is very similar to that in the cat and monkey. These results are discussed in relation to findings in other species and are related to the functions of lemniscal and nonlemniscal somatosensory pathways.

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.

Spinothalamocortical inputs nonpreferentially innervate the superficial and deep cortical layers of SI

Using a combined anterograde and retrograde tracing technique, we examined the distribution pattern of the thalamocortical cells which projected to superficial layers of the hand region of the primary somatosensory cortex (hSI), and quantitatively analyzed the retrogradely labeled cells which putatively contacted terminals of the spinothalamic tract (STT) in the squirrel monkey and the macaque. Less than 25% of the superficial hSI projecting cells were putatively contacted by terminals of cervical enlargement spinothalamic neurons. These cells were primarily located in ventro-posterior lateral, ventroposterior inferior and centrolateral nuclei. Although the number of superficial hSI projecting cells numbered less than 20% of the total hSI projecting cells, their patterns of location and their proportion of overlap with STT terminals within each thalamic nucleus were similar. It is suggested that the spinothalamic nociceptive information input to the cortex equally accesses both superficial and deep SI.

Monoclonal antibody Cat-301 selectively identifies a subset of nuclei in the cat's somatosensory thalamus.

Recently it has been demonstrated that the monoclonal antibody Cat-301 is capable of identifying functionally related neurons in the mammalian visual thalamus. We have examined the possibility that this antibody might display a similar capacity in nonvisual thalamic areas. We demonstrate that in the cat's somatosensory thalamus the distribution of Cat-301-positive cells and neuropil is restricted to a subset of nuclei. These include the ventroposterior medial, ventroposterior lateral, and ventroposterior inferior nuclei. Staining with Cat-301 provides a clear visualisation of the entire somatotopic map within these nuclei. The somatosensory sector of the thalamic reticular nucleus and the perireticular nucleus, which may have a somatosensory sector, are also Cat-301-positive. In contrast, cells that do not express the Cat-301 antigen are located in the ventroposterior oralis nucleus, the ventroposterior shell region, the medial and lateral divisions of the posterior nuclear group, and the inner small cell region adjacent to the thalamic reticular nucleus. In comparison with previous physiological studies, cells that express the Cat-301 antigen most likely represent subpopulations in only a few of the somatic submodality-specific groups. These include cells in the small-field and Pacinian cutaneous-responsive groups, excluding cells in the wide-field cutaneous-, muscle-, joint-, and noxious-responsive groups. Taken together these findings indicate that monoclonal antibody Cat-301 is capable of selectively identifying neurons with distinct functional properties in the mammalian somatosensory thalamus.

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 connections of three representations of the body surface in somatosensory cortex of gray squirrels

The anatomical tracer, wheat germ agglutinin, was used to determine the connections of electrophysiologically identified locations in three architectonically distinct representations of the body surface in the somatosensory cortex of gray squirrels. Injections in the first somatosensory area, S-I, revealed reciprocal connections with the ventroposterior nucleus (VP), a portion of the thalamus just dorsomedial to VP, the posterior medial nucleus, Pom, and sometimes the ventroposterior inferior nucleus (VPI). As expected, injections in the representation of the face in S-I resulted in label in ventroposterior medial (VPM), the medial subnucleus of VP, whereas injections in the representation of the body labeled ventroposterior lateral (VPL), the lateral subnucleus of VP. Furthermore, there was evidence from connections that the caudal face and head are represented dorsolaterally in VPM, and the forelimb is represented centrally and medially in VPL. The results also support the conclusion that a representation paralleling that in VP exists in Pom, so that the ventrolateral part of Pom represents the face and the dorsomedial part of Pom is devoted to the body. Because connections with VPI were not consistently revealed, the possibility exists that only some parts or functional modules of S-I are interconnected with VPI. Two separate small representations of the body surface adjoin the caudoventral border of S-I. Both resemble the second somatosensory area, S-II, enough to be identified as S-II in the absence of evidence for the other. We term the more dorsal of the two fields S-II because it was previously defined as S-II in squirrels (Nelson et al., '79), and because it more closely resembles the S-II identified in most other mammals. We refer to the other field as the parietal ventral area, PV (Krubitzer et al, '86). Injections in S-II revealed reciprocal connections with VP, Pom, and a thalamic region lateral and caudal to Pom and dorsal to VP, the posterior lateral nucleus, Pol. Whereas major interconnections between S-II and VPI have been reported for cats, raccoons, and monkeys, no such interconnections were found for S-II in squirrels. The parietal ventral area, PV, was found to have prominent reciprocal interconnections with VP, VPI, and the internal (magnocellular) division of the medial geniculate complex (MGi). The pattern of connections conforms to the established somatotopic organization of VP and suggests a crude parallel somatotopic organization in VPI. Less prominent interconnections were with Pol.(ABSTRACT TRUNCATED AT 400 WORDS)

A thalamic terminus of the lateral cervical nucleus: the lateral division of the posterior nuclear group.

The submodality and receptive field properties of single units in the lateral cervical nucleus (LCN) of barbiturate anesthetized cats were studied with glass microelectrodes. In other experiments, a region of the posterior thalamus containing neurons with properties comparable to those seen in the LCN was examined with tungsten microelectrodes. The responses of most units in the LCN reflected a major input from large myelinated afferent fibers innervating guard hairs but no input from Pacinian afferent fibers. The large size of the receptive fields indicated that excitatory input converged selectively from afferent fibers serving hairs over large areas of the body. In the posterior thalamus rapidly adapting neurons characterized by very large receptive fields and driven by the movement of guard hairs were observed to a region identified histologically as the rostral extension of the lateral division of the posterior nuclear group (POl). Caudally this region was located immediately adjacent to the dorsolateral part of the ventroposterior inferior nucleus (VPI). In more rostral parts of the thalamus it was located more dorsally and the ventroposterior lateral nucleus intervened between it and the VPI. This region was less than 1 mm wide in the frontal plane but extended rostrocaudally for several millimeters. Horseradish peroxidase injected into the region of the VPI and the POl labeled many cells in the LCN and the caudal pole of the dorsal column nuclei demonstrating that neurons in the LCN relay information to this part of the thalamus. These data, plus previous experiments showing that the VPI receives a major projection from the caudal poles of the dorsal column nuclei, suggest that the rostral portion of the POl receives an important afferent supply from the LCN. The responses of neurons in the POl appear to arise from specific classes of sensory receptors and cannot be considered less precise or more primitive than responses observed in the ventroposterior nucleus of the thalamus.

The ventroposterior inferior nucleus in the thalamus of cats: a relay nucleus in the Pacinian pathway to somatosensory cortex.

The ventroposterior region of the thalamus of mongrel cats was searched to locate zones activated by somatic stimuli. By using stimuli that selectively excited Pacinian corpuscles, areas activated by this class of afferent fibers were differentiated from regions activated by other classes of cutaneous mechanoreceptors. The results showed that Pacinian inputs excite neurons in the ventroposterior inferior nucleus (VPI) of the thalamus, whereas other more dorsal zones within the ventroposterior thalamus receive inputs from other mechanoreceptor classes. This definition of the VPI tended to be larger and to extend further lateral than some published descriptions. Horseradish peroxidase (HRP) was injected into ventroposterior zones shown by electrophysiological recordings to receive inputs from Pacinian afferents. Subsequently, labeled cell bodies were observed in the caudal poles of the dorsal column nuclei, a region previously shown to be activated by Pacinian afferents. Very few labeled cells were found in the central region of these nuclei, a region previously shown to be activated by other classes of cutaneous mechanoreceptors. Electrophysiological recordings were used to locate a small portion of the second somatosensory cortex driven by Pacinian stimuli. When HRP was injected into this region cell bodies in the VPI and the lateral part of the posterior group were labeled, but few or no labeled cells were found in ventroposterior lateral nucleus. We hypothesize that the VPI receives Pacinian information from a cytoarchitecturally distinct region in the caudal poles of the dorsal column nuclei. Further, we suggest that a major cortical target for the VPI is a subdivision of the second somatosensory cortex. These studies do not exclude the possibility that Pacinian inputs have other thalamic and cortical targets.

The organization and connections of somatosensory cortex in primates

The use of microelectrode mapping methods, in conjunction with studies of cortical architecture, patterns of connections, and the response properties of single neurons, has led to a greatly increased understanding of the functional organization of somatosensory cortex in mammals. All mammals appear to have the traditionally defined first and second somatosensory representations, SI and SII, but at least some mammals have additional body representations as well. Furthermore, the area traditionally regarded as “SI” in monkeys actually includes four functionally distinct representations. Within “SI,” four striplike architectonic fields, areas 3a, 3b, 1, and 2 have long been recognized. It is now clear that areas 3b and 1 form separate parallel and largely mirror‐image representations of the body surface, each with major inputs from a single representation of the body surface in the ventroposterior nucleus (VP) of the thalamus. VP appears to relay information from both slowly and rapidly adapting peripheral neurons to area 3b, while information from rapidly adapting neurons is relayed to area 1. Neurons with properties that could reflect Pacinian receptor influences are found in area 1, but not area 3b. The anatomical pathways mediating Pacinian‐like responses are unknown. Area 3a contains a representation of largely deep receptors, probably muscle spindle afferents, relayed from part of the thalamus dorsal to VP. Area 2 forms a representation of both deep and cutaneous body receptors. The somatosensory cortex receives input from other regions as well, including the anterior pulvinar and ventroposterior inferior nucleus of the thalamus. Other portions of the somatosensory cortex are less well understood.

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.

Thalamic connections of the insula in the rhesus monkey and comments on the paralimbic connectivity of the medial pulvinar nucleus.

Thalamic connections of the insula in the rhesus monkey were studied with axonal transport methods. Tritiated amino acid injections limited to the insula revealed autoradiographic label in the principal and parvicellular components of the ventroposterior medial nucleus, the ventroposterior inferior nucleus, the oral and medial pulvinar nuclei, the nucleus reuniens, the parvicellular and magnocellular components of the medial dorsal nucleus, the centromedian-parafasicularis nuclei, and the reticular nucleus. In additional animals, tritiated amino acids and horseradish perioxidase injections were made within different regions of the insula. Although the injection sites in these additional cases may have included minor extensions into claustrum and adjacent structures, several tentative conclusions emerged with respect to the antero-posterior gradient in insulothalamic connectivity. The anterior insula appears to have a more extensive relationship with the ventroposterior medial complex, the medial dorsal nucleus, the centromedian-parafasicularis nuclei and with some midline nuclei. In contrast, the posterior insula is more extensively connected with the ventroposterior inferior nucleus, the oral and medial pulvinar nuclei, and the suprageniculate nucleus. The patterns of insulothalamic connections support conclusions derived from observations on the cortical connectivity of the primate insular cortex indicating that the anterior insula is related to olfactory, gustatory, and viscero-autonomic behavior, whereas the posterior insula is related to auditory-somesthetic-skeletomotor function (Mesulam and Mufson, '82b). The medial pulvinar nucleus has extensive connections with many paralimbic cortical regions including the insula as well as with high order polymodal association cortex. These findings suggest that the medial pulvinar may provide a region for the convergence of multisensory association input with limbic information.

The connections of cortical somatosensory areas I and II with separate nuclei in the ventroposterior thalamus in the raccoon

The thalamocortical afferents to cortical somatosensory areas I (SI) and II (SII) were investigated in the raccoon using the horseradish peroxidase technique. The purpose of this study was to determine if the cell bodies of origin for thalamocortical afferents to these cortical regions were localized in the same or different nuclei in the ventroposterior region of the thalamus. Horseradish peroxidase was injected into subdivisions of SI or SII and after post-injection survival periods of 12–72 hours the horseradish peroxidase in the tissue was reacted with the chromogens dihydrochlorobenzidine or tetramethylbenzidine in the presence of hydrogen peroxide. The results show that SI and SII receive projections from neurons in separate and distinct nuclei in the ventroposterior thalamus. Following injections into subdivisions of area I, a topographical distribution of retrogradely-labelled cell bodies was observed in the ventrobasal complex. Following injections of horseradish peroxidase into subdivisions of area II, a topographical distribution of labelled cell bodies was observed in the ventroposterior inferior nucleus. No labelled cell bodies were observed in the ventrobasal complex. The thalamocortical connections of somatosensory cortices I and II in raccoon are compared with those in other animals and it is suggested that these two cortical areas may be involved in differential processing of tactile information.

Regional segregation of neurons responding to quickly adapting, slowly adapting, deep and pacinian receptors within thalamic ventroposterior lateral and ventroposterior inferior nuclei in the squirrel monkey ( Saimiri sciureus)

The ventroposterior nuclei and the ventroposterior inferior nucleus of the thalamus were mapped in squirrel monkeys ( Saimiri sciureus) using low-impedance platinum-iridium microelectrodes. The body site and receptor category from which the optimum response was obtained was recorded every 50 μm in orthogonal penetrations through these nuclei. The results suggest that the neurons are grouped according to submodality in this area and that discrete and spatially separate volumes of the thalamus are devoted to input from deep, Pacinian, cutaneous rapidly adapting and cutaneous slowly adapting receptors. The Pacinian responses were limited to the ventroposterior inferior nucleus and deep responses were limited to a region just above the ventroposterior nuclei. Within ventroposterior nuclei the cutaneous rapidly adapting and cutaneous slowly adapting volumes were interspersed in a pattern that could not be readily defined. The results indicate that there is more than one representation of the body in this area of the thalamus. Discontinuities in receptive field sequences make it difficult to see how any representation can be a spatially continuous map of the body.

Vestibular projections to the monkey thalamus: An autoradiographic study

Vestibulothalamic projections were studied in the monkey ( Macaca mulatta) by injecting anterograde tracer substances (radioactive leucine and proline) into the vestibular nuclear complex. Terminal labelling was found bilaterally mainly in the nucleus ventroposterior lateralis pars oralis (VPL 0) and to a lesser extent in the nucleus ventroposterior inferior (VPI) and nucleus ventralis lateralis pars caudalis (VL c). The labelling was sparse, and scattered over wide areas. The vestibular origin of this projection was confirmed by injecting retrograde tracer substances (horseradish peroxidase and 125I wheat germ agglutinin) into VPL 0. In the autoradiographic study no labelling was found in the posterior group.

Nucleus ventroposterior inferior (VPI) as the ventibular thalamic relay in the rhesus monkey. I. Field potential investigation.

In order to investigate the thalamic relay of the vestibulo-cortical pathway, field potentials were recorded in the rhesus thalamus under pentobarbital anesthesia. Short latency responses (2.5 msec on the average) upon stimulation in isolation of the vestibular nerve were recorded in the inferior ventroposterior nucleus (VPI). These potentials were abolished after transection of the vestibular nerve but were not affected by total cerebellectomy. Projection of VPI neurons to the primary vestibular cortex was demonstrated by antidromic stimulation. Field potentials with latencies of those observed in the vestibular cortex (about 5 msec) in response to vestibular nerve stimulation were recorded in other areas of the thalamus (ventrobasal, ventrolateral, posterior group, including magnocellular medial geniculate nuclei). Thus, the VPI rather than the other nuclei with long latency responses is likely to be the thalamic relay in the vestibulo-cortical path. The close topographical relationship between vestibular and somatic areas in the cortex is parallelled in the thalamus, the VPI being closely related to VPL and VPM nuclei.