Cutaneous Receptive Field Organization in the Ventral Posterior Nucleus of the Thalamus in the Common Marmoset

P. Wilson,1 P. D. Kitchener,2 and P. J. Snow1

 1Cerebral and Sensory Functions Unit, Department of Anatomical Sciences, University of Queensland, St. Lucia, Brisbane, Queensland 4072; and  2Department of Anatomy and Cell Biology, The University of Melbourne, Parkville, Melbourne, Victoria 3052, Australia


    ABSTRACT
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

Wilson, P., P. D. Kitchener, and P. J. Snow. Cutaneous Receptive Field Organization in the Ventral Posterior Nucleus of the Thalamus in the Common Marmoset. J. Neurophysiol. 82: 1865-1875, 1999. The organization of cutaneous receptive fields in the ventroposterior (VP) thalamus of the common marmosets (Callithrix jacchus) was determined from single-unit recordings, and these data were correlated with the cytochrome oxidase (CO) histochemistry of the thalamus in the same animals. Under continuously maintained ketamine anesthesia, the receptive fields of a total of 192 single units were recorded from the right VP thalamus using 2 MOmega glass microelectrodes. After the receptive fields were mapped, the brains were reacted for CO histochemistry on 50-µm coronal frozen sections through the entire VP thalamus. The majority of units were localized to the CO-reactive regions that define the medial and lateral divisions of VP (VPm and VPl). Apart from the expected finding of the face being represented in VPm and the body in VPl, reconstructing the electrode tracks and unit locations in the histological sections revealed a general association between discrete regions of CO reactivity and the representation of specific body regions. Some low-threshold cutaneous units were apparently localized to VPi (the CO weak regions dorsal, ventral, and interdigitating with, the CO regions of VP). These VPi units were clearly part of the same representational map as the VPl and VPm units. We conclude that the low-threshold cutaneous receptive fields of the marmoset are organized in a single continuous representation of the contralateral body surface, and that this representation can most simply be interpreted as being folded or crumpled into the three-dimensional space of VP thalamus. The folded nature of the body map in VP may be related to the folded nature of VP as revealed by CO histochemistry.


    INTRODUCTION
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

The existence of neuronal representations of the body surface in the form of somatotopic maps has emerged as a fundamental feature of somatosensory organization. Such mappings have been demonstrated at all levels of the somatosensory system in many mammalian species (for reviews see Mountcastle 1984; Snow and Wilson 1991). The tactile receptors of the body surface are represented at the thalamic level as a single, continuous and complete mapping of the contralateral body located in a central or "core" region of the ventroposterior thalamus (VP). Other modalities are represented in approximately concentric shells around this core (Jones 1984, 1985), but there is also evidence that both tactile and nociceptive modalities are superimposed in the single VP representation (reviewed by Mountcastle 1984; see also Apkarian and Shi 1994; Brüggemann et al. 1994; Chung et al. 1986). In apparently all mammals, the central region of the VP complex is composed of a medial division, VPm, which represents the face, and a lateral division, VPl, which represents the body.

Detailed electrophysiological maps of cutaneous input to the VP thalamus have been made in two species of primate: the macaque monkey (Jones and Friedman 1982; Jones et al. 1982; Loe et al. 1977; Mountcastle and Henneman 1952; Poggio and Mountcastle 1963; Pollin and Albe-Fessard 1979; Rausell and Jones 1991a) and the squirrel monkey (Apkarian and Shi 1994; Brüggemann et al. 1994; Dykes et al. 1981; Kaas et al. 1984). Studies of the macaque have reported that the face is represented, in VPm, in a collection of rostrocaudally oriented rods that are largely isorepresentational; that is, the neurons encountered in an anterior to posterior electrode penetration tend to have receptive fields on the same region of the body (Jones et al. 1986; Rausell and Jones 1991a). Although these rods are extended in a somewhat curved rostrocaudal orientation, not all rods run the entire length of VPm or VPl (Jones 1993), and rods may not be separate throughout their entire course (Rausell and Jones 1991a,b). Regions of isorepresentation are also evident in the squirrel monkey (Kaas et al. 1984), but, in general, the body representation appears to be more continuous rather than organized into rod subdomains. Kaas et al. (1984) found a single systematic representation of the entire body surface of the squirrel monkey, and, although there appeared to be discontinuities in this map, these could be explained by considering the representation to be folded. Thus the two-dimensional map of the body surface of the squirrel monkey appeared to be accommodated in VP by splitting and folding (Kaas et al. 1984). One aim of the current work was to describe the somatotopy of the body representation in the VP thalamus of the common marmoset.

Complementary to the issue of the functional organization of the VP thalamus is the question of the relationship between the functional organization and the cytoarchitectural and cytological parcellation of the thalamus. The rostrocaudal rods of largely isorepresentational neurons in VPm of the macaque have been shown to correspond to cytoarchitecturally conspicuous rods of densely packed cells (Rausell and Jones 1991a). These rods could also be delineated by their immunoreactivity for parvalbumin and CAT301 antibody, and by their strong cytochrome oxidase (CO) reactivity (Jones et al. 1986; Rausell and Jones 1991a,b). Between these CO-reactive regions of VPm and VPl is a noncytochrome oxidise-reactive matrix that contains numerous calbindin immunoreactive neurons (Rausell and Jones 1991a,b). In the VPl of macaques, the organization of the CO-reactive domains into rods is similar but less rigid than in VPm (Rausell et al. 1992).

Krubitzer and Kaas (1992) have suggested that, in the marmoset monkey, the CO-weak region ventral to, as well as interdigitating with, VPl and VPm are structurally and functionally extensions of the inferior ventroposterior nucleus (VPi). Thus VPi forms something of a shell around VP, with the additional feature of thin tracts of VPi projecting into and through VPm and VPl. These tracts of VPi within VPm and VPl in the marmoset have been suggested (Krubitzer and Kaas 1992) to be similar to what has been described in the macaque monkey VP thalamus as a matrix of non-CO-reactive cell-sparse regions lying between the CO-reactive rods of VPm and VPl (Rausell and Jones 1991a,b; Rausell et al. 1992). In the macaque monkey the lateral region of VPm abuts VP, but the ventral region of VPm abuts another noncytochrome reactive region called VMb (basal VPm); the functional relationship between VMb and VPi is not clear. In the squirrel monkey the VP thalamus has not been so thoroughly studied in terms of its cytochemistry, but has been described in cytoarchitectural terms to consist of cell-dense blocks separated by cell-sparse septa (Kaas et al. 1984). It may be the case, although it has not been investigated, that the cell-sparse septa in the squirrel monkey VP thalamus might also be extensions of VPi. This cytochemical division of VP thalamus is also seen in nonprimates; in the raccoon Herron et al. (1997) have shown that the small, calbindin-positive, parvalbumin negative neurons that comprise VPi are continuous with the septal regions separating domains of larger, parvalbumin-positive neurons of VPl and VPm.

The relationship between VPi and the cutaneous representation in VP has been examined in the macaque monkey: Rausell and Jones (1991a) made microelectrode recording from VPm and correlated the unit locations with the CO histology, but they considered their method to have insufficient resolution to be able to determine whether any of the multiunit recordings of neurons with cutaneous receptive fields were located in the matrix regions of VPm. Because the intercalation of VPi into VPm and VPl seems to be most prominent in the marmoset, we have sought, in addition to defining the shape and orientation of cutaneous representation in the marmoset VP thalamus, to determine the relationship of this representation to the cytological parcellation of VP as demonstrated by CO histochemistry.

A preliminary account of this work has been presented in abstract form (Kitchener et al. 1997).


    METHODS
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ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

Electrophysiological recordings were made from three adult common marmosets (Callithrix jacchus) that were obtained from the breeding colony maintained by the department of Anatomic Sciences at the University of Queensland. Anesthesia was induced with 4% halothane in air and maintained with intramuscular ketamine (Ketalar, 30 mg/kg) and promethazine (Promex, 1 mg/mg), in addition, animals received intramuscular injections of cortisol (Decadron, 0.25 ml) and penicillin (Austrapam, 25 mg). After intubation and cannulation to facilitate maintenance of positive pressure ventilation to maintain expired carbon dioxide between 3.5 and 4.0% and monitor blood pressure, animals were placed in a stereotaxic frame, and a small region of the skull directly above the VP thalamus was removed with the aid of a dental drill.

With reference to the stereotaxic atlas of the marmoset brain (Stephan et al. 1980), glass microelectrodes (2 MOmega ) filled with 2% pontamine sky blue in 0.5 M sodium acetate were introduced vertically into the thalamus (through the overlying cortex) while providing gentle tactile stimulation to the body surface. All of the units reported in this study were activated by low-threshold cutaneous stimuli. Single units were identified by their invariant action potential shape in successive activation of their receptive field with light touch followed by further characterization of the extent of the field with a fine paintbrush. The stereotaxic coordinates of the recording location and the location of receptive field on the body were recorded for each single unit identified. During the recording, several depth reference marks were created in the thalamus by the electrophoretic injection of pontamine sky blue and would be detected in the histological sections. After the electrophysiological mapping the animals where killed by ventricular perfusion with 0.1 M phosphate buffered saline (PBS) followed by 4% paraformaldehyde in 0.1 M phosphate buffer. The brains were removed and processed for Nissl histology and CO histochemistry, following the method of Wong-Riley (1979) on alternate 50-µm coronal frozen sections through the entire VP thalamus.

The anatomic locations of the identified cutaneous units were determined in two ways. First, for the purpose of determining the orientation of the somatotopic map, the unit depths were (with the aid of the pontamine sky blue depth markers, as shown in Fig. 1) related to the stereotaxic coordinates of the electrode tracks. From these measures a three-dimensional model of all the positions of the units in each of the brains was created (see Figs. 2-4). A second method of identifying unit locations was employed to enable the matching of the unit locations with the CO histochemistry. To optimize accuracy, it was considered necessary to reconstruct the actual trajectories of each electrode track through the thalamus (rather than the trajectories based on the assumption of a perfectly vertical path from the stereotaxic coordinates at the brain surface). Electrode tracks often showed slight deviations from the vertical axis when traveling through the brain tissue. In many instances the electrode tracks were very easy to observe due to the fact they filled with red blood cells, which subsequently stained darkly during the CO histochemistry (Fig. 1). Also, the possible distortion of the tissue due to sectioning meant that the imposition, onto the tissue sections, of an orthogonal grid based on the stereotaxic coordinates would lead to inaccuracies in estimating the true position of recording site. Camera lucida drawings were made of every section that contained part of the VP region. These drawings recorded the CO regions and electrode tracks and the depth markers, if present. By using the depth markers the position of every identified unit was matched to its position on the appropriate electrode track (examples are shown in Fig. 8). Two types of freehand sketches where drawn to illustrate the likely representation of the body surface based on the location unit recording: the first type (Fig. 5, A-C) illustrates the major subdivisions of the body as three-dimensional volumes; the second type (Fig. 5, D-F) shows how the unit data could be incorporated into a continuous representation. Rhinoceros (1.0 Beta) 3-D modeling software (McNeel and Associates) was used to illustrate the concept of how a folded two-dimensional map could give rise to the body representation in VP (Fig. 6).



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Fig. 1. Cytochrome oxidase (CO) reacted coronal sections of the ventroposterior (VP) thalamus showing electrode tracks (small arrows in A and B) and a depth marker formed by pontamine sky blue ionophoresis (large arrow in B). Electrode tracks indicated by arrows in these particular sections are particularly easy to visualize due to the accumulation of red blood cells that have become stained during the CO histochemistry.



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Fig. 2. Receptive field and unit location data from animal 1/3. Unit locations in the 3-dimensional space of VP are shown as diamonds on the electrode tracks (dotted vertical lines) of a series of mediolateral slices. Thin solid lines link these unit locations to the receptive fields shown on the figurines---the end of the line shows the approximate center of the receptive field. Receptive field overlaps are shown in darker shading with increasingly dark shading indicating greater numbers of overlapping fields. The mediolateral dimension has been expanded to allow all locations to be seen; the true proportions (and dimensions) of the volume are shown in the top left, and the orientation is given by the indicator on the left.



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Fig. 3. Receptive field and unit location data from animal 2/3. Unit locations in the 3-dimensional space of VP are shown as diamonds on the electrode tracks (dotted vertical lines) of a series of mediolateral slices. Thin solid lines link these unit locations to the receptive fields shown on the figurines---the end of the line shows the approximate center of the receptive field. Receptive field overlaps are shown in darker shading with increasingly dark shading indicating greater numbers of overlapping fields. The mediolateral dimension has been expanded to allow all locations to be seen; the true proportions (and dimensions) of the volume are shown in the top left, and the orientation is given by the indicator on the left.



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Fig. 4. Receptive field and unit location data from animal 3/3. Unit locations in the 3-dimensional space of VP are shown as diamonds on the electrode tracks (dotted vertical lines) of a series of mediolateral slices. Thin solid lines link these unit locations to the receptive fields shown on the figurines---the end of the line shows the approximate center of the receptive field. Receptive field overlaps are shown in darker shading with increasingly dark shading indicating greater numbers of overlapping fields. The mediolateral dimension has been expanded to allow all locations to be seen; the true proportions (and dimensions) of the volume are shown in the top left, and the orientation is given by the indicator on the left.



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Fig. 5. Pictorial views of the representation in the VP thalamus based on unit locations from 3 marmosets (as shown in Figs. 2-4). For each animal, the left side (A-C) shows units with receptive fields located on the major body regions grouped together into volumes. An alternative summary of the somatic representation is to incorporate the unit location data into a single continuous representation such that the body is magnified and distorted in accordance with the unit locations (shown in D-F).



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Fig. 6. Schematic representations of the somatotopic representation of cutaneous inputs in the VP thalamus. This figure indicates how a hypothetical 2-dimensional map (shown top left and as a slab top right) with the head in the medial part of VP (VPm) and the body in the lateral part (VPl) might be folded and compressed (right side figures) to form a representation in 3 dimensions similar to the map derived from the unit locations (bottom left). The folding pattern illustrated by the 3 right side figures is loosely based on the data obtained from reconstructing the CO-reactive regions (see Fig. 7): there is a major division between VPm and VPl, and there is a major groove running rostrocaudally that separates the hindlimb and forelimb regions (see the arrow in Fig. 7). A groove running lateral to medial has been added to account for the bending of the hindlimb and tail representations (as shown in Fig. 5).

To reconstruct the CO regions, Fig. 8 was produced to show a semitransparent reconstruction of the entire CO-stained region. Camera lucida drawings of the sections were digitized (Agfascan), and Adobe Photoshop software was used to render the digital images transparent and give a gray scale value of 5%. The gray scale value of 5% was chosen because there were 20 CO sections that encompassed the unit recording sites; thus regions that had CO staining in every section would be black (20 × 5% = 100% gray scale: black), whereas regions where, for example, only two sections had CO staining would be 10% gray. All sections were overlaid in the correct alignment; the resulting image was of a semi-transparent volume (representing the CO-stained region) seen from a directly rostral viewpoint. This technique could be considered to be a type of spatial analogue of a running average filter as used with time series data. In addition to creating the combined image of all sections stacked rostral to caudal, the order of the sections was reversed to produce the caudal view.


    RESULTS
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ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

Cutaneous receptive fields and the body representation in VP

The three animals yielded 192 single units (57, 60, and 75) for which the position in the thalamus and their receptive field on the skin was recorded. The majority of units (160) had receptive fields on the body rather than on the head. These data are presented in Figs. 2-4. All three animals showed the same general somatotopic pattern: the head was represented medially (in VPm), and the body representation is more lateral and lies such that its rostrocaudal axis runs approximately lateral to medial (in VPl). The face representation is medial to the rest of the head, the lips more ventral, and the upper face more dorsal. The digits are represented over a comparatively large area rostrally and laterally, with the foot more lateral than hand. Although it was clear that digit one ("thumb") was located most medially and digit 5 was located most laterally, it was not obvious how the dorsal and palmar surfaces are organized in this representation. The tail representation is very lateral, the leg lateral and caudal, chest and abdomen caudal and more medial, arm caudal and more medial than chest.

At least two types of organization can be suggested that might allow the constellation of receptive fields in the VP thalamus to be viewed as part of a more simple pattern. One method would be to see whether the units form natural groupings such that particular regions contained units representing discrete body regions. In Fig. 5, A-C, the receptive fields shown in Figs. 2-4 are grouped into the major body regions by joining up these regions as solid shapes. Another approach would be to conceptualize the units as forming a single continuous body map. Although the latter approach may not seem inherently valid from the distributions shown in Figs. 2-4, it is known from numerous studies that the body surface is represented in continuous two-dimensional maps, not only in conspicuously layered regions of the somatosensory system, but also in the more compact regions such as the VP thalamus. Figure 5, D-F, shows the body surface distorted (in terms of scaling and folding) to accommodate the general pattern of the unit locations in Figs. 2-4. The continuous representations of all three animals are broadly similar, with the differences in their form most likely due to differences in the number of samples obtained in each animal rather than different orientations of the representational maps. Based on these figures (5, D-F) of the approximate orientation of the body representation, a composite drawing was made using this information from all three animals (Fig. 6, bottom left). This composite shows that if the units form a single and continuous representation of the body, then this representation is somewhat folded and there were clearly larger areas of VP devoted to digits than to any other body regions. Figure 6 diagrammatically represents the concept of how a folded two-dimensional map could produce the type of body representation found in the VP thalamus.

CO histochemistry

The distribution of CO reactivity in the marmoset VP thalamus has been described in detail by Krubitzer and Kaas (1992), who defined VP as consisting of patches of cytochrome reactive regions (VPm and VPl) with cell-sparse intervening (septal) regions, VPi, that are weakly or nonreactive for CO and are an extension of a cell sparse region ventral to VPm and VPl. Our results concur with the observations of Krubitzer and Kaas (1992) in that the extent of septal encroachment from the dorsal and ventral CO poor regions is highly variable: in some sections these septa completely divide areas of CO reactivity where other septal regions only make small thin intrusions in to the ventral regions of VPm and VPl. To further characterize the relationship between the CO-stained and nonstained regions, the entire series of sections through the right VP thalamus of one animal was reconstructed with the aid of camera lucida drawing (Fig. 7). To reconstruct the CO regions, the camera lucida sections were converted to semi-transparent digital images and superimposed so that the major folds could be visualized (as described in METHODS). This reconstruction is shown in Fig. 7. There is no evidence of completely isolated areas or "islands" of CO-reactive tissue; despite the appearance of such islands in single sections, there was usually an isthmus of CO-reactive tissue rostrally or caudally that linked regions together. There is a prominent septum running the entire rostrocaudal extent of the ventral part of the CO-reactive region in VPl (see the curved arrow in Fig. 7). The division between the CO-reactive regions of VPm and VPl does not appear to be bridged at any point (as indicated by the line in Fig. 7).



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Fig. 7. Views in the rostrocaudal axis of the entire series of CO-reactive regions of coronal sections of the VP thalamus for a single marmoset. All of the 20 sections have a gray-scale value of 5%; increasingly dark areas indicate increasing more overlap with the CO-reactive regions of other sections (as described in METHODS). Despite the variation in the shape of the reactive regions, the separation of lateral and medial division (curved line) and the ventral groove in VPl (arrow) are consistent features. Slightly different views are obtained depending on whether sections are stacked from the most caudal (A) or from the most rostral (B).

Relation of the body map to CO parcellation

In all cases the electrode tracks could be identified in the histological sections, and in most cases the entire track could be seen; failure to visualize the entire track was no doubt due to part of it coinciding with the edge of the cut section. However, as each electrode track was represented in two or three coronal sections (due to a slight difference in the plane of section from the vertical plane of the stereotaxic coordinates), the entire trajectory of all tracks could, with the additional guidance of the pontamine blue depth markers, be accurately determined. Relating the electrode tracks to the histological sections showed that the electrode penetrations covered a region of the thalamus that included VP with the exception of the most medial region (Fig. 8). The reconstruction of the locations of the 192 cutaneous units revealed 17 to be located outside the CO-reactive regions. Ten units were dorsal to the CO regions, 1 was ventral, and 6 were in the septal fingers of VPi that divided the CO-reactive regions. All of these units located to VPi were clearly part of the same body representation as the adjacent units in the same tracks. Figure 8 shows examples of coronal sections where the CO pattern and the electrode tracks are related to the unit positions (and depth markers). As can be seen in the examples shown in Fig. 8, tracks that ran through large regions of VPi did not appear to encounter any more or any fewer units than other tracks that where confined to the CO-reactive regions.



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Fig. 8. Camera lucida drawings of the CO-reactive regions of coronal sections of VP thalamus. The precise location of cutaneous units was determined by reconstructing the electrode tracks and by reference to the pontamine sky blue depth markers (shown as crosses). Note that contiguous body regions tend to have units within the same cytochrome oxidise reactive region, but also that in all 5 sections shown some units appear to be localized to the non-CO-reactive regions. A-D are from the same animal; the electrode tracks in the section shown in E are somewhat curved and oblique to the dorsoventral axis due to the distortion of this brain during histological processing.

Location of unit positions in the electrode tracks of the CO-stained sections allows the issue of whether the regions or divisions of VPm and VPl reflect functional grouping of the body representation to be examined. As noted above, there are several major divisions of the CO-reactive regions caused by the intrusion of VPi. In the example shown in Fig. 6, there is a prominent and near complete separation of VPm and VPl and also a large septal area that runs through about the middle (of the mediolateral axis) of ventral VPl; this septal region appears to divide, at about the forearm, receptive fields from digits receptive fields. However, the fact that arm receptive fields were seen in one block and digit receptive fields in the next is probably due to the fact that the hand is represented more medially rather than due to the segregation of CO-reactive regions per se. For example, in Fig. 8E there is a track that runs down the medial edge of the lateral block of VPl. As the microelectrode passed along this track it recorded receptive fields on shoulder, forearm, dorsal hand, and finally digit 1.


    DISCUSSION
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

The topographic mappings, which are a salient feature of the somatosensory system, and indeed a general organizational principle in many areas of the nervous system, are most easily appreciated when mapped to sheet or layered neural structures (such as the neocortex, or the laminae of the spinal dorsal horn). Although not as easy to demonstrate, somatotopic maps are also present in three-dimensional volumes such as the dorsal column nuclei of the brain stem and the ventroposterior thalamus (see Snow and Wilson 1991 for review). By combining recordings of single cutaneous units from three animals, we have been able to determine the representation of the body surface in the VP thalamus of the marmoset.

Form and orientation of the cutaneous representation in VP of the marmoset

The broad somatotopy of the cutaneous representation in VP of the marmoset follows, not surprisingly, the pattern that appears to be similar in all mammals (Jones 1985). The body is laid out so that its rostrocaudal axis lies in the mediolateral axis; the tail may be an exception, however, as its proximodistal extent is represented caudal to rostral in the extreme lateral VPl. There is an association of the proximodistal axis of the body with the rostrocaudal extent of VPl, but there did not appear to be an obvious correspondence between the dorsoventral axis of the body and any axis of the representation in VP. The importance of the hands and feet, and especially the digits, for monkeys predicts the finding that the digits on the hindlimb and especially the forelimbs have a comparatively large territory in VPl. Both the individual and the combined data suggest that the low-threshold cutaneous receptive fields of the marmoset can be summarized as forming a single continuous representation of the contralateral body surface. This body map is not only magnified in certain places (most notably the digits) but also is folded or crumpled into the three-dimensional space of the ventral posterior thalamus.

The picture that has emerged from the present mapping of the cutaneous representation in the marmoset appears to have much in common with the cutaneous representation in the squirrel monkey (Dykes et al. 1981; Kaas et al. 1984). In squirrel monkeys there is a single, systematic representation of the body surface that occupies most or all of VP. In addition to finding long sequences of overlapping receptive fields, Kaas et al. (1984) also found sequences of isorepresentation and abrupt discontinuities in receptive field location when making electrode penetrations. However, a single continuous representation was seen to be the best fit to the data when all the locations were considered: the abrupt discontinuities were the result of folds in the body representation so that discontinutities would be found when the electrode crossed from one fold to another. The representation we have found in the marmoset VP thalamus can also be viewed as a single map that appears to be twisted and folded. In this regard the marmoset body representation in VP appears to be like that of the squirrel monkey, and also, perhaps, similar to the rat where the representation of the forepaw has been shown to be folded in the form of a closed fist with the palm and wrist caudal to the digits, which are more rostral and extending through much of the VP in the dorsoventral axis (Angel and Clarke 1975).

There was no evidence in our recordings from marmoset VP of the isorepresentational rods of the type that have been found in the macaque VPm thalamus (Jones et al. 1986; Rausell and Jones 1991a). Although the representation of the face in the body of the macaque monkey is clearly organized into rods, it seems reasonable to infer from the published data that beyond the grouping into rods, there exists an overall single, systematic representation of the body as found in other animals. The segregation of this representation into rods seem to be a further level of order imposed on this mapping and is not evident in our data on the marmoset. However, it must be acknowledged that the VPl of the macaque is not so rigidly organized into the rod pattern seen in VPm, and there exists several large septal regions that divide VPl into regions (Rausell et al. 1992). These larger divisions of VPl regions have reduced CO reactivity (Rausell et al. 1992) and thus could be similar to the major divisions (described below) in the marmoset VPl and in the cytoarchitectonic domains of the squirrel monkey VPl.

CO and functional grouping

In addition to the general pattern and orientation of the cutaneous representation in the marmoset thalamus, we also sought to relate this map to the CO-reactive regions in VP. First, we consider the issue of whether there is a grouping of body regions into distinct CO-reactive regions. Second, given that we found a folded and somewhat twisted representation, the question arises as to whether the folds in the body map corresponds to the folds in the CO-reactive patches.

Our data support the idea that the segregation of CO-reactive regions are related to the segregation of the cutaneous map into body regions. We found no examples of units within the same CO-reactive region with receptive fields on widely separated locations on the body surface. It would appear that the marmoset VP thalamus has an approximately similar degree of compartmentalization of body regions as the squirrel monkey. Although the CO reactivity of the squirrel monkey VP was not examined in the study by Kaas et al. (1984), it seems reasonable to suppose that, as in macaque and marmosets, this CO reactivity is directly associated with the cytoarchitectural parcellation into cell dense and cell sparse regions. Kaas et al. (1984) were able to relate the somatotopic map obtained in their squirrel monkeys to the cytoarchitecture of VP in the same animals and concluded that cell-poor laminae divided and grouped the VP into five major divisions related to the hand, foot, trunk, and tail (all in VPl) and the face (in VPm). These cell-sparse regions also represented the folds in the map that produced the major discontinuities in the receptive field progression in an electrode penetration. A similar, although less complex subdivision of VP appears to exist in the prosimian primate Galago senegalensis: Pearson and Haines (1980) have described a major division (a sheet like laminae of fibers) that segregates forelimb and hindlimb input of VPl.

The subdivision of the macaque and marmoset VP thalamus into cytochrome-rich domains separated by CO-weak septal regions has also been demonstrated in the raccoon thalamus (Doetsch et al. 1988; Wiener et al. 1987). In the enlarged hand representation of the raccoon VP thalamus, five cytoarchitecturally distinct laminae correspond to the representations of the five fingers (Welker and Johnson 1965; Wiener et al. 1987); thus there is a clear association in the raccoon between cytoarchitectural and functional subdivision of VP. CO-reactive patches (in the form of barreloids) have been described in the ventrobasal thalamus of a number of nonprimate mammals including mice and young rats (see Jones and Diamond 1995 for review) and marsupials (reviewed by Waite and Weller 1997). In the animals that exhibit thalamic barreloids, each barreloid represents an individual vibrissa on the face or a region of glabrous skin and projects to similarly functionally dedicated CO-reactive barrels in somatosensory cortex. Unlike primates (and raccoons), however, the spaces or septa between these CO-reactive barreloids in the rat and mouse thalamus seem not to correspond to a separate neural region but instead comprise mainly glia and axons, and these animals do not appear to posses a VPi region (Jones 1985).

Although there appears to be a segregation of body regions to discrete CO-reactive regions in VPm and VPl of the marmoset, our histological data suggest that the division of these CO-reactive regions by septa of VPi is not necessarily as complete as it appears on single coronal sections. Reconstruction of serial sections showed that most of the islands that appeared in sections were joined at some point (in other sections) to the adjacent CO-reactive regions. The incursions of septal regions into VPm and VPl is probably greater than might be indicated from coronal sections as coronal sections show rostrocaudally and mediolaterally oriented septa more clearly than they can show septal incursion that runs in the coronal plane. This is compounded by the fact that the width of these septa are often <100 µm and the section thickness is 50 µm. With regard to the question posed above, as to whether the folds in the body map corresponds to the folds in the CO patches, only a very general answer can be given. Our summary of the cutaneous representation (Fig. 5) suggests that there are several folds in the map: this could be taken as a reflection of the fact that there are several major folds in the overall pattern CO reactivity (Fig. 6). However, beyond this generalization, little more can be said. A more definitive answer will require a significantly more detailed map (or partial map) of the cutaneous representation, very accurately matched with a complete reconstruction of the CO-reactive domains of VP thalamus.

The matrix of the macaque appears, on cytological grounds at least, to be similar to the non-CO-reactive regions within VP described in other primate and nonprimate species, but the connectivity of these regions may differ between species. The matrix of the macaque VP projects to superficial layers of SI (Rausell and Jones 1991b), whereas the regions of VPi within VP of the marmoset thalamus project to SII (Krubitzer and Kaas 1992). Unfortunately, the issues of whether the marmoset VPi projects to superficial SI, and whether macaque VP matrix projects to SII, have not been explicitly investigated. In macaque monkeys retrograde labeling from injections of tracer into SII labels, in addition to large number of VPi neurons, a small number of neurons in VP, but only when large volumes of tracer were injected into SII (Friedman and Murray 1986). Recently, Jones (1998) has suggested that the non-CO-reactive regions of the thalamus comprise neurons projecting to superficial layers of comparatively wide areas of cortex, irrespective of cortical architectonic boundaries.

There may also be differences in the afferent projections to the VP subdivisions of different species, but as with the efferent connections, the extent to which these reflect species differences or different experimental approaches is unclear. In the macaque, the rod domains of VP receive projections predominantly from the principle trigeminal and the dorsal column nuclei (respectively), and thus are said to convey lemniscal information (Rausell and Jones 1991b; Rausell et al. 1992). The matrix region within VP receives caudal trigeminal and spinothalamic tract inputs and thus appears to be a separate channel for nonlemniscal input (Apkarian and Hodge 1989; Boivie 1979; Gingold et al. 1991; Rausell and Jones 1991b). In cats, raccoons, and squirrel monkeys, VP and VPi receive both lemniscal and nonlemniscal inputs (Chung et al.1986; Dykes et al. 1988; Herron and Dykes 1986; Herron et al. 1997). The clear segregation of the efferent projections of VPl (and VPm) and VPi to SI and SII in the marmoset (Krubitzer and Kaas 1992) does not appear to be associated with a segregation of lemniscal and nonlemniscal pathways through the thalamus to the cortex. Electrophysiological mapping of marmoset cortex have indicated that both SI and SII receive low-threshold cutaneous input (Krubitzer and Kaas 1990; Zhang et al. 1996), and recent physiological experiments have provided evidence for parallel pathways of cutaneous information from the thalamus to SI and SII (Zhang et al. 1996).

An important question regarding the cutaneous representation we have described in the marmoset is whether VPi contains low-threshold cutaneous units. In the macaque monkey VPm, the association between function groupings of units and the CO-reactive regions is very marked (although possibly less so in VPl), with essentially isorepresentaional CO-reactive rods (Jones et al. 1986; Rausell and Jones 1991a). Detailed retrograde labeling studies (Darian-Smith and Darian-Smith 1993; Darian-Smith et al. 1990; Rausell and Jones 1995) have revealed that, although the somatotopic order of the rods is reflected in the somatotopy of SI, considerable convergence and divergence outside a simple somatotopic relationship exist between thalamic and cortical somatosensory representation. The functional nature of neurons in the matrix between rods of the macaque VP thalamus is not known. The microelectrode recordings made by Rausell and Jones (1991a) were considered by the authors as having a resolving power of not better than 100 µm, and thus not able to unambiguously identify units or multiunit locations as being in the matrix regions between the rods. However, because they did not find any low-threshold units in the larger matrix regions dorsomedial and ventrolateral to VPm, they suggest that this could indicate that the matrix between the rods does not contain neurons driven by low-threshold inputs.

In the present study, the method of reconstructing electrode tracks to find locations of single units, and the fact that the septal regions of VPm and VPl of the marmoset are more prominent, allows us to tentatively conclude that a small proportion of low-threshold cutaneous units were located outside the CO-reactive regions of VP. The occurrence of low-threshold units in these septal regions could indicate that VPi does not differ greatly from the CO-reactive regions of VP in terms of afferent input. Alternatively, these low-threshold VPi units may reflect the presence of neurons that are more typical of those within the CO-reactive regions, but that lie outside the histologically defined boundary. In this regard, it is interesting to note that although the matrix is the only region of the macaque VP thalamus to contain the small calbindin-immunoreactive cells, both rods and matrix contain larger parvalbumin-immunoreactive cells (Rausell et al. 1992).

Although we took great care to make the reconstructed position of the cutaneous units as accurate as possible, the spatial resolution of the microelectrode recording and the accuracy of the reconstruction (due to any distortions of the brain through multiple electrode penetrations, freezing, and sectioning) no doubt introduces small inaccuracies, and the distance of any cell in the septal part of VPi to the nearest CO-reactive is a matter of <100 µm (and often <30 µm.). Because we have no means of determining the exact spatial resolution of our methods, our conclusion, that some low-threshold units were located in the regions of VPi interdigitating with VPm and VPl, is somewhat tentative. This uncertainty about resolution would not, however, appear to be a problem for the 10 low-threshold cutaneous units found dorsal to the CO-reactive regions; the majority of these units were located >300 µm from the dorsal extent of the CO reactivity. Interestingly, one unit was located to the region clearly ventral to VPm and VPl. An indirect argument could be made that, if the septal regions of VPm and VPl are functionally equivalent extensions of VPi that lies ventral to VPm and VPl, then the fact that we found only one clear example of a cutaneous unit located ventral to the CO-reactive region in VPl would suggest that there are few such units in VPi (including those parts embedded in VPm and VPl). This issue might be resolved by labeling single functionally identified units. Also, immunocytochemical studies could be employed to ascertain whether the matrix of macaque VP is equivalent to the non-CO-reactive regions of the marmoset VP.

In summary, we have described the cutaneous representation in the marmoset VP thalamus as a single continuous map, with enlarged digit representations, which is distorted into what appears to be folds. The simplest scheme that relates this map to the CO-reactive regions of VP is that the folds in the functional map correspond to the topology of the CO-reactive region. Although the functional nature of neurons outside the CO-reactive regions was not studied in detail, at least some of these neurons appeared to have low-threshold cutaneous receptive fields with locations commensurate with the single somatic representation formed by units in the CO-reactive regions.


    ACKNOWLEDGMENTS

We thank E. Hutton for help with the data analysis.

This work was supported by the National Health and Medical Research Council of Australia.

Present address of P. Wilson and P. J. Snow: School of Biological Sciences, University of New England, Armidale, New South Wales 2351, Australia.


    FOOTNOTES

Address reprint requests to P. D. Kitchener.

The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

Received 19 January 1999; accepted in final form 15 June 1999.


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ABSTRACT
INTRODUCTION
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DISCUSSION
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0022-3077/99 $5.00 Copyright © 1999 The American Physiological Society