Organization of somatosensory cortex in the laboratory rat (Rattus norvegicus): Evidence for two lateral areas joined at the representation of the teeth

Lateral somatosensory areas have not been explored in detail in rats, and theories on the organization of this region are based largely on anatomical tracing experiments. We investigated the topography of this region by using microelectrode recordings, which were related to flattened cortical sections processed for cytochrome oxidase (CO). Two lateral somatosensory areas were identified, each containing a complete representation of the body. A larger, more medial representation formed a mirror image of S1 along the rostrocaudal axis of the head region corresponding to the previously identified secondary somatosensory area (S2). A smaller, more lateral representation formed a mirror image of S2 along the rostrocaudal axis of the forelimb and hindlimb regions and likely corresponds to the parietal ventral area (PV) identified in other mammals. We also investigated the representation of the dentition and identified regions of cortex responsive to tooth stimulation. The lower incisor representation was rostral to the lower lip region of S1, and the upper incisor representation was lateral to the buccal pad region of S1. The upper and lower incisors flanked the tongue representation. An additional large region of far lateral cortex responded to both incisors. Finally, five CO-dense modules were consistently identified rostral and lateral to the S1 face representation, which we refer to as OM1, OM2, OM3, FM, and HM. These modules closely correspond to the physiologically identified areas representing the lower incisor (OM1) and tongue (OM2) regions of S1 and the mixed tooth (OM3), forelimb (FM1), and hindlimb (HM) representations of S2 and PV.     

Somatosensory and motor representations in cerebral cortex of a primitive mammal (Monodelphis domestica): a window into the early evolution of sensorimotor cortex

To examine the potential early stages in the evolution of sensorimotor cortex, electrophysiological studies were conducted in the primitive South American marsupial opossum, Monodelphis domestica. Somatosensory maps derived from multiunit microelectrode recordings revealed a complete somatosensory representation of the contralateral body surface within a large region of midrostral cortex (primary somatosensory cortex, or S1). A large proportion ( approximately 51%) of S1 was devoted to representation of the glaborous snout, mystacial vibrissae, lower jaw, and oral cavity (the rostrum). A second representation, the second somatosensory area (or S2), was found adjacent and caudolateral to S1 as a mirror image reversed along the representation of the glabrous snout. A reversal of somatotopic order and an enlargement of receptive fields marked the transition from S1 to S2. Mapping of excitable cortex was conducted by using intracortical microstimulation (ICMS) techniques, as well as low-impedance depth stimulation and bipolar surface stimulation. In all three procedures, electrical stimulation resulted in movements confined strictly to the face. Specifically, at virtually all sites from which movements could be evoked, stimulation resulted in only vibrissae movement. ICMS-evoked vibrissae movements typically occurred at sites within S1 with receptive fields of the mystacial vibrissae, lower jaw, and glaborous snout. Results were similar using low-impedance depth stimulation and bipolar surface stimulation techniques except that the motor response maps were generally larger in area. There was no evidence of a motor representation rostral to S1. Examination of the cytoarchitecture in this cortical region (reminiscent of typical mammalian somatosensory cortex) and the high levels of stimulation needed for vibrissae movement suggest that the parietal neocortex of Monodelphis is representative of a primitive sensorimotor condition. It possesses a complete S1 representation with an incomplete motor component overlapping the S1 representation of the face. It contains no primary motor representation. Completion of the motor representations within S1 (trunk, limbs, tail) as well as the emergence of a primary motor cortex rostral to S1 may have occurred relatively late in mammalian phylogeny.     

Corticocortical projections to representations of the teeth,tongue, and face in somatosensory area 3b of macaques

We placed injections of anatomical tracers into representations of the tongue, teeth, and face in the primary somatosensory cortex (area 3b) of macaque monkeys. Our injections revealed strong projections to representations of the tongue and teeth from other parts of the oral cavity responsive region in 3b. The 3b face also provided input to the representations of the intraoral structures. The primary representation of the face showed a pattern of intrinsic connections similar to that of the mouth. The area 3b hand representation provided little to no input to either the mouth or the face representations. The mouth and face representations of area 3b received projections from the presumptive oral cavity and face regions of other somatosensory areas in the anterior parietal cortex and the lateral sulcus, including areas 3a, 1, 2, the second somatosensory area (S2), the parietal ventral area (PV), and cortex that may include the parietal rostral (PR) and ventral somatosensory (VS) areas. Additional inputs came from primary motor (M1) and ventral premotor (PMv) areas. This areal pattern of projections is similar to the well‐studied pattern revealed by tracer injections in regions of 3b representing the hand. The tongue representation appeared to be unique in area 3b in that it also received inputs from areas in the anterior upper bank of the lateral sulcus and anterior insula that may include the primary gustatory area (area G) and other cortical taste‐processing areas, as well as a region of lateral prefrontal cortex (LPFC) lining the principal sulcus. J. Comp. Neurol. 522:546–572, 2014. © 2013 Wiley Periodicals, Inc.     

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

The ventral posterior nucleus of thalamus sends highly segregated inputs into each digit representation in area 3b of primary somatosensory cortex. However, the spatial organization of the connections that link digit representations of areas 3b with other somatosensory areas is less understood. Here we examined the cortical inputs to individual digit representations of area 3b in four squirrel monkeys and one prosimian galago. Retrograde tracers were injected into neurophysiologically defined representations of individual digits of area 3b. Cortical tissues were cut parallel to the surface in some cases and showed that feedback projections to individual digits overlapped extensively in the hand representations of areas 3b, 1, and parietal ventral (PV) and second somatosensory (S2) areas. Other regions with overlapping populations of labeled cells included area 3a and primary motor cortex (M1). The results were confirmed in other cases in which the cortical tissues were cut in the coronal plane. The same cases also showed that cells were primarily labeled in the infragranular and supragranular layers. Thus, feedback projections to individual digit representations in area 3b mainly originate from multiple digits and other portions of hand representations of areas 3b, 1, PV, and S2. This organization is in stark contrast to the segregated thalamocortical inputs, which originate in single digit representations and terminate in the matching digit representation in the cortex. The organization of feedback connections could provide a substrate for the integration of information across the representations of adjacent digits in area 3b. J. Comp. Neurol. 521:3768–3790, 2013. © 2013 Wiley Periodicals, Inc.     

Topography,architecture, and connections of somatosensory cortex in opossums: Evidence for five somatosensory areas

Microelectrode maps of somatosensory inputs were related to cortical architecture and patterns of cortical connections to provide evidence for five subdivisions of the somatosensory or sensorimotor cortex in North American opossums (Didelphis marsupialis). Microelectrode recordings revealed three systematic representations of the body surface. A large mediolaterally oriented representation was identified as the primary somatosensory area (S1) by its relative position, somatotopy, architecture, and connections. S1 represented the hindlimb, trunk, forelimb, and face in a mediolateral sequence. Two additional representations of cutaneous receptors were found caudolateral to S1, each with face representations adjacent to the border of lateral S1 and other body-part representations progressing more caudally toward the auditory cortex. We identified the more dorsal field as the second somatosensory area (S2) and the more ventral field as the parietal ventral area (PV). Tracers injected into S1 labeled neurons and terminals in architectonically distinct fields rostral and caudal to S1, the somatosensory caudal area (SC) and the somatosensory rostral area (SR). Movements could be evoked by microstimulation from sites scattered over S1, SR, and the frontal cortex, but thresholds were high and uncharacteristic of motor cortex. S2 and PV merged caudally with the cortex responsive to auditory stimuli, possibly A1, and neurons in some caudal recording sites in PV were activated by both auditory and cutaneous stimuli. Primary (V1) and secondary (V2) visual areas were also identified by microelectrode mapping, architecture, and connections. In addition, at least part of the cortex between V2 and the somatosensory cortex had visual connections. Thus, most of the dorsolateral cortex of opossums appears to be somatosensory, auditory, or visual. © 1996 Wiley-Liss, Inc.     

Chemoarchitecture of layer 4 isocortex in the American water shrew (Sorex palustris)

We examined the chemoarchitecture of layer 4 isocortex and the number of myelinated nerve fibers of selected cranial nerves in the American water shrew (Sorex palustris). This study took advantage of the opportunity to examine juvenile brain tissue, which often reveals the most distinctive cortical modules related to different sensory representations. Flattened cortical sections were processed for the metabolic enzyme cytochrome oxidase, revealing a number of modules and septa. Subdivisions related to sensory representations were tentatively identified by performing microelectrode recordings in a single adult shrew in this study, combined with microelectrode recordings and anatomical findings from a previous investigation. Taken together, these results suggest that characteristic chemoarchitectonic borders in the shrew neocortex can be used to delineate and quantify cortical areas. The most obvious subdivisions in the water shrew include a relatively small primary visual cortex which responded to visual stimuli, a larger representation of vibrissae in the primary somatosensory cortex, and a prominent representation of oral structures apparent in the more rostral-lateral cortex. A presumptive auditory area was located in the far caudal cortex. These findings for the cortex are consistent with counts from optic, auditory and trigeminal nerves, suggesting that somatosensory inputs dominate the shrew's senses whereas visual and auditory inputs play a small role in navigation and in finding prey. More generally, we find that shrews share unusual features of cortical organization with moles, supporting their close taxonomic relationship.     

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.     

Corticocortical projections to area 1 in squirrel monkeys (Saimiri sciureus)

Cortical area 1 is a non‐primary somatosensory area in the primate anterior parietal cortex that is critical to tactile discrimination. The corticocortical projections to area 1 in squirrel monkeys were determined by placing multiple injections of anatomical tracers into separate body part representations defined by multiunit microelectrode mapping in area 1. The pattern of labeled cells in the cortex indicated that area 1 has strong intrinsic connections within each body part representation and has inputs from somatotopically matched regions of areas 3b, 3a, 2 and 5. Somatosensory areas in the lateral sulcus, including the second somatosensory area (S2), the parietal ventral area (PV), and the presumptive parietal rostral (PR) and ventral somatosensory (VS) areas, also project to area 1. Topographically organized projections to area 1 also came from the primary motor cortex (M1), the dorsal and ventral premotor areas (PMd and PMv), and the supplementary motor area (SMA). Labeled cells were also found in cingulate motor and sensory areas on the medial wall of the hemisphere. Previous studies revealed a similar pattern of projections to area 1 in Old World macaque monkeys, suggesting a pattern of cortical inputs to area 1 that is common across anthropoid primates.     

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.     

Organization of somatosensory cortex and distribution of corticospinal neurons in the eastern mole (Scalopus aquaticus)

The somatotopic organization of somatosensory cortex of the eastern mole (Scalopus aquaticus) was explored with multiunit microelectrode recordings from middle layers of cortex. The recordings revealed the presence of at least parts of two systematic representations of the body surface in the lateral cortex. One of the representations appears to be primary somatosensory cortex (S1), and it contained cytochrome oxidase dark regions, separated by light septa that formed isomorphs with some body parts. The rostral portion of this presumptive S1 cortex contained a face representation with a series of barrel-like cytochrome oxidase dark ovals that corresponded to the vibrissae on the snout. In caudolateral S1, light septa outline the palm and digits of the forepaw. Cortex caudal to S1, in the expected region of auditory cortex, responded to vibration, suggesting a modification of auditory cortex. Injections of wheat germ agglutinin-horseradish peroxidase into the cervical enlargement of the spinal cord revealed two dense foci of cortical cells that project to the spinal cord. The focus medial to the face region in S1 may correspond to primary motor cortex (M1). The second focus was coextensive with the somatosensory representation of the forelimb and the trunk in S1. The dense corticospinal projections from the forelimb representation of S1 and motor cortex may reflect sensorimotor specializations related to digging behaviors in moles. J. Comp. Neurol. 378:337–353, 1997. © 1997 Wiley-Liss, Inc.     

Double representation of the body surface within cytoarchitectonic areas 3b and 1 in "SI" in the owl monkey (Aotus trivirgatus).

Microelectrode multiunit mapping studies of parietal cortex in owl monkeys indicate that the classical “primary” somatosensory region (or “SI”) including the separate architectonic fields 3a, 3b, 1, and 2 contains as many as four separate representations of the body rather than one. An analysis of receptive field locations for extensive arrays of closely placed recording sites in parietal cortex which were later related to cortical architecture led to the following conclusions: (1) There are two large systematic representations of the body surface within “SI”. Each is activated by low threshold cutaneous stimuli; one representation is coextensive with Area 3b and the other with Area 1. (2) While each of these representations contain regions of cortex with topological or “somatotopic” transformations of skin surface, the representations have many discontinuities where adjoining skin surfaces are adjoining in the representations. Thus, the representations can be considered as composites of somatotopically organized regions, but cannot be accurately depicted by simple continuous homunculi. Lines of discontinuity often cut across dermatomes and seldom follow dermatomal boundaries, i.e., neither cutaneous representation constitutes a systematic representation of dermatomal skin fields. (3) While the two cutaneous fields are basically similar in organization and are approximate mirror images of each other, they differ in important details, i.e., lines of discontinuity in the representations and the sites of representations of different specific skin surfaces differ significantly in the two representations. (4) The two cutaneous representations also differ in size and in the relative proportions in each representation differ, they cannot both be simple reflections of overall peripheral innervation density. (5) All or part of Area 2 contains a systematic representation of deep body structures. These conclusions are consistent with a view of the anterior parietal region as containing functionally distinct fields at least partially related to different subsets of receptor populations and coding or representing different aspects of somatic sensation. We suggest that the “SI” region of primates be redefined as a parietal somatosensory strip, the Area 1 representation as the posterior cutaneous field, and, for reasons of probable homology with “SI” of other mammals, the Area 3b representation as SI proper.     

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

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

Body surface maps in the somatosensory cortex of rabbit

The organization of somatosensory maps was examined in rabbits with the aid of microelectrode multi-unit recording techniques. Two complete maps of the contralateral body surface are identified in the parietal cortex. The first map, S I, is found entirely on the lateral convexity of the hemisphere and closely resembles S I described in the rat (Welker, '71, '76). It is organized in a complex, though systematic, fashion with the representations of the hindlimb and tail located caudomedially. These representations are followed laterally in sequence by those of the trunk and forelimb and then the representation of the head. Within the head representation the lips are found rostrally, the vibrissae caudomedially, and the displaced representation of the pinna of the ear is located caudolaterally. Unlike the disposition in most other mammals, the dorsal midline of the trunk is represented along the caudal border of S I. Within S I, the representations of the circumoral surfaces, including the lips, philtrum, nose, and vibrissae, are emphasized, occupying approximately 86.4% of the map. It is suggested that S I is contained within a single major koniocortical region, here called the medial parietal area, or Pm. The several previously described parietal regions (Rose, '31; Fleischhauer et al., '80) are interpreted as subregions that are related to particular representations of portions of the body surface. The second map, S II, is located lateral to S I in a region here called the lateral parietal area or Pl. S II shares a common border with S I along the representations of the philtrum, bridge of the nose, and top of the head. The body is oriented in an erect conformation with the head located rostrally and medially and the hindlimb and tail located caudally and laterally.     

Organization of the somatosensory cortex of the star-nosed mole

The nose of thestar-nosed mole consists of a star-like array of 22 fleshy appendages that radiate from the nostrils and are moved about to explore the environment. The surface of each appendage, or ray, is densely packed with bulbous receptor organs (Eimer's organs) that are highly responsive to tactile stimulation. Here, we report that these rays have corresponding morphological specializations in somatosensory cortex. Using a stain for the metabolic enzyme, cytochrome oxidase (CO), to reveal subdivisions of cortex, we disclosed a complex pattern of CO-dense stripes or bands separated by sharp lines or septa of low CO staining. Multiunit microelectrode recordings of neural activity evoked by light tactile stimuli in somatosensory cortex of anesthetized moles allowed us to mark some of the bands and other CO-dark regions with small electrolytic lesions and later relate recording results to the CO pattern. The results suggest that the primary somatosensory cortex, S1, has an unusual ventrolateral location and orientation with representations of mouth, nose rays, facial vibrissae, forepaw, and trunk in a rostrocaudal sequence. Within this presumptive S1, the 11 rays of the contralateral nose are represented as a rostral-to-caudal cortical pinwheel of 11 stripes. Cortex ventral to the primary set of stripes contains a second rostrocaudal representation of the rays as a mirror image of the first. This second set of stripes may be part of the second somatosensory area, S2. A third pattern of CO stripes appears to merge partially with caudal stripes of the first two patterns, so that a full pattern of 11 stripes is not obvious. This representation may correspond to the ventral somatosensory area, VS, of other mammals. An extensive area of cortex separated from the nose by a large septum was responsive to stimulation of the forelimb. Auditory cortex is unusually caudal in this mole, and the presumptive primary visual area is relatively small. These specializations of somatosensory cortex in star-nosed moles may be more patent examples of the consequences of more general factors in brain development. The observations are consistent with the general rule that the terminations of sensory projections with discorrelated activity segregate. © 1995 Willy-Liss, Inc.     

Five topographically organized fields in the somatosensory cortex of the flying fox: microelectrode maps, myeloarchitecture, and cortical modules.

Five somatosensory fields were defined in the grey-headed flying fox by using microelectrode mapping procedures. These fields are: the primary somatosensory area, SI or area 3b; a field caudal to area 3b, area 1/2; the second somatosensory area, SII; the parietal ventral area, PV; and the ventral somatosensory area, VS. A large number of closely spaced electrode penetrations recording multiunit activity revealed that each of these fields had a complete somatotopic representation. Microelectrode maps of somatosensory fields were related to architecture in cortex that had been flattened, cut parallel to the cortical surface, and stained for myelin. Receptive field size and some neural properties of individual fields were directly compared. Area 3b was the largest field identified and its topography was similar to that described in many other mammals. Neurons in 3b were highly responsive to cutaneous stimulation of peripheral body parts and had relatively small receptive fields. The myeloarchitecture revealed patches of dense myelination surrounded by thin zones of lightly myelinated cortex. Microelectrode recordings showed that myelin-dense and sparse zones in 3b were related to neurons that responded consistently or habituated to repetitive stimulation respectively. In cortex caudal to 3b, and protruding into 3b, a complete representation of the body surface adjacent to much of the caudal boundary of 3b was defined. Neurons in this area habituated rapidly to repetitive stimulation. We termed this caudal field area 1/2 because it had properties of both area 1 and area 2 of primates. In cortex caudolateral to 3b and lateral to area 1/2 (cortex traditionally defined as SII) we describe three separate representations of the body surface coextensive with distinct myeloarchitectonic appearances. The second somatosensory area, SII, shared a congruent border with 3b at the representation of the nose. In SII, the overall orientation of the body representation was erect. The lips were represented rostrolaterally, the digits were represented laterally, and the toes were caudolateral to the digits. The trunk was represented caudally and the head was represented medially. A second complete representation, PV, had an inverted body representation with respect to SII and bordered SII at the representation of the distal limbs. The proximal body parts were represented rostrolaterally in PV. Finally, caudal to both SII and PV, an additional representation, VS, shared a congruent border with the distal hindlimb representation of both SII and PV. VS had a crude topography, and receptive fields of neurons in VS were relatively large. Many neurons in VS responded to both somatosensory and auditory stimulation.     

Organization of somatosensory cortex in monotremes: In search of the prototypical plan

The present investigation was designed to determine the number and internal organization of somatosensory fields in monotremes. Microelectrode mapping methods were used in conjunction with cytochrome oxidase and myelin staining to reveal subdivisions and topography of somatosensory cortex in the platypus and the short-billed echidna. The neocortices of both monotremes were found to contain four representations of the body surface. A large area that contained neurons predominantly responsive to cutaneous stimulation of the contralateral body surface was identified as the primary somatosensory area (SI). Although the overall organization of SI was similar in both mammals, the platypus had a relatively larger representation of the bill. Furthermore, some of the neurons in the bill representation of SI were also responsive to low amplitude electrical stimulation. These neurons were spatially segregated from neurons responsive to pure mechanosensory stimulation. Another somatosensory field (R) was identified immediately rostral to SI. The topographic organization of R was similar to that found in SI; however, neurons in R responded most often to light pressure and taps to peripheral body parts. Neurons in cortex rostral to R were responsive to manipulation of joints and hard taps to the body. We termed this field the manipulation field (M). The mediolateral sequence of representation in M was similar to that of both SI and R, but was topographically less precise. Another somatosensory field, caudal to SI, was adjacent to SI laterally at the representation of the face, but medially was separated from SI by auditory cortex. Its position relative to SI and auditory cortex, and its topographic organization led us to hypothesize that this caudal field may be homologous to the parietal ventral area (PV) as described in other mammals. The evidence for the existence of four separate representations in somatosensory cortex in the two species of monotremes indicates that cortical organization is more complex in these mammals than was previously thought. Because the two monotreme families have been separate for at least 55 million years (Richardson, B. J. [1987] Aust. Mammal. 11:71–73), the present results suggest either that the original differentiation of fields occurred very early in mammalian evolution or that the potential for differentiation of somatosensory cortex into multiple fields is highly constrained in evolution, so that both species arrived at the same solution independently. © 1995 Willy-Liss, Inc.     

Ipsilateral cortical inputs to the rostral and caudal motor areas in rats

Rats have a complete body representation in the primary motor cortex (M1). Rostrally there are additional representations of the forelimb and whiskers, called the rostral forelimb area (RFA) and the rostral whisker area (RWA). Recently we showed that sources of thalamic inputs to RFA and RWA are similar, but they are different from those for the caudal forelimb area (CFA) and the caudal whisker area (CWA) of M1 (Mohammed and Jain [2014] J Comp Neurol 522:528–545). We proposed that RWA and RFA are part of a second motor area, the rostral motor area (RMA). Here we report ipsilateral cortical connections of whisker representation in RMA, and compare them with connections of CWA. Connections of RFA, CFA, and the caudally located hindlimb area (CHA), which is a part of M1, were determined for comparison. The most distinctive features of cortical inputs to RWA compared with CWA include lack of inputs from the face region of the primary somatosensory cortex (S1), and only about half as much inputs from S1 compared with the lateral somatosensory areas S2 (second somatosensory area) and the parietal ventral area (PV). A similar pattern of inputs is seen for CFA and RFA, with RFA receiving smaller proportion of inputs from the forepaw region of S1 compared with CFA, and receiving fewer inputs from S1 compared with those from S2. These and other features of the cortical input pattern suggest that RMA has a distinct, and more of integrative functional role compared with M1. J. Comp. Neurol. 524:3104–3123, 2016. © 2016 Wiley Periodicals, Inc.     

Impaired learning-dependent cortical plasticity in Huntington's disease transgenic mice

Huntington's disease (HD) is a genetically transmitted neurodegenerative disorder. The neuropathology in HD is a selective neuronal cell death in several brain regions including cortex. Although changes in synaptic plasticity were shown within the hippocampus and striatum of HD transgenic mice, there are no studies considering neocortical synaptic plasticity abnormalities in HD. We examined the impact of the HD transgene upon learning-dependent plasticity of cortical representational maps. The effect of associative learning, in which stimulation of a row of vibrissae was paired with appetitive stimulus, upon functional representations of vibrissae in the barrel cortex, was investigated with 2-deoxyglucose brain mapping in presymptomatic R6/1 HD mice. In wild-type mice, cortical representation of the row of vibrissae involved in the training was expanded, while in HD mice the representation of this row was not expanded. The results suggest that presymptomatic R6/1 HD transgenic mice show deficits in plasticity of primary somatosensory cortex.     

The development of vibrissae representation in subcortical trigeminal centers of the neonatal rat.

In every station of the trigeminal system of the young rat, the segmented activity of the mitochondrial enzyme succinic dehydrogenase (SDH) clearly delineates the representation of the mystacial vibrissae. In the trigeminal complex of the medulla, three parallel representation can be seen, two in the spinal trigeminal nucleus and one in the principal trigeminal nucleus. In the next station, the ventrobasal complex of the thalamus, a single representation occurs. Likewise, layer IV of somatosensory cortex contains one representation of the vibrissae. Further, neonatal damage to the mystacial vibrissae results in anomalies within each representation. The present study delineates both the normal development of subcortical trigeminal stations and the aberrant organization seen after vibrisse removal. The results of a similar study on somatosensory cortex (Killackey and Belford, '79) and the present data allow the comparison of the development of each of the five vibrissae representations in the trigeminal system. In the brainstem, each of the three trigeminal complex representations are present at birth, although the pattern becomes more distinct over the first several days of life. Interestingly, vibrissae removal at birth induces an aberrant pattern that is distinct by postnatal Day 3. Although details are not equally discernible in each representation, the abnormalities appear to be similar. The SDH segmentation in the ventrobasal complex develops during postnatal Days 1 through 4. At Day 1, portions of the matrix of high density SDH activity break up into bands. Clusters can be discerned within these bands on Day 2. By Day 4 the pattern is sharply delineated. Vibrissae removal at birth results in anomalies that are a part of the initial development of segmentation, not a later reorganization. Comparison of the present data with that of our previous studies indicates that there is a sequential development of the central somatosensory structures related to the vibrissae, beginning with the most peripheral station. Further, there are many similarities in the development of each station. There are also differences which are particularly important in comparing the trigeminal nuclei with the later stations. The unique features in the abnormal development of the trigeminal nuclei are likely due to their direct connections with the periphery.