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The purpose of this study was to examine the pattern of postnatal brain growth in two wild canid species: the coyote (Canis latrans) and gray wolf (Canis lupus). Adult regional and total brain volume differences were also compared between the two species as well as within each species by sex. Three-dimensional virtual endocasts of endocranial airspace were created from computed tomography scans of 52 coyote skulls (28 female, 24 male; 1 day to 13.4 years) and 46 gray wolf skulls (25 female, 21 male; 1 day to 7.9 years). Age was known in coyotes or estimated from dentition patterns in wolves. The 95% asymptotic growth of the endocranium is completed by 21 weeks in male and 17.5 weeks in female coyotes and by 27 weeks in male and 18.5 weeks in female wolves. These ages are well before age at first reproduction (coyote – 40.4 weeks; wolf – 91.25 weeks). Skull growth as measured by centroid size lags behind endocranial growth but is also completed before sexual maturity. Intra- and interspecific comparisons of brain volumes in the adult wolves and coyotes revealed that relative anterior cerebrum (AC) volume was greater in males than females in both species. Relative brain size was greater in the coyote than in the wolf as was relative cerebrum volume. However, relative AC volume and relative cerebellum and brainstem volume was greater in the wolf than coyote. One explanation for the increased AC volume in males compared to females may be related to the role of social information processing. However, additional data are needed to determine the correspondence between regional volumes and functional differences either between or within these species. Nonetheless, these findings provide important baseline data for further studies on wild canid brain variations and development.

The purpose of this study was to examine the pattern of postnatal brain growth in two wild canid species: the coyote (Canis latrans) and gray wolf (Canis lupus). Adult regional and total brain volume differences were also compared between the two species as well as within each species by sex. Three-dimensional virtual endocasts of endocranial airspace were created from computed tomography scans of 52 coyote skulls (28 female, 24 male; 1 day to 13.4 years) and 46 gray wolf skulls (25 female, 21 male; 1 day to 7.9 years). Age was known in coyotes or estimated from dentition patterns in wolves. The 95% asymptotic growth of the endocranium is completed by 21 weeks in male and 17.5 weeks in female coyotes and by 27 weeks in male and 18.5 weeks in female wolves. These ages are well before age at first reproduction (coyote - 40.4 weeks; wolf - 91.25 weeks). Skull growth as measured by centroid size lags behind endocranial growth but is also completed before sexual maturity. Intra- and interspecific comparisons of brain volumes in the adult wolves and coyotes revealed that relative anterior cerebrum (AC) volume was greater in males than females in both species. Relative brain size was greater in the coyote than in the wolf as was relative cerebrum volume. However, relative AC volume and relative cerebellum and brainstem volume was greater in the wolf than coyote. One explanation for the increased AC volume in males compared to females may be related to the role of social information processing. However, additional data are needed to determine the correspondence between regional volumes and functional differences either between or within these species. Nonetheless, these findings provide important baseline data for further studies on wild canid brain variations and development.

The purpose of this study was to examine if differences in social life histories correspond to intraspecific variation in total or regional brain volumes in the African lion (Panthera leo) and cougar (Puma concolor). African lions live in gregarious prides usually consisting of related adult females, their dependent offspring, and a coalition of immigrant males. Upon reaching maturity, male lions enter a nomadic and often, solitary phase in their lives, whereas females are mainly philopatric and highly social throughout their lives. In contrast, the social life history does not differ between male and female cougars; both are solitary. Three-dimensional virtual endocasts were created using computed tomography from the skulls of 14 adult African lions (8 male, 6 female) and 14 cougars (7 male, 7 female). Endocranial volume and basal skull length were highly correlated in African lions (r = 0.59, p < 0.05) and in cougars (r = 0.67, p < 0.01). Analyses of total endocranial volume relative to skull length revealed no sex differences in either African lions or cougars. However, relative anterior cerebrum volume comprised primarily of frontal cortex and surface area was significantly greater in female African lions than males, while relative posterior cerebrum volume and surface area was greater in males than females. These differences were specific to the neocortex and were not found in the solitary cougar, suggesting that social life history is linked to sex-specific neocortical patterns in these species. We further hypothesize that increased frontal cortical volume in female lions is related to the need for greater inhibitory control in the presence of a dominant male aggressor.

Reliable brain volume measurements are crucial in identifying factors that influence the course of brain evolution. Here, we demonstrate the potential for using virtual endocasts (VEs) to examine inter- and intraspecific variation in brain volume in members of the family Hyaenidae. Total endocranial volume (adjusted for body size) and anterior cerebrum volume (adjusted for endocranial volume) were greater in the spotted hyena, the most gregarious of the species, than in the other hyaenids, all of which are less gregarious. An intraspecific analysis of spotted hyenas revealed that anterior cerebrum volume is significantly larger in males than females, although total endocranial volume does not differ between the sexes. Greater total endocranial and anterior cerebrum volume of spotted hyenas, relative to those of other hyena species, may be related to increased neural processing mediating cognitive demands associated with a complex social life. These data demonstrate that computed tomographic (CT) technology can be used to create VEs in species for which actual brains are rare or unavailable, and suggest that this approach can be applied systematically to explore intra- and interspecies brain variations in studies of brain evolution.

The social brain hypothesis posits that the demands of living in complex social groups require increased neural processing, and that this underlies the expansion of brain areas involved in mediation of complex social behavior. However, much of the support for the social brain hypothesis is derived from comparative studies in primates. If large brains evolved as a result of selection pressures imposed by life within complex societies, as the social brain hypothesis predicts, then gregarious nonprimate species should possess large brains and exhibit comparable expansion of brain areas mediating social behavior. Our purpose here was to test a prediction of the social brain hypothesis – that increased brain size is related to social complexity – by examining species in the carnivore family Hyaenidae. Hyaenidae contains 4 extant species that span a spectrum of social complexity: the aardwolf (Proteles cristata) is solitary during the nonbreeding season, and forms monogamous pairs during the breeding season; the striped hyena (Hyaena hyaena) lives solitarily or in small groups; the brown hyena (Parahyaena brunnea) lives in groups of up to 14 individuals; and the spotted hyena (Crocuta crocuta) lives in complex hierarchically organized groups containing up to 90 animals. Computed tomography was used to create three-dimensional endocasts based on serial analysis of coronal sections of the adult endocranium. The largest brain volume, relative to body size, is found in the spotted hyena. We found no significant variation in relative brain volume among striped hyenas, brown hyenas, and aardwolves. The spotted hyena also possesses a larger anterior cerebrum volume relative to total brain volume than is found in the other hyena species; this region is composed primarily of frontal cortex. These data are consistent with the idea that expansion of the frontal cortex is driven by the demands of processing cognitive information associated with complex social lives, but other factors may drive the evolution of large brains in hyaenids.

The purpose of this study was to examine developmental and individual variation in total endocranial volume and regional brain volumes, including the anterior cerebrum, posterior cerebrum and cerebellum/brain stem, in the spotted hyena (Crocuta crocuta). The spotted hyena is a highly gregarious animal noted for living in large, hierarchically organized groups. The social lives of male and female spotted hyenas do not differ until after puberty, when males disperse from the natal group, while females remain philopatric. Here we sought to determine whether the divergent life histories of male and female spotted hyenas are linked to differences in brain size or organization. Three-dimensional virtual endocasts were created using computed tomography from 46 spotted hyenas skulls (23 females, 22 males, 1 unknown sex) ranging in age from 1 day to 18 years. Brain volume and skull length were highly correlated (r = 0.91), and both reached asymptotic values by 34 months of age. Analyses of total endocranial volume (relative to skull length) and cerebellum/brain stem volume (relative to total endocranial volume) revealed no sex differences. However, relative anterior cerebrum volume, comprised mainly of frontal cortex, was significantly greater in adult males than adult females, and relative posterior cerebrum volume was greater in adult females than adult males. We hypothesize that the demands of neural processing underlying enhanced social cognition required for successful male transfer between matriarchical social groups at dispersal may be greater than cognitive demands on philopatric females.

Recent neurophysiological studies indicate a role for reticulospinal neurons of the pontomedullary reticular formation (PMRF) in motor preparation and goal-directed reaching in the monkey. Although the macaque monkey is an important model for such investigations, little is known regarding the organization of the PMRF in the monkey. In the present study, we investigated the distribution of reticulospinal neurons in the macaque. Bilateral injections of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) were made into the cervical spinal cord. A wide band of retrogradely labeled cells was found in the gigantocellular reticular nucleus (Gi) and labeled cells continued rostrally into the caudal pontine reticular nucleus (PnC) and into the oral pontine reticular nucleus (PnO). Additional retrograde tracing studies following unilateral cervical spinal cord injections of cholera toxin subunit B revealed that there were more ipsilateral (60%) than contralateral (40%) projecting cells in Gi, while an approximately 50:50 ratio contralateral to ipsilateral split was found in PnC and more contralateral projections arose from PnO. Reticulospinal neurons in PMRF ranged widely in size from over 50 microm to under 25 microm across the major somatic axis. Labeled giant cells (soma diameters greater than 50 microm) comprised a small percentage of the neurons and were found in Gi, PnC and PnO. The present results define the origins of the reticulospinal system in the monkey and provide an important foundation for future investigations of the anatomy and physiology of this system in primates.

The ability to break open large bones has evolved independently in only three groups of carnivorous mammals, all of which have robust teeth, vaulted foreheads, and pronounced sagittal crests. One unusual skull feature, present in bone-cracking members of the family Hyaenidae, is a caudally elongated frontal sinus, hypothesized to function in resistance to bending and stress dissipation during bone-cracking. In the present study, we used finite element (FE) analysis to examine patterns of stress distribution in the spotted hyena (Crocuta crocuta) skull during unilateral biting, and inquire about the functional role of the fronto-parietal sinus in stress dissipation. We constructed and compared three FE models: (1) a ‘normal’ model of an adult Crocuta skull; (2) a model in which the caudal portion of the fronto-parietal sinus was filled with bone; and (3) a model in which we flattened the sagittal crest to resemble the plate-like crests of other mammals. During biting, an arc of stress extends from the bite point up through the vaulted forehead and along the sagittal crest. Our results suggest that pneumatization of the hyena's skull both enhances its ability to resist bending and, together with the vaulted forehead, plays a critical role in evenly dissipating stress away from the facial region. © 2008 The Linnean Society of London, Biological Journal of the Linnean Society, 2008, 95, 246–255.

Large brains and great intelligence are metabolically costly, but the social complexity hypothesis suggests that these traits were favored nonetheless in primates by selection pressures associated with life in complex societies. If so, then cognitive abilities and nervous systems with primatelike attributes should have evolved convergently in nonprimate mammals living in large, elaborate societies in which social dexterity enhances individual fitness. Societies of spotted hyenas (Crocuta crocuta) are remarkably similar to those of cercopithecine primates with respect to size, structure, and patterns of competition and cooperation. These similarities set an ideal stage for comparative analysis of social intelligence and nervous system organization. As in cercopithecine primates, spotted hyenas use multiple sensory modalities to recognize their kin and other conspecifics as individuals, they know that some group-mates are more valuable social partners than others, they recognize 3rd-party kin and rank relationships among their clan-mates, and they use this knowledge adaptively during social decision-making. Examination of the available data strongly suggests convergent evolution of intelligence between spotted hyenas and primates. Evidence that less gregarious members of the family Hyaenidae lack some of the cognitive abilities found in spotted hyenas would provide further support for the idea that social complexity favors enhancement of intelligence in mammals.

The goal of the present study was to determine if the medial agranular cortex (AGm) that is thought to contain the supplementary motor area in the rat receives the pallidothalamocortical pathway. A double labeling paradigm was employed whereby injections of the anterograde tracer Phaseolus vulgaris-leucoagglutinin (PHA-L) into the entopeduncular nucleus (EN) were combined with injections of the retrograde tracer cholera toxin subunit B into either the rostral or caudal part AGm in the rat. We found that the pallidothalamic projections occupied a crescent shaped arc within the thalamic ventral anterior-ventral lateral nuclear complex (VAL) and ventral medial nucleus (VM). Additional efferents projected to LD/LP, MD, Pf and CL. Dense projections were also found to LHb. In addition, we found that a wide band of cells extending from rostral thalamus including VAL and VM through caudal thalamus including MD, Po, LD/LP, Pf and CL projected to either the rostral or caudal parts of the AGm. The greatest coincidence of the pallidothalamic efferents and AGm thalamocortical cells was found in VAL and VM where numerous PHA-L varicosities were found in close apposition to CTB labeled cells. Synaptic contacts between VAL thalamocortical neurons and pallidal terminals were verified in the electron microscope. These results demonstrate for the first time the pallidothalamocortical pathway by way of VAL to the AGm in the rat.

In the present study, we determined where thalamic neurons projecting to the pre-supplementary motor area (pre-SMA) are located relative to pallidothalamic and cerebellothalamic inputs and nuclear boundaries. We employed a triple-labeling technique in the same owl monkey (Aotus trivirgatus). The cerebellothalamic projections were labeled with injections of wheat germ agglutinin conjugated to horseradish peroxidase, and the pallidothalamic projections were labeled with biotinylated dextran amine. The pre-SMA was identified by location and movement patterns evoked by intracortical microstimulation and injected with the retrograde tracer cholera toxin subunit B. Brain sections were processed sequentially using different chromogens to visualize all three tracers in the same section. Alternate sections were processed for Nissl cytoarchitecture or acetylcholinesterase chemoarchitecture for nuclear boundaries. The cerebellar nuclei primarily projected to posterior (VLp), medial (VLx), and dorsal (VLd) divisions of the ventral lateral nucleus; the pallidum largely projected to the anterior division (VLa) of the ventral lateral nucleus and the parvocellular part of the ventral anterior nucleus (VApc). However, we also found zones of overlapping projections, as well as interdigitating foci of pallidal and cerebellar label, particularly in border regions of the VLa and VApc. Thalamic neurons labeled by pre-SMA injections occupied a wide band and were especially concentrated in the VLx and VApc, cerebellar and pallidal territories, respectively. Labeled thalamocortical neurons overlapped cerebellar inputs in the VLd and VApc and overlapped pallidal inputs in the VLa and the ventral medial nucleus. The results demonstrate that inputs from both the cerebellum and globus pallidus are relayed to the pre-SMA.

The purpose of the present study was to determine if the cortical efferents from homologous body regions of the supplementary motor area (SMA) and the primary motor cortex (MI) project to separate or to overlapping regions in the striatum. In order to investigate the dual corticostriatal projections, we employed an anterograde double labeling paradigm in which two tracers could be simultaneously detected in the same histological section. Prior to the injections, the forelimb representation in the two cortical motor areas was identified by using intracortical microstimulation in four Japanese monkeys (Macaca fuscata). Multiple injections of biotinylated dextran amine (BDA) were made into the forelimb regions of MI and wheat germ agglutinin conjugated horseradish peroxidase (WGA-HRP) was injected into the arm region of the SMA. In additional animals, the tracers were reversed such that BDA was injected into the SMA and WGA-HRP was injected into the MI. The tissue was processed sequentially using different chromogens in order to visualize both tracers in a single section. We analyzed the distribution of the ipsilateral anterograde label. The striatal labeling from each cortical area basically consisted of a wide band of patchy dense labeling interrupted by lighter labeling. The SMA striatal projections were located mainly within the putamen, distributing from the level of the anterior commissure to the most posterior extent of the putamen. At an intermediate level, the label spread obliquely from the ventrolateral edge of the putamen dorsomedially as far as the lateral edge of the caudate nucleus. The label from the MI was observed in comparable portions of the putamen, although the SMA projections were shifted more anterior and dorsomedial to the MI projections and the heaviest projections from the SMA and the MI were separately located. On the basis of the double anterograde labeling technique, we found considerable overlap mainly in the central portion of the putamen from the SMA and MI forelimb representation. These results suggest that the homologous body regions of the SMA and MI send widespread, and substantially overlapping projections, to portions of the striatum. © 1996 Wiley-Liss, Inc.

The present study investigated whether increased relative brain size, including regional brain volumes, is related to differing behavioral specializations exhibited by three member species of the family Procyonidae. Procyonid species exhibit continuums of behaviors related to social and physical environmental complexities: the mostly solitary, semiarboreal and highly dexterous raccoons (Procyon lotor); the exclusively arboreal kinkajous (Potos flavus), which live either alone or in small polyandrous family groups, and the social, terrestrial coatimundi (Nasua nasua, N. narica). Computed tomographic (CT) scans of 45 adult skulls including 17 coatimundis (9 male, 8 female), 14 raccoons (7 male, 7 female), and 14 kinkajous (7 male, 7 female) were used to create three-dimensional virtual endocasts. Endocranial volume was positively correlated with two separate measures of body size: skull basal length (r = 0.78, p0.01) and basicranial axis length (r = 0.45, p = 0.002). However, relative brain size (total endocranial volume as a function of body size) varied by species depending on which body size measurement (skull basal length or basicranial axis length) was used. Comparisons of relative regional brain volumes revealed that the anterior cerebrum volume consisting mainly of frontal cortex and surface area was significantly larger in the social coatimundi compared to kinkajous and raccoons. The dexterous raccoon had the largest relative posterior cerebrum volume, which includes the somatosensory cortex, in comparison to the other procyonid species studied. The exclusively arboreal kinkajou had the largest relative cerebellum and brain stem volume in comparison to the semi arboreal raccoon and the terrestrial coatimundi. Finally, intraspecific comparisons failed to reveal any sex differences, except in the social coatimundi. Female coatimundis possessed a larger relative frontal cortical volume than males. Social life histories differ in male and female coatimundis but not in either kinkajous or raccoons. This difference may reflect the differing social life histories experienced by females who reside in their natal bands, and forage and engage in antipredator behavior as a group, while males disperse upon reaching adulthood and are usually solitary thereafter. This analysis in the three procyonid species supports the comparative neurology principle that behavioral specializations correspond to an expansion of neural tissue involved in that function.

Mammalian brain volumes vary considerably, even after controlling for body size. Although several hypotheses have been proposed to explain this variation, most research in mammals on the evolution of encephalization has focused on primates, leaving the generality of these explanations uncertain. Furthermore, much research still addresses only one hypothesis at a time, despite the demonstrated importance of considering multiple factors simultaneously. We used phylogenetic comparative methods to investigate simultaneously the importance of several factors previously hypothesized to be important in neural evolution among mammalian carnivores, including social complexity, forelimb use, home range size, diet, life history, phylogeny, and recent evolutionary changes in body size. We also tested hypotheses suggesting roles for these variables in determining the relative volume of four brain regions measured using computed tomography. Our data suggest that, in contrast to brain size in primates, carnivoran brain size may lag behind body size over evolutionary time. Moreover, carnivore species that primarily consume vertebrates have the largest brains. Although we found no support for a role of social complexity in overall encephalization, relative cerebrum volume correlated positively with sociality. Finally, our results support negative relationships among different brain regions after accounting for overall endocranial volume, suggesting that increased size of one brain regions is often accompanied by reduced size in other regions rather than overall brain expansion.

The purpose of this study was to examine if differences in social life histories correspond to intraspecific variation in total or regional brain volumes in the African lion (Panthera leo) and cougar (Puma concolor). African lions live in gregarious prides usually consisting of related adult females, their dependent offspring, and a coalition of immigrant males. Upon reaching maturity, male lions enter a nomadic and often, solitary phase in their lives, whereas females are mainly philopatric and highly social throughout their lives. In contrast, the social life history does not differ between male and female cougars; both are solitary. Three-dimensional virtual endocasts were created using computed tomography from the skulls of 14 adult African lions (8 male, 6 female) and 14 cougars (7 male, 7 female). Endocranial volume and basal skull length were highly correlated in African lions (r = 0.59, p0.05) and in cougars (r = 0.67, p0.01). Analyses of total endocranial volume relative to skull length revealed no sex differences in either African lions or cougars. However, relative anterior cerebrum volume comprised primarily of frontal cortex and surface area was significantly greater in female African lions than males, while relative posterior cerebrum volume and surface area was greater in males than females. These differences were specific to the neocortex and were not found in the solitary cougar, suggesting that social life history is linked to sex-specific neocortical patterns in these species. We further hypothesize that increased frontal cortical volume in female lions is related to the need for greater inhibitory control in the presence of a dominant male aggressor.

Reliable brain volume measurements are crucial in identifying factors that influence the course of brain evolution. Here, we demonstrate the potential for using virtual endocasts (VEs) to examine inter- and intraspecific variation in brain volume in members of the family Hyaenidae. Total endocranial volume (adjusted for body size) and anterior cerebrum volume (adjusted for endocranial volume) were greater in the spotted hyena, the most gregarious of the species, than in the other hyaenids, all of which are less gregarious. An intraspecific analysis of spotted hyenas revealed that anterior cerebrum volume is significantly larger in males than females, although total endocranial volume does not differ between the sexes. Greater total endocranial and anterior cerebrum volume of spotted hyenas, relative to those of other hyena species, may be related to increased neural processing mediating cognitive demands associated with a complex social life. These data demonstrate that computed tomographic (CT) technology can be used to create VEs in species for which actual brains are rare or unavailable, and suggest that this approach can be applied systematically to explore intra- and interspecies brain variations in studies of brain evolution.

If the large brains and great intelligence characteristic of primates were favoured by selection pressures associated with life in complex societies, then cognitive abilities and nervous systems with primate-like attributes should have evolved convergently in non-primate mammals living in large, elaborate societies in which social dexterity enhances individual fitness. The societies of spotted hyenas are remarkably like those of cercopithecine primates with respect to size, structure and patterns of competition and cooperation. These similarities set an ideal stage for comparative analysis of social intelligence and nervous system organization. As in cercopithecine primates, spotted hyenas use multiple sensory modalities to recognize their kin and other conspecifics as individuals, they recognize third-party kin and rank relationships among their clan mates, and they use this knowledge adaptively during social decision making. However, hyenas appear to rely more intensively than primates on social facilitation and simple rules of thumb in social decision making. No evidence to date suggests that hyenas are capable of true imitation. Finally, it appears that the gross anatomy of the brain in spotted hyenas might resemble that in primates with respect to expansion of frontal cortex, presumed to be involved in the mediation of social behaviour.

In the present study, we determined the anatomic relationships between somatosensory and motor pathways within ventrolateral (VL) thalamic nuclei of the motor thalamus of macaque monkeys. In labeling experiments, four macaque monkeys (Macaca mulatta) received injections of biotinylated dextran amine and wheat germ agglutinin conjugated to horseradish peroxidase into the cerebellar nuclei or internal segment of the globus pallidus and cervical segments of the spinal cord, respectively. Each tracer was visualized in brain sections by sequentially using a different chromogen. Labeled terminals were plotted and superimposed on adjacent brain sections processed for Nissl substance, acetylcholinesterase, and the antigens for calbindin and Cat-301 to reveal thalamic nuclei. The labeled cerebellar terminals were distributed throughout the posterior VL (VLp), whereas the labeled pallidothalamic terminals were concentrated in the anterior VL and the ventral anterior nucleus. The spinothalamic input was directed mostly to the ventral posterior complex and cells just caudal to it. In addition, the patches of spinothalamic terminations intermingled and partly overlapped with the cerebellothalamic, but not with the pallidothalamic terminations within VLp. The regions of overlap of somatosensory and cerebellar inputs within the VLp of the present study appear to correspond to the reported locations of the tremor-related cells in parkinsonian patients. Thus, the overlapping spinothalamic and cerebellar inputs may provide a substrate for the altered activity of motor thalamic neurons in such patients.

The purpose of the present study was to determine the interrelationship between the thalamic afferents arising from the cerebellum (Cb) and the internal segment of the globus pallidus (GPi) with the neurons projecting to the primary motor cortex (MI) and to the supplementary motor area (SMA). We combined fluorescent retrograde tracers with a double anterograde labeling technique. Multiple injections of a combination of Diamidino Yellow and Fast Blue were made into either the MI or SMA hand/arm representation as determined by intracortical microstimulation. In the same animal, biotinylated dextran amine was injected into the GPi and horseradish peroxidase conjugated to wheat germ agglutinin was injected into the contralateral cerebellar nuclei. The results revealed that the cerebellar and pallidal thalamic territories are largely separate. The ventral anterior nucleus (VA) and the ventral lateral nucleus pars oralis (VLo) contained a greater density of pallidal labeling while a greater density of cerebellar label was observed more caudally in the ventral posterior lateral nucleus pars oralis (VPLo) as well as in nucleus X (X). Moreover, we observed that the greatest coincidence of retrograde cell labeling was within the pallidal thalamic territory following the SMA injections and within the cerebellar thalamic territory following the MI injections. However, interdigitating foci of pallidal and cerebellar label were also observed particularly in the ventral lateral nucleus pars oralis (VLo) and the ventral lateral nucleus pars caudalis (VLc). In both VLo and VLc, we additionally observed coincidence between the cerebellar labeling and SMA projection neurons as well as between pallidal labeling and MI projection neurons. These data suggest that while MI primarily receives inputs originating from Cb and SMA primarily receives inputs originating from GPi, it also appears that MI and SMA receive secondary afferents arising from GPi and Cb, respectively.

We investigated the interrelationship between the supplementary motor area (SMA) thalamocortical projection neurons and the pallidothalamic and cerebellothalamic territories in the monkey (Macaca fuscata) using a combination of three tracers in a triple labeling paradigm. Thalamic labeling was analyzed following injections of the anterograde tracers, biotinylated dextran amine (BDA) into the internal segment of the globus pallidus (GPi) and wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) into the contralateral cerebellar interpositus and dentate nuclei. In addition, the retrograde tracer cholera toxin subunit B (CTB) was injected into the physiologically identified hand/arm representation of SMA. The tissue was processed sequentially using different chromogens in order to visualize all three tracers in a single section. We found that the SMA thalamocortical neurons occupied a wide band extending from the ventral anterior nucleus pars principalis (VApc) through the ventral lateral nucleus pars oralis (VLo) and the ventral lateral nucleus pars medialis (VLm) and into to the ventral lateral nucleus pars caudalis (VLc) including a portion of ventral posterior lateral nucleus pars oralis (VPLo) and nucleus X. The heaviest CTB labeling was found in VLo with dense plexuses of BDA labeled pallidothalamic fibers and swellings often observed superimposed upon retrogradely labeled CTB cells. In addition, dense foci of cerebellothalamic WGA-HRP anterograde label were observed coinciding with the occasional retrogradely CTB labeled neurons in VLc and transitional zones between VApc, VLo and VPLo. Our light microscopic results suggest that the SMA receives thalamic inputs with afferents largely derived from GPi and minor inputs originating from the cerebellum.

Although the rat medial agranular cortex (AGm) has been implicated in a variety of motor functions, the source of the afferents impinging upon thalamic neurons projecting to the AGm is not directly known. The main purpose of this study was to determine whether the AGm is a major recipient of the nigrothalamocortical pathway. This issue was addressed by two sets of experiments. First, the organization of the nigrothalamic projections was studied by light and electron microscopy following injections of the anterograde tracer Phaseolus vulgaris-leucoagglutinin (PHA-L) into the pars reticulata of the substantia nigra (SNR). The major finding of this study was the disclosure of a heretofore unknown projection to the rostromedial part of the ventral anterior-ventral lateral complex (VAL). This projection originates exclusively from the ventral portion of the SNR and is comparable in strength to the well-known nigrothalamic projection to the ventromedial nucleus (VM). Electron microscopic examination revealed differences in the synaptic organization of nigral terminals in the VAL and the VM. A large proportion of the labeled terminals in the VAL was involved in axosomatic synapses, whereas, in the VM, the axosomatic synapses were rare, and 67% of nigral terminals were found in contact with thin dendrites. To assess a possible disynaptic nigrothalamocortical pathway to the AGm, a double-labeling strategy combining PHA-L injections in the SNR and pressure injections of the retrograde tracer, cholera toxin subunit B (CTB) in the AGm was employed. The greatest density of CTB-labeled neurons was found in the rostral and central portion of the VAL, coincident with the nigrothalamic labeling originating from the ventral SNR. Electron microscopic analysis confirmed that some of the PHA-L-labeled terminals established synaptic contacts with the CTB-labeled cell bodies and large dendrites. In conclusion, our findings indicate that there exist two different nigrothalamocortical pathways through the motor thalamus in the rat. The SNR-VAL-AGm cortical projection may play a role in oculomotor functions, whereas the SNR-VM-cortical pathway has been implicated in arousal mechanisms. J. Comp. Neurol. 391:506–525, 1998. © 1998 Wiley-Liss, Inc.

To address the question of segregated projections from the internal segment of the globus pallidus (GPi) and the cerebellar nuclei (Cb) to the thalamus in the monkey, we employed a double anterograde labeling strategy combining the anterograde transport of horseradish peroxidase conjugated to wheat germ agglutinin (WGA-HRP) with biotinylated dextran amine (BDA) transport. The tissue was processed sequentially for WGA-HRP, and then BDA immunohistochemistry using two different chromogens. Since the two labels were easily distinguishable on the same histological section, the interrelationship between the cerebellar and pallidal projection systems could be directly evaluated. We found that both the cerebellothalamic and pallidothalamic label consisted of dense plexuses of labeled fibers and swellings in a patch-like configuration. The patches or foci of labeling were distributed either as dense single label or as interdigitating patches of double label. We found dense single label in the central portion of the ventral anterior nucleus pars principalis (VApc) and the ventral lateral nucleus pars oralis (VLo) following the GPi injections or in the central portion of the ventral posterior lateral nucleus pars oralis (VPLo) and nucleus X (X) following the cerebellar nuclei injections. Complementary interdigitating patches of WGA-HRP and BDA labeling were found primarily in transitional border regions between thalamic nuclei. On occasion, we found overlap of both labels. We observed a gradient pattern in the density of the pallidothalamic and cerebellothalamic projections. The pallidothalamic territory included VApc, VLo, and the ventral lateral nucleus pars caudalis (VLc), with the density of these projections decreasing along an anterior to posterior gradient in the thalamus. Occasional patches of pallidal label were found in VPLo and nucleus X. Conversely, the density of cerebellothalamic projections increased along the same gradient, with the cerebellothalamic territory extending anteriorly beyond the cell-sparse zones of VPLo, X, and VLc to include VLo and VApc also. These data suggest that although the cerebellar and pallidal projections primarily occupy separate thalamic territories, individual thalamic nuclei receive differentially weighted inputs from these sources. © 1996 Wiley-Liss, Inc.

The question of segregated circuitry through the basal ganglia has been a key issue regarding the functional organization of this system. Accumulated evidence has suggested that sensorimotor, association, and limbic cortical areas project in a segregated manner onto three distinct striatal regions, and this segregation is maintained through the basal ganglia to the thalamus (Parent and Hazrati, 1995). Moreover, within the sensorimotor circuit, different motor cortical subdivisions have been proposed to establish the segregated cortico-basal ganglia-thalamo-cortical loops (Alexander and Crutcher, 1990; Strick et al., 1995). That is, individual motor cortical areas project to topographically distinct portions in the striatum, and this segregated arrangement is maintained throughout the loop back to the same cortical areas.

In the present study, we compared the distribution of thalamocortical afferents of cortical area 4 to that of cortical area 6 in the dog, using fluorescent tracers. Multiple injections of combinations of two dyes (diamidino yellow dihydrochloride, Evans blue, fast blue, granular blue) were made into either the anterior and posterior sigmoid gyri or into the medial and lateral regions of the anterior sigmoid gyrus in the anesthetized dog. We found that the thalamic afferents of areas 4 and 6 arise from topographically organized bands of cells that traverse several thalamic nuclei and extend throughout the rostrocaudal extent of the thalamus. The most medial band included area 6-projecting neurons in the anterior nuclei, the rhomboid nucleus, the ventral anterior nucleus (VA), ventromedial nucleus (VM) and mediodorsal nucleus (MD). Within this band, cells projecting to medial area 6 a alpha tended to be more numerous in the anterior nuclei, anterior parts of VA and VM and anterior and caudal parts of MD. Fewer cells in MD but more cells in caudal parts of VA and VM projected to lateral area 6 a beta. Lateral bands of cells in central through lateral parts of VA and VL projected topographically to lateral area 4 on the anterior sigmoid gyrus and lateral through medial parts of postcruciate area 4. The most lateral band of cells in VL continued ventrally into the zona incerta. Area 4 also received input from VM and the central lateral (CL) and centrum medianum (CM) nuclei.(ABSTRACT TRUNCATED AT 250 WORDS)

The distribution of nigrothalamic and cerebellothalamic projections was investigated in the dog by a double labeling strategy combining the anterograde transport of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) and tritiated amino acids. Following tritiated amino acid injections into the substantia nigra pars reticulata (SNr) and WGA-HRP injections into the contralateral cerebellar nuclei, we found that the nigrothalamic and cerebellothalamic afferents distribute to three main targets: the central portion of the ventral anterior nucleus (VA) and the ventral lateral nucleus (VL), the internal medullary lamina (IML) region, which includes the paralaminar VA, the mediodorsal nucleus (MD) and the central lateral nucleus (CL), and finally the ventromedial nucleus (VM). We observed three distribution patterns of labeled fibers: (a) Dense single label was observed in the central portion of VA following the SNr injections and in VL following the cerebellar nuclei injections. (b) A complementary pattern consisting of alternating foci of nigral and cerebellar label was found in the IML region. This pattern was also observed in the caudal intralaminar nuclei where cerebellar label predominated in the centrum medianum (CM), while the parafascicular nucleus (Pf) primarily contained nigral label. (c) An overlapping pattern of autoradiographic and WGA-HRP label was found in the lateral half of the VM. Overall, the distribution of nigrothalamic and cerebellothalamic projections was widespread throughout much of rostrocaudal thalamus. However, the pattern of projections varied along a continuum from lateral to medial thalamus. In lateral thalamus, nigral and cerebellar projections distributed to separate nuclei while in medial thalamus, the projection pattern changed to focal and complementary in the IML and overlapping in VM. Taken together, these thalamic projections may constitute crucial links in different functional channels involved in alerting and orienting mechanisms associated with motor behavior. Our findings also suggest that the organization of motor thalamic afferents in the dog shares similarities with the segregated and parallel circuitry characteristic of primates as well as with the overlapping and converging circuits of rodents and other carnivores.

The distribution of nigrothalamic projections was studied in the dog by using the autoradiographic tracing method. On the basis of a systematic series of tritiated amino acid injections into different portions of the substantia nigra, we found that the rostral three-fourths of the substantia nigra pars reticulata (SNr) gives rise to widespread thalamic projections. The nigrothalamic label, occupied a longitudinal band extending from rostral ventral anterior nucleus (VA) through to caudal mediodorsal nucleus (MD). In the dog, VA is histochemically identified as an acetylthiocholinesterase (AChE)-negative region and is distinct from the adjacent ventral lateral nuclei which stain positively for AChE. In rostral thalamus, the dense autoradiographic label was observed in rostral VA within the AchE-negative region lying alongside the mammillothalamic tract (MT). The adjacent AChE-positive ventral lateral nuclei did not contain autoradiographic label. In addition, homogeneously distributed silver grains were observed throughout the ventromedial nucleus (VM) bilaterally. More caudally, as the ventral lateral thalamic compartment emerges, label was also observed within the internal medullary lamina region, including the paralaminar portion of VA, the central lateral nucleus (CL), and the paralaminar portion of MD. Finally, in caudal thalamus, the central portion of MD, as well as the parafascicular nucleus (Pf), contained autoradiographic label. The overall nigrothalamic distribution observed in the dog was similar to the distribution of nigrothalamic projections in monkeys with one exception. Unlike that of primates, the distribution of nigral efferents is to the whole extent of VM in the dog as in other non-primate species. Overall, we found that nigral efferents primarily project to three main thalamic targets: the VA/MD region, VM, and the internal medullary lamina region, which includes dorsal CL and paralaminar VA and MD. We propose that these three nigrothalamic territories may constitute critical links subserving different functional channels.

The cytoarchitecture and thalamic afferents of cortical area 6 located on the anterior sigmoid gyrus were mapped and analyzed in the dog by means of cytoarchitectonic, horseradish peroxidase (HRP), and autoradiographic methods. Cytoarchitectonically, area 6 consists of medial and lateral subdivisions that correspond, respectively, to areas 6aα and 6aβ in the cat. In the dog, area 6aα is characterized by a wide layer III, the merging of borders between layers III and V, the presence of small-to-medium-size pyramidal cells in layer V, and a pallisade arrangement of cells in layer VI. Area 6aβ appears more stratified, with a relatively acellular layer present between layers V and VI and the presence of large pyramidal cells in layer V. Neither area 6aα nor 6aβ contains a layer IV. Data obtained from injections of HRP into areas 6aα or 6aβ revealed that labeled thalamic neurons were distributed in a longitudinal band extending from the rostral part of the ventral anterior nucelus (VA) through the caudal part of the mediodorsal nucleus (MD). Labeled cells were observed in the ventral and ventral medial thalamic nuclei as well as in several of the intralaminar nuclei including the central lateral, central medial, parafascicular, and centrum medianum nuclei. A few labeled cells were also located in the suprageniculate nucleus. The densest thalamic labeling was present in VA and MD following injections into area 6aα. Equivalent or even larger injections into area 6aβ resulted in much less thalamic labeling. The band of labeled cells also extended into the hypothalamus, zona incerta, amygdala, claustrum, periaqueductal gray of the midbrain, and the nucleus of Darkschewitsch. Results from autoradiographic experiments showed that area 6 subdivisions receive a loosely organized topographic input from VA. Injections of tritiated amino acids were made into selected regions of VA and into the caudal part of MD, areas in which the largest numbers of HRP-labeled cells were located. Area 6aα receives afferents primarily from the rostromedial part of VA and the caudal part of MD while area 6aβ receives its principal input from the caudal and lateral parts of VA with minimal input from MD. Axons originating from VA terminate in both layers I and III of area 6 while those originating from the caudal part of MD terminate only in layer III. The results of this study show that area 6 in the dog may be divided cytoarchitectonically into two distinct regions, each of which receives a characteristic pattern of thalamic inputs. The organization of these inputs, as well as those from other, nonthalamic subcortical areas, lends anatomical support for the functional division of area 6 into an electrophysiologically defined medial eye movement field and a lateral supplementary motor area.

Corticothalamic projections from postcruciate area 4, located on the rostral part of the posterior sigmoid gyms, were traced with the autora-diographic technique in the dog. Injections of tritiated amino acids were made into the lateral and medial parts of area 4 in regions corresponding to the forelimb and hindlimb areas of the primary motor cortex, respectively. In cases with injections placed in the lateral part of areas, dense accu-mulations of label were present in the lateral part of the ventral anterior nucleus (VA), the central part of the ventral lateral nucleus (VL), the ventral half of the ventral posterior inferior nucleus (VPI), the caudal part of the central lateral nucleus (CL), and the centrum medianum (CM). Lighter label was also present in the lateral part of the cytoarchitectonically distinct VL region bordering the ventrobasal complex (VB), as well as in the ventro-lateral part of the mediodorsal nucleus (MD), and in the lateral posterior nucleus (LP). In one case in which the injection site involved an adjacent part of area 3a, label was also seen ventrally in the medial division of the posterior nuclear group (POm). However, no detectable differences in VL, MD, or intralaminar labeling patterns were noted between this case and the four other cases with injections confined to the lateral part of area 4. In two cases with injections restricted to the medial part of area 4, dense label was present in the lateralmost part of VL, the ventral part of VPI, the caudal part of CL, and CM. Lighter label was also present in the VL region bordering the dorsolateral edge of VB and in LP. An additional case in which the injection also involved the rostral border of area 3a showed a similar pattern cf thalamic labeling. Projections from both the lateral and medial parts of area 4 were also noted in the subthalamic nucleus, zona incerta, and nucleus of Darkschewitsch. These results suggest that Corticothalamic projections from postcruciate area 4 to VL are organized topographically such that projections from the lateral part of area 4 project centrally within VL while those from the medial part of area 4 project more laterally. Both parts of area 4 also project top-ographically to a cytoarchitectonically distinct region of VL located im-mediately adjacent to VB, In contrast, the projections to the intralaminar nuclei do not appear to be topographically organized. The data from cases involving spread of the injection into area 3a suggest that projection pat-terns from area 3a to ventral, intralaminar, and medial thalamic nuclei are similar to those from area 4. However, it appears that at least the lateral part of area 3a also projects to POm.

Contralateral corticothalamic projections from cytoarchitectonic area 6 in the raccoon were studied using the autoradiographic tracing technique. Following injections of tritiated amino acids, accumulations of silver grains were present over both the ipsilateral and contralateral ventral medial, central lateral, paracentral, central medial, parafascicular and mediodorsal thalamic nuclei. These nuclei are also known to receive a number bilateral subcortical motor inputs. The additional presence of bilateral area 6 inputs suggests that these thalamic nuclei may be critically invovled in the bilateral control of movement.

The canine ventral lateral nucleus was defined on the basis of Nissl cytoarchitecture, acetylthiocholinesterase (AChE) chemoarchitecture, and the distribution of cerebellar afferents. It consists of at least two subdivisions: a dorsal and rostral division (VLd) that stains intensely for AChE and contains densely packed cells, and a larger principal division (VL), located ventrocaudally, that stains moderately for AChE and contains a mixture of cells with moderate packing density. Results from autoradiographic, silver degeneration, and AChE material indicate that both divisions receive input from the deep cerebellar nuclei.

The purpose of this study was to determine the pattern of thalamic projections of the primary motor cortex (MI) in the raccoon, a carnivore species noted for neural specialization of sensorimotor function. Following electrophysiological identification of circumscribed regions of MI, injections of horseradish peroxidase (HRP) or HRP combined with tritiated amino acids were made in 15 animals. Labeled thalamic cells were found predominantly in the ventral lateral nucleus (VL). For a given cortical injection site within MI, labeled neurons in VL formed a crescent-shaped band which extended in a dorsoventral direction when viewed in coronal sections. These bands were topographically organized. Following an injection of the MI hindlimb area in the medial part of the posterior cruciate gyrus, both retrogradely labeled neurons and anterograde label formed a thin band at the lateral edge of VL while an injection of the MI face area in the lateral part of the anterior cruciate gyrus resulted in both anterograde and retrograde label in medial VL and the principal division of the ventromedial nucleus (VMp). An injection of the MI forepaw area localized to the rostral and central part of the posterior cruciate gyrus resulted in a band of labeled neurons occupying the dorsal extent of VL and continuing into the ventrolateral aspect of the ventral anterior nucleus (VA). In contrast, an injection of the MI forepaw area which was localized to the caudal extent of the posterior cruciate gyrus resulted in a wide and diffuse band of labeled neurons and anterograde label in the ventral portion of VL. All injections of MI produced cell labeling in the paracentral nucleus (PC) and the central lateral nucleus (CL) of the intralaminar group. These results demonstrate that VL is the primary thalamic dependency of MI in the raccoon. Labeled cells were not observed in the ventrobasal complex. The MI pattern of thalamic connectivity observed in the present study suggests that while differences exist in the regional specialization of sensorimotor structures among species, there appears to be little variation in the overall organization of thalamocortical relations.