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Somatic sensory, auditory and visual areas of cerebral neocortex were mapped in anesthetized capybaras using surface macroelectrode-evoked potential recording methods. The cortical motor area was mapped using electrical stimulation methods. The results of these experiments in the largest living rodent were similar to those found for the cortical sensory and motor areas of guinea pigs, a small rodent in a closely related family. The representation of the perioral skin in SI cortex was relatively large in capybaras and guinea pigs. In capybara, several cortical sulci reliably demarcate different cortical projection areas from one another. Quantitive neuroanatomical comparisons of volumes and neuron numbers in several major prosencephalic nuclei revealed that all nuclear masses are larger in capybara than in guinea pig, but that different nuclei are enlarged to different degrees. Possible causes of larger brains in larger animals are discussed.

Somatic sensory (SI) cortex was mapped in hyraxes by micromapping methods developed and used earlier in slow lorises and llamas. The somatotopic pattern of organization was similar in its general features to that found in other mammals. The perioral and intraoral representations are relatively large in SI of hyrax and exhibit considerable distortion. Sulci are reliable landmarks demarcating boundaries of SI in hyrax as they are in other mammals.

In order to gain insight into the functional properties of the tail in North American beavers (Castor canadensis) we (1) examined morphological features of skeletal, muscular, vascular, cutaneous and neural structures; (2) determined the segmental organization of spinal roots and certain stimulus-response features of receptive fields of single dorsal root fibers; (3) mapped the main somatic sensory area (SI) of the cerebral neocortex, and (4) attempted to relate these findings to observations of tail behavior in living beavers. The behavioral observations revealed the beaver capable of forceful yet discrete movements of the tail in water. A morphological correlate of these motor skills was the distinct segmentally organized pattern of serial muscle tendon arrangements and spinal sensory and motor innervation. Neither the receptors innervating the scales or hairs of the tail, the stimulus-response properties of single dorsal root afferents, nor the representation of the tail in SI suggested unusual cutaneous sensory capabilities associated with the morphological and behavioral specializations exhibited by the beaver's tail.

Members of the order Sirenia are unique among mammals in being the only totally aquatic herbivores. They display correspondingly specialized physiological, behavioral and anatomical features. There have been few reports concerning sirenian neuroanatomy, and most of these have consisted of gross anatomical observations. Our interest in Sirenia stems from the desire to understand neuroanatomical specializations in the context of behavior and the effort to elucidate trends in mammalian brain evolution. The architecture of frontal regions of cerebral cortex was investigated in several brains of the Florida manatee, Trichechus manatus latirostris. Through observation of sections stained for Nissl substance or myelinated fibers, several distinct cortical areas were identified on the basis of laminar organization. These range from areas with poorly defined laminae to those having 6 well-defined layers, some of which exhibit sublayers. Two cortical areas exhibit pronounced cell clusters in layer VI, and these stain positively for acetylcholinesterase and cytochrome oxidase. We hypothesize that these clusters may be involved in perioral tactile bristle function. Certain of our findings are consistent with previous observations in the literature on the brains of dugongs. On the basis of their lamination patterns, these frontal cortical areas appear to be organized into concentric zones of allocortex, mesocortex and isocortex.

Microelectrode mapping methods and strategies were developed to map fine details of the somatotopic organization of somatic sensory cortex (SI) in slow loris. In-depth, multiple-unit recording, and threshold natural stimulation of skin and hari revealed three modality-specific cortical areas. The centrally located one was activated by extremely light touch and the somatopic organization of peripheral projections to it was mapped in detail. As many as 25 punctures/mm2 were made in delineating these details. The microelectrode mapping method used revealed smaller receptive fields, lesser degrees of overlap, a more differentiated pattern of peripheral projections, and a more precise correlation of electrophysiological results with underlying cytoarchitectonic details than did the usual macroelectrode surface recording method. These results and the correlations found between different physiologically defined areas and cortical sulci are discussed as they pertain to methodological advances in studies of cerebral localization of function.

The somatic sensory cortex (SI and SII) was mapped in llamas using microelectrode mapping methods developed earlier in a study of SI of the slow loris. Projections to SI from the llama's prehensile browsing lips were differentially enlarged when compared to those reported for sheep. In llama, SII was reversed in its mediolatreal pattern from that reported for SII in most other mammals. Fissural landmarks reliably demarcated different projections within SI, between SI and SII and between SI or SII and other surrounding nonsensory areas. The use of microelectrode mapping methods in different mammals to determine gyral and fissural homologies is discussed.