Multiple somatosensory areas in the anterior parietal cortex of the California ground squirrel (Spermophilus beecheyii)

Multiunit electrophysiological recording techniques were used to explore th e somatosensory cortex of the California ground squirrel (Spermophilus beec heyii). Cortex rostral and caudal to the primary somatosensory area (SI) co ntained neurons that responded to stimulation of deep receptors and to musc le and joint manipulation. The region of cortex rostral to SI was termed th e rostral field (R) because of possible homologies with a similar field des cribed in other mammals. Cortex caudal to SI had neurons that responded to stimulation of deep receptors and has been termed the parietal medial area (PM), as in previous investigations in squirrels. Like SI, both R and PM co ntained a complete or almost complete representation of the body surface, a lthough the receptive field size for clusters of neurons in these regions w as somewhat larger than those for clusters of neurons in SI. Electrophysiol ogical recording results were correlated with histologically processed tiss ue that had been sectioned tangentially. Although SI was clearly identified as a myelin-dense region, both R and PM stained much less densely for myel in. Our results indicate that as in a number of other mammals including mon otremes, marsupials, carnivores, and primates, the anterior parietal cortex of the California ground squirrel contains multiple representations of the sensory epithelium. This work, as well as a growing body of studies of som atosensory cortex organization in a variety of mammals, indicates that ante rior parietal fields other than SI existed early in mammalian evolution, an d were present in the common ancestor of all mammals. (C) 2000 Wiley-Liss, Inc.