Organization of sensory cortex in a Madagascan insectivore, the tenrec (Echinops telfairi).

We identified subdivisions of somatosensory cortex, and the borders and extents of auditory and visual cortex in Madagascan tenrecs (Echinops telfairi) by using microelectrode recording techniques and cortical myeloarchitecture. There was evidence for three distinct somatosensory fields. The primary somatosensory area (S1) contained an orderly representation of the contralateral body surface that stained darkly for myelin. Neurons were activated by light touch, and receptive fields were often small, especially for the snout. Immediately rostral to S1, a lightly myelinated rostral field (R) also contained a representation of the contralateral body, although the internal topography was not fully determined. Neurons in R responded to manipulations of body parts and tissue displacements. A small, moderately myelinated area lateral to S1 was termed PV/S2 because it possessed features that were similar to both the parietal ventral area (PV) and the second somatosensory area (S2) in other mammals. Neurons in PV/S2 responded to light tactile stimulation. A densely myelinated oval of cortex caudal to PV/S2, the auditory area (A), contained neurons that responded to clicks, and the densely myelinated caudomedial visual area (V) contained neurons that were activated by stimulation of one or both eyes. Some characteristics of V were similar to the primary visual area (V1) described in other mammals. A visual area located in rostromedial cortex (RV) contained neurons that were highly responsive to visual stimulation. Area RV may be a specialization of tenrecs or an elaboration of a visuomotor field that has been retained in most extant mammals. The results support the view that most of the neocortex of primitive mammals was composed of a few sensory areas. J. Comp. Neurol. 379:399-414, 1997.