Organization of sensory cortex in the east African hedgehog (Atelerix albiventris)

We investigated the organization of neocortex in the East African hedgehog (Atelerix albiventris) with microelectrode recordings from sensory areas th at were later correlated with cytochrome oxidase patterns in sections of fl attened cortex. The location of corticospinal projecting neurons was also e xamined and related to sensory areas by making small injections of wheat ge rm agglutinin-horseradish peroxidase into the spinal cord. Our goals were t o determine how hedgehog cortex is organized, how much sensory areas overla p, and to compare results with recent findings in other insectivores. Evide nce was found for three separate topographically organized somatosensory ar eas, two visual areas, and a caudolateral auditory area. A medial somatosen sory area corresponded to S1, the primary somatosensory area, whereas two l ateral areas partially encircled auditory cortex and corresponded to the pa rietal ventral area (PV) and the secondary somatosensory area (S2). Primary visual cortex (V1) was delineated by a caudomedial cytochrome oxidase dark oval, and a more lateral visual area between V1 and somatosensory cortex c orresponded to V2, or area 18. Two patches of corticospinal projecting cell s were found primarily overlapping S1 and S2. Some bimodal auditory and som atosensory responses were found in parts of PV and S2, but for the most par t, areas had relatively sharp histochemically apparent and physiologically defined borders. The present results indicate that the caudal neocortex of hedgehogs has only a few sensory areas, corresponding to those commonly fou nd in several other small-brained mammals. Hedgehog cortical organization d iffers significantly in somatotopy, number, and position of fields from tha t of closely related shrews and moles. Thus, clear specializations occur, e ven within the order Insectivora. (C) 2000 Wiley-Liss, Inc.