Interhemispheric connections of somatosensory cortex in the flying fox

The interhemispheric connections of somatosensory cortex in the gray-headed flying fox (Pteropus poliocephalus) were examined. Injections of anatomica l tracers were placed into five electrophysiologically identified somatosen sory areas: the primary somatosensory area (SI or area 3b), the anterior pa rietal areas 3a and 1/2, and the lateral somatosensory areas SII (the secon dary somatosensory area) and PV (pairetal ventral area). In two animals, th e hemisphere opposite to that containing the injection sites was explored e lectrophysiologically to allow the details of the topography of interconnec tions to be assessed. Examination of the areal distribution of labeled cell bodies and/or axon terminals in cortex sectioned tangential to the pial su rface revealed several consistent findings. First, the density of connectio ns varied as a function of the body part representation injected. For examp le, the area 3b representation of the trunk and structures of the face are more densely interconnected than the representation of distal body parts (e .g., digit 1, D1). Second, callosal connections appear to be both matched a nd mismatched to the body part representations injected in the opposite hem isphere. For example, an injection of retrograde tracer into the trunk repr esentation of area 3b revealed connections from the trunk representation in the opposite hemisphere, as well as from shoulder and forelimb/wing repres entations. Third, the same body part is differentially connected in differe nt fields via the corpus callosum. For example, the D1 representation in ar ea 3b in one hemisphere had no connections with the area 3b D1 representati on in the opposite hemisphere, whereas the D1 representation in area 1/2 ha d relatively dense reciprocal connections with area 1/2 in the opposite hem isphere. Finally, there are callosal projections to fields other than the h omotopic, contralateral field. For example, the D1 representation in area 1 /2 projects to contralateral area 1/2, and also to area 3b and SII. J. Comp . Neurol. 402:538-559, 1998. (C) 1998 Wiley-Liss, Inc.