Thalamic projections to the anterior suprasylvian and posterior sigmoid cortex: an HRP study of the "vestibular areas" of the cerebral cortex in the cat.

We have confirmed electrophysiologically the existence of an oligosynaptic vestibular projection to the cortex surrounding the rostral end of the anterior suprasylvian sulcus ( ASsS ). However, we failed to confirm a similar projection to area 3a in the posterior sigmoid gyrus. We studied the thalamic projections to each of these cortical regions by injecting small amounts of HRP in the cortex and looking for neurons retrogradely labeled throughout the thalamus. The exact location of the cortical injections was assessed cytoarchitectonically. The heaviest neuronal labeling after injections in the banks of ASsS was obtained in Po (including in this complex GMmc ). A moderate number of projections was found from VPi, VPm and VPl (the labeling in the latter being particularly prominent in a case injected in the lower bank of ASsS ), and also from VL. Occasional labeled neurons were found in the rostro-ventral part of LP. After injections in area 3a in the posterior sigmoid gyrus, which affected to a minor degree either area 3b or 4, many labeled cells appeared in the rostral and dorsal part of VPl, and in the central and lateral parts of VL. Fewer labeled cells were found in VPi, Po and LP. In most cases some occasional labeled cell was observed also in the intralaminar nuclei and in Vm.