Functional significance of the fibrous laminae in the ventrobasal complex of the thalamus of slow loris

Investigations on the thalamocortical relationships were carried out on nine slow lorises. The electrophysiological data obtained from our previous experiments on the somatotopic organization within the Sm I and Sm II cortical regions were utilized for the placement of selective cortical ablations. The thalamic regions exhibiting retrograde degenerative changes were reconstructed. The projection of fibres to the electrophysiologically defined Sm I cortex is largely confined to the ventrobasal complex of the thalamus. Moreover, this region of the thalamus in slow loris is subdivided into a lateral, an intermediate and a medial portion by two main fibrous septa. The medial septum is found between the pars medialis and pars lateralis of the ventrobasal complex separating the head representation from the forelimb representation while the lateral septum, situated within the pars lateralis delineates the forelimb representation from that of the hind limb. A correlation is found to exist between these fibrous septa and the limiting sulci found within the somatic‐sensory cortex separating functional subdivisions. Moreover, a group of cells lying ventromedial to the pars medialis showed degenerative changes in the material where Sm II cortical region was ablated indicating the projection from this group of cells of at least some fibres to the second somatic‐sensory cortex. Another interesting feature that emerged from the results of the present study is the possible connection of n. subparacentralis with the paleoclaustrum, and/or the overlying allocortex.