Thalamocortical neurons projecting to the areas surrounding the anterior and middle suprasylvian sulci in the cat. A horseradish peroxidase study.

The thalamic projections to the cortical area surrounding the anterior (ASs) and middle (MSs) suprasylvian sulci in the cat were studied using horseradish peroxidase histochemistry. The medial MSs area (medial wall of the MsS) receives thalamic afferents from the pulvinar-lateralis posterior (Pul-LP) complex, posterior nucleus (Pn) and medial interlaminar nucleus (MIN) and C laminae of the dorsal lateral geniculate nucleus (GLd). Groups of labeled cells in the Pul-LP complex are arranged to form four different neuronal bands oriented dorsoventrally. The four neuronal bands are located, laterally to medially, in the lateral and medial parts of Pul and in the lateral and central parts of LP. The ventral ends of the first (in the lateral Pul) and the third (in the lateral LP) bands extend to MIN and Pn, respectively. The locations of the four bands in the Pul-LP complex appear to correspond to the terminal fields of afferents from four different regions (retina, pretectum, visual cortex and superior colliculus) related to visual function. The anterior part of the lateral MSs area (lateral wall of the MSs) receives thalamic fibers from the lateral and intermediate divisions of the posterior nuclear group (PO1 and POi), while its posterior part receives fibers from the dorsal and medial parts of LP and POi. Both walls of the ASs receive fibers from the ventrolateral part of LP as well as from the anterior part of PO1 and the PO1-POm border. In addition, the ASs-MSs area receives fibers from the central lateral nucleus (CL). It is of interest to correlate the locations of cells projecting to the ASs-MSs area with the pathways terminating in thalamic regions. It is clear that, from the hodological point of view, the ASs-MSs area is functionally segregated, which is in agreement with previous studies we have made corticocortical connections.