Thalamic branched and unbranched axons to feline polysensory cortex.

Thalamic afferents to polysensory neocortex of the suprasylvian, anterior lateral, and pericruciate gyri were demonstrated by retrograde axonal transport of wheat germ agglutinin lectin-bound horseradish peroxidase (HRP) and nuclear yellow (NY). Marker injections were placed in areas identified as responsive to auditory stimuli through the use of click-evoked potentials. Both HRP and NY labeled neurons were found in close proximity or in overlapping regions of the rostral intralaminar nuclei and the adjacent ventral lateral nucleus of the thalamus. While thalamic neurons projecting to different polysensory neocortical areas were often in overlapping fields, double labeling was rare. These results indicate that similar auditory responses seen in different cortical polysensory areas are mediated by mainly separate but topographically related populations of thalamocortical neurons. Similarities in auditory response patterns seen across widely separated areas of polysensory neocortex may be due to common inputs relayed through overlapping or adjacent areas of the thalamus.