Ventroposterior thalamic regions projecting to cytoarchitectonic areas 3a and 3b in the cat.

The adequate stimulus and body site that excited neurons in cat cortical somatosensory areas 3a and 3b were recorded using low-impedance tungsten microelectrodes. Horizontal penetrations provided a good correlation between the electrophysiological and cytoarchitectonic data. Responses best driven by cutaneous stimuli were replaced with responses driven by manipulation of deep tissue at, or very near, the border between areas 3a and 3b. Following functional identification of these areas horseradish peroxidase was injected into one of them. Injections into area 3a labeled neurons in a rostral and dorsal cap of the ventroposterior thalamus. It was suggested that this region is a distinct nucleus termed the ventroposterior oralis nucleus (VPO). Injections into the forelimb portion of area 3b labeled neurons in the ventroposterior lateral nucleus (VPL). With both vertical and horizontal microelectrode trajectories through the ventroposterior thalamic nuclei, inputs from deep structures presumed to be muscles were consistently located in the VPO nucleus and cutaneous inputs activated neurons in the VPL. The existence of several functionally-distinct subdivisions within the somatosensory nuclei of the thalamus supports the hypothesis of parallel processing and relay of somatosensory information at this level of the pathway.