Thalamic projections to visually responsive regions of parietal cortex.

Thalamic efferents to visually responsive regions of parietal cortex of the cat were investigated by experimental retrograde tracing techniques. Two classes of photically evoked potentials recorded from the middle suprasylvian gyrus could be distinguished on the basis of waveform and latency. Type 1 responses were recorded from the Clare-Bishop area at the lateral border of the suprasylvian gyrus and Type 2 responses were recorded from association response areas on the crown of the suprasylvian gyrus. Studies of retrograde degeneration in kittens and adult cats and retrograde transport of intracortically injected horseradish peroxidase indicate that afferents to the electrophysiologically identified Clare-Bishop area originate in the lateroposterior nucleus, the posterior nucleus, and the medial interlaminar nucleus of the lateral geniculate body. Afferents to electrophysiologically identified association response areas originate in the lateroposterior nucleus, ventroanterior nucleus, the pulvinar, the laterodorsal nucleus, and the central lateral nucleus. Studies of orthograde degeneration following placement of parietal lesions indicate a close reciprocity of corticothalamic and thalamocortical projections.