Thalamocortical neurons projecting to superficial and to deep layers in parietal, frontal and prefrontal regions in the cat.

Superficial HRP applications and deep injections were performed on symmetric foci of the same cortical region in primary somatosensory, motor and prefrontal areas. Retrogradely labeled neurons were analyzed for body size distribution and intensity of labeling. Neurons in the ventral posterior and ventralis lateralis nuclei projecting to layer I of the somatic sensory and motor cortices are smaller in size and less intensely labeled than neurons projecting to deeper layers. Neurons projecting to the prefrontal cortex from the ventromedialis and mediodorsalis nuclei had the same soma size and topography irrespective of the cortical layer affected by the HRP, although they varied in number and in intensity of labeling.