A quantitative comparison of the hominoid thalamus. IV. Posterior association nuclei-the pulvinar and lateral posterior nucleus.

Nuclear volumes, nerve cell densities, numbers of neurons, and volumes of nerve cell perikarya in the thalamic association complex, the pulvinar and lateral posterior nuclei (Pu-LP) were compared among two gibbons, one gorilla, one chimpanzee, and three humans. The human Pu has approximately twice as many neurons as do the great apes, whereas the human and gorilla LP have a similar number. The numbers of neurons in the human Pu and combined Pu-LP complex were predictable from the ape data. Nevertheless, a shift in perikaryal sizes from a unimodal to a bimodal population distinguished the human specimen. It is hypothesized that during human evolution Pu expanded in proportion to the rest of the brain, but that not all parts of Pu expanded equally.