Cortical topography of thalamic intralaminar nuclei.

The cortical projection of the thalamic intralaminar nuclei (ILN) has been studied by injecting little amounts of horseradish peroxidase (HRP) on the cerebral cortex of the rat. All of the cortical areas (except area 17) receive ipsilateral projections from at least one nucleus of the ILN. The nucleus centralis lateralis had the largest number of labeled neurons, principally after injections in frontal, temporal and occipital cortical areas. The nucleus paracentralis presented only moderate numbers of HRP positive neurons from the frontal cortex, and very few from parietal and temporal areas. The nucleus parafascicularis showed labeled somata after frontal injections as well as parietal and temporal areas. In comparison to the other ILN, the amount of labeled neurons in this nucleus is relatively small. The nucleus centralis medialis presented the least number of labeled neurons regardless of injected area. Its cortical efferents remain restricted to small areas of the dorsal aspects of frontal, anterior cingular and temporal cortices. Each of the ILN contains neurons which connect with more than one cortical zone, according to a characteristic topographic distribution.