Afferent connections of the thalamic intralaminar nuclei in the cat

Afferents to the central lateral (CL), paracentral (PC) and central medial (CE) intralaminar nuclei (ILN) from cortical and subcortical sites were studied in the cat. We utilized stereotaxically guided injections of HRP into the CL and PC nuclei and tritiated leucine injections into various visual, parietal and limbic areas of cortex to demonstrate these connections. In studying the relatively weak visual cortical projections to the ILN, we demonstrated projections from areas 19, 20a, 21a, 21b, AMLS, PMLS and PLLS. However, our HRP injections into the ILN often revealed only a few labeled cells in any of the above areas; therefore conclusions regarding the absence of projections to ILN from remaining visual cortical areas should be made cautiously. The ILN receive heavier projections from the frontal eye fields, cingulate cortex, splenial cortex, insular cortex, somatosensory areas SI and SII, auditory areas SF, AII, and Ep, and parietal areas 5 and 7. The most robust projections appear to be from from frontal eye fields, cingulate and parietal areas. No topography was apparent in the projections to the ILN. All cortical projections originate ipsilaterally from layers V and VI. Heavy subcortical projections to the ILN originate in the pretectum, superior colliculus, reticular formation, and periaqueductal grey. Fewer afferents arise from several other brainstem and thalamic nuclei.