Efferent projections of the thalamic intralaminar nuclei in the cat.

Efferent projections of the central lateral (CL), paracentral (PC) and central medial (CE) intralaminar nuclei (ILN) to cortical and subcortical sites were studied in the cat. The combined methods of electrophysiologically guided cortical injections of tritiated leucine and stereotaxic injections of horseradish peroxidase (HRP) into the CL and PC nuclei were utilized. Additionally, fluorescent double-labeling techniques demonstrated patterns of intralaminar axon collateralization. We found that the ILN project ipsilaterally to all visual cortical areas except area 17. Projections to visual cortex are not arranged topographically or retinotopically. The ILN also project to the frontal eye fields (areas 6 and 8), anterior cingulate gyrus, suprasylvian fringe of the auditory cortex, insular cortex, parietal areas 5 and 7, caudate nucleus and claustrum. We noted especially heavy projections to the frontal eye fields and parietal areas 5 and 7. Fibers from the ILN terminate in cortical layers I and VI, and at the layer III-IV border. The demonstration of collateralization of ILN axons to two separate cortical areas implies that the same neuronal message may pass from the ILN to multiple cortical areas. It is concluded that the ILN may mediate a general cortical activation and may play a role in attention to visual, auditory and somatosensory (especially nociceptive) stimuli.