Studies of the cat's medial interlaminar nucleus: A subdivision of the dorsal lateral geniculate nucleus

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Abstract
The medial interlaminar nucleus (MIN) of the cat lies medial to the laminated region of the dorsal lateral geniculate (lamLGN). This latter region includes the A and C laminae. As does lamLGN, MIN receives direct retinal input and projects to various visual cortical areas. We examined the MIN of 15 normal adult cats with electrophysiological and anatomical techniques.Autoradiographs processed from cats that had one eye injected with tritiated fucose and proline indicate that MIN is composed of at least two laminae, one for each eye. The area which receives input from the ipsilateral eye is a small central region surrounded dorsally, medially, and ventrally by a larger crescent shaped region that receives input from the contralateral eye. This pattern was also evident from electrophysiological recording experiments.Extracellular recordings from 102 single‐units in MIN indicate that these cells have properties essentially identical to lamLGN Y‐cells. That is, they had short latencies to orthodromic stimulation of the optic chiasm and antidromic stimulation of the visual cortices, responded in a phasic manner to the presentation of a standing contrast within the receptive field center, responded to rapidly moving visual stimuli, and showed non‐linear spatial summation properties typical of lamLGN Y‐cells. We discovered two difference between MIN cells and lamLGN Y‐cells. First the mean receptive field center size of MIN cells is considerably larger than that of lamLGN Y‐cells, and second, MIN cells do not have the non‐dominant eye inhibitory receptive fields found for many lamLGN Y‐cells.Cell size measurements indicate that while the mean cell size in MIN is approximately 30% greater than in the A laminae of lamLGN, the distribution of MIN cell sizes extends over the full range of cell sizes in the A laminae. Since the A laminae are comprised mostly of X‐ and Y‐cells, this suggests that, although Y‐cells on average are larger than X‐cells, considerable overlap exists in their size distribution. No differences between the ipsilateral and contra lateral terminal zones were found on any measure.Since MIN cells share most or all the fundamental features of lamLGN Y‐cells, we suggest that these cell groups should be considered subpopulations of a more general group of geniculate Y‐cells. Accordingly, we refer to these two subpopulations as lamLGN Y‐cells and MIN Y‐cells.