Cat medial interlaminar nucleus: retinotopy, relation to tapetum and implications for scotopic vision

The medial interlaminar nucleus (MIN) of the cat was electrophysiologically mapped in sufficient detail to resolve individual laminae and to allow reconstruction of isoazimuth and isoelevation lines in coronal, sagittal, and horizontal planes. The electrophysiologically defined laminar pattern was in agreement with that revealed anatomically in the same animal, as well as with the general pattern revealed by anatomical methods in several unmapped nuclei. The MIN is made up of three distinct layers, each receiving inputs from one hemi-retina but none representing an entire hemifield. We confirm the findings of Guillery et al. (19) that the contralateral hemifield is represented in layers 1 and 2 through the contralateral and ipsilateral eyes, respectively, and that layer 3 represents the ipsilateral hemifield through the contralateral eye. Elevation and absolute value of azimuth are represented continuously through the MIN. When an isoazimuth line crosses the border between layer 3 and either layer 1 or 2, the absolute value of azimuth is maintained but the sign of the azimuth changes. Adjacent points on either side of this border represent mirror symmetrical visual directions on opposite sides of the vertical meridian. This indicates that the distance from the vertical meridian is an independently coded parameter within the geniculate complex. There is virtually no nasotemporal overlap in any layer of the MIN. The function relating magnification (mm3 per steradian) to eccentricity is strikingly similar to the function relating retinal ganglion cell density to eccentricity, suggesting that a constant fraction of retinal ganglion cells project to the MIN at all eccentricities. Most of the volume of each MIN layer is devoted to lower visual fields. Analysis of the geniculate retinotopic maps of Sanderson (46) reveals no equivalent bias toward lower visual fields in the dorsal lateral geniculate nucleus. The MIN represents a region of retina roughly coincident with the tapetum, suggesting a role of the MIN in dim-light vision.