The cat medial interlaminar nucleus (MIN) receives inputs almost exclusively from tapetal retina, suggesting that the MIN has a special role in dim-light vision. In this study we compared the sensitivities of cells in the MIN with those in layers A and magnocellular C of the lateral geniculate nucleus (LGNd), using drifting sinusoidal gratings to determine contrast thresholds as a function of spatial frequency and retinal adaptation level over the entire scotopic range. 2. About one-half of the cells recorded in the MIN and layer A had brisk responses that could be nulled by properly positioned, counterphased sinusoidal gratings, and were classified as X cells. The rest of the cells in the MIN and layer A, as well as all cells recorded in layer C, were Y cells. 3. MIN cells had higher contrast sensitivity than layer A cells for low spatial frequencies (0.15 cycles/deg and below) over a wide range of adaptation levels, both overall and for separate comparisons within X or Y cells. Layer C Y cells were intermediate in sensitivity between MIN and layer A Y cells. For low spatial frequencies, Y cells as a group were more sensitive than X cells, whereas the reverse was true for high spatial frequencies. 4. These data enable one to determine the lowest adaptation level at which stimuli of a given contrast can be detected for a given structure. At the lowest spatial frequencies, the MIN can function at adaptation levels approximately 1 log unit below layer A, averaged over all stimulus contrasts. In contrast, the tapetum lowers luminance threshold by at most 0.16 log unit. 5. For scotopic conditions and eccentricities within 15 degrees of the area centralis, contrast sensitivity decreases with eccentricity for low spatial frequencies and remains flat or slightly increases for high spatial frequencies. This relationship, which is opposite to that found for photopic vision, is strongest for MIN Y cells. 6. These data support the hypothesis that the retinal conflict between sensitivity and acuity is ameliorated in the CNS through separate thalamic relays with different degrees of afferent convergence. MIN cells have higher luminance sensitivity than layer A cells, but at the expense of acuity. Layer C appears to occupy an intermediate position in this trade-off.
