SPATIAL AND TEMPORAL CONTRAST SENSITIVITIES OF NEURONS IN LATERAL GENICULATE-NUCLEUS OF MACAQUE

The discharges of single neurons in the parvocellular and magnocellular laminae of the macaque''s [Macaca fascicularis, Macaca mulatta] lateral geniculate nucleus (l.g.n.) were recorded with glass-insulated tungsten micro-electrodes. Linearity of spatial summation was examined using the test devised by Hochstein and Shapley. Parvocellular units (2 of 272)and 6 of 105 magnocellular units showed clearly non-linear spatial summation. A quantitative index of non-linearity did not suggest the the existence of a distinct non-linear class of magnocellular unit. Spatial contrast sensitivity to moving gratings was measured by a tracking procedure in which contrast was adjusted to elicit a reliable modulation of discharge. With the exception of cells that were driven by blue-sensitive cones, measurements of contrast sensitivity did not reveal distinct subgroups of parvocellular units. All had low sensitivity, and those with receptive fields in the fovea could resolve spatial frequencies of up to 40 cycles deg-1 [degree]. Magnocellular units had substantially higher sensitivity, but poorer spatial resolution. The higher sensitivities of magnocellular units led to their giving saturated responses to stimuli of high contrast. Responses of parvocellular units were rarely saturated by any stimulus. At any 1 eccentricity, the receptive fields of parvocellular units had smaller centers than did those of magnocellular units. Receptive fields of magnocellular units driven by the ipsilateral eye had larger receptive fields than did those driven by the contralateral eye. Parvocellular units were most sensitive to stimuli modulated at temporal frequencies close to 10 Hz; magnocellular units were most sensitive to stimuli modulated at frequencies nearer 20 Hz. The loss of sensitivity, as temporal frequency fell below optimum, was more marked in magnocellular than parvocellular units. Changes in temporal frequency altered the shapes of the spatial contrast sensitivity curves of both parvocellular and magnocellular units. These changes could be explained by supposing that center and surround have different temporal properties and that the surround is relatively less sensitive to higher temporal frequencies.