THE RESPONSE OF MACAQUE GANGLION-CELLS AND HUMAN OBSERVERS TO HETEROCHROMATICALLY MODULATED LIGHTS - THE EFFECT OF STIMULUS SIZE

Psychophysical sensitivity of human observers closely resembles respon sivity of retinal ganglion cells of the magnocellular (MC-) pathway as a function of the relative phase of heterochromatically modulated lig hts. The MC-pathway phase effect is absent if the receptive field cent re alone is stimulated. Here we confirm this physiological result, and show that the psychophysical phase shift is also abolished with small stimuli. The space constant of the psychophysical effect is consisten t with a surround diameter for MC-pathway cells in the fovea of about 50 min arc, about 10 times estimated centre diameter. On changing reti nal illuminance, the amplitude of the physiological and psychophysical phase shifts also changed in a parallel manner. These experiments sup port the hypothesis that the physiological origin of psychophysical ph ase shifts is in the MC-pathway, and indicate the spatial frequency (c . 2 c/deg) below which the psychophysical phase shift should become ap parent.