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 responsivity of retinal ganglion cells of the magnocellular (MC-) pathway as a function of the relative phase of heterochromatically modulated lights. The MC-pathway phase effect is absent if the receptive field centre 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 consistent with a surround diameter for MC-pathway cells in the fovea of about 50 min arc, about 10 times estimated centre diameter. On changing retinal illuminance, the amplitude of the physiological and psychophysical phase shifts also changed in a parallel manner. These experiments support the hypothesis that the physiological origin of psychophysical phase shifts is in the MC-pathway, and indicate the spatial frequency (c. 2c/deg) below which the psychophysical phase shift should become apparent.