Models of binocular luminance interaction evaluated using visually evoked potential and psychophysical measures: a tribute to M. Russell Harter.

We determined subjects' responses to sine-wave modulated lights employing visually evoked potentials (VEPs) and psychophysical thresholds in a series of experiments. The stimuli had the same temporal frequency and mean luminance in each eye but the phase difference between the two eyes was varied so that phase was either the same (dioptic) or different (dichoptic) in the two eyes. The data were fit by a model which had two binocular pathways, one which summed monocular nonlinear elements and a second which had a nonlinearity following the combination of monocular linear elements. In the second channel the outputs of the monocular linear elements were summed at low luminance while at higher luminance levels they were subtracted. Based on variations in the threshold data with temporal frequency, the pathway which summed nonlinear monocular elements was identified with the magnocellular (M) pathway, and the pathway which combined monocular linear elements prior to a binocular nonlinear element was identified with the parvocellular (P) pathway.