RESPONSES OF MACAQUE GANGLION-CELLS AND HUMAN OBSERVERS TO COMPOUND PERIODIC WAVE-FORMS

We measured responses of macaque retinal ganglion cells to different p eriodic waveforms (sinusoidal, square, rapid-on and rapid-off sawtooth waveforms) for both luminance and equiluminant chromatic modulation. We analyzed the responses with a peak-to-trough detector. At low frequ encies, on-center and off-center magnocellular (MC-) pathway cells sho wed a ten-fold higher responsivity to the rapid-on and rapid-off sawto oth respectively. Red-on (+L-M) and green-on (+M-L) parvocellular (PC- ) pathway cells showed a four-fold greater responsivity to rapid red-o n and rapid green-on equiluminant chromatic sawtooth waveforms respect ively. At an equivalent retinal eccentricity, we measured psychophysic al thresholds for luminance stimuli and chromatic stimuli. We conclude d that luminance sawtooth sensitivities from psychophysics are consist ent with selective detection through MC-pathway on- and off-center cha nnels in the visual system. The differences between the compound perio dic waveforms seen in the PC-pathway cell data did not occur in the ps ychophysics. In a second analysis, cell responses to sinusoidal modula tion were used to predict the linear response to square-wave and sawto oth waveforms. PC-pathway cells showed linear temporal behavior over a wide range of contrasts, but MC-pathway cells displayed linear behavi or only for low-contrast luminance modulation. Using these linear fits , we implemented a model incorporating central low-pass filtering in t he MC- and PC-pathways before the peak-to-trough detector. This model captured better the time scale and relative sensitivity to periodic wa veforms found in the psychophysical data.