Temporal resolution for disparity modulation may be limited by the speed of response modulation in V1

Abstract

The human ability to perceive modulation of disparity over time is relatively poor (Norcia and Tyler, 1984), compared with the ability to detect luminance modulation. In order to examine the physiological basis of this poor temporal resolution of stereopsis, we quantified responses to disparity modulation in 54 disparity selective V1 neurons from four awake behaving monkeys. Stimuli were random dot stereograms in which disparity varied as a sinusoidal function of time. The neurons modulated their response at the temporal frequency of the stimulus. Since for most cells the mean firing did not vary significantly as a function of temporal frequency, we analyzed changes in modulation amplitude. Temporal frequency tuning in response to disparity modulation was only weakly correlated with temporal frequency tuning in response to drifting luminance gratings, and had lower temporal frequency high cutoffs on average. Note that the temporal frequency tuning in response to the drifting gratings was obtained from mean firing rates. This difference in temporal frequency tuning could therefore be explained if the temporal response for changing spike rate (the time constant of the output), is slower than that for responses supporting mean firing (presumably reflecting the time constant of some input elements). The temporal frequency high cut for disparity modulation was negatively correlated both with the response latency and the speed of the response onset (rise time to 60% peak), supporting the idea that the output time constant limits temporal resolution for disparity. Mean high cut temporal frequency in response to disparity modulation was 10Hz, similar to the values reported in human psychophysical studies. This suggests that the human temporal resolution for detecting modulation of disparity is limited by the temporal frequency up to which disparity selective V1 neurons are able to modulate their response.