Temporal impulse response function of the visual system estimated from ocular following responses in humans

Early visual processing functions as a set of spatiotemporal image filters. Our ability to sense changes in retinal images is determined by these filters along the temporal axis. In this study, we developed a paradigm to identify the kernel of the temporal filters based on ocular following responses (OFRs) to two-frame apparent motion stimuli. We first conducted two experiments to acquire fundamental data. In the first experiment, in which a quarter wavelength step of a sinusoidal grating was presented with various inter-stimulus intervals (ISIs), we found that OFRs were reversed by the ISI, which is consistent with previous findings. In the second experiment, a quarter wavelength step of a sinusoidal grating was applied with various durations of the initial image frame (motion onset delays; MODs); we found that longer exposure to the initial image reduced OFRs. Parameters of motion energy model involving temporal filters were optimized so that the model could reproduce the dependence of OFRs on ISIs and MODs. We were then able to successfully obtain quantitative estimates of the biphasic temporal filters with optimal frequencies in 6-8Hz. This method is completely objective and will thus be applicable to a wide range of human subjects and model animals.