Human temporal impulse response speeds up with increased stimulus contrast

It is well known that raising mean luminance speeds-up the visual response to temporal change. At higher mean luminance, the temporal impulse response function (IRF) becomes more transient or biphasic. An analogous effect is observed physiologically when stimulus contrast is increased, at constant mean luminance. As stimulus contrast is raised, the temporal response to flicker advances in phase and becomes more transient (bandpass). The MC (magnocellular) retinal ganglion cells manifest this temporal contrast gain control, but the PC (parvocellular) cells do not. We show psychophysically that the temporal response in humans speeds-up in an analogous manner as stimulus contrast is raised. Low spatial-frequency gratings, of suprathreshold contrast, were presented as pairs of pulses, separated by brief delays. Responses became more transient with increasing contrast in both our motion task (direction discrimination) and in our flicker task ('agitation' discrimination), mimicking the temporal contrast gain control seen in the physiological studies. Results could be modeled with a nonlinearity, in which the IRF shortens with increasing contrast.