Abstract
The temporal sensitivity of the visual system has been extensively investigated outside the foveola, the 1 degree region of the retina specialized for high-acuity vision. Outside this region, it is well established that the temporal contrast sensitivity function changes little with increasing eccentricity, when stimuli are properly enlarged to account for cortical magnification. Much less is known about temporal sensitivity within the foveola. In this region, mapping sensitivity is challenged by the incessant presence of fixational eye movements, which shift the stimulus on the retina by an amount as large as the foveola itself. Despite these experimental difficulties, it is commonly assumed that the foveola responds weakly to moving and temporally modulated stimuli, an assumption that conflicts with the recent observation that retinal motion from eye movements enhances foveal sensitivity. Here we took advantage of recent advances in gaze-contingent control and localization of the center of gaze to map temporal sensitivity across the foveola. Human observers (N=5) were presented with a 10 arcmin square probe temporally modulated with a contrast-reversal sinusoidal function (frequency: 0, 1, 5, 10 and 15 Hz). Contrast thresholds were estimated at 5 foveal eccentricities (0, 10, 20, 30, 60 arcminutes). A custom apparatus for gaze-contingent display counteracted the consequences of eye movements and maintained the probe stabilized at the desired retinal location. Our data show that the foveola is more sensitive to non-zero temporal frequencies. Although individual differences exist in how sensitivity varies across eccentricity, all subjects at all foveal eccentricities exhibited a band-pass sensitivity function. Contrast sensitivity increases with temporal frequency up to a peak in the 1-5 Hz range and then declines so that it is greatly attenuated above 10 Hz. These results challenge the traditional idea that the fovea responds more strongly to stationary and unchanging stimuli.