Temporal sensitivity in the central fovea

The human retina is a spatially inhomogeneous structure, with striking differences in the size, density, and distribution of photoreceptors across locations. These differences are particularly evident the further away one moves from the center of vision (foveola), where diverse neuronal populations are assumed to feed into overall similar processing mechanisms. This idea receives support from measurements of contrast sensitivity at various eccentricities: once stimuli are enlarged to compensate for differences in the density of photoreceptors, both spatial and—with some exceptions—temporal sensitivity remain constant across the visual field. Although more difficult to study, inhomogeneities also occur within the foveola, both in the layout of receptors and in the measured spatial and temporal sensitivity. Here we investigated whether temporal contrast sensitivity varies across the foveola. Human observers (N=6) detected a 10 arcmin square presented at various distances from the preferred locus of fixation (PLF) on the horizontal meridian (0, 10, 20, 30, 60 arcminutes). To compensate for the normal instability of visual fixation, eye movements were recorded at high resolution via Dual Purkinje Imaging, and stimuli were stabilized on the retina by means of a custom system for gaze-contingent display control. At each foveal location, we measured the contrast sensitivity function as the probe flickered at various temporal frequencies (~0, 1, 5, 10, 15 Hz). In selected participants, an adaptive optics scanning laser ophthalmoscope (AOSLO) enabled the estimation of cone density and cone-to-cone spacing. Contrast sensitivity measured with a fixed stimulus size was a bandpass function of temporal frequency that varied little across eccentricities. However, sensitivity measurements at distinct eccentricities diverged once rescaled by the individual cone density or cone-to-cone spacing, suggesting that temporal sensitivity is not uniform within the foveola.