Oculomotor influences on extrafoveal sensitivity

The human eyes are always in motion, alternating rapid gaze shifts (microsaccades) with slow smooth movements (drifts) even when attending to a single point. In the fovea, these fixational eye movements (FEM) have been shown to enhance sensitivity in complementary spatial frequency ranges, in a way that is consistent with their reformatting of spatial patterns into temporal signals: the luminance modulations from microsaccades and drifts emphasize low and high spatial frequencies, respectively. Outside the fovea, however, the perceptual roles of FEM remain unclear. In the periphery, views range from general functions (‘refreshing’ percepts) to no function because of the little FEM motion relative to receptive fields size. Here we show that FEM lead to similar perceptual consequences inside and outside the fovea. Human observers (N=6) were asked to report the orientation (±45°) of a full-field grating while maintaining fixation. The grating was either at high (10 cpd) or low (0.2 cpd) spatial frequency. To restrict stimulation to the peripheral visual field, a circular gray patch (diameter 15° or 23°) remained stationary on the retina centered on the line of sight. We compared performance in the presence and absence of the retinal motion caused by FEM. In the latter condition, eye movements were counteracted in real-time by moving the stimulus on the display via a custom apparatus. Our results show that, also outside the fovea, drifts and microsaccades selectively improve sensitivity to high and low spatial frequencies, respectively. On average performance dropped by approximately 10% at high frequency when retinal motion was eliminated and improved by a similar amount at low frequency in the trials with microsaccades. Together, these results indicate that FEM operate uniformly throughout the visual field, reformatting luminance patterns into spatiotemporal signals that enhance contrast sensitivity in complementary ranges of spatial frequencies.