Consequences of eye movements for spatial selectivity

What determines spatial tuning in the visual system? Standard views rely on the assumption that spatial information is directly inherited from the relative position of photoreceptors and shaped by neuronal connectivity.1,2 1,2 However, human eyes are always in motion during fixation,3-6 3-6 so retinal neurons receive temporal modulations that depend on the interaction of the spatial structure of the stimulus with eye movements. It has long been hypothesized that these modulations might contribute to spatial encoding,7-12 7-12 a proposal supported by several recent observations.13-16 13-16 A fundamental, yet untested, consequence of this encoding strategy is that spatial tuning is not hard-wired in the visual system but critically depends on how the fixational motion of the eye shapes the temporal structure of the signals impinging onto the retina. Here we used high-resolution techniques for eye-tracking17 17 and gaze-contingent display control18 18 to quantitatively test this distinctive prediction. We examined how contrast sensitivity, a hallmark of spatial vision, is influenced by fixational motion, both during normal active fixation and when the spatiotemporal stimulus on the retina is altered to mimic changes in fixational control. We showed that visual sensitivity closely follows the strength of the luminance modulations delivered within a narrow temporal bandwidth, so changes in fixational motion have opposite visual effects at low and high spatial frequencies. By identifying a key role for oculomotor activity in spatial selectivity, these findings have important implications for the perceptual consequences of abnormal eye movements, the sources of perceptual variability, and the function of oculomotor control.