Information, movement, and adaptation in human vision

Our eyes are never still. Even when fixating, they exhibit small, jittery motions. While it has long been argued that these fixational eye movements (FEMs) aid the acquisition of visual information, a complete theoretical description of their impact on the information available in the early visual system has been lacking. Here, we consider the drift component of FEMs. We describe these FEMs as a diffusive process and build a minimal theoretical model of the early visual response, including the critical process of adaptation (a fading response to a fixed image). We establish the effect of FEMs on the mutual information between a visual stimulus and this response. Our approach identifies the key dimensionless parameters that characterize the effect of fixational eye movements and reveals the regimes in which this effect can be beneficial, detrimental, or negligible. Taking parameter values appropriate for human vision, we show that the spatiotemporal couplings induced by fixational eye movements can explain the qualitative features of the human contrast sensitivity function as well as classic experiments on temporal integration. To our knowledge, the consideration of fixational eye movements in this latter context is unique, and our results suggest the need for future experiments to determine the mechanisms by which spatial and temporal responses are coupled in the human visual system.