Abstract
Our visual system contains a built-in contradiction: when we fixate our gaze on an object of interest, our eyes are never still. Instead we produce, several times a second, small fixational eye movements of which we are unaware. We recently showed that fixational microsaccades counteract Troxler fading and drive visibility during fixation (Martinez-Conde et al., Neuron 2006). Here we set out to determine whether microsaccades may also counteract the perceptual filling-in of artificial scotomas. Several studies have shown that artificial scotomas positioned within peripheral dynamic noise can fade from perception (that is, the surrounding dynamic noise appears to fill-in the scotoma). Because the dynamic noise is continuously refreshed, this filling-in effect cannot be explained by simple low-level adaptation mechanisms, such as those that may underlie classical Troxler fading. We asked subjects to indicate, via button press/release, whether an artificial scotoma presented on a dynamic noise background was visible or invisible at any given time. The subjects' eye movements were simultaneously measured with a high precision video system. No previous studies have established a direct correlation between eye-movement dynamics to the perception of filling-in, with tight timing. We found that increases in microsaccade production counteracted the perception of filling-in (and correlated to the visibility of the artificial scotoma), whereas decreased microsaccades allowed perceptual filling-in to take place. Our results show that microsaccades do not solely overcome low-level adaptation mechanisms but also contribute to maintaining second-order visibility during fixation.
This study was funded by Dana Foundation and Barrow Neurological Foundation. XGT is a Caja Madrid postdoctoral fellow.