Saccade control without a fovea

Abstract

Prior research has suggested that two types of fast eye movements (FEMs) can be distinguished behaviorally. Foveating saccades respond to salient peripheral targets by directing the target image to the fovea. Non-foveating saccades include other FEMs such as nystagmus quick phase, saccades without visual stimuli and visually-directed saccades that direct target images to eccentric retina. Foveating saccades have a shorter initiation latency and are faster than non-foveating saccades. Following adaptation to central scotoma, patients tend to use preferred retinal loci for fixation (PRL). If PRL acquire the foveal characteristic of a retino-motor center then visually guided saccades would acquire the properties of foveating saccades. Using an objectively-calibrated 2-dimensional search coil, we measured saccades in response to salient, unpredictable targets. The saccades of normal observers were compared to the saccades of patients with long-standing macular scotomas. Although the saccades of patients consistently directed images to PRL, the saccades still had the latency and dynamic characteristics of non-foveating saccades. Moreover, the non-foveating saccades of patients were found to be less accurate than foveating saccades, showing a range effect (larger saccades undershoot with greater error than do smaller saccades). Apparently, patients with macular scotoma suppress rather than adapt a foveating saccade mechanism.