Perisaccadic mislocalization in slow saccades

Abstract

Despite frequent saccadic gaze shifts during visual exploration we perceive the outer world as stable. Under laboratory conditions, however, space constancy can be challenged: Briefly flashed stimuli are systematically mislocalized around the onset of saccadic eye movements. One well-established pattern of perisaccadic mislocalization is an anisotropic localization bias towards the saccade target which has been termed a compression of apparent positions (e.g., Ross et al. 1997, Lappe et al. 2000). The dynamics of perisaccadic mislocalization vary considerably between subjects. Recently, we observed that a large part of inter-individual variance in perisaccadic compression strength can be explained by corresponding variations in mean saccadic peak velocity (Ostendorf et al. 2007). The observed correlation may indicate a modulation of visual reafferent processing by extraretinal signals associated with the oculomotor command, leading to stronger perceptual compression with higher saccade velocities. Alternatively, differential retinal stimulation due to a variable displacement speed may result in different degrees of spatio-temporal uncertainty for peri-saccadically flashed stimuli. This could, at least partially, explain an association between individual mislocalization patterns and corresponding saccade velocities as well (Brenner et al. 2006). Here, we aimed at dissociating the oculomotor command and the velocity of corresponding saccades in order to better discriminate between these two hypotheses. To this end, we examined perisaccadic mislocalization in one patient with an autosomal dominant spinocerebellar ataxia (SCA Type 2) who exhibited markedly slowed saccades. We compared the dynamics of perceptual mislocalization in this patient with a control group of ten healthy subjects. A clear correlation between perisaccadic mislocalization strength and mean saccadic peak velocity was observed in the control group. The patient however exhibited a disproportionally strong compression with respect to her mean saccadic peak velocity. The observed dissociation supports the notion that extraretinal signals contribute to the phenomenon of perisaccadic mislocalization.