Cancellation of gaze stabilizing mechanisms during human smooth pursuit: Indications for the involvement of an extra-retinal reference

Abstract

When our eyes are tracking a target that is moving in front of a structured background, background image motion is induced in the opposite direction. Usually, such global image motion is an appropriate stimulus to drive gaze stabilizing mechanisms, which in turn would counteract the voluntary eye movement. We have previously demonstrated, that during human smooth pursuit eye movements (SPEM) the sensitivity for full-field motion is reduced strongly — but only in the direction opposite to the eye movement (Vision Res. 2001; 41:1685–94). Although these results provided an explanation for the accuracy of SPEM despite self-induced motion the mechanisms underlying this asymmetry in global motion processing remained unclear.

To come up with an answer we employed brief and unexpected background motion (20°/s) during horizontal SPEM (10°/s). Background motion could be either ‘in-phase’ or ‘counter-phase’ with respect to the direction of SPEM. In exp. 1 the SOA of background motion was varied between 0, 100 and 200ms with respect to target motion onset (thereby varying relative motion between target and background during SPEM initiation). As in our previous experiments a background induced perturbation in SPEM velocity occurred in ‘in-phase’ conditions, only. Therefore this asymmetry significantly depended on the phase relation between background motion and SPEM direction (p<0.001) but not on the SOAs and related relative motion (p>0.5; ANOVA2). In exp. 2 the pursuit target was briefly extinguished during background motion. Although now any relative motion between target and background was absent the effect of ‘phase’ was still present (p<0.001) and did not significantly differ from trials without target extinction (p>0.5; ANOVA2).

The asymmetry in global motion processing cannot solely be attributed to a visual mechanism, i.e. relative motion. This implies that the cancellation of gaze stabilizing mechanisms critically depends on extra-retinal information on SPEM direction.

Footnotes

 Supported by DFG (GK Neurobiologie to AL; Heisenberg to UJI)