Journal of Vision - Motion perception during smooth pursuit eye movements: object and background motion perception depend on different non-retinal signals on eye velocity, by Lindner, Haarmeier, & Thier

The perception of target (=object) motion during steady-state smooth-pursuit eye movements (SPEM) in darkness must rely on non-retinal information on eye velocity ("reference signal", RS-object) as significant target image slip is missing. On the other hand, an eye movement reference signal (RS-background) is also needed in order to guarantee the perception of a stable background despite pursuit induced background image slip. This is the result of a subtractive comparison of background image slip and RS-b. We have recently shown that RS-b is continuously re-calibrated, based on visual information on its adequacy. Here we used our ability to change the size of RS-b in order to investigate, whether target and background motion perception share a common reference signal. Six human subjects participated in two experiments, in which we compared the perception of pursuit-induced background movement (experiment a) with the perception of object motion (experiment b) during SPEMs. In each experiment, "test" trials, used to measure the percept of object and background motion respectively, were presented randomly interleaved with a much larger number of "constant stimuli" (CS) trials, in which SPEM was carried out over structured backgrounds moving at constant velocities. These CS trials, mimicking a mismatch between RS-b and the retinal signal, allowed us to manipulate the size of RS-b, thereby altering the percept of background motion (Lindner et al. 2000). In accordance with our earlier observations, we found that the velocity of the background associated with subjective stationarity was shifted towards the velocity of the CS, reflecting a change in the size of the RS-b. However, perceived target velocity turned out to be independent of the size and direction of the CS. These findings indicate that the percept of object and background motion during SPEM depends on distinct reference signals.