Task schematic and exemplary results.
A, Trials started with a fixation cue and a target 10 degree to the left (black lines). After 400 ms of stimulus overlap (shaded area), the fixation was switched off, serving as go signal to perform a saccadic eye movement to the target (exemplary eye trace in horizontal plane, gray line). Saccade onset triggered a target blank for 250 ms (JUMP task), after which the target reappeared with unpredictable offset. Subjects indicated apparent jump direction. B, In the VISUAL task, subjects kept initial fixation while the peripheral target and an additional flanker object (vertical bar) was presented at one of three distances [at 1.2, 2.8 (this trial) and 4.4 deg left of target; blue continuous, dashed and dotted lines, respectively]. The fixation dot then disappeared and 200 ms later, the target-flanker pair was briefly switched off and reappeared in the center field of view, mimicking the visual consequences of an intervening saccade to the target. Critically, the target-flanker distance was now changed (shown only for actual flanker position with fainter color corresponding to the unchanged relative position) and subjects reported the apparent change in relative position of the flanker with respect to the target. C, In the COMBI task, stimuli and instructions were identical to the JUMP task, apart from the additional flanker object that was presented together with the target at one of three target-flanker distances [at 1.2, 2.8 (this trial) and 4.4 deg left of target; green continuous, dashed and dotted lines, respectively]. D-F, Psychometric functions of one naïve subject. Proportion of trials in which subject reported an apparent target jump in saccade direction (forward), plotted against relative displacement levels. Negative values refer to target displacements against saccade direction. Circle sizes represent the number of trials for a given target jump. Cumulative Gaussians were fitted to perceptual response data, separately for different conditions [JUMP (D), VISUAL (E), COMBI (F)] and the three different target-flanker distances in VISUAL and COMBI condition (1.2 deg, continuous lines; 2.8 deg, dashed lines; 4.4 deg, dotted lines). Raw data for medium and large distance are not shown for the sake of clarity.