Motion perception of saccade-induced retinal translation

Abstract

Active visual perception relies on the ability to interpret correctly retinal motion signals induced either by moving objects viewed with static eyes or by stationary objects viewed with moving eyes. A motionless environment is not normally perceived as moving during saccadic eye movements. It is commonly believed that this phenomenon involves central oculomotor signals that inhibit intrasaccadic visual motion processing. The keystone of this extraretinal theory relies on experimental reports showing that physically stationary scenes displayed only during saccades, thus producing high retinal velocities, are never perceived as moving but appear as static blurred images. We, however, provide evidence that stimuli optimized for high-speed motion detection elicit clear motion perception against saccade direction, thus making the search for extraretinal suppression superfluous. The data indicate that visual motion is the main cue used by observers to perform the task independently of other perceptual factors covarying with intrasaccadic stimulation. By using stimuli of different durations, we show that the probability of perceiving the stimulus as static, rather than moving, increases when the intrasaccadic stimulation is preceded or followed by a significant extrasaccadic stimulation. We suggest that intrasaccadic motion perception is accomplished by motion-selective magnocellular neurons through temporal integration of rapidly increasing retinal velocities. The functional mechanism that usually prevents this intrasaccadic activity from being perceived seems to rely on temporal masking effects induced by the static retinal images present before and/or after the saccade.

Fig 1.

Methods. (a) Eye position signal for a typical 6° saccade. (b) Eye velocity profile (left ordinate) and corresponding retinal temporal frequency for a 0.18 c/° grating presented continuously (right ordinate). (c) Schematic of experimental method. A static grating is displayed for 19 ms on each trial around the time of saccade latency. The difference between grating and saccade onsets (t) is +10 ms in trial 1 and −15 ms in trial n. Gratings of optimal low spatial frequency appearing near saccade onset (t ≈ 0 ms) produce vivid motion perception against the saccade direction.

Fig 2.

Results of experiment 1 for three observers (LQ was a naïve subject). (a) Probability of perceiving motion for static gratings presented at different times relative to saccade onset. Motion perception occurs with low spatial frequencies (circles and diamonds), whereas it is abolished with high spatial frequencies (triangles). Error bars show standard errors (). (b) Distributions of the durations of saccades kept in the analysis. (c) Distributions of peak velocities.

Fig 3.

Results of experiment 2 for two observers. Contrast of the 0.18 c/° grating was randomized across trials (from 10% to 100% in steps of 10%). Only 100% contrast data are presented here (squares) to allow comparison with fixed 100% contrast data replotted from Fig. 2a (circles). Error bars show standard errors.

Fig 4.

Results of experiment 3 for three observers (EP was a naïve subject). Stimuli and methods were the same as in experiment 1 except for the duration of the grating (25, 37.5, or 50 ms). Data from experiment 1 (duration = 19 ms) have been included in this analysis. Grating onset could be either before saccade onset (Left) or within the saccade (Right). Abscissa indicates the proportion of the saccade duration that was stimulated by the grating. Note the reversed axis for right-hand graphs. Error bars show standard errors.