Abstract
During active visual exploration, saccadic eye movements rapidly shift the visual image across the human retina. Although these high-speed shifts occur at a high rate and introduce considerable amounts of motion smear during natural vision, our perceptual experience is oblivious to it-a phenomenon known as saccadic omission. Using tachistoscopic displays of natural scenes, we rendered saccade-induced smear highly conspicuous. By manipulating perisaccadic display durations, we studied the dynamics of smear in a time-resolved manner, assessing discrimination performance of smeared scenes, as well as smear amount and orientation. Both measures showed distinctive, U-shaped time courses throughout the saccade, indicating that generation and reduction of smear occurred during saccades. Moreover, low spatial frequencies and orientations parallel to the direction of the ongoing saccade were identified as the predominant visual features accessible in motion smear. We explain these findings using computational models that assume no more than saccadic velocity and human contrast sensitivity profiles and present a motion-filter model capable of predicting observers’ perceived amount of smear based on their eyes’ trajectories, suggesting a direct link between perceptual and saccade dynamics. Replays of the visual consequences of saccades during fixation led to virtually identical results as actively making saccades, whereas the additional simulation of perisaccadic contrast suppression heavily reduced this similarity, providing strong evidence that this suppression explained neither our findings nor the phenomenon of omission. Saccadic omission of motion smear may be understood as emerging naturally from the interplay of the retinal consequences of saccades and early visual processing.
We rarely become aware of the immediate sensory consequences of our own saccades, that is, a massive amount of motion blur as the entire visual scene shifts across the retina. In this paper, we applied a novel tachistoscopic presentation technique to flash natural scenes in total darkness while observers made saccades. That way, motion smear induced by rapid image motion (otherwise omitted from perception) became readily observable. With this setup we could not only study the time course of motion smear generation and reduction, but also determine what visual features are encoded in smeared images. Low spatial frequencies and, most prominently, orientations parallel to the direction of the ongoing saccade. Using some cool computational modeling, we show that these results can be explained assuming no more than saccadic velocity and human contrast sensitivity profiles. To demonstrate that motion smear is directly linked to saccade dynamics, we show that the time course of perceived smear across observers can be predicted by a parsimonious motion-filter model that only takes the eyes’ trajectories as an input. And the best thing about it: This works even if no saccades are made and the visual consequences of saccades are merely replayed to the fixating eye! In the name of open science, all modeling code, as well as data and data analysis code, is again publicly available.
Richard Schweitzer
Postdoc in Vision Science
Passionate about psychophysics, eye tracking, M/EEG, and computational modeling.