Abstract
During the active visual exploration of the world rapid eye movements, so-called saccades, shift the visual image across the human retina. Although these high-speed shifts introduce considerable amounts of intra-saccadic motion smear, our perceptual experience is oblivious to it – a phenomenon termed saccadic omission. Yet, remaining unaware of saccade-induced motion smear during natural vision does not entail that it cannot be perceived in principle. Here we triggered tachistoscopic displays of natural scenes upon the onset of saccades to render smear, that would otherwise be omitted from perception, highly conspicuous. By systematically manipulating display durations we studied the dynamics of smear in a time-resolved manner, assessing identification performance of smeared scenes, as well as the perceived smear amount and direction. Both measures showed distinctive, U-shaped time courses throughout the saccade, indicating that not only generation but also reduction of perceived smear occurred during saccades. The spectral content of the scene strongly modulated these time courses, as did presenting only parts of the scene of varying sizes. Using reverse-regression analyses we further identified low spatial frequencies and orientations parallel to the direction of the ongoing saccade as the predominant visual features encoded in motion smear. We show that this finding can be explained by early visual models that assume no more than saccadic velocity and human contrast sensitivity profiles. Finally, we 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. Saccadic omission of motion smear may be conceptualized as a parsimonious visual mechanism that emerges naturally as a consequence of 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. In the name of open science, all modeling code, as well as data and data analysis code, is again publicly available. The manuscript currently under review.
Richard Schweitzer
Postdoc in Vision Science
Passionate about psychophysics, eye tracking, M/EEG, and computational modeling.