intrasaccadic perception
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.
In this piece we showed that the visual traces that moving objects induce during saccades can facilitate secondary saccades in both accuracy and saccade initiation latency. Secondary saccades are typically prompted when one saccade does not entirely reach a target or when the saccade target is displaced in mid-flight. Our results provide evidence against the widely acknowledged notion that our brains preemptively discard visual information which reaches the eye during saccades. The paper has received some peer and media attention, such as a well-written commentary by Jasper Fabius and Stefan van der Stigchel, as well as articles in Nature Research Highlights, AAAS, New Scientist, or Vozpópuli (see the Rolfslab's blog post for the full list). Notably, this study is the first one to apply the new TrackPixx eye tracking system, for which I have written a Matlab toolbox.
Is intra-saccadic vision merely an epiphenomenon or could visual information that reaches the eye during saccades be used by the visual system? That was the question of my cumulative doctoral dissertation, which features not only a synopsis of all studies conducted up to this point, but also a review of the saccadic-suppression and motion-streak literature to put these findings into context. The dissertation has been awarded two prizes – the Humboldt Prize and the Lieselotte Pongratz-Promotionspreis by Studienstiftung des Deutschen Volkes (see also the short movie).
Whenever we make a saccade to an object, that object will travel from the periphery to the fovea at extremely high velocities. Depending on the visual features of the object, such motion can induce streaks, that may serve as visual clues to solve the problem of trans-saccadic object correspondence. Using a high-speed projection system operating at 1440 fps, we investigated to what extent human observers are capable of matching pre- and post-saccadic object locations when their only cue was an intra-saccadic motion streak, and compared their performance during saccades to a replay of the retinal stimulus trajectory presented during fixation. Note that a toolbox for parsing Eyelink EDF files was implemented in R to analyze this series of experiments, which can be found here.
To study intrasaccadic vision, we need stimulus manipulations that occur strictly during saccades. Due to the brief durations of saccades, this can prove a difficult task, as various system latencies (eye tracker, refresh cycle, video delay, and some more) have to be considered. While most of these delays are hardware-dependent, one opportunity to alleviate timing issues in gaze-contingent eye-tracking paradigms is applying an efficient online saccade detection! In this paper we described such an algorithm, validated it in simulations and experiments, and made it publicly available so that it can be used with a range of different programming languages.
