An adaptive algorithm for fast and reliable online saccade detection

Abstract

To investigate visual perception around the time of eye movements, vision scientists manipulate stimuli contingent upon the onset of a saccade. For these experimental paradigms, timing is especially crucial, because saccade offset imposes a deadline on the display change. Although efficient online saccade detection can greatly improve timing, most algorithms rely on spatialboundary techniques or absolute-velocity thresholds, which both suffer from weaknesses: late detections and false alarms, respectively. We propose an adaptive, velocity-based algorithm for online saccade detection that surpasses both standard techniques in speed and accuracy and allows the user to freely define the detection criteria. Inspired by the Engbert-Kliegl algorithm for microsaccade detection, our algorithm computes two-dimensional velocity thresholds from variance in the preceding fixation samples, while compensating for noisy or missing data samples. An optional direction criterion limits detection to the instructed saccade direction, further increasing robustness. We validated the algorithm by simulating its performance on a large saccade dataset and found that high detection accuracy (false-alarm rates of < 1%) could be achieved with detection latencies of only 3 ms. High accuracy was maintained even under simulated high-noise conditions. To demonstrate that purely intrasaccadic presentations are technically feasible, we devised an experimental test in which a Gabor patch drifted at saccadic peak velocities. Whereas this stimulus was invisible when presented during fixation, observers reliably detected it during saccades. Photodiode measurements verified that-including all system delays-the stimuli were physically displayed on average 20 ms after saccade onset. Thus, the proposed algorithm provides a valuable tool for gaze-contingent paradigms.

Intra-saccadic motion streaks as cues to linking object locations across saccades

“…Indeed, showed that contrast sensitivity to a grating drifting at only 100 dva/s decreased by almost one log unit when its spatial frequency was increased from 0.1 to 0.7 cpd, and that gratings drifting at 800 dva/s could only be detected if their SF was below 0.1 cpd. Moreover, Schweitzer and Rolfs (2019) found that Gabor patches (0.5 cpd and 0.5 dva SD Gaussian aperture) rapidly drifting within their aperture (thus not producing any motion streaks) became impossible to detect during fixation when drift velocities came close to or exceeded saccadic peak velocities (∼400 dva/s). Finally, Castet et al (2002) presented striking evidence that intra-saccadic gratings (with orthogonal orientation to the saccade direction, saccadic peak velocities around 280 dva/s) induced motion percepts if their SFs were 0.04 and 0.18 cpd, but not if their SF was 1.81 cpd, because the latter induced temporal frequencies of ∼500 Hz, which cannot be resolved by motion detectors.…”

“…A horizontal rightward saccade was only detected when its direction was in the range of 360°± 25°, whereas a leftward saccade had to be in the range of 180°± 25°. A more thorough description of the algorithm can be found in Schweitzer and Rolfs (2019).…”

“…With a mean online saccade detection latency (online detection relative to saccade onset detected offline) of 12.5 ms (Experiment 1), 12.5 ms (Experiment 2), 12.3 ms (Experiment 3), and 11.4 ms (Experiment 4), we achieved physical stimulus onsets of ∼27 ms after the onset of the saccade (Figures A1 and A2 in the Appendix). This latency already includes a deterministic video latency of the ProPixx projection system used which occurs because the presentation of the ProPixx projector (unlike a CRT monitor) updates only once the entire signal because the refresh of the graphics card is transferred (the duration of one refresh cycle at 120 frames per second; personal communication, Peter April, June 2018; see also Schweitzer & Rolfs, 2019).…”

“…To compute the physical onset and offset of the stimulus movement, we added the deterministic video latency of the projector of 8.3 ms (mentioned above) to the respective time stamps of the synchronization with the vertical blank (median latency for online saccade detection, 12 ms; median latency for physical stimulus onset relative to online saccade detection, 15 ms; median saccade duration, 62 ms). As it is crucial to determine the exact time of presentation during the saccade, we validated our procedure in a separate experiment using photodiode measurements (Schweitzer & Rolfs, 2019). Averaged across movement durations and participants, stimulus movements finished 24 ms (Experiment 1, SD = 6.7 ms), 25 ms (Experiment 2, SD = 6.6 ms), 29 ms (Experiment 3, SD = 6.7 ms), and 14 ms (Experiment 4, SD = 5.8 ms) before saccade offset.…”

Intra-saccadic motion streaks jump-start gaze correction

“…After successful fixation, an exogenous cue was presented to indicate the saccade target: The target stimulus -one of the six presented stimuli and one of the two types of noise patches -was enlarged linearly up to twice its initial size for 25 ms and then decreased for 25 ms until the initial size was restored. Saccades were detected online using the algorithm described by Schweitzer & Rolfs (2019) with parameters k=2, λ=10, and θ=40, on both eyes. As soon as the saccade was detected, the cued stimulus moved 30 deg in a clockwise (CW) or counterclockwise (CCW) direction for 14.6 msamounting to a distance traveled of 5.2 dva at a velocity of approximately 360 dva/s -or remained in its pre-saccadic location.…”