An adaptive algorithm for fast and reliable online saccade detection

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, as saccade offset imposes a deadline on the display change. Although efficient online saccade detection can greatly improve timing, most algorithms rely on spatial-boundary techniques or absolute-velocity thresholds, which both suffer from their respective weaknesses: late detections and false alarms. 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 detection criteria. Inspired by the Engbert-Kliegl-algorithm for microsaccade detection, our algorithm computes two-dimensional velocity thresholds from variance in 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 milliseconds. High accuracy was maintained even under simulated high-noise conditions. To demonstrate that purely intra-saccadic presentations are technically feasible, we devised an experimental test, in which a Gabor patch drifted at saccadic peak velocities. While this stimulus was invisible when presented during fixation, observers reliably detected it during saccades. Photodiode measurements verified that - including all system delays - stimuli were physically displayed on average 20 ms after saccade onset. Thus, the proposed algorithm provides valuable tool for gaze-contingent paradigms.