Figure caption. (A) Overall SRT distribution for all 22 participants of correct (―) and incorrect (- - -) trials in the Bright target condition (red) and the Dim target (blue) condition. Shaded area denotes the standard error across participants. (B) Cross-species comparison of the average minimal visual SRT as a function of target luminance of our human participants to the previously published (Marino et al. 2015) awake monkey experimental minimal visual SRT (▲) for 7 luminance levels. The filled markers (●/■) represent the statistical method of determining the human minimal visual SRT (stats) (n = 17; 9 female), the open markers (○/□) depict the human minimal visual SRT when assessing the cumulative distribution qualitatively (kink) (n = 16; 9 female).
We describe a novel behavioral method to accurately discriminate anticipatory (i.e., saccades are not generated by visual input) from visually triggered saccades and to identify the minimal visual saccadic reaction time (SRT). We compare this method with previously published electrophysiological finding in monkeys (Marino et. al. 2015). This behavioral method can be used to calculate a feasible lower bound cut-off for latencies of visually triggered saccades within a certain experimental context or participant group. We apply this method to compute the minimal visual SRT for two different saccade target luminance levels. Three main findings are presented: 1) the minimal visual SRT for all participants was 46 ms shorter for bright targets than for dim targets, 2) the transition from non-visually triggered to visually triggered saccades occurred abruptly, independent of target luminance, and 3) although the absolute minimal visual SRTs varied between participants, the response pattern (response to bright targets being faster than to dim targets) was consistent across participants. Based on these results, we argue that differences in the minimal visual SRT can easily occur when stimuli vary in luminance or other saliency features. Applying an absolute cut-off (i.e., 70 to 90 ms) that approaches the minimal neuronal conduction delays, which is general practice in many labs, may result in the wrongful inclusion of saccades that are not visually triggered. It is suggested to assess the lower SRT bound for visually triggered saccades when piloting an experimental set-up and before including saccades based on particular latency criteria.
Jessica Heeman, Stefan Van der Stigchel, Douglas P. Munoz & Jan Theeuwes (2019) Discriminating between anticipatory and visually triggered saccades: Measuring minimal visual saccadic response time using luminance. Journal of Neurophysiology.