**Participants.** 10 participants will be recruited through word of mouth and campus mailing lists. Each participant will complete two sessions of approximately 1.5 hours (plus one hour prior to the experimental session for preparing the EEG recording). Participants will receive 8 Euros per hour and 2 Euros as a bonus for the completion of both sessions. We will obtain written informed consent from all subjects prior to inclusion in the study. **Apparatus.** The experiment will take place in a dimly lit, sound-attenuated, and electromagnetically shielded cabin. A Propixx DLP Projector (Vpixx Technologies, Saint-Bruno, QC, Canada) running at a temporal resolution of 1440 frames per second and a spatial resolution of 960 x 540 pixels projects into the cabin onto a 200 x 113 cm screen (Celexon HomeCinema, Tharston, Norwich, UK). The projector is connected to the experimental host-PC via a Datapixx3 (Vpixx Technologies, Saint-Bruno, QC, Canada). Observers will be seated at a distance of 180 cm away from the projection screen with their head supported by a chin rest. Stimulus display will be controlled using the PsychProPixx function from PsychToolbox (Pelli, 1997; Kleiner et al., 2007) running in Matlab 2016b (Mathworks, Natick, MA, USA) on a custom-build desktop computer with an Intel i7-2700K eight-core processor, 8 GB working memory, and a Nvidia GTX 1070 Ti graphics card, running Ubuntu 18.04.1 (64-bit) as operating system. Eye tracking will be performed using an Eyelink 1000+ desktop base system, tracking participants’ dominant eye at a sampling rate of 2000 Hz. Tracking will be controlled during the experiment using the Eyelink Toolbox (Cornelisen, Peters, & Palmer, 2002). We will collect EEG data (64 EEG + 2 Mastoid + 4 EOG, see figure below) from an actiChamp EEG amplifier (Brain Products, Gilching, Germany) at a sampling rate of 2500 Hz. To synchronize eye tracking and EEG data, we apply a DB-25 Y-splitter cable to simultaneously send triggers of 1.1 ms duration to the Eyelink host computer and EEG host computer. To temporally align EEG and Eyelink data, we will use the EYE-EEG Toolbox (Dimigen, Sommer, Hohlfeld, Jacobs, & Kliegl, 2011) in EEGLAB (Delorme & Makeig, 2004). ![EEG-EOG montage][1] *Figure 1.* EEG and EOG montage. We will record from 70 electrodes in total (64 EEG, 2 Mastoid, 4 EOG). Reference electrode is left mastoid (not shown). **Stimuli.** The target stimulus will be random noise patches (pixel noise) of low spatial frequencies, that is bandpass-filtered from 0.25 to 1 cycles/dva with 100% contrast. Bandpass-filtered noise patches, initially of 3 dva diameter, will be enveloped in a Gaussian window with a standard deviation of 0.5 dva. The same noise patch will be used for each participant in all trials. Fixation markers will be white squares with black center of 0.4 dva width and height. **Procedure.** Each trial will start with the presentation of the fixation marker (left or right side of screen at 8 dva horizontal eccentricity relative to screen center) and the target stimulus (right or left side of screen at 8 dva horizontal eccentricity, respectively) displayed on a grey background (Figure 2, 1st panel). After 400 ms of successful fixation within a circular boundary with a radius of 2 dva around the center of the fixation marker (trial abort after 2 seconds of not fixating or 100 refixations), the fixation dot will disappear, constituting the cue for the participant to make a saccade to the target stimulus (Figure 2, 2nd panel). As soon as the saccade is detected online (see *Online saccade detection*), the target will be displaced to 4 different locations at 6 dva eccentricity around the initial target location (see dashed placeholders in Figure 2, 3rd panel, and *Design*) or remain at its initial location. Depending on whether the stimulus movement should produce an intra-saccadic motion streak, the displacement will either be a linear, continuous movement of the stimulus or a blank screen for 25 ms duration (36 frames at 1440 fps). After the primary saccade is concluded, the participant will make a secondary saccade to the displaced target stimulus. If this secondary saccade is not concluded by 450 ms after stimulus movement offset (gaze position inside of a 3-dva boundary around the final stimulus location), feedback will be presented to the participant that a secondary saccade has not been registered (see video of task procedure). Furthermore, feedback will be given if gaze position did not fall within a 2.5 dva circular boundary around the saccade target ('Saccade did not reach the target area'), if 2 or more saccades were made ('Please make one saccade (you made XX)'), or if a wrong response key was pressed ('Wrong response key pressed.'). Trials, in which any of these events were detected, will be appended to the list of trials in each run. One session will be subdivided into 4 runs, each containing 2 blocks of 250 trials. After all session are concluded, participants will be asked about their thoughts on the purpose of the experiment and, most importantly, whether they were aware of motion streaks during saccades. ![Task procedure.][2] *Figure 2*. Illustration of task procedure. Video of task procedure at 50 fps with the mouse cursor representing gaze position: https://osf.io/7qxp6/ **Online saccade detection.** Online saccade detection will be performed using the algorithm described by Schweitzer & Rolfs (2019) with the parameters θ=30, λ=8, and k=3. In each trial, all samples collected since the beginning of the fixation onset were used to compute the two-dimensional velocity thresholds. **Design.** Each experimental session will consist of 2000 trials (500 trials per run), thus there will be 4000 trials in total. In each experimental cell, there will be 200 repetitions. The number of trials is determined by the following experimental factors: - Movement direction (5 levels). Noise patches can be displaced upwards, downwards, in saccade direction, against the saccade direction, or remain static. - Availability of continuous stimulus movement (2 levels). Stimuli will travel for 25 ms (36 frames at 1440 fps) across a distance of 6 dva, amounting to a speed of ~240 dva/s. Their movement is either continuous (i.e., producing a streak) or apparent (i.e., blanked and re-appearing at target location after 25 ms). - Saccade direction (2 levels). Saccades (16 dva instructed horizontal amplitude) will be either rightward or leftward. **References** Cornelissen, F. W., Peters, E. M., & Palmer, J. (2002). The Eyelink Toolbox: eye tracking with MATLAB and the Psychophysics Toolbox. Behavior Research Methods, 34(4), 613-617. Delorme, A., & Makeig, S. (2004). EEGLAB: An open source toolbox for analysis of single-trial eeg dynamics including independent component analysis. Journal of Neuroscience Methods, 134(1), 9–21. Dimigen, O., Sommer, W., Hohlfeld, A., Jacobs, A. M., & Kliegl, R. (2011). Coregistration of eye movements and eeg in natural reading: Analyses and review. Journal of Experimental Psychology: General, 140(4), 552. Engbert, R., & Mergenthaler, K. (2006). Microsaccades are triggered by low retinal image slip. Proceedings of the National Academy of Sciences, 103(18), 7192-7197. Kleiner, M., Brainard, D., Pelli, D., Ingling, A., Murray, R., & Broussard, C. (2007). What’s new in Psychtoolbox-3. Perception, 36(14) Pelli, D. G. (1997). The videotoolbox software for visual psychophysics: Transforming numbers into movies. Spatial Vision, 10(4), 437–442. Schweitzer, R., & Rolfs, M. (2019). An adaptive algorithm for fast and reliable online saccade detection. Behavior Research Methods. doi: 10.3758/s13428-019-01304-3 [1]: https://mfr.de-1.osf.io/export?url=https://osf.io/jasrk/?action=download&mode=render&direct&public_file=False&initialWidth=848&childId=mfrIframe&parentTitle=OSF%20%7C%20Channel_locs_64Ch_EOG.jpg&parentUrl=https://osf.io/jasrk/&format=1200x1200.jpeg [2]: https://mfr.de-1.osf.io/export?url=https://osf.io/kyz96/?action=download&mode=render&direct&public_file=False&initialWidth=848&childId=mfrIframe&parentTitle=OSF%20%7C%20instruction_figure_SDEC.png&parentUrl=https://osf.io/kyz96/&format=600x600.jpeg