**Participants**
We will collect data from ten naive participants, recruited through campus mailing lists. Each participant will be asked to complete three sessions of about 45 min each. They receive 7 Euros per session and a bonus of 4 Euros for completing all three sessions, resulting in a total of 25 Euros.
**Aparatus**
Stimuli will be projected onto a standard 16:9 (200 x 113 cm) video-projection screen (Celexon HomeCinema, Tharston, Norwich, UK), mounted on a wall, 270 cm in front of the participant. The projector is a ProPixx (Vpixx Technologies, Saint-Bruno, QC, Canada) running at 1440 Hz vertical refresh and a resolution of 960 x 540 pixels. The experimental code is implemented in MATLAB (Mathworks, Natick, MA, USA), using the Psychophysics and Eyelink toolboxes (Kleiner et al., 2007; Cornelissen et al., 2002) and runs on a Dell Precision T7810 Workstation with a Debian 8 operating system. Eye movements are recorded via an EyeLink 2 head-mounted system (SR Research, Osgoode, ON, Canada) at a sampling rate of 500 Hz. Responses are collected with a standard keyboard.
**Procedure**
Participants will complete three sessions of data collection, each consisting of 1400 trials, distributed over 10 blocks (140 trials per block). Each trial is preceded by a fixation check: a fixation spot (diameter: 0.15 degrees of visual angle, dva) is displayed at the center of the screen, and once fixation is detected in an area of 2.0 dva around the fixation spot for at least 200 ms, the trial starts. The fixation spot stays on the screen throughout the trial. After 50 to 100 ms of fixation, a Gabor stimulus appears either left or right of the screen center (chosen randomly on each trial), ramping up from zero to full contrast in 100 ms. Once at full contrast, the stimulus rapidly moves—in a curved trajectory such that it passes the center above or below the fixation spot with a maximum distance of 30% of the horizontal movement amplitude—towards the other side of fixation, following a certain velocity profile (see below), before ramping back to zero contrast in 100 ms. Once the stimulus has disappeared, the observer presses one of two buttons to indicate whether the stimulus moved in an upward or a downward curvature.
**Stimuli**
Stimuli were vertically-oriented Gabor patches (1 cycle per degree, sigma of the envelope: 1/3 deg), traveling on a motion path corresponding to an arc of a circle with a radius chosen such that the maximum deviation from a straight line was exactly 30% (reached at the center of the screen, right above or below fixation). The horizontal velocity is constant throughout the movement, corresponding either to the peak velocity of a saccade at that amplitude, a slowed-down, or a sped-up version of it (peak velocity multiplied by factors of 1/4, 1/3, 1/2, 1/1.5, 1/1.25, 1, 1.25). The peak velocity is determined using the parameters of the standard main sequence described by Collewijn (1988). The amplitude of the movement will be either 4, 6, 8, 10, or 12 dva.
Each combination of movement amplitude (4, 6, 8, 10, 12 dva), movement velocity (1/4, 1/3, 1/2, 1/1.5, 1/1.25, 1, or 1.25 times the saccadic peak velocity of a given amplitude), curvature direction (up or down), and motion direction (leftward or rightward), occurrs once per block of 140 trials (total number of blocks: 30). For all analyses, we intend to collapse across curvature directions and motion direction, resulting in a total of 120 trials per data point.
**Data analysis**
We will confirm successful fixation during each trial off-line, using standard procedures for (micro)saccade detection (Engbert & Mergenthaler, 2006). Trials with saccades larger than 1 dva during stimulus presentation will be excluded, as will trials with missing data (e.g. due to blinks).
All remaining trials will be included in subsequent analyses. For each combination of motion amplitude and velocity, stimulus visibility will be assessed by computing each observer's ability to correctly identify the curvature of the stimulus (up or down). We will compute both the percentage of correct reports and a bias-free measure of visual sensitivity (*d'*). Manual reaction times will be evaluated to rule out any speed-accuracy tradeoffs. Fixational eye movements may be inspected for potential traces of conscious detection (see White & Rolfs, 2016). Given the rapid succession of events in a trial, however, we do not expect a high rate of microsaccades.
