Eyelink 1000 plus

Pupil size and eye position were continuously recorded at a sampling frequency of 500 Hz using an EyeLink 1000 Plus (SR Research).

Stimuli were generated using MATLAB (MathWorks, Natick, MA) and the Psychophysics Toolbox.^{76 ,77} Stimuli were presented at a viewing distance of 57 cm on a 523 × 300 mm large ViewPixx3D Full Monitor (VPixx Technologies) with a spatial resolution of 1920 × 1080 pixels and a refresh rate of 120 Hz. Eye movements were recorded with an Eyelink-1000 Plus device (SR Research, Ottawa, ON, Canada) in tower-mount configuration using the EyeLink Toolbox.⁷⁸ (link) Only the left eye was tracked.
Stimuli were presented on a gray background. The initial fixation cross was presented in black color in the center of the screen with a width and height of 0.5 degrees of visual angle (dva). The 5 possible saccade target positions were given by white circles with a radius of 0.25 dva arranged in a circle equally spaced around the center of the screen (radius = 10 dva). The selection of a position was determined in two steps. First, gaze is required reach the target area, which means it has to exceed a circle around the center of the screen (radius = 7.5 dva) and second the position closest to gaze was selected.

Testing occurred at the University of Montreal (Montreal, Canada) and at the Centre of Neuroscience Research of Lyon (CNRL; Lyon, France) with similar apparatus for eye-movement recording. Participants sat in a dark room 57 cm away from a high-speed computer screen (at CNRL: 15.7*11.8 inches, Visual Stimulus Generator ViSaGe, Cambridge Research System, Rochester, UK; at the University of Montreal, 20.5*11.5 inches, VIEWpixx 3D, VPixx Technologies, Montreal, Canada). Head movements were restricted with chin and forehead rests during the task. An eye-tracker, set in a binocular tower-mount, recorded eye movements (at CNRL: ViSaGe, Cambridge Research System, Rochester, UK, frequency: 250 Hz; at the University of Montreal: EyeLink 1000 Plus, SR Research, Kanata, Canada, frequency: 1000 Hz).

Participants placed their heads in a chin and forehead rest and viewed stimuli at a distance of ∼64 cm at 29.7 × 22.3 degrees visual angle. The experiment was controlled via Psychtoolbox (Kleiner et al., 2007 ) and MATLAB (MathWorks, Natick, MA, USA). Gaze data were acquired using an EyeLink 1000 Plus eye tracker (SR Research, Ottawa, Canada) at a frequency of 2 kHz.

Each trial consisted of a drift correction screen and a stimuli screen (see Fig. 1). The drift correction screen was presented as a small white target on a gray background (#999999). The participant would gaze directly at the target and confirm by pressing the space key. Next, two video stimuli appeared on the right and left sides of the screen (each video occupied 16.5% of the screen space with 480 × 270 px resolution). During the 2000 ms presentation of the stimuli, eye movements were recorded using EyeLink 1000 Plus (SR Research). After that, a new trial began with the appearance of a drift correction screen.Figure 1

Trial structure.