Eyelink 1000 desktop mount system

Eye movements were recorded using an EyeLink 1000 Desktop mount system (SR-Research, Ottawa, ON, Canada). It was equipped with the 2000 Hz camera upgrade, allowing for binocular recordings at a sampling rate of 1000 Hz for each eye. Data from the right eye were analysed. The experiment was implemented with the SR Research Experiment Builder software.
Each participant viewed 150 colour photographs of real-world scenes (Fig. 1a), which were presented in random order. The scene images were displayed on a 21-inch CRT monitor at a screen resolution of 800 × 600 pixels (width × height). Head position and viewing distance were fixed at 90 cm from the screen using a chin rest. Accordingly, scenes subtended 25.78° × 19.34°. Before the onset of each scene, a central fixation check was performed. Afterwards, the scene was displayed for 6 s during which participants were free to move their eyes. To provide a common task across participants, they were informed that, on a given trial, they would view a real-life scene and that this may be followed by a question asking them to recall a specific detail of the scene. On 30 trials, a test question asking about the presence or absence of a particular object appeared after scene presentation to probe participants’ scene encoding.

Using the EyeLink® 1000 Desktop Mount system (SR Research Ltd., Ontario, Canada), eye position and eye movements were determined by measuring the corneal reflection and dark pupil with a video-based infrared camera and an infrared reflective mirror. The eye tracker had a spatial resolution of 0.01° of the visual angle, and the signal was sampled and stored at a rate of 1000 Hz. Although the viewing was binocular, the recording was monocular (a standard procedure in eye-tracking studies). Calibration and validation of the measurements were performed before each experimental session.

Twenty-two participants (mean age: 32.5 years; 11 women, 11 men) with normal or corrected-to-normal vision took part in the study. Eighteen participants reported to be right-handed (4 left-handed, all women). All participants gave their informed consent before the experiment. The Psychology Department at the University of Edinburgh granted ethics approval for the study, which conformed to the Declaration of Helsinki.
Stimuli were presented on a 21-inch CRT monitor at a viewing distance of 90 cm, with each scene image subtending a visual angle of 25.78° horizontally × 19.34° vertically. The experiment was implemented in SR Research Experiment Builder. Participants used a four-button Microsoft Sidewinder controller to indicate that they had found the target object.
Eye movements were recorded with an SR Research EyeLink 1000 Desktop mount system, which was equipped with the 2000 Hz camera upgrade, allowing for binocular recordings at a sampling rate of 1000 Hz for each eye. A chin rest with head support minimized head movement. We placed the chin rest such that the participant’s midpoint between the centers of the eyes was aligned with the vertical midline of the screen. We adjusted table height to place the participant’s straight-ahead view at the midpoint of the screen.

Eye tracking was done using an Eyelink 1000 Desktop Mount system (SR Research, Ontario Canada) recording the right eye at 1000 Hz. Participants placed their head on a headrest supported under the chin and forehead to keep a stable position. Participants underwent a 13 point calibration/validation procedure at the beginning of each run (1 Baseline run, 1 Conditioning run and 3 runs of Generalization). The average mean-calibration error across all runs was Mean = 0.36°, Median = 0.34°, SD = 0.11. 91% of all runs had a calibration better than or equal to. 5°.
Fixation events were identified using commonly used parameter definitions (Wilming et al., 2017 (link)) (Eyelink cognitive configuration: saccade velocity threshold = 30°/second, saccade acceleration threshold = 8000° per second², motion threshold = 0.1°). Fixation density maps (FDMs) were computed by spatially smoothing (Gaussian kernel of 1° of full width at half maximum) a 2D histogram of fixation locations, and were transformed to probability densities by normalizing to unit sum. FDMs included the center 500 × 500 pixels, including all facial elements where fixations were mostly concentrated (~95% of all fixations).