All stimuli were generated using Matlab (MathWorks) and Psychtoolbox 3 (Brainard, 1997 (link); Pelli, 1997 (link)) running on an Apple Macintosh computer. Stimuli were displayed on two 21-inch CRT monitors (Sony Trinitron Multiscan G520) with a spatial resolution of 1280 × 1024 pixels and a frame rate of 75 Hz. Each eye of the observer viewed only one monitor at a viewing distance of 42 cm using a haploscopic setup. The two monitors were positioned to the left and right of the observer with their screens facing each other. Observers viewed the screens through two mirrors placed at 45° angles in front of the eyes. Head position was stabilized with a chin-and-forehead rest. Responses were recorded with mouse clicks and keyboard button presses.
Throughout all experimental procedures, the background of the screen was dark gray (14.6 cdm−2). The fixation point was a black (1.11 cdm−2) annulus with a white (103.5 cdm−2) center with a diameter of 0.5° of visual angle and was placed vertically in the center of the screen and horizontally 7.7° left or right from the center of the screen. The fixation point was visible to both eyes. In addition, a centered red square frame (side length 25°) was presented to both eyes to help observers fuse the two screen's images.
Throughout all experimental procedures, the background of the screen was dark gray (14.6 cdm−2). The fixation point was a black (1.11 cdm−2) annulus with a white (103.5 cdm−2) center with a diameter of 0.5° of visual angle and was placed vertically in the center of the screen and horizontally 7.7° left or right from the center of the screen. The fixation point was visible to both eyes. In addition, a centered red square frame (side length 25°) was presented to both eyes to help observers fuse the two screen's images.