Our research followed the tenets of the Declaration of Helsinki and was approved by the institutional review boards at the University of Massachusetts Boston and the Schepens Eye Research Institute.
Two authors (GL and TG) and two naïve subjects (ET and RP) participated in the study. Stimuli were presented on a 21-inch Dell P1130 monitor with a resolution of 1024 × 768 pixels and a refresh rate of 100 Hz. The monitor spanned 44° × 33° at the observation distance of 36 cm. Eye movements were recorded using an EyeLink II eye-tracker system (SR Research, Ottawa, Canada), which samples at 500 Hz and has an average accuracy of 0.5°. All experiments were performed in a normally lit room (688 lux). Screen background was black (6.5 cd/m2), and stimuli included a fixation marker (1° crosshair, red, 33 cd/m2), a saccade target (1° round dot, red, 33 cd/m2), and a flashed bar (1°, 4°, or 8° wide and 0.5° high, white, 120 cd/m2).
Each trial started with fixation at 10° left of screen center (
Figure 1). After a random delay (1000 to 2000 ms), the fixation cross disappeared and the saccade target appeared at 10° right of screen center for 600 ms, at which point observers made a saccade to the saccade target. Sometime between 100 ms before and 100 ms after the anticipated saccade onset, a horizontal bar was flashed for one frame centered at one of six possible locations along the horizontal midline (−10, −5, 0, 5, 10, and 15° relative to the horizontal screen center). Following their saccades, participants reported the perceived location of the bar by using a standard PC mouse to click at the two end points of the perceived bar. The cursor was only visible during the report phase.
A baseline test was also performed in which the identical procedure was followed, except that subjects instead fixated on the cross at −10° during the presentation of the flashed bar.