The experimental program was run in Matlab (Math-Works, Natick, MA), using the Psychophysics toolbox (Brainard, 1997; Pelli, 1997; version 3.0.8) on Windowsrun PCs. Participants performed the task in dimly lit rooms. Stimuli were presented on 1900 CRT monitors set at 1024 768 resolution and 85 Hz refresh rate. Using a head-rest, participants’ distance from the display was fixed at about 45 cm.