Visual Stimulus Generation

All visual stimuli were displayed at a resolution of 1024x731 pixels and a video frame rate of 100 Hz on either a Nanao T550i or Eizo T550 monitor. Look-up tables were used to correct for nonlinearities in the relation between input voltage and phosphor luminance in the monitors. Stimuli were monochromatic and were generated by one of two different systems. Gratings for basic characterization (orientation, spatial and temporal frequency, and size) and for the surround timing experiments were presented by a Cambridge Research Systems VSG 2/2 board (Kent, UK) running on an Intel x86-based host computer. All other grating and dot stimuli were presented by a Silicon Graphics workstation. The mean luminance of the display was approximately 33 cd/m$^2$ when displaying gratings. All of the gratings were presented in a circular aperture surrounded by a gray field of the average luminance. Gratings were shown at 100% contrast except when two gratings were combined to make a plaid (where they were each shown at 50% contrast) or when contrast was an experimental variable.