In the non-memory vigilance control block, the trial sequence was identical to the memory block with the exception of the probe stimulus. On control trials, in place of a probe stimulus, after the delay period, a grating stimulus (30 ms) was presented followed by a backward mask (a plaid, tilted at 45°, 120 ms). The grating stimulus was a vertically oriented square-wave grating with a maximum 3 cycles per degree of contrast, subtending 4° of visual angle. Subjects were instructed not to remember the target stimulus but instead to determine whether the grating was tilted clockwise (50% of trials, randomly interleaved) or counterclockwise from the vertical. Performance difficulty was dynamically adjusted on a trial-by-trial basis using five possible magnitudes of orientation changing logarithmically from 0.3° to 5° to the vertical. Trial-by-trial adjustments on control trials were based on subjects’ performance on memory trials.
