Figure 1. Timing and synthesis of visual and auditory trials.

The stimuli in both modalities were created from the same random sequence (duration 6.25 s) filtered to have uniform power between 0–80 Hz. For the visual stimulus, the sequence described the luminance of a peripheral disc. For the auditory stimulus, the sequence described the loudness of a 1000 Hz carrier tone. Oddball targets were presented on a random 20% of trials to maintain a state of vigilance. The target was a square appearing at the center of the disk with a barely noticeable decrease of luminance for the visual stimulus, and a slight decrease in the carrier frequency for the auditory stimulus. For both visual and auditory trials, the target duration was 1 s, and the onset timing was randomized evenly along the stimulus, excluding the very first and last seconds.