Subjects adjusted the rate of a repeating toneburst to match that of a concurrently flashing light, or vice versa. Flashes were viewed in luminous or wholly dark surrounds. Matches usually departed from veridical rate matches, and always were affected in the same direction by changes in surround luminance. Matches were a function of whether subjects controlled the visual or auditory stimulus; also, subjects usually reported "driving" of flash rates by auditory rates when they controlled tone rate, but not when they controlled flash rate.When an observer views a repeatedly flashing light and simultaneously hears a repeated clicking sound at repetition rates around 4-10 Hz, the apparent rates of the auditory and visual stimuli may appear markedly different from each other, even when the flash and click rates are identical and are produced in synchrony by means of a single pulse-train generator. While making informal observations of this effect, the authors noted that the apparent rate of the flashes instantaneously increased when the room in which they were viewed was darkened, and decreased again when lights were restored. These effects were duplicated in pilot studies in which subjects matched the rate of a flashing light by adjusting the rate of an independently variable clicking sound.There are a number of studies in the literature that deal with the apparent rate or frequency of repetitive visual and/or auditory stimuli (Forsyth