Reaction times in the detection of gratings by human observers: A probabilistic mechanism

Abstract

Reaction times were measured for sinusoidal gratings which were flashed on with various temporal waveforms. The contrast was close to threshold. At low spatial-frequencies, the reaction times were grouped just after any sudden transient in the stimulus, even when this was at the end of the stimulus. At higher spatial-frequencies, the reaction times were not related to the time of sudden changes in contrast but were distributed throughout the body of the stimulus; the longer the stimulus duration, the greater was the chance that the stimulus would be detected. These results can be explained if a stimulus can be detected at any time when the visual system's response to it is moderately high and not simply at the time when the response is greatest. At low spatial-frequencies, the channels have transient step-responses; at higher frequencies, the responses are sustained.