Auditory-visual temporal integration measured by shifts in perceived temporal location.

The perceived time of occurrence of a visual stimulus may be shifted towards the onset of an auditory stimulus occurring a short time later. The effect has been attributed to auditory-visual temporal integration although an unknown portion of the shift may be explained by the different processing times of visual and auditory stimuli. Here, perceived onset time is measured in a novel way that separates and compares the magnitude of these effects. Participants observed either a sequence consisting of a visual stimulus followed by an auditory stimulus and then another visual stimulus or the reverse. The temporal location of the second stimulus was varied systematically between the onset of the first and third stimuli, which were separated by a fixed duration. Two timescales were used: a short timescale that allowed for auditory-visual temporal integration to occur, and a long timescale that did not. Psychometric curves were fitted for both timescales, to the percentage the first interval was perceived is shortest, as a function of first interval duration. For the long timescale condition the point of subjective equality (PSE) of the two interval lengths was consistent with the different processing latencies. When visual and auditory stimuli occurred within 125 ms significant additional shifting of the PSE occurred. These results indicate that temporal integration shifts the perceived timing of a visual stimulus by an amount much larger than can be explained differential processing latencies.