Perceptual Coherence : Hearing and Seeing

408 Perceptual Coherence
then any pair of inputs will generate different output excitations, depending
on the amount of temporal overlap of the pair. 5
Why then do we perceive simultaneity at most distances? One possibility
is that there is a long temporal integration window. Based on single-cell
recordings, if the auditory and visual neural signals occur within 100 ms of
each other, that input will be sufficient to fire neural cells that integrate firings
in different modalities (Meredith, Clemo, & Stein, 1987). This implies
that the source of the auditory and visual stimulus energy could be +/−
30 m or more from each other, based on the overlap of activation patterns
(Meredith et al., 1987). Sugita and Suzuki (2003) have suggested an alternative
to a wide integration duration, namely that the integration region
shifts as a function of the perceived distance of the source. In their task, observers
judged whether an LED light source positioned from 1 to 50 m in
front of them was presented before or after a burst of white noise presented
by headphones. They found that when the LEDs were 1 m away, the noise
needed to be delayed by 5 ms to be perceived as synchronous, but if the
LEDs were 40 m away, the noise had to be delayed by 106 ms to be perceived
as synchronous. The participants thus expected the sound to occur
later relative to the light as the distance increased. There are huge but stable
individual differences in this sort of task. With 17 participants, the range of
synchrony judgments was 170 ms (Stone et al., 2001).
Perceived Rhythmic Synchrony (Temporal
Ventriloquism)
In the research described below, the auditory input is a series of discrete
short tones while the visual input may be a continuous light or a series of
discrete short light flashes. Here, the onset of a visual target is perceived to
occur at the onset of a sound even if the timings are quite different. This illusionary
percept has been termed temporal ventriloquism to create a parallel
to the classical term spatial ventriloquism, in which a sound is perceived
to come from the spatial location of the visual object. 6
One type of temporal ventriloquism occurs when the amplitude modulation
of a tone induces an illusionary modulation of a light. For example, if a
single visual flash is presented simultaneously with two or more short auditory
beeps, the light flash appears to oscillate on and off two times (Shams,
Kamitani, & Shimojo, 2000). The oscillation occurs even if the beep onset
is delayed by 70 ms, but disappears if the onset delay is 100 ms or more.
5. This is yet another example of a population code, analogous to those for orientation or
auditory frequency. The perceptual information is found in the distribution of the responses,
not the magnitude of any single output.
6. For temporal ventriloquism, the observer’s task is to judge the occurrence or rhythm of
an event. For spatial ventriloquism, the observer’s task is to judge the location of an event.