Perceptual Coherence : Hearing and Seeing

412 Perceptual Coherence
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Figure 9.17. Perceived location and onset asynchrony jointly determine spatial
ventriloquism. Using a single LED and one loudspeaker (short noise burst or
1000 Hz tone), the light and sound were perceived to come from the same position
as long as the actual spatial discrepancy was less than 4° regardless of the temporal
asynchrony. In similar fashion, the light and sound were perceived to come from
different positions as long as the actual spatial discrepancy was greater than 8°
regardless of the temporal asynchrony. Adapted from “Temporal and Spatial
Dependency of the Ventriloquism Effect,” by D. A. Slutsky and G. H. Recanzone,
2001, NeuroReport, 12, 7–10.
(e.g., a speaking face), spatial ventriloquism will occur at far greater temporal
disparities and spatial discrepancies. 8
Ventriloquism can serve the useful purpose of separating two intermixed
auditory messages. Driver (1996) created a particularly clever demonstration
of this. Participants had to shadow (verbally repeat the words of ) one
message composed of random words that was presented interleaved with
another random word message spoken by the same voice coming from
one loudspeaker. Only the visual lip movements on one television corresponding
to the target speech sounds specified the relevant message. In the
control condition, the loudspeaker and video monitor displaying the lip
movements were presented at the same spatial position. In the experimental
condition, the video monitor was spatially offset so that the lip movements
came from a different location (sketched in figure 9.18). The surprising
outcome was that performance was better in the experimental condition in
which the spoken words and lip movements were spatially offset.
8. These outcomes resemble those for the formation of auditory streams. Even though onset
asynchrony was the most important cue for auditory segregation, other factors would determine
segregation if the onset asynchrony was ambiguous (10–20 ms), not long enough to
dominate. Here, spatial disparity was the most important cue for visual capture, and temporal
disparity was only important if the spatial disparity was ambiguous.