Reverse ventriloquism
We can be tricked into trusting our ears over our
eyes.
03 February 2004
LAURA NELSON
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| Who said that? Follow your ears, not your
eyes. |
| © Corbis |
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Ventriloquists should be jealous. Scientists have explained their trick
in detail, and have managed to produce the reverse effect - where people are
tricked into believing their ears over their eyes1.
Researchers have long known that people place different amounts of faith
in their different senses. This is exploited by ventriloquists, who fool us
into thinking sound is coming from someplace it isn't by relying on the fact
that people use their vision more heavily than their hearing to locate the
source of a sound. This is because the eye's retina is very sensitive to the
direction of light that hits it, while the ear isn't so sensitive to the
direction of a noise.
"The cinema is the classic ventriloquist effect," says David Burr at the
University of Florence in Italy, who co-authored the study. We assume the
voices are coming from the actors on the screen, he explains, instead of from
the loudspeaker somewhere else in the room.
It is commonly thought that vision will always dominate over hearing in
such situations. But Burr and colleagues have now shown this isn't true.
Instead, both visual and auditory cues are weighted by the brain, with one cue
compensating for the other when it is less reliable: "What you're more
confident about, you give more weight," says Burr.
More than meets
the eye
The researchers first assessed how good our eyes are at noticing small
movements in stimulus, as compared to how good our ears are at the same thing.
To do this, they first flashed two circles onto a computer screen, one after
the other, and asked viewers which circle was further to the left. They then
played two clicks through loudspeakers, one after the other, and asked
listeners which sound came from further to the left. As expected, people were
much better at locating the circles than they were at locating the sound.
The team then combined visual and auditory stimuli into a single
experiment -- a circle and a click happened at the same time, followed by a
second set of a circle and a click. The participants were asked which set of
stimuli was further to the left.
Unbenownst to the participants, the clicks and circles weren't
necessarily coming from the same place. Sometimes the circles moved to the
left, while the sounds moved to the right, or vice versa.
When the circles were small and defined, participants relied on their
sight rather than their hearing to gauge the direction of movement. But,
intriguingly, when the circles were large and fuzzy, observers were more likely
to believe their ears over their eyes, and used the sound to judge the
direction of movement. The authors call this effect "reverse ventriloquism",
and it occurs because large, blurred blobs are perceived as being less
trustworthy than small, sharply defined ones.
Using the data gathered from such experiments, Burr's team was able to
calculate how much weight the brain puts on visual and auditory stimuli in
different situations - though they don't yet know how the brain manages to do
so. When both stimuli were perfectly evenly weighted, the team found that the
two senses would work together to produce a better judgement of motion than
when only one sense was called upon.
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