A brain region that processes sight and sound simultaneously in monkeys could hold the key to explaining how ventriloquists create the illusion that their puppets can speak.
"The prevailing wisdom among brain scientists has been that each of the five senses—sight, hearing, smell, touch and taste—is governed by its own corresponding region of the brain," said study team member Jennifer Groh, a neurobiologist at Duke University in Durham, N.C. "Now we are beginning to appreciate that it's not that simple."
The finding, detailed online this week in the journal for the Proceedings of the National Academy of Sciences, offers new insights into how the brain builds a complete picture of the world based on input from our senses. It could also help shed light on synesthesia, a rare neurological condition in which two or more senses are intertwined.
Brain's Grand Central
By studying rhesus monkeys, the researchers found that the inferior colliculus, a tiny structure in the brain known to be important for hearing, can respond simultaneously to visual input from the eyes and sound information from the ears.
The inferior colliculus is less than a half-inch in diameter and is one of several early stops for signals flowing from the ear to the cortex, the analytical part of the brain that stitches sense stimuli together into coherent thoughts. Previous studies had shown that the inferior colliculus also receives inputs from nerve cells in the retina.
The structure is located in one of the most primitive areas of the brain, suggesting the ability to multitask with the senses has deep evolutionary roots.
"Our results show that there are interactions between the sensory pathways that occur very early in the process, which implies that the integration of the different senses may be a more primitive process and one not requiring high-level brain function," Groh said.
Experiments by Groh's team show that about 64 percent of the neurons in the inferior colliculus can transmit visual as well as auditory signals.
"This means that visual and auditory information gets combined quite early, and before the 'thinking part' of the brain can make sense of it," Groh said.
Fooling the brain
This has implications for ventriloquism. In ancient times, the ability to throw one's voice was associated with magic and witchcraft and the ability to commune with the dead. Nowadays, it is mainly viewed as a neat trick, and aspiring magicians can learn it by following instructions found on the Web.
While the trick is no longer a secret, how ventriloquism fools the brain has remained unknown. The new study suggests that the association between the voice and the moving mouth of the puppet dummy is made before the viewer consciously thinks about it.
"The eyes see the lips moving and the ears hear the sound, and the brain immediately jumps to the conclusion about the origin of the voice," Groh explained.
The same process also explains why the words being spoken by someone on TV appear to be coming out of their mouths, even though the television speakers are located to the side of the set, Groh said.
The team suspects there are undiscovered regions in the brain where other senses are processed simultaneously. The findings suggest that everyone is a "synesthete" to some degree.
"It must be the case that some amount of intermingling of the senses is normal," Groh told LiveScience. "And there must be something different about synesthetes, maybe in the degree to which the connections are intertwined."
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