There's a brain region in monkeys that processes sight and sound simultaneously and offers new insights into how the brain builds a complete picture of the world based on input from our senses.
The finding, detailed online this week in the journal for the Proceedings of the National Academy of Sciences, could hold the key to explaining how ventriloquists create the illusion their puppets can speak and help shed light on synesthesia, a rare neurological condition in which two or more senses are intertwined.
"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 researchers found by studying rhesus monkeys 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.
"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. "This means that visual and auditory information gets combined quite early, and before the 'thinking part' of the brain can make sense of it."
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.