In a study of monkeys, researchers at Duke University Medical Center have found that the area of the brain that processes sounds entering the ears, inferior colliculus, also appears to process stimulus entering the eyes.
The researchers found that auditory and visual information is processed together before the combined signals make it to the brain's cortex, the analytical portion of the brain that assembles the stimuli from all the senses into coherent thoughts.
"In our experiments, we found that this structure, which had been assumed to mainly process auditory information, actually responds to visual information as well," said Dr. Jennifer Groh, a neurobiologist in Duke's Center for Cognitive Neuroscience.
"In fact, about 64 percent of the neurons in the inferior colliculus can carry 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," the researchers added.
Writing about the new findings in the online edition of the Proceedings of the National Academy of Sciences, the researchers said that it might be due to the same process that ventriloquism seems to work.
"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 said.
The researchers say that their findings provide new insights into how the brain takes in and assembles a multitude of stimuli from the outside world.
"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. The view has been that each of these areas processes the information separately and sends that information to the cortex, which puts it all together at the end," said Dr. Groh.
"Now, we are beginning to appreciate that it's not that simple. 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 functioning," Groh added.
The researchers are now conducting experiments to determine whether or not one of the senses influences how the other is perceived.