14 DAY TRIAL // In many cases what you see is not what you hear and what you hear is not what you see. This is how many tricks work, like ventriloquists making people believe the dummy is speaking. Now, a monkey brain has revealed us how this can happen: due to a brain nucleus involved in sight and sound simultaneously.
"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. Now we are beginning to appreciate that it's not that simple," said Jennifer Groh, a neurobiologist at Duke University in Durham, N.C. and co-author of the research published in the journal for the Proceedings of the National Academy of Sciences.
The new research could explain synesthesia, a rare neurological disease combining two or more senses. In synesthesia, some people, for example, perceive colors when musical notes are played. One very common form of synesthesia is the grapheme-color synesthesia, in which letters or numbers (graphemes) are highlighted with particular colors.
The team investigated the brain of rhesus monkeys, discovering that the inferior colliculus, a minute nucleus (less than 0.5 in (1 cm) in diameter) involved in hearing can be activated at the same time by visual and auditive signals.
The inferior colliculus is one of several stations for information flowing from the ear to the cortex, the brain part turning stimuli into sensations. Previous researches had revealed that the inferior colliculus also receives signals from nerves coming from the eye. Colliculus is part of one of the most primitive brain parts, thus multitask ability could be extremely old in mammalian evolution.
"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," said Groh.
The new tests revealed that approximately 64 % of the brain cells in the inferior colliculus can process both visual and 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," said Groh.
This explains ventriloquism, which was associated with magic, witchcraft and the ability to commune with the dead, while now is known to be a mere trick easy to learn by anybody who is interested and now the explanation has come. It seems that the link between the voice and the moving mouth of the dummy occurs before we consciously perceive them as visual and auditory sensations.
"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. 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," explained Groh.
Other brain areas too could be involved in processing simultaneously senses and it appears that everybody has a degree of synesthesia.
"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."