Changes in our early sensory experiences resulting from disease or environmental factors may shape our sensory perceptions

Nov 21 2005

Our brain's ability to combine sensory information from a single event - such as seeing an ambulance and hearing its siren - has been shown to speed our reactions, help us identify objects and heighten our awareness.

New research in animals suggests that it's unlikely that we're born with this ability. Instead, its development may depend on our sensory experiences during the early months of life.

"The way in which this ability develops has profound implications for those who are born blind or deaf, or who suffer from disorders such as autism and dyslexia in which early sensory processes are altered," said Mark Wallace, Ph.D., a neuroscientist at Wake Forest University School of Medicine. "Knowing how these brain circuits mature may one day be used to tailor treatment strategies for those who have problems in basic sensory processes."

Wallace and colleagues presented the results from two related studies at the 35th annual meeting of the Society for Neuroscience in Washington, D.C.

The goal of the studies was to learn more about multisensory integration, which refers to the brain's ability to combine information from our different senses. Although much is now known about how multisensory integration is carried out in "lower" brain regions such as the brainstem, little is known about multisensory integration in "higher" brain regions such as the neocortex - a region responsible for our perceptions.

The researchers studied individual neurons in the neocortex of cats to see how they respond to sight, sound and touch. Surprisingly, they found that many of the neurons could respond to stimuli in several of these senses.

"The neurons responses to combinations of sensory stimuli were often must greater than we predicted," said Wallace. "This suggests that these neurons have the capacity to greatly amplify their signals when confronted with stimuli from multiple senses."

He said this finding may explain how multisensory stimuli can lead to improvements in our perceptions - such as how seeing a friend speaking across a crowded and noisy room can help us better "hear" what he or she is saying.

In addition to studying these neurons in adult cats, the researchers also examined how multisensory neurons mature in the developing brain. They found that immediately after birth, multisensory neurons were not present in the neocortex. Only after several months of development did these neurons first appear, and they were strikingly immature, lacking the ability to amplify their signals. Several weeks later, these neurons began to acquire this multisensory capability.

Next, the researchers examined development in animals raised in an abnormal sensory environment in which lights and sounds were always presented at the same time, but from different locations. Wallace said this arrangement is at odds with the normal world, where sensory cues from a single event typically occur at the same time and place.

"Intriguingly, the neurons in animals raised in this strange sensory world tailored their integration to fit their environment," he said. "The neurons were able to amplify their signals only when visual and auditory stimuli were presented at different locations - locations that were very similar to those experienced during early life."

Wallace said the results show that multisensory processes in brain circuits mature slowly, a finding that dovetails with studies showing that perceptual processes are similarly immature early in human life. He said findings also suggest that the multisensory circuitry is highly malleable and reflects the sensory world of the newborn.

"This implies that changes in our early sensory experiences resulting from disease or environmental factors will likely have a substantial impact on brain development," he said.