Brain's Sensory Signals Active Even in Background Noise If We are Attentive

by Rajashri on Sep 27 2009 1:48 PM

A new study by researchers at the Salk Institute for Biological Studies says that when we pay attention on something, the sensory signals in our brains make themselves heard even in background noise.

While studying visual attention, the researchers have discovered a novel mechanism that explains how incoming sensory signals stand out amidst the constant background rumblings, so they can be reliably processed and passed on.

"We live with the illusion that our visual system processes all the information that is available in the visual scene in a single glimpse. In reality, there is far too much detail in a typical scene for the visual system to take it in all at once. So our perception of the world around us is in a sense pieced together from what we pay attention to," says Dr. John H. Reynolds.

It was already known that paying attention to visual details increases the firing rate of neurons tuned for attended stimulus.

To date, it has been assumed that these attention-dependent increases in neural activity are the primary cause of the improvement in perceptual discrimination that we experience when we focus a sensory stimulus.

But the new findings have revealed that the uptick in the firing rate is only a small part of the story.

"What we found is that attention also reduces background activity. We estimate that this noise reduction increases the fidelity of the neural signal by a factor that is as much as four times as large as the improvement caused by attention-dependent increases in firing rate. This reduction in noise may account for as much as 80% of the attention story," said Dr. Jude Mitchell.

When light hits the retina, visual information is translated into a cascade of nerve impulses sending signals deep into the brain.

And it is in the brain's visual cortex, which resides in the occipital lobe at the back of the skull, that these signals are interpreted and give rise to perception.

But the visual system has limited capacity and cannot process everything that falls onto the retina.

Instead, the brain relies on attention to bring details of interest into focus so that it can select them out from background clutter.

But when the researchers measured the activity of a large population of visual neurons in animals trained to play a simple video game that required rapt attention to a visual stimulus on the screen, they observed that the internal fluctuations or shared noise quieted down, increasing the visibility of the incoming sensory information.

"Attention is an essential part of perception. Brain disorders in which attention fails therefore have devastating effects. Gaining insight into the neural mechanisms of attention is essential if we are to understand the causes of these perceptual deficits and find ways to treat them. By revealing a major new attentional mechanism, Jude has taken a major step toward understanding the neural mechanisms of conscious awareness," said Reynolds.

The study has been published in the journal Neuron.