Rising above the din: Attention makes sensory signals stand out amidst the backg

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Rising above the din

Attention makes sensory signals stand out amidst the background noise in the

brain

http://www.eurekalert.org/pub_releases/2009-09/si-rat092209.php

LA JOLLA, CA—The brain never sits idle. Whether we are awake or asleep, watch TV

or close our eyes, waves of spontaneous nerve signals wash through our brains.

Researchers at the Salk Institute for Biological Studies studying visual

attention have discovered a novel mechanism that explains how incoming sensory

signals make themselves heard 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, " says H.

Reynolds, Ph.D., an associate professor in the Systems Neurobiology Laboratory

at the Salk Institute and senior author of the current study. " 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. "

Researchers had known for some time that paying attention to visual details

increases the firing rate of neurons tuned for attended stimulus. Until now, it

was assumed that these attention-dependent increases in neural activity were the

primary cause of the improvement in perceptual discrimination that we experience

when we focus a sensory stimulus.

The findings of the Salk researchers, published in the September 24, 2009 issue

of the journal Neuron, reveal 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, " says postdoctoral researcher and first author Jude , Ph.D.

" 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. "

When light hits the retina, visual information is translated into a cascade of

nerve impulses sending signals deep into the brain. It is here, 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 it can select them out from background clutter.

In their study, Reynolds, , and former graduate student Kristy Sundberg

asked whether attention, which so efficiently tunes out external distractions,

does the same for the internal racket. Attention generally increases the firing

rate of responsive neurons: The stronger the stimulus, the more impulses are

sent per second, which improves the quality of the signal somewhat. " It's a

little bit like turning up the volume from very low to high on a stereo, " says

Reynolds. " You are not hearing it very clearly at low volume not only because

the signal is weak but because ambient noise is masking the stimulus. As you

increase the volume, the signal becomes clearer. "

But even under the most controlled laboratory conditions, the responses evoked

by identically repeated stimuli vary from trial to trial. " Neurons are very

noisy computing devices, " says . " Each neuron receives input from

thousands of neurons and needs to distinguish the incoming information from the

background noise. "

If each neuron produced random noise that is independent from what its neighbor

neuron is doing, the brain cell on the receiving end could simply pool all

incoming signals and average out the noise. Reynolds compares it to diversifying

risk in a stock portfolio: " If you have a portfolio of stocks whose prices vary

independently, you can reduce fluctuations by dividing your investment among a

large pool of stocks. "

Unfortunately, for neurons this option is off the table since most of the

brain's background noise originates in waves of spontaneous nerve signals that

undulate across a large population of brain cells. Says , " These

fluctuations can't be simply averaged out since they are shared across the

neural population. " To extend the investment analogy, say you put your money

into a pool of real estate investments. Your portfolio is subject to

fluctuations in the real estate market – the correlated fluctuations in the

values of individual investments – no matter how big the pool.

But an interesting thing happened 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. The

internal fluctuations or shared noise quieted down, increasing the visibility of

the incoming sensory information.

" Attention is an essential part of perception, " says Reynolds. " 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. "

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This work was supported in part by the National Institutes of Health and the

National Science Foundation.

About the Salk Institute for Biological Studies

The Salk Institute for Biological Studies is one of the world's preeminent basic

research institutions, where internationally renowned faculty probe fundamental

life science questions in a unique, collaborative, and creative environment.

Focused both on discovery and on mentoring future generations of researchers,

Salk scientists make groundbreaking contributions to our understanding of

cancer, aging, Alzheimer's, diabetes, and cardiovascular disorders by studying

neuroscience, genetics, cell and plant biology, and related disciplines.

Faculty achievements have been recognized with numerous honors, including Nobel

Prizes and memberships in the National Academy of Sciences. Founded in 1960 by

polio vaccine pioneer Jonas Salk, M.D., the Institute is an independent

nonprofit organization and architectural landmark.