WASHINGTON – Whether we are awake or asleep, watch TV or close our eyes, waves of spontaneous nerve signals are bombarding our brains. Now, researchers have discovered a unique mechanism that explains how incoming sensory signals make themselves heard amid constant background din 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 John H. Reynolds, professor at the Salk Institute and senior study author.

“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,” Reynolds says.

Attention generally increases the firing rate of responsive neurons (nerve cells). 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. You are not hearing it very clearly at low volume because ambient noise is masking the stimulus. As you increase the volume, the signal becomes clearer,” says Reynolds.

Researchers had known for some time that paying attention to visual details increases the firing rate of neurons tuned for attended stimulus.

These findings reveal that the up tick in the firing rate is only a small part of the story. “What we found is that attention also reduces background activity,” says study co-author Jude Mitchell.

“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 percent of the attention story,” says Mitchell.

These findings were published in the Thursday issue of Neuron.