In a crowded room of people talking, a few voices singing in unison will quickly drown out the noise. NIMH scientists have discovered that the brain uses a similar principle to enable its neurons engaged in critically important tasks to win out over neurons essentially processing distractions. In the 23 February 2001 Science, Robert Desimone, Ph.D., Pascal Fries, Ph.D., and Laboratory of Neuropsychology (LNP) colleagues, suggest that synchronous neuronal firing may be a fundamental mechanism for boosting the volume of brain signals representing behaviorally relevant stimuli.
Desimone and colleagues pinpointed the responses of brain neurons in monkeys paying attention to a particular stimulus and ignoring nearby distracters. Using electrophysiological recordings, they monitored groups of neurons in the visual processing area at the back of the brain (V4). They discovered that neurons activated by features of the attended stimulus conspicuously synchronized their activity in the gamma (40-90 hz) range, which roughly corresponds to the hum of an electrical power outlet or fluorescent light. Meanwhile, neurons activated by features of the distracters, in effect, continued to speak with disparate voices.
The researchers suggest that such synchronous ensemble firing produces an amplifying effect that telegraphs the oscillating signal representing the attended stimulus to downstream processing areas in the brain. These areas, such as the inferior temporal cortex, will, in turn pass on the signal to brain areas involved in the highest levels of visual cognition and awareness, creating, in effect, a chain reaction through the visual processing pathways.
"Disorders of attention are common components of various mental illnesses," noted Desimone, who serves as both Chief of the LNP and as Director of the NIMH Intramural Research Program. "We're beginning to appreciate that the things that go awry in the brain may have to do with the timing of neuronal signals - with the neuronal choir going out of sync." He suggested that future studies might examine possible underlying mechanisms, such as the role of neuromodulators, such as acetylcholine, in synchronizing activity of neuronal ensembles.
Also participating in the research were John Reynolds, Ph.D., NIMH LNP and The Salk Institute for Biological Studies, and Alan Rorie, Ph.D., NIMH LNP.