Dr. Desimone's most recent work has focused
on the neural mechanisms that promote or limit the ability
to respond selectively to one stimulus seen in a multitude
of stimuli. Humans' ability to perform such selective
filtering is as limited as it is crucial to normal, everyday
function. His lab is attempting to understand the neural
substrate of those limitations and, potentially, the source
of individual differences in this important ability.
Neural systems for visual processing can
focus attention on behaviorally relevant objects, filtering
out competing distractors. Neurophysiological studies
in animals and brain imaging studies in humans suggest
that such filtering depends on top-down inputs to extrastriate
visual areas, originating in structures important for
attentional control. In order to carry out his research
program, Dr. Desimone's laboratory uses a variety of techniques,
including recordings from single and multiple neurons
in various regions of the primate brain, analysis of high-frequency
(gamma band) activity and synchronization in the primate
brain, and sophisticated behavioral paradigms, particularly
ones designed to assess visual spatial attention.
To optimize the limited capacity for selective
filtering of incoming inputs, he suggested that the brain's
attentional mechanisms must give priority to behaviorally
relevant stimuli-usually at the expense of irrelevant
stimuli. He has demonstrated that in several visual areas
of the primate brain, attended stimuli induce enhanced
responses and an enhanced synchronization of rhythmic
neuronal activity in the gamma frequency band (40-70 Hz).
Both of these effects are likely to improve the signal-to-noise
ratio associated with attended stimuli, both within any
one brain region and among different brain regions.
Attention also results in improved behavioral
performance, as assessed by accuracy and shortened reaction
times. Although that result has been replicated many times
over the years, it was not known how reaction times were
related to either response strength or gamma-band synchronization
in visual areas. Dr. Desimone showed that behavioral response
times to a change in stimulus can be predicted by the
degree of gamma-band synchronization among those neurons
in monkey visual area V4 that are activated by the behaviorally
relevant stimulus. When there are two visual stimuli and
monkeys have to detect a change in one stimulus while
ignoring the other, their reactions are fastest when the
relevant stimulus induces strong gamma-band synchronization
before and after the change in stimulus. This enhanced
gamma-band synchronization is also followed by shorter
neuronal response latencies on the fast trials. Conversely,
the monkeys' reactions are slowest when gamma-band synchronization
is high in response to the irrelevant distractor. Thus,
enhanced neuronal gamma- band synchronization and shortened
neuronal response latencies to an attended stimulus seem
to have direct effects on visually triggered behavior,
reflecting a very early neuronal correlate of efficient
visuomotor integration.
