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Abstract:
The prefrontal (PF) cortex is a collection of cortical areas
in the most anterior portion of the frontal lobes. It has long been
associated with high-level, "executive" processes needed for
voluntary goal-directed behavior. Its damage in humans results in
disturbances in a variety of functions, including attention,
memory, response selection, planning, and inhibitory control. The
neural mechanisms it contributes to these processes, however, have
been enigmatic. Complex behavior requires that we use acquired
knowledge to focus on and integrate potentially important sensory
inputs and stored memories so that appropriate actions can be
selected. Here, I will present data from neurophysiological
experiments designed to gain insight into these processes.
One line of research involves the role of the prefrontal cortex
in selective attention, that is, the ability to voluntarily focus
awareness on certain sensory inputs, thoughts, or actions. We find
that many prefrontal neurons had properties consistent with a role
in allocating attention. Before search commenced, they maintained
information about which object was the sought-after target. Soon
after search initiation, their activity reflected selection of the
target, sooner than in similar studies in the visual cortex. This
suggests that the prefrontal cortex may be a source of the top-down
signals that orchestrate the allocation of attention.
Complex behavior, however, depends on more than selectively
attending important sensory inputs. To benefit from past
experience, we also must be able to select (recall) stored
knowledge. We explore this by training monkeys on associative
memory tasks. Over weeks, they learn an association between a
visual cue and another visual stimulus or an action. We have found
that when the cue is presented, many prefrontal neurons began to
convey information about the associated stimulus or action well
before that stimulus appears or the action commences. This ability
to prospectively code a forthcoming stimulus or action by recalling
it from long-term memory is crucial for planning behavior. Once a
sensory input or memory is selected it must be coordinated with
other information common to the goal at hand. We have explored the
role of the PF neurons in integrative functions by studying their
ability to link information about an object's form and color with
its location in visual space. In several studies, we have shown
that when monkeys are explicitly required to link an object's
features with its location, as many as half of the neurons in the
lateral prefrontal cortex convey both visual attributes.
Central to all these tasks, and all complex behaviors, is
control. How do we determine what sensory inputs and memories are
relevant, what needs to be integrated, and what actions are
required? This is addressed in another line of research. Control is
ultimately derived from "rules" acquired through experience and the
ability to form arbitrary associations is fundamental to rule
learning. We have explored the role of the prefrontal cortex in
this process. We found that as monkeys learn an arbitrary
association between a cue and an action, information about them
gradually merges together in prefrontal activity.
Overall, our studies have shown that, consistent with their
putative role in the highest level of cognitive function, PF
neurons have complex response properties that are highly dependent
on, and shaped by, task demands.
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