13 Nisan 2012 Cuma

Quotations from "The Prefrontal Cortex: Minireview"

My quotations from: The Prefrontal Cortex: Minireview
Complex Neural Properties for Complex Behavior
Earl K. Miller*
Department of Brain and Cognitive Sciences and The Center for Learning and Memory
Massachusetts Institute of Technology
Cambridge, Massachusetts 02139
The PF cortex is a collection of cortical areas in the most anterior portion of the frontal lobes (Figure 1). It has long been associated with high-level, “executive” processes needed for voluntary goal-directed behavior. Its damage in humans does not produce a single, characteristic deficit. Rather, it results in disturbances in a variety of functions, including attention, memory, response selection, planning, and inhibitory control.

Studies of the neural basis of PF function in monkeys focused primarily on active short-term, or working memory. They have revealed that when a delay is imposed between a visual stimulus and a response based on it, many PF neurons show sustained stimulus-related activity (Goldman-Rakic, 1994; Fuster, 1995). Because sensory inputs are often fleeting, this short-term maintenance may be fundamental to many cognitive functions. However, complex behavior requires more than temporary storage. Relevant sensory inputs need to be selected and integrated with other information common to the goal at hand. Also needed are the executive mechanisms that determine, for example, which stimuli are relevant and should be selected. Much less is known about the neural basis of these higher functions.
In many views of cognition, the type of executive control thought to be mediated by the PF cortex is synonymous with attentional selection, that is, the ability to voluntarily focus awareness on certain sensory inputs, thoughts, or actions. Selection is necessary because higher-order cognitive functions are severely limited in capacity. Indeed, at a given moment we are aware of only a small fraction of available sensory information.

Given these capacity limitations, the ability to ignore distractions and select behaviorally relevant information is critical. Evidence that the lateral PF cortex is involved in selection processes comes from a number of studies, including a recent study by Rainer et al. (1998b)
Earlier target selection in the PF cortex suggests that the visual cortex may “inherit” target information from the PF cortex. That is, that the PF cortex may be a source of the top-down signals that mediate attentional selection in the visual cortex (Desimone and Duncan.)

Complex behavior, however, depends on more than selecting sensory information. To benefit from past experience, we must be able to select (recall) stored knowledge. Watanabe (1996) demonstrated this ability in PF neurons. He found that when monkeys could predict that a specific reward would appear (e.g., raisins, cabbage, etc.), the activity of many PF neurons reflected that expected reward. This ability to prospectively” code predicted events is thus critical for choosing among response alternatives.

Sensory processing is highly fragmented. Even within a modality, different stimulus attributes may be separately processed. The primate cortical visual system, for example, is thought to analyze the form and color information to identify a stimulus (i.e., what it is) largely from information about stimulus location (i.e., where it is). An area involved in complex behavior, however, needs to have access to diverse information.

Even actions almost invariably require satisfying multiple, diverse constraints. When I search for my coffee cup, for example, I have in mind not only what it looks like but also where it is likely to be. So, somewhere and somehow, diverse information such as what and where needs to come together. Given its role in organizing behavior and its extensive connections, the PF cortex seems a likely region where integration might be evident, particularly when integration is needed for behavior.
Other studies have also found that many PF neurons process both object identity and location.
Interconnections between different PF regions (Figure 1) could result in a population of PF neurons with multimodal properties.

Associative Learning, Rules, Context, and Cognitive Control
The complexity of primate behavior is partially attributable to the fact that primates can acquire new goals and manners of achieving them. Not surprisingly, the PF cortex is thought to be central to this ability. Its executive role in brain function has been hypothesized to result from the acquisition and representation of “rules” that guide goal-directed behavior.

Rule learning depends on forming arbitrary associations between disparate, but behaviorally related, information. We learn that “red” means “stop,” for example. The PF cortex seems well positioned to play a role in associative learning. It is interconnected with all sensory systems, with the motor system, and with limbic structures involved in long-term memory and affect; it is truly “association cortex.”
Thus, PF activity can reflect stimuli, associated actions, and expected consequences.
Cohen and colleagues have suggested how the ability of PF neurons to reflect conjunctions of behaviorally related information might result in mechanisms for executive control. They posit that control emanates from representation of context (Cohen and Servan-Schreiber, 1992). Context is the constellation of task-relevant information needed to guide a given behavior.
Top-down signals from the PF cortex are excitatory and represent the to-be-attended item. These signals increase activity of neurons that process the relevant information and, by virtue of the mutual inhibition, suppress activity of neurons processing irrelevant information. Context representations may act in a similar fashion. However, rather than convey only visual attributes, context representations are thought to be multimodal and include information about stimuli, actions, etc., that have become associated through experience. Thus, they can bias motor as well as sensory processing and also allow appropriate actions to be selected and executed.

Recent 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. They selectively process and integrate information needed for a common behavioral goal. Thus, its extensive connections and the ability of its neurons to be modified by experience may allow the PF cortex to play a role in knitting together behaviorally relevant associations, a process needed for acquisition of “rules” that guide goal-directed behavior. This may result in a representation of context, a template of a previously successful configuration of sensory and response-related information that biases processing in other brain regions in favor of task-relevant information. Such complicated and malleable activity would be expected for a region so closely linked with the complexity and flexibility that are the hallmarks of primate behavior.