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ScienceWeek
NEUROSCIENCE: FRONTAL CORTEX AND COGNITIVE CONTROL
The following points are made by K.R. Ridderinkhof et al (Science 2004 306:443):
1) Flexible goal-directed behavior requires an adaptive cognitive control system for selecting contextually relevant information and for organizing and optimizing information processing. Such adaptive control is effortful, and therefore it may not be efficient to maintain high levels of control at all times. Convergent evidence suggests that the posterior medial frontal cortex (pMFC) and lateral prefrontal cortex (LPFC) are important contributors to cognitive control. The authors focus on the role of the pMFC in performance monitoring, especially in situations in which pMFC activity is followed by performance adjustments.
2) Evaluating the adequacy and success of performance is instrumental in determining and implementing appropriate behavioral adjustments. For instance, detection of a performance error may be used to shift performance strategy to a more conservative speed/accuracy balance. The authors develop the tentative hypothesis that one unified function of the pMFC is performance monitoring in relation to anticipated rewards. The monitored signals may index the failure (errors or negative feedback) or reduced probability (conflicts or decision uncertainty) of obtaining such rewards, and as such signal the need for increased control.
3) Although the pMFC can also be activated by positive events (such as rewards) [1,2], the authors focus on negative events and their consequences. Because errors and conflicts are intrinsically negative, and because unfavorable outcomes are typically more consequential for the regulation of cognitive control than are favorable outcomes, the authors focus on the role of the pMFC in monitoring negative events.
4) Electrophysiological recordings in nonhuman primates implicate the pMFC in monitoring performance outcomes. Distinct neuron populations in the pMFC, particularly in the supplementary eye fields and the rostral cingulate motor area (CMAr), are sensitive to reward expectancy and reward delivery [1,3,4]. In addition, CMAr neurons exhibit sensitivity to unexpected reductions in reward [5]. Likewise, specific groups of neurons in the depth of the cingulate sulcus (area 24c) react to response errors and to unexpected omissions of rewards [5]. These findings are consistent with a role for these neuronal populations in comparing expected and actual outcomes.
5) In summary: Adaptive goal-directed behavior involves monitoring of ongoing actions and performance outcomes, and subsequent adjustments of behavior and learning. The authors evaluate new findings in cognitive neuroscience concerning cortical interactions that subserve the recruitment and implementation of such cognitive control. A review of primate and human studies, along with a meta-analysis of the human functional neuroimaging literature, suggests that the detection of unfavorable outcomes, response errors, response conflict, and decision uncertainty elicits largely overlapping clusters of activation foci in an extensive part of the posterior medial frontal cortex (pMFC). The authors draw a direct link between activity in this area and subsequent adjustments in performance. Emerging evidence points to functional interactions between the pMFC and the lateral prefrontal cortex (LPFC), so that monitoring-related pMFC activity serves as a signal that engages regulatory processes in the LPFC to implement performance adjustments.
References (abridged):
1. M. Shidara, B. Richmond, Science 296, 1709 (2002)
2. B. Knutson, G. W. Fong, C. M. Adams, J. L. Varner, D. Hommer, Neuroreport 12, 3683 (2001)
3. V. Stuphorn, T. L. Taylor, J. D. Schall, Nature 408, 857 (2000)
4. S. Ito, V. Stuphorn, J. W. Brown, J. D. Schall, Science 302, 120 (2003)
5. K. Shima, J. Tanji, Science 282, 1335 (1998)
Science http://www.sciencemag.org
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Related Material:
ON THE FUNCTIONS OF THE HUMAN FRONTAL LOBES
Notes by ScienceWeek:
The human cerebral hemispheres (collectively termed the "cerebrum") represent 85 percent of the brain by weight, and for nearly two centuries one sustained research effort, involving a large number of researchers, has been to identify which parts of the cerebral hemispheres are involved with which mental functions. Such identifications must be made carefully and in context, since essentially every part of the brain is directly or indirectly connected to every other part, with all parts in principle capable of interaction. Still, for certain higher functions, a high degree of localization is apparent.
Apart from their large size in humans, what is most evident about the human cerebral hemispheres is the high degree of convolution, a tortuous array of foldings of tissue, one consequence of which is an enormous increase in surface area. This increase in surface area is of some significance, since the entire convoluted surface of the hemispheres comprises a laminated rind of neurons and supporting cells approximately 2 millimeters thick, the rind called the "cerebral cortex". The total surface area of the cerebral cortex comprises approximately 1.6 square-meters, and it is within this relatively thin layer of neurons that most of the processing for the so-called "higher functions" is accomplished. The convolutions of the cerebrum thus make it possible to have an enormous number of neurons distributed in two dimensions in the cerebral cortex without the necessity for an excessively large head.
Seen in toto, each cerebral hemisphere consists of 4 lobes: frontal, parietal, temporal, and occipital (named after the bones under which they lie), and it has been the frontal lobe, the large fore-part of the brain, which has been the most mysterious in terms of function. The most functionally well-defined region of the frontal lobe is the "primary motor area", which lies at the border between the frontal lobe and the parietal lobe, and which is involved in the voluntary control of movement. The back part of the frontal lobe anterior to the motor region is called the "prefrontal region", and prefrontal cortex is apparently involved in planning complex cognitive behaviors.
In recent years, "*functional imaging" techniques, which are essentially non-invasive and which can be used with healthy and awake human subjects, have become important new approaches to an old problem. In general, the human frontal cortex apparently helps mediate "working memory", a system that is used by the brain for temporary storage and manipulation of information, and that is involved in many higher cognitive functions. Working memory apparently includes two components: short-term storage (on the order of seconds), and executive processes that operate on the contents of storage.
The following points are made by E.E. Smith and J. Jonides (Science 1999 283:1657):
1) The authors present a review of current research concerning the functions of the human frontal lobes as revealed by experiments using *positron emission tomography or *functional magnetic resonance imaging to image subjects while the subjects engage in cognitive tasks designed to reveal processes of interest.
2) The authors report that studies of storage indicate that different frontal regions are activated for different kinds of information: storage for verbal materials activates Broca's area (an area specialized for the production of language) and left-hemisphere prefrontal areas adjacent to primary motor cortex; storage of spatial information activates right-hemisphere prefrontal cortex adjacent to primary motor cortex; storage of object information activates other areas of prefrontal cortex. Selective attention and task management, two of the fundamental executive processes, both activate regions of prefrontal cortex. The authors conclude: "Neuroimaging analyses of executive processes are quite recent, and they have yet to lead to clear dissociations between processes. Perhaps the highest priority, then, is to turn further attention to executive processes and their implementation in frontal cortex."
Science http://www.sciencemag.org
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Notes by ScienceWeek:
functional imaging: In general, in this context, the term "functional imaging" refers to any technique which images neural activity produced by specific behaviors (functions).
positron emission tomography: Positron emission tomography is a technique for producing cross-sectional images of the body after ingestion and systemic distribution of safely metabolized positron-emitting agents. The images are essentially functional or metabolic, since the ingested agents are metabolized in various tissues. Fluorodeoxyglucose and H(sub2)O(sup15) are common agents used for cerebral applications, and in cerebral applications of central importance to the technique is the fact that changes in the cellular activity of the brains of normal, awake humans and unanesthetized laboratory animals are invariably accompanied by changes in local blood flow and also changes in oxygen consumption.
functional magnetic resonance: Magnetic resonance imaging is a technique involving images produced by mobile protons of a tissue excited by the application of a magnetic field, and when used in functional cerebral imaging, the basis of the technique is that it images very small metabolic, blood-flow, and perfusion-diffusion changes in vivo, in real time, and with no risk to the subject.
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