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A Multidisciplinary Approach to Anterior Attentional Functions

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A Multidisciplinary Approach to Anterior Attentional Functions
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  PDFlib PLOP: PDF Linearization, Optimization, ProtectionPage inserted by evaluation versionwww.pdflib.com – sales@pdflib.com  A Multidisciplinary Approach to Anterior Attentional Functions z . T. STUSS,b,r T. SHALLICE,h,d M. P. ALEXANDER,'f AND T. W. PICTONb.C zyx otman Research Institute of Baycrest Centre 3560 Bathurst Street North York, Ontario M6A zyxw E1, Canada zy   Departments of Medicine Neurology) and Psychology University zyx   Toronto Toronto, Canuda University Coilege, London WCIE 6BT, UK SISSA, Trieste Braintree Rehabilitation Hospital Braintree, Massachusetts 02184 zy   Boston University School of Medicine Boston, Massachusetts INTRODUCTION Understanding the functions of the human prefrontal cortex is essential to any understanding of human cognition. This region, which comprises between a quarter and a third of the human cerebral cortex,'.2 is what most readily distin- guishes a primate brain from the brains of other mamrnal~.~ nfortunately, under- standing the cognitive processes carried out by the prefrontal cortex is hampered by several factors: the complexity of these processes; the extensive connections between the frontal lobes and other regions of the brain; the absence of clear animal homologues for many hypothesized prefrontal processes; and the relative rarity of patients with exclusively frontal lesions. Although the neuropsychological approach has been quite successful in the study of posterior brain functions, several factors have impeded its effectiveness in evaluating frontal lobe function. First, theories of prefrontal functions have used terms or concepts that derive from everyday language and are not easily operationalized into experimental paradigms. Second, the clinical tests that show deficits in patients with frontal lobe lesions generally involve multiple components and lack performance measures specific to particular cognitive processes, making a detailed functional analysis of these processes virtually impossible. A third prob- lem is the absence of clinical conditions with specific anatomical relations to the prefrontal cortex, like Korsakoff s psychosis to the mammillary bodies or Parkin- son's disease to the basal ganglia. This work was supported by grants from the Ontario Mental Health Foundation z to D.T.S.) and the Medical Research Council of Canada to D.T.S. and T.W.P.). 191  192 zyxwvutsr NNALS NEW YORK ACADEMY OF SCIENCES A zyxwvuts ourth and more subtle problem arises from the way that theoretical infer- ences in neuropsychology depend upon the pattern of associations and dissocia- tions across different tasks. The correlation between different tasks held to be sensitive to frontal lobe lesions has been low (e.g., ref. 4). One possible reason is that tests demonstrating selective frontal lobe deficits may not show the same results from one session to the next. Performance on a particular task may depend upon learning or adopting an effective strategy, and this can vary from one test session to the next. Indeed, variability of performance is probably characteristic of patients with frontal lesion^.^ A second possibility is that any two frontal tests may each be individually sensitive to frontal lesions but heavily loaded on other nonfrontal processes that differ between the tests.6 A final possibility for why frontally sensitive tests correlate weakly with each other is that they evaluate anatomically and functionally separate systems within the frontal lobes.'.* Proper assessment of this possibility will require detailed anatomical information for each patient. A NEW APPROACH In this paper we present a new approach to the study of frontal lobe functioning. We start with the assumption that there is no single basic frontal proces~.~.'~ here are five steps in the approach. The first is to determine a set of putative frontal processes that are closely related and that are used in many different tasks. The second is to select a set of tasks, each of which loads differently on the different processes. The third is to describe the tasks in the context of a theory of frontal lobe function that provides predictions about underlying cognitive processes and cerebral mechanisms. The fourth is to test individual subjects more than once to estimate reliability and variability. The fifth is to use different experimental meth- ods to provide converging evidence for interpreting distinct frontal processes. The methods we suggest are anatomical (lesion location), neuropsychological (behav- ioral tests), and physiological (event-related potentials or ERPs). ERPs are used to illustrate the physiological approach. Other techniques, such as those measuring cerebral blood flow, are also important sources of conver- gence. Recent developments in source analysis should allow us to localize ERP components generated in the frontal lobes, thereby providing a physiological basis for psychological theories of frontal function. Because extensive ERP evaluations of frontal lobe patients have not yet been made, our ERP discussion is more hypothetical than for the neuropsychological methods. The theoretical framework by which we characterize the component processes is the Supervisory System model of Norman and Shallice.6.'2,'3 The framework is represented diagrammatically in a simplified form in FIGURE . The theory follows the basic tenet of other theories of executive abilities-the separation between routine and nonroutine activities. The theory translates some of Luria's conceptsI4 into cognitive psychology and information-processing theory. Four components for cognitive processing are postulated: (1) cognitive units or mod- ules, 2) schemata, 3) contention scheduling, and 4) supervisory (attentional) system. The first three components are related to routine activities. Basic cogni-  STUSS el zyxwvu l : zyxwvusr NTERIOR ATTENTIONAL FUNCTIONS zyx   93 z r Perceptual Information Supervisory System Schemata1 _ ffector FIGURE 1 Supervisory Systems in human attention. This diagram derives from earlier representations (e.g., ref. 12). The interface between incoming information and behavior occurs through schemata. Their hierarchical arrangement is suggested by the overlapping levels. Schemata are controlled by a Supervisory System and by mechanisms intrinsic to the connectivity of the schema level. The Supervisory System consists of many component processes. This diagram indicates five of these processes that are particularly important in attention: Energization of schemata, Inhibition of schemata, adjustment of Contention scheduling, Monitoring of schema activity, and control of “if-then” Logical processes. The neuroanatomical basis of the different processes represented in the diagram is unknown. Schemata are probably mainly located in parietal and temporal association areas. The Super- visory System probably involves the prefrontal cortices. The diagram is highly simplified. tive operations are carried out in modules or units. Such units are controlled by schemata, which are routine programs for the control of overlearned skills FIG. z  . Even when complex, schemata are still standard and routine. Hierarchies of schemata allow component schemata to be recruited into more complex routine activities. Contention scheduling is the term used to describe the lateral inhibitory mechanisms that control competition between schemata. Schemata are activated by triggers, which can be perceptions or the output of other schemata. The fourth unit is the general executive component, labeled the Supervisory System. This system acts to handle nonroutine behaviors and functions primarily under four circumstances: when there is no known solution to the task at hand; when weakly activated schemata are evoked; when specific selection among schemata is neces- sary; and when inappropriate schemata must be inhibited. It functions by top- down activation or inhibition of schemata. The Supervisory System has in the past been considered in terms of a general unitary process. Research in recent years has, however, suggested that the Super- visory System can be fractionated into component proces~es.~~~~’~~~~ hese find- ings show the importance of looking for associations and/or dissociations among hypothesized processes related to the frontal lobes. FIGURE includes at least five independent supervisory processes: energizing schemata, inhibiting sche- mata, adjusting contention-scheduling, monitoring the level of activity in sche- mata, and control of “if-then’’ logical processes.  194 zyxwvutsr NNALS NEW YORK ACADEMY OF SCIENCES z Schema Activation from Feedback to Supervisory Supervisory System output to System Activation Effector System or other Schemata ateral Inhibition by Perceptual Input or other Schemata (Contention Scheduling) FIGURE 2. Schema interactions. A schema is a network of connected neurons that can be activated by sensory input, by other schemata, or by the Supervisory System. z n turn, it can recruit other schemata to control cognitive processing systems so as to produce its required response(s). In addition, we propose it provides feedback to the Supervisory System about its level of activity. Different schemata compete for the control of thought and behavior by means of contention scheduling, which is probably mediated by lateral inhibition. The representation is highly simplified. A schema contains multiple internal connections, some of which provide internal feedback. One major characteristic of a schema that cannot be represented in a static diagram is the time-course of its activity. Once activated, a schema remains active for a period of time depending upon its goals and processing characteristics. In straightforward reaction-time tasks this can be assumed to be a few seconds (unless further stimuli arrive). More prolonged activity without triggering input requires repeated energization from the Supervisory System. ATTENTION The utility of this approach is illustrated by the example of attention. Recent views of attention have proposed that attention is a system with different compo- nents related to distinct anatomical or physiological bases. This has led to the differentiation of an anterior attentional system centered in the frontal lobe, and a posterior attentional system centered in the parietal lobe. I6-l8 'The posterior system appears to be responsible for the spatial allocation of attention, whereas the anterior attentional system is concerned with the executive control of atten- tion. The nature of this executive control remains unclear. We postulate that the control of attention is shown in the following seven types of tasks: sustaining, concentrating, sharing, suppressing, switching, preparing, and setting of attention (see TABLE ). We define and characterize the processes involved in each task in terms of the Supervisory System model, and then consider the neuropsychological, anatomical, and electrophysiological evidence supporting the existence of these separate supervisory processes. Several of the tasks are considered diagrammatically in terms of the Supervisory System model FIGS. 3-5).
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