Investigaciones relacionadas con la Teoría de la Carga Cognitiva

Luria (e.g. 1973) provided one of the most influential of the earlier descriptions o f frontal lobe function, and many of his theoretical ideas have been incorporated in later theories. Luria (1973) postulated the frontal lobes play an essential role in modulating the activity of 'lower* brain systems when the individual is carrying out complex tasks. He believed the frontal lobes have a crucial role in directing non-routine mental activity requiring the initiation and maintenance of a plan. He suggested the frontal lobes are necessary when forming a plan of activity, executing a complex "program of activity", organising a strategy, and evaluating the action taken. His description of the frontal lobe syndrome included a reduced ability to direct attention appropriately; an increase in "stereotyped" behaviour and habitual routines; and a reduced ability to plan, organise and reflect on a course of action. Luria (1973) believed that studying the performance of frontal lobe patients on tasks requiring complex intellectual activity, such as problem-solving tasks, would be most revealing of the deficits associated with damage to the frontal lobes.

lobes. She hypothesised that frontal lobe lesions in man are associated with two dissociable types of deficit: one in the ability to modulate self-generated plans, the other in the ability to monitor external events. This argument was based on the results of a series of studies comparing the performance of patients with unilateral frontal lobe lesions (i.e. lesions of either the right or left frontal lobe) on a range of tasks. On self-ordered pointing tasks of the type described above (see section 2.2.3.1), which are thought to measure planning ability, there was evidence that the left frontal lobe played a major role. In contrast, Ss with right frontal deficits were more impaired than left frontal Ss on a task o f recency discrimination (deciding which o f two stimuli had been presented more recently), suggesting a failure to monitor external events.

One of the most prominent theories of recent years is was posited by Norman and Shallice (1986), and has been described in detail in a number of other publications (e.g. Shallice, 1982, 1988). This theory elaborates on some of the ideas put forward by Luria (e.g. 1973) described above. The Norman and Shallice (e.g. Shallice, 1982; 1988) model assumes both cognition and action depend upon the 'running* of highly specialised routine schemata, each controlling a specific over-learned action or skill. Schemata may be activated in various ways, such as 'triggers' from perception, or the output o f other schemata, and, since they are activated in ways that are independent of each other, several schemata may run at once. The novel aspect of this model is the inclusion of two processes in the selection of schemata. Contention scheduling is held to involve routine selection between the schemata, while the Supervisory Attentional System (SAS) is required in non-routine or novel situations. The model predicts a specific deficit o f the SAS should not affect the performance of routine tasks even if they require considerable processing resources, but would result in difficulty in coping with novelty, in planning, or in overcoming a strong habitual response. The predicted impairments fit the classical view of frontal lobe dysfunction. If the SAS is inoperative, then behaviour will be controlled by the contention scheduling process, and this will lead to behaviour characterised by perseveration, inability to initiate activities, distractibility, failures of planning, and inability to inhibit habitual patterns o f behaviour. These are all explained as a failure of the SAS to initiate appropriate activities, or interrupt inappropriate ones.

Although the Norman and Shallice (e.g. Shallice, 1982, 1988) model of frontal lobe function described above goes some way to capturing the 'flavour' of frontal lobe

dysfunction, it also has at least one severe limitation. McCarthy and Warrington (1990, p.363) note the single supervisory system proposed by the model is not compatible with evidence of dissociations between different frontal lobe functions (e.g. Milner, 1982; see above). This point has been noted in later work by Shallice and Burgess (1991; 1993) who found evidence of fractionation whilst investigating possible measures of the SAS. As a result, Shallice and Burgess (1991; 1993) have modified the concept of a single resource SAS to encompass the possibility of fractionation. It is now postulated to consist of an unspecified number of sub-components (Shallice & Burgess, 1991; 1993).

Other approaches to frontal lobe function have placed different emphases on the various aspects of behaviour associated with frontal lobe syndrome. For example, Fuster (1989) emphasised the role of the frontal lobes in the temporal organisation of behaviour. He suggested there are at least three cognitive functions that can be identified as specific to the frontal lobes: 1) WM, which permits referral to relevant preceding events; 2) anticipatory set, which uses past experience to anticipate and prepare for future events; and 3) an interference control that inhibits the disruption of goal-directed behaviour by behaviour that is incompatible with it. Fuster argues the unifying purpose of these three components is the temporal organisation of behaviour and the structuring of goal-directed behaviour. The frontal lobes serve to bind together temporally distal events and behaviours for the purpose of goal attainment. Fuster emphasises it is the discontinuity between environmental events, behavioural responses, and their goals which make the role of the frontal lobes essential. In a different approach, Dempster (1991) emphasised the inhibitory function o f the frontal lobes He argued the suppression of irrelevant stimuli or associations may play an important, and often unrecognised, role in determining "intelligent" behaviour. Many tasks require the suppression of task-irrelevant information for effective performance, and the suppression of task-irrelevant thoughts and perceptions may be an important factor in the acquisition of higher-level knowledge structures such as strategies. Dempster suggests individual and group differences in the capacity for inhibition are manifestations of frontal lobe function.

Kimberg and Farah (1993) have criticised existing models of frontal lobe function, including those of Luria (e.g. 1973), Milner (1982), Norman and Shallice (e.g. Shallice, 1982; 1988; Shallice & Burgess, 1991), Fuster (1989) and Dempster (1991) reviewed above, on the basis that no single model proposed thus far is able to account for the full

range o f deficits associated with frontal lobe damage. Instead, each model may provide a good fit to deficits shown on one or two tasks, but be less applicable to deficits on other tasks. Kimberg and Farah (1993) propose a model of their own which, they argue, is capable of explaining a wider range of frontal lobe deficits than earlier models.

Unlike some of the earlier models (e.g. Luria 1973; Shallice, 1982; 1988), Kimberg and Farah (1993) do not posit a central executive or SAS function for the frontal lobes. Instead, the central tenet of their theory is that the frontal lobes are involved in maintaining associations among the elements of WM. These elements include representations of goals, stimuli in the environment, and stored declarative knowledge. It is argued that these representations are intact in frontal patients, but the associations between them are impaired. Frontal lobe damage would therefore be associated with a lowered sensitivity to the mutual relevance of goals and stimulus attributes and to the relations between facts and their contexts. In situations where several sources of information could potentially influence behaviour, connections between internal representations may be critical in determining which is successful. For example, responding to a stimulus in the environment that is relevant to a particular goal. If these connections are weakened by frontal lobe damage, other sources become more important in determining behaviour. Thus, behaviour may be influenced by stimuli irrelevant to current goals. Normally these stimuli would be ignored because of their lack of association in WM with any goals.

The model proposed by Kimberg and Farah (1993) has some advantages over earlier models of frontal lobe function in that it avoids the conceptual difficulties, outlined above, associated with the notion of a single executive or SAS. Furthermore, Kimberg and Farah (1993) have successfully modelled their approach and simulated performance on a range of tasks sometimes impaired by frontal lobe damage. These simulations produced patterns of performance qualitatively similar to those often seen in frontal lobe patients. However, there is still much work that needs to be done in extending this model to other tasks, in exploring dissociations between tasks, and in specifying the way WM functions might be organised within the frontal lobes.

In summary, the neuropsychological approach to understanding reasoning and problem­ solving is closely related to theories of frontal lobe function. The frontal lobes are still

relatively poorly understood in neuropsychology, chiefly because the effect of frontal lobe damage has been observed on a diverse range of tasks. This has resulted in a proliferation of theories, each pertinent to understanding specific aspects of frontal lobe function, but without the capacity to explain the full range of frontal lobe deficits. Nevertheless, the field has made progress in the last thirty years, and it is interesting to note there are increasing links with theories of reasoning from the field of cognitive psychology discussed above (see section 2.2.2). In particular, the central role of some form of WM is emerging from both cognitive and neuropsychological approaches. With regard to this, current models of WM will be explored in more detail in section 2.2.5.2, but first the implications of these approaches to reasoning and problem-solving will be considered in relation to the models of thinking in depression outlined in section 2.1.3.3.