Determinación de nitrógeno básico volátil:

The current literature continues to reflect the debate on whether the frontal lobes play an exclusive role in executive functioning, or whether there is involvement of other brain regions (Andres & Van Der Linden, 1998; Garavan, Ross, Li, & Stein, 2000).

Frontal subcortical circuits that link the frontal cortex of the brain to other brain areas are believed to play an important role in mediating behaviour. These circuits originate from frontal cortical areas including the dorsolateral and orbital frontal regions and the cingulate gyrus (Tekin & Cummings, 2002). Executive problems are believed to arise from lesions to the dorsolateral prefrontal cortex and include difficulties with shifting set, mental flexibility, generating or initiating motor programmes and strategies, attending to information, organizing behaviour, monitoring in working memory, and ordering events within the correct time sequence. Lesions to the orbitofrontal cortex are thought to result in changes of personality, for example increased disinhibition. Apathy and reduced motivation are associated with damage to the anterior cingulate gyrus

(Tekin & Cummings, 2002).

Several studies, including functional imaging studies, have provided support for a link between executive process and the frontal lobes (Burgess & Shallice, 1996a; Burgess & Shallice, 1996b; Miller, 1984; Regard, 1981). Duncan and Owen (2000) employed functional imaging methods to highlight the involvement of three regions of the

prefrontal cortex of the frontal lobes, the mid-dorsolateral, mid ventrolateral and dorsal anterior cingulate cortex and executive functions such as problem solving,

and initial learning of novel tasks. The mid-dorsolateral prefrontal cortex has been found to play a role in the monitoring of information in visual working and spatial memory (Alexander et al., 1986, Monchi, Petrides, Petre, Worsley, & Dagher, 2001; Petrides, 2000a). The mid-ventrolateral prefrontal cortex is involved with executive processes associated with memory processes such as comparing and judging stimuli (Petrides, 2000b) and active retrieval of visual information (Cadoret, Pike & Petrides, 2001).

The anterior cingulate gyrus is located on the medial surface of the frontal lobes (Carter, Botvinick & Cohen, 1999) and is considered to play an important role in the allocation of attention, for example when there is the need to inhibit a prepotent responses or to detect conflicts, motivated attention and error detection (Carter et al., 1999). It is believed that the mid-dorsolateral, mid ventrolateral and dorsal anterior cingulate cortical regions of the frontal lobe collaborate in dealing with complex cognitive demands, lending support to the theory of specialization of function within a particular brain region (Duncan & Owen, 2000). The ACC is also thought to play a role in verbal fluency tasks where several responses are cued or initiated and there is competition in the production of responses. Duncan and Owen describe electrophysical studies in which neurones spread across the dorsal and ventral regions of the frontal cortex are found to adapt the type of information they carry to the demands of the executive task.

A relationship between the frontal lobe and processes of response initiation and suppression has been supported by further functional imaging studies (e.g. Nathaniel- James et al., 1997) and clinical studies using the verbal fluency and Stroop tasks (Frith, Friston, Liddle & Frackowiak, 1991; Pardo, Pardo, Janer & Raichle, 1990; Perret, 1974) (see sections 2.3.4.1 & 2.B.4.2, respectively). Both the anterior cingulate and left

dorsolateral prefrontal cortex have been implicated (Barch, Braver, Sabb & Noll, 2000; Frith et al., 1991; Warburton et al., 1996).

Further experimental and clinical evidence suggests that the frontal lobes play a central role in set shifting ability (Metzler & Parkin, 2000; Monsell, 2003), more specifically, the dorsolateral region of the prefrontal cortex (Milner, 1963; Milner, 1964; Monsell, 2003; Sandson & Albert, 1984; Stuss et al., 2000). Patients with frontal lobe lesions have often been observed to have difficulty switching from use of a previous strategy to use of a novel strategy when sorting a series of cards on the WCST (Kimberg & Farah,

1993). ‘Stuck-in-set’ perseveration is considered to be related more specifically with the left frontal cortex (Monsell, 2003; Sandson & Albert, 1984).

Contrary to the above findings, however, executive function has also been associated with a ‘distributed set of neural networks’ (Andres & Van Der Linden, 2001). It has been considered that ‘supervisory attentional functions, including inhibition (Norman & Shallice, 1986), are sustained by a cortical-subcortical network, and not by a localised region such as the frontal lobes’ (p. 39, Andres & Van der Linden, 1998).

Several studies have provided little evidence to support the exclusive involvement of the frontal lobes in response inhibition (Anderson, Damasio, Jones & Tranel, 1991). A review of the literature by Andres and Van Der Linden (1998) in which studies

employed ‘classical tests’ of inhibition, provide contradictory evidence to this exclusive frontal involvement. Andres and Van Der Linden invited a group of patients with lesions exclusively located in the frontal lobes to complete tests of inhibition. The authors found that although the patients were slower than the control group on these tasks, they were

not less accurate.

Andres and Van Der Linden (2001) examined the performance of patients with focal lesions located exclusively in the frontal lobes on executive tests such as planning, rule detection and inhibition previously employed by Shallice and others (Shallice, 1982; Burgess & Shallice, 1996a; Burgess & Shallice, 1996b). The findings showed that although this patient group was slower than the control group on two of the executive measures, the groups did not differ in terms of accuracy of performance on all three executive tasks as might be expected in patients with lesions exclusive to the frontal lobe. Shallice and Burgess found that significantly more errors and slower responses latencies were produced by the patient group who had lesions that were not exclusively located in the frontal cortex when compared to the control group (Burgess & Shallice, 1996b). This suggests that there is not necessarily an exclusive relationship between the frontal cortex and executive processes and that there may be relative sparing of some executive processes in patients with damage to the frontal lobe (Andres & Van Der Linden, 2001). Andres & Van der Linden (2002) suggest that the frontal cortex ‘cannot serve, on its own, as the CE (Central Executive) of the brain’ (p. 843). Neuroimaging studies by Garavan, Ross, Li and Stein (2000) have provided further evidence for activation of cortical areas outside of the frontal lobes during performance on executive tests. Involvement of the parietal lobe has also been implicated (Miyake et al., 2000).

Brain regions outside of the frontal cortex have also been implicated in the process of set switching through brain imaging studies. These areas include the parietal lobes,

cerebellum, basal ganglia and other subcortical brain structures (Gotham et al., 1988; Monsell, 2003).

There have been several competing theories of executive function that have been proposed by those working in the field of cognitive and neuropsychology. This section will provide an overview of models of executive function to date,