Frontal cortical lesions and their effect on the control of memory retrieval and general tests of executive functions

Many major claims about the functions of the frontal cortex originate from patient studies examining the effects of extensive damage to the anterior part of the brain, including subcortical and extra-frontal regions. In general, the claims are that damage to the frontal cortex results in generalized impairments of executive functions that can be measured by specific tests or observed in specific behaviours. However, the attribution of deficits to the frontal cortex per se based on examination of patients with anterior brain damage that includes massive extra-frontal damage is a serious problem leading to potentially incorrect interpretation of frontal cortical function. This thesis tested the validity of some of these claims about frontal cortical functions by examining the performance of patients with well-documented lesions restricted to the frontal cortex and contrasting it with that of patients with temporal lobe lesions and neurologically intact individuals matched for age and education.. The patients had undergone brain surgery at the Montreal Neurological Hospital for the removal, in the frontal cortex, of a low-grade cerebral tumour or cortical excision for the relief of focal epileptic seizures. A few patients had damage following a stroke.Study 1 showed that the Frontal Assessment Battery (FAB; Dubois et al., 2000), which is a short battery assumed to be a sensitive measure of frontal cortex executive dysfunction, but only validated on patients with a variety of neurodegenerative conditions causing widespread brain damage, is not a sensitive measure of frontal cortex executive dysfunction. Only performance on the verbal fluency subtest was specifically sensitive to damage to the frontal cortex, more specifically to the left dorsomedial frontal cortex. In Study 2, the presence of utilization behaviour (Lhermitte, 1983), defined as the impulse to grasp and use a presented object although it is not contextually appropriate to use it that has been commonly attributed to the frontal cortex, did not differ in patients with frontal cortical lesions in comparison with patients with temporal lobe lesions and normal control subjects. These results suggest that, in previous studies, the impairment on the FAB and the exhibition of utilization behavior by patients may have been due to widespread brain dysfunction or to frontal cortical damage in conjunction with other damage, but not solely to frontal cortical damage. In addition, the claim that memory for context depends on the frontal cortex only under certain circumstances, such as when memory traces require active disambiguation (Petrides, 2002, 2005) was examined. Subjects performed a mnemonic context retrieval task in which the stability with which items (words) and contexts (abstract coloured backgrounds) entered in relationships with one another was manipulated in order to recruit disambiguation processes (Study 3). Patients with lesions to the left dorsomedial frontal cortex were impaired mnemonic context retrieval, regardless of the stability of the relationships between items and contexts. In contrast, lesions to the right ventrolateral prefrontal cortex caused impairment only on when the relationships between items and contexts were unstable, suggesting that this region is critical for active retrieval processes (Petrides, 2002, 2005) necessary to disambiguate information existing in multiple and unstable ways in memory during context retrieval.In conclusion, the lesion specificity and the inclusion of patient and normal control groups in the present work demonstrated that no generalized executive or behavioural impairments, as measured by the FAB and observed in utilization behaviour, result from lesions restricted to the frontal cortex, but that precise regions of the frontal cortex contribute specifically to particular cognitive processes.

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