Métodos de clasificación

4.4.1. The Hayling test

Executive functions related to response initiation and inhibition were assessed in the current study using the Hayling sentence completion test. It was hypothesised that functional changes in activation of orbitofrontal and ventromedial cortical areas in chronic opiate users, would lead to deficits on tasks purported to tap inhibitory control subserved by these areas.

The first part of the Hayling test measures response initiation, by requiring the participant to complete a sentence with an automatic response, which is a word that is highly cued by the sentence context. It was hypothesised that methadone participants’ response initiation abilities would be comparable to healthy, matched controls, as there was no evidence in extant literature to suggest that chronic opiate abuse is associated with impaired automatic responding. As predicted, no significant difference was found between the methadone and control groups in mean response time and most participants performed within the ‘average’ range. Burgess and Shallice (1996a) found that patients with frontal lobe lesions were significantly slower than controls and patients with posteriorly-located lesions. The current findings suggest that, with respect to initiation of automatic responses, the methadone group do not exhibit deficits similar to those associated with the consequences of frontal lobe injury reported by Burgess and Shallice

(1996a). Indeed, the methadone group mean time to respond was 15 seconds less than the frontal lesion group mean in the Burgess and Shallice (1996a) study. However, the finding that frontal patients showed longer response times was not replicated in a study by Andrés and van der Linden (2001) who found no evidence that frontal patients differed from controls on the initiation section. The frontal group mean (in the Andrés & Van der Linden, 2001) was within 5 seconds of the methadone mean in the current study. A lack of consistency in finding a ‘frontal deficit’ on this measure suggests that, speculations on the functional-anatomical intactness of the methadone group would be inappropriate.

The second part of the Hayling is purported to measure response inhibition, defined as the ‘inability to suppress the most salient response’ (Burgess & Shallice, 1996a: p.263). Participants had to inhibit a well-leamed response, which was highly cued by the sentence context. Both groups took longer on the inhibition section than the initiation part. There was a trend towards methadone participants taking slightly longer than controls to give a response, but the difference was non-significant. Most participants performed in the ‘average’ range, although there was greater variability in the methadone participants’ performances.

Burgess and Shallice (1996a) derived a measure of ‘additional thinking time’ associated with the second part of the Hayling compared to the first, because they hypothesised that inhibition performance was partly influenced by the ability to initiate a response. Therefore, they calculated the difference in response time between initiation and inhibition sections for each participant and found a significant frontal-control difference.

There was a trend, in the current study, of longer additional thinking times in the methadone group, although the trend disappeared once the influence of age, years of education and depression score were covaried. It seems that the methadone participants tended to require longer times when asked to inhibit automatic responses, but this tendency was related to more general variance between the groups.

An error in the inhibition section was defined as the inability to inhibit automatic responses that were semantically related to the sentence and were divided into two subtypes; category a errors were defined as straightforward, automatic, completions of the sentence, whereas category b errors were defined as being only semantically-related. The current study found the methadone group made significantly more category b errors and tended to make more category a errors too, compared to controls. However, these error scores were comorbid with age, years of education and depression score, as the differences were non-significant once the influence these factors were covaried out. The finding that error score was influenced by age, replicates findings by Burgess and Shallice (1996a), who also found age to be a source of covariance, although the frontal impairment in error score remained after the effects of age were accounted for in their study.

Importantly, unlike the control group, there was wide variation among methadone participants in scaled error scores. A bimodal distribution was observed in the methadone groups’ error scores, whereby a sub-group (25%) performed in the ‘abnormal’- ‘impaired’ range and another sub-group (75%) performed in the ‘average’- ‘good’ range. This observation of wide inter-group heterogeneity has been observed in

other studies of neuropsychological functioning in methadone users (Davis, Liddiard & McMillan, 2002), which raises the question of whether there is a sub-set of severely cognitively impaired opiate users, whose cognitive deficits are well-represented by models of executive dysfunction in frontal patients. Unfortunately, the small sample size in the current study means that it is impossible to meaningfully analyse these inter-group variations.

Overall, the results supported the hypothesis that response initiation abilities are not impaired in the methadone group. The hypothesis that the methadone group would be impaired on the response inhibition section of the Hayling was not supported in the current study. Wide variability in the performance of methadone participants on the inhibition section was found however, and revealed that a sub-group of methadone users exists that who demonstrate severely impaired inhibitory control. This finding warrants further investigation.

4.4.2. Verbal fluency

It was hypothesised that the methadone group would show significantly reduced word fluency compared to healthy controls, as a result of reduced inhibitory control in the methadone group. This hypothesis was supported by the results, which indicated that methadone participants produced fewer words compared to controls. However, there is much debate in the literature regarding the underlying mechanisms governing word fluency abilities, some have posited fluency as mediated by initiation abilities (Ramier & Hacaen, 1970 cited in Burgess & Shallice, 1996a), whereas others take the view that it taps inhibitory processes because it requires the suppression of habitual meaning-based

word search strategies (Perret, 1974). In the current study, a significant negative correlation between fluency and the response time on the inhibition section of the Hayling was found, whereby lower fluency was associated longer response latencies on the Hayling (inhibition), which supports the view that both tasks may tap similar underlying inhibitory processes.

Qualitative differences were observed between the groups in their tendency to use organised approaches to the fluency task. Word fluency tasks have been viewed as measuring an individuals’ strategy generation abilities (Estes, 1974), good strategies include using the same initial stem (e.g. content, contend, contain) or deriving word from a themed category (e.g. sew, stitch, seam) (Lezak, 1995). Informal examination of the words produced by participants in the current study suggested that, methadone participants did not use strategies as often as control participants. Methadone participants showed a greater tendency to pick random words from memory, which, it is proposed, is a less efficient means of achieving the goal of the task.

Superior verbal fluency performance has been related to higher intelligence (particularly verbal intelligence), however, the group difference remained significant once the influence of premorbid intelligence and current non-verbal reasoning were covaried out.