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New approaches to distinct profiles: biases in information processing
So far, this review has focused on the performance of cognitive and neuropsychological tasks employing neutral materials — those that are not emotionally relevant to the patient's condition, i.e. materials not seemingly positive or negative in affective or emotional tone. This exclusion of affective material effectively removes mood from the experimental dynamic; in order to assess the possible relationship between mood and cognition in the affective disorders, we must consider studies incorporating affective material in the experimental design. In patients with depression, empirical studies of mood-congruent biases in information processing are abundant, with biases reported in evaluative processes, social judgements, decision-making, attention and memory (Clark & Teasdale, 1982; Blaney, 1986; Gotlib & Cane, 1987; Mogg et al, 1995; Bradley et al, 1996). One of the earliest studies examined the recall of past experiences in patients who were clinically depressed and healthy control participants (Lloyd & Lishman, 1975). The results indicated that when patients with depression were required to recall pleasant or unpleasant experiences from their past in response to various cue words (e.g. ‘house’, ‘table’), patients recalled unpleasant memories more quickly than pleasant ones as the severity of depression increased.
In light of these findings, it seemed reasonable to suppose that if differences in cognitive functioning in mania and depression do indeed exist, they will emerge on tasks involving the interaction between cognitive and affective (or emotional) processing. We attempted to address this hypothesis by administering a novel ‘affective go/no-go’ task to patients with mania and depression, and to healthy controls matched for age and premorbid intelligence (Murphy et al, 1999). This task required both attentional and affective processes for its successful completion. Specifically, subjects were required to respond to target words of either positive or negative affective tone by tapping the space bar of a computer keyboard as quickly as possible, and to inhibit this response to words of the competing affective category. As shown in Fig. 2, both groups of patients exhibited attention and response biases — in mania towards the positive stimuli and in depression towards the negative stimuli. In addition, patients with mania — but not those with depression — were impaired in their ability to inhibit behavioural responses and focus attention. These findings were particularly interesting against a background of similar impairments on conventional neuropsychological tests of memory and executive function (see above).
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Mean response times for ‘happy’ and ‘sad’ targets in the affective go/no-go task for patients with mania (black bars), depression (white bars) and control subjects (shaded bars). Bars represent one standard error of the mean (s.e.m.). Data taken from Murphy et al (1999).
Neuroimaging studies of the neural regions that underlie cognitive processing of affective meaning suggest that medial and orbitofrontal prefrontal cortex (PFC) are particularly involved (Beauregard et al, 1997; Teasdale et al, 1999). In line with these findings, Murphy et al (1999) concluded that performances in mania and depression were most likely to differ on cognitive tasks subserved by functioning of the orbital/ventromedial regions of PFC. Indeed, Drevets et al (1997) found that the subgenual PFC, which lies in the ventromedial PFC, is differentially activated during periods of mania and depression. The disinhibited response often observed in mania, but not in depression, provides further evidence for differential performance on tasks requiring ventromedial prefrontal functioning, as patients with medial or ventral prefrontal damage are similarly impaired on ‘go/no-go’ tasks (Drewe, 1975; Malloy et al, 1993).
At first glance it might seem puzzling that patients with mania and depression in the study by Murphy et al were differently impaired on the ‘affective go/no-go’ task but not on the Tower of London test of planning, tasks both thought to be subserved by PFC. This apparent inconsistency may be explained by the functional and anatomical distinctions between the dorsolateral and orbital/ventromedial regions of PFC that have been postulated in recent years. It is now known that tasks such as the WCST and the Tower of London test activate a neural network that includes important areas such as dorsolateral regions of PFC (Berman et al, 1986; Baker et al, 1996). These regions have numerous connections with cortical systems involved in information processing. In contrast, tasks that assess ability to make decisions and reverse associations between stimulus and reward are thought to be subserved by ventromedial regions (Rahman et al, 1999; Rogers et al, 1999), which are more extensively connected with limbic structures (Pandya & Yeterian, 1996). As a result, it is possible that this inconsistency is related to the different neural pathways subserving cognitive function in these two tasks.
To the best of our knowledge, no other studies have compared information processing biases in mania and depression. The mood-congruent bias observed in depression is consistent with many depression studies demonstrating biases of memory and attention (see above), but this may be the first demonstration of a positive attentional bias in mania. In this context, it is worth noting that a recent study demonstrated a bias for processing negative information in bipolar mania (Lyon et al, 1999). While such results may seem directly contradictory to the findings reported above, the authors suggested that negative bias may be limited to implicit tests of affective orientation; the ‘go/no-go’ task used by Murphy et al and described here surely taps affective bias more explicitly.
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