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We cannot possibly process all the information that is available. By allocating the limited attentional resources that we have to specific stimuli, some information is selected for further processing at the expense of other information. Aberrations in the regulation of attention occur in a wide range of psychiatric disorders.

First, severe disorders like schizophrenia or attention-deficit hyperactivity disorder are accompanied by a general deficit in attentional selection. Patients display attentional deficits that are largely independent of the semantic content of the information to be processed. A good illustration is provided by the systematic work of Shakow and colleagues who sought to identify the earliest stage at which cognitive deficits occur in schizophrenics. This work revealed that schizophrenic patients perform at a normal level on repetitive motor tasks and tasks measuring visual acuity, but display a slowing of speed when they have to react to a signalled target stimulus/60 This accords well with Kraepelin' clinical observation that a general attentional dysfunction is one of the core disturbances in schizophrenia.

In many other conditions, however, attentional disruptions are not general but domain specific, because they depend on the content (or 'valence' or 'meaning') of the information to be processed. As there is no reason to suppose that such an 'attentional bias' is accessible to introspection, researchers adopted paradigms from experimental psychology for documenting and unravelling the phenomenon. Typically, several groups of patients and healthy controls are asked to perform a task during which distractors, which vary in content (e.g. emotionally neutral versus emotionally negative), are presented. To the degree that attention is selectively allocated to emotionally negative material, for example, performance on the primary task will deteriorate more when negative distractors are presented than when the distractors are neutral. When certain patients are more distracted by, for example, specific negative distractors than by neutral distractors, relative to healthy or psychiatric controls, it is inferred that they display a domain-specific 'attentional bias'. While a wide variety of such tasks is available, by far the best studied is the so-called emotional Stroop test. Patients are seated before a computer screen and are presented with a sequence of words varying in colour. Words are sometimes emotionally neutral and sometimes emotionally valent (e.g. related to threat). The subject' task is to ignore the word' meaning and to name its colour. Findings with this paradigm rank among the most robust data in the cognitive psychology of emotional disorders. Anxious patients typically slow down when colour naming emotionally provocative words relative to neutral words. Healthy controls typically do not. Attentional bias, measured with this approach, has been documented in addiction and eating disorders, but most notably in anxiety disorders, i.e. phobias, panic disorder, social phobia, generalized anxiety disorder, post-traumatic stress disorder, and obsessive-compulsive disorder. (61,6, and 63) The most intriguing aspect of this phenomenon is not that patients selectively attend to issues of personal concern, but that they are apparently unable to abstain voluntarily from selective attention to threat. After all, they do try to ignore the meaning of the distracting word content. In psychology, such involuntariness is seen as a feature of 'automatic processing', which is contrasted with 'controlled processing' which does require willful effort. (64)

Another psychological feature distinguishing controlled from automatic processing is awareness. Whereas controlled processing (e.g. driving a car by a novice driver) requires awareness of the information to be processed and of the operations to be carried out, automatic or automatized processing (e.g. driving a car by an experienced driver) does not require such awareness. Thus the question arises as to whether anxiety-related non-volitional selective attention to threat requires consciousness of the material that is processed. The answer to this is negative. There are good reasons to believe that even when anxiety patients cannot possibly be aware of the information that is presented to them, they preferentially process information with a negative content. Evidence comes mainly from a modification of the Stroop test. Words to be colour named are presented for an ultrashort period of time, say 20 ms, after which they are replaced by a coloured mask that effectively blocks the after-image of the word, preventing conscious recognition. These masked words are emotionally neutral or negative, and the subject is asked to name the colour of the mask as quickly as possible. Again, anxious patients selectively slow down when a mask is preceded by a threat word as opposed to neutral words. This selective attention to cues that are not consciously identified has been observed with high-trait anxious subjects, (6 66 and 67) with phobics,(68) and with patients suffering from generalized anxiety disorder/6.9) Selective attending to preconscious threat cues in anxiety not only affects ongoing low-level cognitive operations, but also seems to activate the sympathetic branch of the autonomic nervous system; fearful subjects display increased electrodermal activity when confronted with masked presentations of threat pictures, whereas controls do not. (7°)

While attentional bias has been observed in several disorders, a curious exception to this rule is depression. Depressed patients tend to selectively remember negatively valenced information ('memory bias'), but they typically do not selectively attend to threat. (71> This may be because depressed people are preoccupied with (perceived) loss in the past and not, like anxious people, with (perceived) future harm. From a functionalistic position, it seems that selective attention is to be expected for those cues that are relevant for immediate action, like approach or avoidance. In line with this implication from general emotion theory, it was found that attentional bias also occurs for emotionally positive cues, given that the cues relate to highly desirable immediate action. For example, subjects who fast for 24 h selectively attend to positively valenced cues related to eating. (72)

Theoretically and clinically, a crucial question is the causal status of attentional bias: is it related to the genesis and/or maintenance of disorders or is it a result of these disorders? The empirical evidence that has accumulated favours both interpretations. Thus several studies have found that attentional bias disappears or is reduced after successful cognitive behaviour therapy.(73,7 and 75) The finding that cognitive behaviour therapy affects not only behaviour and self-reported complaints but also objective manifestations of information processing, while encouraging for cognitive behaviour therapy, suggests that attentional bias results from anxiety. Obviously, the relevance of attentional bias would be rather limited if there were an epiphenomenal and superficial feature of the disorder, appearing and disappearing with the waxing and waning of the syndrome. Still, there is more to attentional bias than this. MacLeod and Hagan, (65,) using the preconsciously operating 'masked' Stroop test in a prospective study, observed that future distress could be predicted from a currently present tendency to selectively attend to threat. This was replicated in a cross-sectional study.(67) Of particular interest is that the prediction of distress from current attentional bias was unrelated to current anxiety levels. Independent of current anxiety, attentional bias was a predictor of subsequent symptoms. This suggests that anxiety and attentional bias are reciprocally related; anxiety may foster attentional bias, but in and of itself attentional bias may be a vulnerability factor. A therapeutic implication would be that reducing attentional bias may help in reducing anxiety disorders or preventing relapse after successful treatment. While promising observations have been reported, (76) no controlled data are yet available.

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