Distractibility in Schizophrenia and Organic Cerebral Disease

Recent investigations of cognitive disorder in schizophrenia (Payne and Hewlett, 1960; Payne, Caird and Laverty, 1964) have indicated that schizophrenic performance in cognitive tests is characterized by a tendency to use very loose, “overinclusive” concepts. This phenomenon which Cameron (1944) termed “overinclusive thinking”, has been re-interpreted by Payne (1961) in terms of an impairment of a hypothetical filter mechanism which normally excludes irrelevant stimuli from consciousness and so allows attention to be directed towards the task in hand. McGhie and Chapman (1961) and Chapman (1966) have presented clinical evidence based on the subjective reports of schizophrenic patients supporting the hypothesis that a primary disorder in this disease is an impairment in the selective and inhibitory functions of attention. Chapman and McGhie (1962) and McGhie, Chapman and Lawson (1965a and b) have substantiated and elaborated this clinically derived hypothesis in an experimental setting where the subjects were required to perform various psychomotor and short-term memory tasks with and without distracting stimuli. The results of these investigations, while supporting the hypothesis in a general way, were nevertheless equivocal. The distraction effect in schizophrenia appeared to be confined to those situations where adequate performance involved the processing of a substantial amount of information. This finding was explained in terms of Broadbent's (1958) theory of the human operator as a limited capacity information channel. It appeared that distraction affected the performance of schizophrenic patients only in situations where the limited information channel was fully occupied in handling relevant aspects of the task—for example, in tracking and short term memory tests. Under these conditions the assimilation of irrelevant information produces overloading of the information channel and a consequent breakdown of performance. On the other hand, in those tasks requiring little information processing—for example, in repetitive psychomotor tasks—the channel is operating well below capacity, the assimilation of irrelevant information does not lead to overloading, and consequently does not have any detrimental effect on performance. An analogue of this situation occurs in normal subjects, in whom division of attention between two sources does not necessarily lead to impairment of performance if the informational requirements of each source are small and total information load does not exceed the critical limit (Broadbent and Herron, 1962). For the schizophrenic patient attention tends to be divided between the relevant and the irrelevant, yet impairment in performance need not occur if the informational requirements of the task are kept small.