In recent years an increasing number of workers investigating schizophrenic behaviour have concluded that many of the symptoms found in schizophrenia are related to a disturbance in the selective and inhibitory functions of attention. One of the earliest statements of this argument is found in Norman Cameron's (1938, 1939, 1944) concept of “over-inclusion”, which he used to describe the schizophrenic patient's tendency to include many elements irrelevant to the central idea in his thinking. Shakow (1962) reached the following conclusions in summarizing his own psychological studies of schizophrenia—“It is as if, in the scanning process which takes place before the response to stimulus is made, the schizophrenic is unable to select out the material relevant for optimal response. He apparently cannot free himself from the irrelevant among the numerous possibilities available for choice.” Weckowicz and Blewett (1959), in their studies of alterations in perceptual constancy in schizophrenic patients, interpreted their findings as suggesting that the patient's basic difficulty was that of “an inability to attend selectively or to select relevant information“. Venables and his colleagues (1959, 1962, 1963), in a series of studies on the arousal level of schizophrenic patients, also concluded that many of the behavioural abnormalities demonstrated were due to variations in the range of attention. In a series of investigations carried out by Payne and his colleagues (1960, 1961, 1963) to develop Cameron's concept of over-inclusive thinking in schizophrenia, the authors utilized Broad-bent's (1958) model of selective attention to postulate that this form of thought disorder is basically due “to a defect in some hypothetical central filter mechanism, the function of which

is to screen out irrelevant data both internal … and external … to allow for the most efficient processing of incoming information”.