Three groups of participants were selected for study. One was a group of 20 patients resident in a mental hospital. There were substantial differences among the participants in age and in how long they had been in hospital but all had been diagnosed as sufferlng from a paranoid form of the disorder and all were receiving the same medication (Phenothiazine). A second group ( "normals") consisted of 20 undergraduates who took part in the study as a course requirement for their degree in psychology. The third group was included in order to control for the possibility that hospitalization might itself influence behaviour. This group of 20 participants ("controls") was recruited from the surgical ward of a general hospital. All had recently undergone orthopaedic surgery. Details of the participants are presented in Table l.
Table l: Group means and ranges
All participants were tested individually on the following two-phase task. In phase l they were told to listen to a tape-recording of a short story (5 min in duration). Superimposed on the recording were 30 brief (3-second) bursts of white noise occurring at random intervals. Phase 2 tested how well the participants had learned to ignore thc noise. The tape was played again but this time a point was added to a counter placed in front of the participant after each presentation of the noise. The participant's task was to guess the rule by which points were being awarded. Subjects were told to raise a hand each time they thought the award of a point was just about to occur. Previous work has shown that normal participants will continue to ignore the noise in this second phase and thus take longer to recognise the rule than control participants who have not experienced the first phase.
The number of noise presentations on which participants raised a hand was scored. (Two schizophrenic participants who apparently failed to understand the instructions and made no responses were excluded from the experiment at this stage). The results of phase 2 are shown in Table 2.
Table 2: Mean number of noise presentations with a response
The schizophrenics and controls did not differ on this measure (t = 1.7, df = 36) but each scored less than normals (schizophrenics v. normals, t = 3.9, df = 36; controls v. normals, t = 3.8, df = 38, p < .05). These results seemed to indicate rather surprisingly that both schizophrenics and controls had effectively learned to ignore the noise as a result of phase-l training. The good performance of the normal participants indicated that they were still attending to the noise in the second phase of training.
In order to explain the results produced by the schizophrenic group the experimenters turned to the hypothesis that the drug taken by these participants might dramatically enhance the ability to "tune out" irrelevant stimuli. They were unable to test this suggestion with schizophrenic patients since all were on medication. Accordingly they recruited 20 undergraduate volunteers, 10 of whom received a dose of Phenothiazine and 10 a placebo before listening to the tape used in the previous experiment. This study had only one phase of testing and the measure of attention to the noise was the number of occasions on which the noise evoked a marked skin conductance response. Any such responses that occurred during intervals between noises were also scored. The results are shown in Table 3.
Table 3: Mean number of skin conductance responses
|During noise||During intervals|
The number of responses to the noise was significantly greater in the placebo group than in the drug group (t = 3.8, df = 18, p < .05).
The experimenters concluded that Phenothiazine increases a participant's ability to ignore an irrelevant stimulus and that the performance of the schizophrenic participants in the first experiment (which suggested that they were better able than normals to learn to ignore the irrelevant noise) was a consequence of the fact that their medication more than compensated for their basic attentional deficit.