Controls High Symptoms
5.4.3 Group differences on cognitive tests requiring response suppression
test compared to the controls and the ‘high symptom’ group had significantly longer response times for the response suppression condition compared to the response initiation condition than the controls. None of the groups differed significantly on the number of errors made on the Hayling test. These results lend some support to previous studies which have found impaired performance on the Hayling test in patients with schizophrenia (Nathaniel-James and Frith, 1996). Both patient groups produced significantly fewer words in the alternating word fluency task compared to the controls. This deficit on tests of word fluency also replicates previous findings in schizophrenia (Liddle and Morris, 1991, Frith 1992, Beatty et al., 1993, Crawford et al., 1993, Abbruzzese et al, 1995). However, unlike previous studies reporting deficits on the Stroop (Mahurin et al., 1998; Liddle and Morris, 1991, Jaquet et al., 1997) or random number generation (Rosenberg et al., 1990) in patients with schizophrenia, neither of the two patient groups differed significantly from the controls on the Stroop or RNG tasks. The absence of such significant group differences may partly relate to the fact that the present sample were not highly symptomatic at the time of study, with only 7 of the
14 patients having relatively high positive and negative ratings.
More importantly in relation to the aims of the present study was the finding that performance on the cognitive tasks requiring suppression of habitual responses correlated with performance on the go no-go tasks. Word fluency was negatively associated with errors in the CRT condition of Study 5, suggesting that poorer word fluency correlated with higher errors on the go no-go task. A larger ‘Stroop’ effect, that is greater susceptibility to interference from the colour words was associated with more omission errors in the SRT condition of Study 4. Higher count score 1, indicative of a failure to suppress habitual counting during random number generation was associated with greater anticipation errors in the SRT condition o f Study 4. These correlations
suggest that the go no-go tasks and the cognitive tasks may both be tapping some common ability to inhibit or suppress inappropriate responses for selection or release of the appropriate response.
Servan-Schreiber et al. (1996) used a variation of the CPT in patients with schizophrenia. They varied the interstimulus interval (ISI) between 750 ms and 5 s and the percentage of targets to non-targets to 80% to 20%. The rule was to respond when the letter X appeared, if it was preceded by an A (AX). On 10% of the trials X appeared, preceded by a different letter (BX) and in 10% of the trials A appeared, followed by a different letter (AY). The authors propose that the two cognitive impairments reported in schizophrenia, dysfunction of working memory (Weinberger et al., 1986) and impaired inhibition (Manschreck et al., 1988), can be explained by a single mechanism - an impairment in maintaining contextual information over time. According to this model, patients with schizophrenia with a mild impairment in maintaining contextual information would show an increase in BX errors on long ISIs, reflecting a working memory deficit, whereas patients with severe impairments in maintaining contextual information would show an increase in AY errors for the short ISIs, reflecting impaired inhibition. Unmedicated patients tested during their first episode exhibited working memory impairments, whereas the unmedicated ‘multi episode’ patients (those recently admitted who had a long history of relapse) exhibited impaired inhibition. Other reports of intact inhibition but impaired memory in first episode schizophrenia has been reported (Hutton et al., 1998). These findings and the results of the current study suggest that impaired performance on no-go tasks may be dependent on duration of illness and symptom severity. In the present study, significant group differences in RTs were found between patients with high positive and high negative symptoms relative to controls, while differential effects of condition on RTs
was seen for patients with high negative signs relative to normals. The effect of symptom severity is clearly an issue that requires further investigation using a larger number of patients with more severe symptom ratings. For example, it would be interesting to compare patients with very high ratings of positive symptoms with patients with very high ratings of negative signs and also to compare a large group of patients with very high symptom ratings to a group of patients with very low symptom ratings.
Another possible explanation for the lack of major differences between the patients and controls may relate to the ratio of go to no-go trials. As we were interested in a failure of inhibition in schizophrenia, the current study was designed to increase the tendency to make errors of commission. Thus an 80% go to 20% no-go ratio was chosen, based on the rationale that the increased preparedness to respond on 80% of the trials should promote high errors of commission. However, it is also possible that with only 20 % of the trials requiring response inhibition, sufficient sampling of the no-go performance was not undertaken. In effect, if the key factor determining the success or failure of response inhibition is the degree of uncertainty, a task with a 50/50 ration of go to go-no trials would be optimal. This issue can be examined more fully in future studies by systematic variation of the proportion of go to no-go trials across blocks from 80/20, to 50/50 to 20/80 for example.