According to the aberrant salience account of schizophrenia, the positive symptoms of the condition – the hallucinations and delusions – come about because patients see meaning where there is none. The neurotransmitter dopamine, associated with learning and reward, is found in excess in patients with schizophrenia, and it is this chemical anomaly that is thought to underlie their tendency to misinterpret the meaning of things.
The aberrant salience account was proposed by psychiatrist Shitij Kapur and while it has provoked a lot of interest, few studies have attempted to directly test its predictions. Now Jonathan Roiser and colleagues have shown that patients experiencing delusions show more aberrant salience in a learning task than do patients whose symptoms are in remission – a finding entirely consistent with Kapur’s account.
On each trial, 20 patients on medication for schizophrenia and 17 healthy controls had to press a button as fast as possible in response to a black square appearing on a computer monitor. Sometimes participants were rewarded for responding quickly to this square and sometimes they weren’t. Crucially, the reward schedule wasn’t completely random, and the likelihood of a trial being rewarded could sometimes be predicted by an image (e.g. a household item) flashed on-screen before the black square. However not all images were predictive in this way. Some were irrelevant, and a key feature of the task was whether or not participants would learn which images were predictive and which weren’t.
If participants responded more quickly after predictive images than irrelevant ones, then this would indicate they had learned correctly – a sign of so-called “adaptive salience”. By contrast, speedier responding after irrelevant images would indicate that they’d read predictive meaning where there was none – a sign of “aberrant salience”. After the testing, the participants were also asked to report which images they thought were predictive and which weren’t, thus providing another, more explicit, measure of adaptive and aberrant salience.
Overall, the medicated patients with schizophrenia showed no more aberrant salience than the controls. That is, they were no more likely to believe an irrelevant image signalled a forthcoming reward. Crucially, however, among the patients, those who were still experiencing delusions showed more evidence of aberrant salience than those whose symptoms were in remission.
Moreover, the patients showed reduced adaptive salience relative to the controls. This is also consistent with Kapur’s account, which predicts that patients treated with anti-psychotic medication will show impaired learning as a side-effect of their medication. A final supportive finding was that control participants who scored higher on a test of schizophrenia-like experiences also demonstrated increased aberrant salience.
A finding not predicted by Kapur’s account was that patients with more negative symptoms of schizophrenia – lethargy and lack of emotion – also tended to display more aberrant salience.
“…these data are consistent with the hypothesis that schizophrenia patients with delusions exhibit aberrant salience,” Roiser’s group concluded. “The aberrant salience hypothesis warrants further investigation in unmedicated patients with schizophrenia.”
J. P. Roiser, K. E. Stephan, H. E. M. den Ouden, T. R. E. Barnes, K. J. Friston, E. M. Joyce (2008). Do patients with schizophrenia exhibit aberrant salience? Psychological Medicine, 39 (02) DOI: 10.1017/S0033291708003863