Support for the Aberrant Salience hypothesis of Psychosis
Last week I wrote about the aberrant salience theory of psychosis, and luckily, this week itself a new study has surfaced that corroborates that theory with some preliminary evidence.
What Rosier et al did was to administer a Salience Attribution Test to both patients with Schizophrenia and normal controls, and to look for differences in the adaptive and aberrant salience. It is important to realize that most of the patients were medicated on anti-psychotics, and as per the theory advocated by Shitij Kapur, the anti-psychotics would dampen the normal adaptive salience too as psychosis is due to hyper reactivity of dopamine system and anti-psychotics are supposed to work by attenuating that behavior. More specifically, the predictions were:
It has been hypothesized that dopamine antagonists reduce both adaptive and aberrant salience, and that in the absence of effective treatment patients with schizophrenia exhibit aberrant salience (Kapur, 2003). Therefore, our first prediction was that that medicated patients with schizophrenia would exhibit reduced adaptive salience relative to controls, representing an undesirable side-effect of anti-psychotic medication. Our second prediction was that medicated patients with schizophrenia would exhibit equivalent aberrant salience to controls, representing the beneficial effect of anti-psychotic medication, which is hypothesized to normalize aberrant salience from a previously elevated level (Kapur, 2003). Our third prediction was that those patients with persistent positive symptoms, in whom medication is not entirely effective, would exhibit greater aberrant salience than patients without positive symptoms. Our fourth prediction was that in the controls, individual differences in aberrant salience would be related to the personality trait of schizotypy, considered to be an index of psychosis proneness (Chapman et al. 1994; Claridge, 1994; Stefanis et al. 2004).
All of their predictions were supported by the test results. The SAT paradigm is really simple and depends on reaction time measures following CS+ and CS-; with CS+ reaction times quantifying adaptive salience and CS- reaction times quantifying aberrant salience attribution. Read the methods section for more on the SAT.
Interestingly in patients, those with persisting delusions as well as those high on Negative symptoms exhibited higher aberrant salience as compared to patients/ controls without any delusional symptoms.Also, in controls the introverted anhedonia subscale of schizotypy correlated signficantly with the aberrant salience, thus indicating a role for negative symptom formation/ explanation too as apart of the aberrant salience. This is how the authors interpret their findings:
Aberrant salience and positive symptoms of schizophrenia
One explanation of increased aberrant salience in patients with positive symptoms concerns aberrant dopamine signalling. Contemporary accounts of reward learning suggest that phasic dopamine firing codes reward prediction errors (Schultz et al. 1997), for example, those arising from temporal difference models of reinforcement learning (Dayan & Balleine, 2002). Such models elegantly account for changes in both the firing patterns of ventral tegmental area dopamine neurons in monkeys (Schultz, 1997), and ventral striatal responses in humans (Pessiglione et al. 2006; Seymour et al. 2007), as reward-learning progresses. If phasic dopamine release signals reinforcement prediction errors, any large stochastic fluctuation in dopamine release may disrupt learning about stimulus–reinforcement associations, generating a state in which motivational salience could be misattributed to neutral stimuli, or what might be termed a ‘false-positive’ phasic dopamine signal; such events have been proposed to result in positive symptoms (Kapur, 2003).
In the present study, patients for whom medication had effectively eliminated positive symptoms actually exhibited significantly less aberrant salience than controls, supporting the hypothesis that the beneficial effects of antipsychotic medications on positive symptoms are related to their ability to dampen-down aberrant salience (Kapur, 2003). However, independent of symptoms at the time of testing, the patients with schizophrenia exhibited significantly less adaptive salience than controls. Antipsychotic medication has long been considered to exacerbate negative symptoms in schizophrenia, which may be related to reduced adaptive salience [see discussion below and Schooler (1994) ]. Our findings support the suggestion of Kapur (2003) that this may be a necessary corollary to the beneficial effect of antipsychotic medication on positive symptoms.
Previous studies suggest that antipsychotic medication does not necessarily normalize abnormal dopamine signalling in psychotic patients. For example, functional neuroimaging studies have shown dopamine dysregulation in both medicated and unmedicated patients (Hietala et al. 1995; Abi-Dargham, 2004; McGowan et al. 2004). Therefore persistent symptoms in medicated patients might still be related to aberrant salience. Furthermore, the only other study investigating stimulus–reinforcement learning for appetitive outcomes in psychosis found that both medicated and unmedicated patients responded more quickly to a CS? than controls, a finding interpreted as aberrant salience (Murray et al. 2008). This study also reported that patients exhibited reduced haemodynamic correlates of reward prediction errors in the ventral striatum relative to controls, consistent with other findings in medicated patients (Juckel et al. 2006; Jensen et al. 2008). Nevertheless it will be important to confirm our findings in unmedicated patients.
Aberrant salience and negative symptoms of schizophrenia
Although positive symptoms were associated with increased aberrant salience, our data also suggest a link between aberrant salience and negative symptoms. Aberrant salience correlated not only with negative symptoms in the patients, but also with O-LIFE introvertive anhedonia, which relates to reduced interest and social withdrawal, in the controls. If dopamine transmission is dysregulated in psychosis (Abi-Dargham, 2004), it is possible that ‘false negatives’ in the phasic dopamine signal might occur, i.e. a reinforcement-related stimulus fails to elicit a sufficiently large phasic dopamine response. False negatives would decrease the value of motivationally salient stimuli, possibly leading to symptoms such as avolition, apathy and social withdrawal. Consistent with this explanation, other studies that investigated responses to emotionally salient images in medicated patients with schizophrenia reported decreased responding for (Heerey & Gold, 2007) and ventral striatal responses to (Taylor et al. 2005) positive emotional stimuli relative to controls.
This explanation is also consistent with data from a functional magnetic resonance imaging study investigating the effects of d-amphetamine on reward processing in healthy volunteers. Knutson et al. (2004) found that amphetamine administration paradoxically decreased the magnitude of phasic ventral striatal haemodynamic responses in response to a CS+ that signalled reward (i.e. increasing the potential for a false negative). In the same study, amphetamine administration caused significant phasic haemodynamic responses in the ventral striatum following CS+ that signalled potential monetary loss, an effect that was absent under placebo, possibly reflecting a loss of specificity of dopamine signalling (i.e. increasing the potential for a false positive). The aberrant salience model might therefore explain both positive and negative symptoms by appealing to a common neurobiological mechanism, namely a loss of signal:noise ratio in the mesolimbic dopamine system, possibly as a result of increased tonic dopamine activity (Grace, 1991; Winterer & Weinberger, 2004).
I believe they are on to something, but the explanation for negative symptoms is still not fully fleshed out or convincing. and of course one has to remember that these results are juts with 20 patients so need to be replicated before being put to use/ accepted as orthodoxy.
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
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