Impaired effort allocation in patients with schizophrenia
Introduction
A growing body of evidence suggests that reduced motivation and goal-directed behavior may occur in schizophrenia without concomitant alterations of hedonic responsivity (Gard et al., 2007, Gold et al., 2008, Barch and Dowd, 2010, Folley and Park, 2010). In preclinical models, effort-based decision-making paradigms that assess the willingness to invest greater effort in order to obtain larger or preferred rewards have repeatedly implicated disruption of corticostriatal dopamine (DA) as a possible substrate for motivational impairments (Salamone and Correa, 2012, Treadway and Zald, 2013); indeed, potentiation or attenuation of DA signaling can increase or decrease effort expenditure for rewards in both rodents (Salamone et al., 2007, Floresco et al., 2008, Bardgett et al., 2009) and humans (Venugopalan et al., 2011, Wardle et al., 2011).
In the context of schizophrenia, however, it appears unlikely that negative symptoms are mediated by a global reduction in striatal DA given robust evidence for striatal DA elevations as a mechanism underlying symptoms of psychosis (Fusar-Poli and Meyer-Lindenberg, 2012, Howes et al., 2013). An alternative explanation is that both positive and negative symptoms result from irregular (as opposed to simply enhanced or reduced) striatal DA release that may not respond appropriately to meaningful reward incentives. Consistent with this model, several recent studies of effort-based decision-making in patients with schizophrenia have found no evidence for a global reduction of effort expenditure, but rather an apparent failure to mobilize effort in response to maximally rewarding cues (Fervaha et al., 2013, Gold et al., 2013, Barch et al., 2014). In the present study, we adopted a similar methodology in an attempt to replicate these prior findings and extend them with a more direct investigation into the utilization of reward magnitude and probability information in guiding effort-based choice in schizophrenia patients.
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Participants and procedure
All subjects provided written informed consent, and all study procedures were approved by the Vanderbilt Institutional Review Board. 13 outpatients with schizophrenia (SZ) participated in the study, and data from 15 healthy control subjects (HC) were drawn from a prior published study (Treadway et al., 2012). All patients were recruited from private-care facilities in Nashville, TN, and completed the Structured Clinical Interview for DSM-IV (First et al., 2005) to confirm diagnosis. Exclusion
Main effects of the EEFRT
Consistent with prior studies using the EEfRT, there were significant main effects of probability (Huynh–Feldt F(2, 39.7) = 9.69, p = 0.001) and reward value (Huynh–Feldt F(3, 64.9) = 15.31, p < 0.00001), such that all subjects were more likely to choose the high effort option when the reward value and probability of receiving reward were higher. There were no group differences in average reaction time between HC (M = 1.32, SD = 0.44) and SZ (M = 1.10 SD = 0.71) (t25 = 1.02, p = 0.319), and no differences in
Discussion
In this study, we replicated prior findings suggesting that SZ patients show significant impairment in the ability to allocate physical effort resources in pursuit of rewards. This is in keeping with three prior independent effort-based decision-making studies, all of which found that SZ did not make fewer high effort choices than HC on average, but generally failed to select the high effort option for the most rewarded trials (Fervaha et al., 2013, Gold et al., 2013, Barch et al., 2014).
Our
Role of funding source
This study was supported by the National Institute of Mental Health (NIMH) grants F31MH087015, K99MH102355 (MTT), and T32 MH018921-21A1 (JSP), and a Brain & Behavior Research Foundation NARSAD grant (SP).
Contributors
MTT, DHZ and SP designed the study, MTT and JSP collected and analyzed the data, MTT, JSP, DHZ, and SP wrote the paper.
Conflict of interest
The authors report no conflict of interest related to this work, financial or otherwise.
Acknowledgments
The authors report no conflict of interest related to this work, financial or otherwise. This study was supported by the National Institute of Mental Health (NIMH) grants F31MH087015, K99MH102355 (MTT), and T32 MH018921-21A1 (JSP), and a Brain & Behavior Research Foundation NARSAD grant (SP). The authors wish to thank Lindsey McIntosh and Heath Nichols for help with recruitment and symptom assessment.
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