Retest reliability of reward-related BOLD signals
Introduction
Over the last 20 years, functional magnetic resonance imaging (fMRI) has made immense contributions to the understanding of the neural foundations of reward processing in humans. Reward processing plays a central role for basic cognitive functions like reinforcement learning and decision making, and is thus of fundamental importance for neuroscientific studies in the fields of learning theory, economic decision making (neuroeconomics) and social neurosciences (because social decisions appear to be grounded in the same reward processing brain structures as financial decisions; Fehr and Camerer, 2007).
The fundamentals of reward processing were first identified in animal studies, which showed that dopaminergic midbrain neurons increase their activity when a cue signals an upcoming reward or when an unexpected reward actually occurs (reward prediction error, RPE) (Schultz et al., 1997). Secondary reward signals occur in the primary projection sites of those midbrain neurons, i.e. the ventral striatum (VS) and the orbitofrontal cortex (OFC). FMRI studies have shown that BOLD signal responses in humans parallel those findings to a high extent: BOLD activity in the midbrain (D'Ardenne et al., 2008) and in its projection sites (Pagnoni et al., 2002, Rolls et al., 2008) increases with reward predictions and scales positively with the RPE. It shows temporal characteristics as expected from temporal difference models of reinforcement learning (O'Doherty et al., 2003). FMRI studies have demonstrated that a variety of different rewards can induce such reward-related brain activity. The strength of the brain's response to a reward correlates better with the subjective value of a reward for that individual (individual preference) than with objective reward magnitude (Kable and Glimcher, 2007, O'Doherty et al., 2006, Tobler et al., 2007). Together, these results suggest that activity in reward processing brain areas can be interpreted as a surrogate biomarker of individuals' preferences (Knutson et al., 2009). There are also promising results suggesting that inter-individual differences in reward-related BOLD signals are related to personality traits (Beaver et al., 2006) and to genetic polymorphisms that affect dopamine metabolism (Cohen et al., 2005, Jocham et al., 2009).
Further establishing these links requires better knowledge about the reliability of the respective measures. This subject has been the topic of a recent scientific debate that has evolved regarding the meaningfulness of across-subject correlations between BOLD signals and other individual measures such as personality traits, preferences or attitudes (Poldrack and Mumford, 2009, Vul et al., 2009). The expected correlation between two measures A and B is theoretically limited by measures' reliability, where the upper limit of correlation is given by the reliability index: √(relA ⁎ relB) (Nunnally, 1970). Therefore, when using the BOLD signal as an indicator for individual characteristics, it is essential to know its reliability, i.e. the stability of inter-individual differences in the magnitude of BOLD contrasts over time. Given the central role of reward processing for many fields of cognitive neuroscience and the potential use of reward-related BOLD signals as biomarkers for individuals' preferences, we therefore specifically analyzed the test–retest reliability of reward-related BOLD responses.
Section snippets
Subjects
Eight female and 17 male subjects, aged 19–35 (mean = 26.12, SD = 3.98), were scanned during two sessions separated by 7.7 days on average (range 7–13 days). The time of day of each session was kept fairly constant for each subject (median daytime differences between the sessions: 18 min, range 0–260 min). Subjects had no history of psychiatric or neurological disorders. Two female subjects had to be excluded from the entire analysis, one due to an incidental finding of a brain abnormality and the
Behavioral data
Behavioral data revealed no gross differences in subject's response behavior between both sessions: neither the percentage of missed responses, mean response times nor amount of rewards differed significantly between sessions 1 and 2 in any of the paradigms (p > 0.05, see Table 2), except that mean response times in Session 2 of Paradigm C were significantly lower than in session one (p = 0.015). Percentage of improbable guesses in Paradigm A and percentage of incorrect responses in Paradigm B also
Discussion
This study determined retest reliabilities of BOLD signals related to reward processing. This was based on the consideration that these signals might be used as surrogate markers for individuals' preferences, but that such an application requires sufficient reliability. Despite highly significant main effects of reward contrasts and good reliabilities of motor-related activity, we found that retest reliabilities of different reward contrasts in the ventral striatum and the orbitofrontal cortex
Acknowledgments
We would like to thank Courtney B. Phillipps, Florian Mormann and Jason Aimone for valuable comments on the manuscript.
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These authors have equally contributed to the study.