Dopamine signaling predicts individual differences in reinforcement learning in humans

The tradeoff between short-term and long-term consequences is inherent in many important decisions, as retirement savings or health behavior. Different neural accounts have been proposed to underlie intertemporal choice behavior via differences in valuation or selfcontrol mechanisms. Recent studies showed that imagining specific future events reduced reward delay discounting and was associated with increased prefrontal-mediotemporal coupling. We investigated whether temporal lobe epilepsy patients who suffer from hippocampal sclerosis exhibit increased discounting of future rewards, dependent on the side of the sclerosis. Methods: Up to now, 12 patients with right (RMTL), 8 patients with left (LMTL) and 11 patients with bilateral (BMTL) hippocampal sclerosis have been included together with 30 patients with extra temporal lobe epilepsy (ETL). The groups did not differ with respect to age or gender. We elicited time-preferences using three different lists of choices. Subjects had to decide between a sooner and an increasing later amount of money in each row. The three lists differed with respect to the time horizons. Future discounting was measured by taking the switching points from the sooner to the later option. To control for effects of general risk aversion we also conducted an incentivized risk experiment. Subjects had to decide between a lottery with a winning probability of 0.5 and an increasing amount of a safe option. Again the switching point served as a measure for preference. In both time and risk experiments we adapted a similar design as in Dohmen et al. (American Economic Review, 2009). Results: MTL patients with leftor bilateral hippocampal sclerosis showed higher discounting of future rewards compared to right – MTL and ETL patients (mean switching points: LMTL 16.1; BMTL: 18.2; RMTL: 9.7; ETL 10.9, Kurskal-Wallis-Test of independent groups p=0.034). Conclusions: Our results show that patients with bilateral or pure left sided hippocampal sclerosis discount future rewards more steeply, highlighting the importance of the medial temporal lobe in intertemporal choices. Two processes could underlie this effect: first, damage to the hippocampus itself – which is involved in episodic future thinking (Addis et al., Human Neurosci., 2012; Schacter et al., Nat. Neurosci., 2007); second, the inhibition of the neural self-control mechanism through interictal discharges to the left lateral prefrontal cortex (LPFC). Further studies will have to investigate these effects in more detail. Acknowledgements: BW is funded by a Heisenberg-Grant of the German Research Council (WE 4427/3-1)