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Our analysis also identified a region in the dorsal hippocampus as responding preferentially in the d = today condition. However, the mean event-related response in these voxels was qualitatively different from that in the other regions identified by the β analysis (fig. S2). To confirm this, for each area we conducted paired t tests comparing d = today with d = 2 weeks and d = 1 month at each time point after the time of choice. All areas showed at least two time points at which activity was significantly greater for d = today (P < 0.01; Bonferroni correction for five comparisons) except the hippocampus, which, by contrast, is not significant for any individual time point. For these reasons, we do not include this region in further analyses. Results are available in (21) (fig. 52).
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One possible explanation for increased activity associated with choice sets that contain immediate rewards is that the discounted value for these choice sets is higher than the discounted value of choice sets that contain only delayed rewards. To rule out this possibility, we estimated discounted value for each choice as the maximum discounted value among the two options. We made the simplifying assumption that subjects maintain a constant weekly discount rate and estimated this value based on expressed preferences (best-fitting value was 7.5% discount rate per week). We then regressed out effects of value from our data with two separate mechanisms. First, we included value as a separate control variable in our baseline GLM model and tested for β and δ effects. Second, we performed a hierarchical analysis in which the effect of value was estimated in a first-stage GLM; this source of variation was then partialed out of the data and the residual data was used to identify β and δ regions in a second-stage GLM. Both of these procedures indicate that value has minimal effects on our results, with all areas of activation remaining significant at P < 0.001, uncorrected.
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Difficulty was assessed by appealing to the variance in preferences indicated by participants. In particular, when the percent difference between dollar amounts of the options in each choice pair was 1% or 3%, subjects invariably opted for the earlier reward, and when the percent difference was 35% or 50%, subjects always selected the later, larger amount. Given this consistency in results, we call these choices "easy." For all other differences, subjects show large variability in preference, and we call these choices "difficult" (Fig. 3A). These designations are further justified by analyzing the mean response time for difficult and easy questions. Subjects required on average 3.95 s to respond to difficult questions and 3.42 s to respond to easy questions (Fig. 3B) (P < 0.005). We assume that these differences in response time reflect prolonged decision-making processes for the difficult choices. Based on these designations, we calculated mean blood oxygen level dependent (BOLD) responses for easy and difficult choices (Fig. 3C).
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Because difficulty was associated with longer RT, it was necessary to rule out nonspecific (i.e., non-decision-related) effects of RT as a confound in producing our results. We performed analyses controlling for RT analogous to those performed for discounted value as described above (29). This is a conservative test because, as noted above (30), we hypothesize that at least some of the variance in RT was related to the decision-making processes of interest. Nevertheless, these analyses indicated that removing the effects of RT does not qualitatively affect our results.
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This prediction requires only that we assume that activity in each system reflects its overall engagement by the decision and, therefore, its contribution to the outcome. Specifically, it does not require that we assume that the level of activity in either system reflects the value assigned to a particular choice.
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Our results are also consistent with the hypothesis that the fronto-parietal system inhibits the impulse to choose more immediate rewards. However, this hypothesis does not easily account for the fact that this system is recruited even when both rewards are substantially delayed (e.g., 1 month versus 1 month and 2 weeks) and the existence of an impulsive response seems unlikely. Therefore, we favor the hypothesis that fronto-parietal regions may project future benefits (through abstract reasoning or possibly "simulation" with imagery), providing top-down support for responses that favor greater long-term reward and allowing them to compete effectively with limbically mediated responses when these are present.
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We thank K. D'Ardenne, L. Nystrom, and J. Lee for help with the experiment and J. Schooler for inspiring discussions in the early planning phases of this work. This work was supported by NIH grants MH132804 (J.D.C), MH065214 (S.M.M.), National Institute on Aging grant AG05842 (D.I.L.), and NSF grant SES-0099025 (D.I.L).
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