Proefschrift_Holstein

Chapter 5

effect on switch trials (Age x Reward: r (ρ) = - 0.367, p < 0.001; figure 5.2b ). Inspection of figure 5.2b shows that these correlations were driven by Reward effects in young, but not old participants. Young, but not old participants, showed a reward-related increase in speed-over- accuracy on switch trials, and a reward-related decrease in speed-over-accuracy on repeat trials. Thus, the main task effects, observed across the group as a whole and described above, were driven by younger participants and were absent in older participants. The degree to which a promised reward affected task switching did not change with age in terms of response times (Age x Reward x Task switching: r (ρ) = -0.038, p = 0.714) or accuracy (Age x Reward x Task switching: r (ρ) = -0.178, p = 0.053). We validated the SAT effects in a subgroup of participants who all received the same reward size (i.e. group A, C and D, table 5.1 ). Age-dependent effects in this subsample, which was not confounded by differential reward size, resembled those observed in the large sample. A negative correlation was observed between Age and the effect of Reward on Task switching (Age x Reward x Task switching: r (ρ) = - 0.360, p = 0.003; figure 5.3a ). This three-way interaction was again due to a positive correlation between Age and the Reward effect on repeat trials (Age x Reward: r (ρ) = 0.277, p = 0.022) and a negative correlation between Age and the Reward effect on switch trials (Age x Reward: r (ρ) = -0.373, p = 0.002). The result from this continuous analysis with Age as a covariate was confirmed by a between-group independent samples Mann-Whitney U test with Age as a between-subject factor. For this analysis, the subgroup (N = 68, mean age 30.29, range 14 – 67 years) was split into two groups based on the median age (25 years old; youngest group: N = 35, mean 16.23 (SE 0.48) years old; oldest group: N = 33, mean 45.18 (SE 2.29) years old; U = 7.114, p < 0.001). A significant Age x Reward x Task-switching interaction was revealed (U = -2.755, p = 0.006), which was due to a Reward x Task switching interaction in the younger group (W = -3.849, p < 0.001), but not in the older group (W = -1.885, p = 0.059) ( table 5.2, figure 5.3b ). In sum, aging was accompanied by diminished effects of Reward on Task switching in terms of SAT. This effect was confirmed in a smaller subsample, corrected for reward size. Age-related changes in response deadlines and earned rewards We hypothesized that age-related changes in task switching would be grounded in motivational changes. Increasing age was indeed associated with cognitive changes: We observed age-related changes in task switching. In addition, we observed smaller reward effects with age, both across repeat and switch trials and as a function of task switching. In a supplementary analysis, we assessed whether this reward-related deficit in terms of behavior was accompanied by changes in total earnings. Surprisingly, we observed an age-related increase in the total reward earned on the rewarded task-switching paradigm. This effect was observed in the large sample (n=118) (Age x Total reward: r (ρ) = 0.581, p < 0.001), and in the subgroup of 68 participants in which the maximum bonus did not vary across participants (Age x Total reward: r (ρ) = 0.309, p =

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