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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 =