Neural sensitivity to absolute and relative anticipated reward in adolescents

Abstract

Adolescence is associated with a dramatic increase in risky and impulsive behaviors that have been attributed to developmental differences in neural processing of rewards. In the present study, we sought to identify age differences in anticipation of absolute and relative rewards. To do so, we modified a commonly used monetary incentive delay (MID) task in order to examine brain activity to relative anticipated reward value (neural sensitivity to the value of a reward as a function of other available rewards). This design also made it possible to examine developmental differences in brain activation to absolute anticipated reward magnitude (the degree to which neural activity increases with increasing reward magnitude). While undergoing fMRI, 18 adolescents and 18 adult participants were presented with cues associated with different reward magnitudes. After the cue, participants responded to a target to win money on that trial. Presentation of cues was blocked such that two reward cues associated with $.20, $1.00, or $5.00 were in play on a given block. Thus, the relative value of the $1.00 reward varied depending on whether it was paired with a smaller or larger reward. Reflecting age differences in neural responses to relative anticipated reward (i.e., reference dependent processing), adults, but not adolescents, demonstrated greater activity to a $1 reward when it was the larger of the two available rewards. Adults also demonstrated a more linear increase in ventral striatal activity as a function of increasing absolute reward magnitude compared to adolescents. Additionally, reduced ventral striatal sensitivity to absolute anticipated reward (i.e., the difference in activity to medium versus small rewards) correlated with higher levels of trait Impulsivity. Thus, ventral striatal activity in anticipation of absolute and relative rewards develops with age. Absolute reward processing is also linked to individual differences in Impulsivity.

Figure 2. Group differences in reward processing in the right VS.

A) Effect of absolute reward magnitude processing. Adults compared to adolescents demonstrated a more linear increase in anticipatory reward activation as a function of absolute reward magnitude at the point of peak activation (6 seconds post-stimulus onset; p = .02). B) Relative reward activity in VS to $1 reward. Based on paired t-tests for each age group, results showed that adults (p = .007) but not adolescents (p = .34) demonstrated significant differences in VS activity at 6 seconds post-stimulus onset to the $1 reward depending on the relative context.

Figure 1. Behavioral performance on the modified MID Task.

As described in greater detail in the text, adults and adolescents were faster and more accurate on high reward trials.

Figure 3. Group differences in reward processing in the left VS.

A) Effect of absolute reward magnitude processing. There was a trend for adults compared to adolescents to demonstrate a more linear increase in anticipatory reward activation as a function of absolute reward magnitude at the point of peak activation (6 seconds post-stimulus onset; p = .07). B) Relative reward activity in VS to $1 reward. Based on paired t-tests for each age group, results showed that adults showed greater activity at 6 seconds post-stimulus onset in the $1 amid $.20 alternatives condition (p = .01). The same effect was not significant for adolescents (p = .65).

Figure 4. Group differences in reward processing in the mesial PFC.

Only hit trials were included for mesial PFC time courses. A) Effect of absolute reward magnitude processing in mesial PFC. There was a main effect at 8 s as a function of absolute reward value (p = .01). However, there were no significant interaction effects between absolute reward value and age group at any time point (ps>.05). B) Relative reward activity to the $1 reward in mesial PFC. A paired t-test indicated that adults did show a difference in the degree to which mesial PFC activity decreased as a function of relative reward value at Time 0 (p = .01). However, none of the interaction effects between relative reward value and age group were significant at any time point (ps>.05).

Figure 5. Anticipated absolute reward magnitude activity in adolescents and adults for the whole brain analyses.

Results are shown on a coronal slices for the VS (y = 8). Although the direct comparison between the two groups was not significant, the within group analyses show that the adults demonstrated strong, bilateral activation in the VS and other adjacent regions whereas the effect was much weaker in the adolescents. P-value threshold set to .005 as in other whole brain analyses.