Tuesday, 28 November 2017

Barkley and Levenson's neural representation of expected value in the adolescent brain (2014)



Background/context

Subjective value = the value an individual gives to a stimulus

Expected value = sum of all possible outcomes of a choice

Ontogenetic differences = differences in biological development

The adolescent reward system is hyperactive but this may confounded by differences in how teens look at money compared to adults

Barkley-levenson's 3 hypotheses

1- Adolescents will exhibit greater behavioural sensitivity to EV than adults
2- Neurobiology Vs. activation will modulate in proportion to increasing EV more for adolescents than adults
3- Adults behaving like adolescents in terms of gambling will not exhibit hyperactive striatal activation


Aim

To investigate whether adolescents attach more value to rewards than adults do


Method

Quasi-experiment with independent measures design, lab conditions. 
IV= adult or adolescent. 
Dv= neural activation in brain using fmri during asimple gambling game


Procedure

Sample-

19 right handed adults aged 25-30 and 22 right handed adolescents aged 13-17 recruited through adverts at UCLA


Intake session-

P's visited lab for intake session and neuroimaging, and provided informed consent

Each p asked to provide their primary money source, and amount of spending money per month

Significant difference in spending money between adolescents ($52) and adults ($467)

P's familiarised with scanner by a mock scanner, and paid $20 for the intake session, they were informed they could use it as spending money in the lab session, and informed that they could win $20 more OR lose the original $20 in the gambling fMRI task


Actual session-

A week later, p's returned for the fMRI session and completed a gambling task, then presented with a series of gambles with a 50% probability of gaining amount shown on 1 side of the spinner OR losing the amount shown on the other side

Gain and loss amounts ranged between -$20 and +$20

144 trials, 24 gain only and 24 loss only

order of the stimuli was counter balanced across p's and for each trial, p's decided whether or not to gamble that money and were informed that one of the trials they accepted would be played for with real money, this encouraged a choice on each trial that was consistent with the p's gambles


Findings

20 adolescents and 17 adults in the final sample because the others didn't produce useful data

Acceptance rates didn't change in either group when there was no risk involved in the trials, suggesting adolescents behave similarly to adults when there is no risk involved

Across all p's all trials will positive EV's accepted more than ones with negative EV

No significant difference between groups in % of EV + trials accepted, % of EV0 trials accepted and % of EV trials accepted

Increased EV = increased likelihood of an acceptance response and even more activation in the VS in adolescents than adults, as EV increased

Amount of disposable income didn't have a effect on relationship between EV and acceptance rates

no significant differences between groups' reaction time 

Increasing EV had a stronger influence over gambling choices in adolescents

Activation in some brain areas increased with increasing EV, while activation in other brain areas decreased with increasing EV


Conclusion

Value of available options = greater influence in adolescents, even with constant observation and sub values

Maturational changes in neural representation of valuation during adolescence = more robust in the Ventral striatum

Neural differences in sensitivity to EV change across development

Hyper-activation of reward circuitry in adolescence = normative ontogenetic shift = adolescents do more risk taking behaviour


Evaluation

Risk that p's may drop out of study because it was longitudinal, but it was ethically sound and FMRI can cause discomfort and claustraphobia

Adolescents may fall under social desirability effects

Small sample size and lacked ecological validity
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