Barkley-Levenson & Galvan

• What was the background?

  • Adolescents have heightened sensitivity to reward and an increased response in the ventral striatum when they anticipate and receive an unexpected or expected reward
  • Unclear if sensitivity towards reward a genuine phenomenon or extraneous variable as previously used money as reward and perhaps they value money more than adults as they have less experience of it
• BACKGROUND: What was meant by subjective value?

  • Subjective value is the value an individual places on a stimulus
  • We value choices by working out subjective value of each option and select one with the highest subjective value
• BACKGROUND: What was meant by expected value?

  • Sum of all possible outcomes of choice multiplied by probabilities
  • When expected value was increased in adults there is corresponding increase in activity in the ventral striatum, midbrain prefrontal cortex and dorsolateral prefrontal cortex
  • However previous research has not investigated development of this response so it is unclear if it is true of adolescents
• What were the aims?

  • Whether adolescents more sensitive than adults to increased expected value (EV) (by accepting more gambles)
  • Whether adolescents would show greater ventral striatum activation as EV increases compared to adults
  • Whether adolescents would show heightened ventral striatum response even after matching with adults on no. of gambles accepted (when adults act like adolescents would still not show increased VS activity)
• What was the method?

  • Quasi experiment - lab based
  • IV - adult or adolescent
  • DV - No. of gambles in spinner game accepted by ppt and activity in ventral striatum
• What was the sample?

  • 19 adults and 22 adolescents
  • Self selecting
  • Recruited through posters and online and also drew upon database of people previously taken part in research at UCLA
  • Deemed healthy by self report, no psychiatric disorders or developmental delays
  • No ppts took medication for psychiatric disorders and no metal in bodies (for scan)
• Outline intake session

  • Informed consent - adults own & adol. legal guardians + own agreement
  • Gave details of monthly spending & where money came from - adult $467, adol $52.50
  • Thought to influence value placed on monetary rewards & wished to control this
  • Given $20 as "playing money" - told could win or lose 20
  • Gave ppts sense of ownership of money and also prevents house money effect - recklessly gamble as don't believe money is really theirs
• PROCEDURE: How was spinner set up in fMRI session

  • fMRI session
  • Each half presented amount between 5-20
  • 1 half showed amount could gain and other lose
  • Which side was loss or gain was counterbalanced - ext. v - reliability
• PROCEDURE: How were trials set up in fMRI session

  • 192 gambles per ppt
  • 48 fixed trials
  • 24 manipulated to only be gain and 24 only loss
  • Other 144 mixed and distributed among fixed trials
  • Ppts knew they were gain or loss trials so choosing to gamble not risk
  • EVs of mixed gambles ranged from loss or gain of $7.50 - risk
  • Gain only from $6 to $9
  • Loss only from - $6 to - $9
  • Decide on each whether prepared to accept gamble for real money
  • At end 1 of accepted randomly chosen & played for real money added or taken from participation payment
  • Thoroughly trained in task before doing it - rel.
• What were the results?

  1:

  • Using Hierarchical Linear Modelling found increasing EV - more likely to accept gamble - sig. greater in adol.
  • Disposable income amount no effect
  • No risk trials gamble acceptance no change
  • When no risk adol. behave same as adults

  2:

  • As EV increase - increase in superior medial prefrontal cortex and decreased activation in amygdala, hippocampus
  • Adol. greater activation to increased EV in left ventral striatum but not for right VS

  3: (adult behaving like adolescent)

  • Found difference in activation in left ventral striatum between adults & adol. remains
• What were the conclusions?

  • Value of available options greater influence in adolescence vs adult choices when objective value & subjective choice held constant
  • Maturational changes in neural representation of valuation during adolescence most robust in ventral striatum
  • Neural differences in sensitivity to EV change across development
  • Hyper activation of reward circuitry in adolescents may be normative ontogenic shift due to greater valuation in adolescent brain - not just methodological issue w/ using money as reward
  • Adolescents behave similarly to adults when no risk