8/01/2006

Developmental Change in the Neural Mechanisms of Risk and Feedback Perception

Effective decision making involves diverse skills, including the estimation of risk, constant monitoring of feedback, and task-set maintenance - all of which undergo rapid developmental shifts even before adolescence. What aspects of decision making change during this time, and is it possible to localize these functions to their hardware components in the brain? This is the question explored in the first fMRI study of decision making in children younger than 12, by Leijenhorst, Crone and Bunge.

According to these authors, recent neuroimaging evidence suppors the idea that risk estimation & anticipation can be localized to the orbitofrontal cortex (OFC) and the anterior cingulate cortex (ACC), while ventrolateral prefrontal cortex (vlPFC) is activated by negative performance feedback. (These results are surprising, given that ACC is typically considered a "conflict detector" or "error monitor;" one would expect ACC, and not necessarily vlPFC, to be engaged by negative feedback.)

One useful task for investigating decision making is the Iowa Gambling Task, in which healthy subjects learn to forego immediate gains for larger (but delayed) rewards. The task itself usually consists of four stacks of cards; if a card is drawn from two of these (A or B), the participant receives a large reward (e.g., $100), whereas a card drawn from the other two stacks (C or D) provides only a small reward ($50). However, stacks A and B each contain 10% penalty cards, in which the participant will lose $1250; C and D contain a similar card, but only for $250. On the whole, then, stacks C and D are "good" stacks, while A and B are "bad" decks; to succeed on this task, however, participants must be capable of maintaining their task-set, monitoring feedback, and estimating risk.

Typically, subjects with OFC damage perseverate on the bad decks, and furthermore do not show the typical stress reaction (via galvanic skin response) to hovering over the "bad" decks. One interpretation of this evidence is that OFC subserves the estimation of risk, and may be specifically engaged when "Reversal learning" is required (i.e., changing responses on the basis of risk estimation). Other imaging evidence shows that DLPFC is engaged when subjects decide to forego immediate gain for future reward (perhaps because it is maintaining the choice to wait).

Given the different developmental trajectories of these brain regions, Leijenhorst, Crone and Bunge created a child-adapted version of the gambling task to differentiate risk estimation and feedback processing in 9-12 year olds and young adults. They used the "cake task" in which subjects are presented with a display containing 9 slices of cake, and must decide which of two types of cake (chocolate or strawberry) is most likely to be selected by the computer (which choses at random). The task is designed such that there are high-risk decisions (in which three or four pieces differed in flavor from the others) and low-risk decisions (in which only one or two pieces differed in flavor from the others), as well as positive feedback (when they select the type of cake that is randomly selected by the computer) and negative feedback (when they don't select the type of cake randomly selected by the computer).

[for the fMRI haters in the audience, I highly suggest that you read Section 2.4 of this paper, which details every step of the fMRI data analysis - this particular study was quite well done. For now, I'll just say that they did an event-related design with over 120 trials on each of 26 subjects. The authors performed ROI analyses of OFC, VLPFC, DLPFC, medial PFC/ACC, and midbrain, by contrasting high-risk, positive feedback trials with low-risk, positive feedback trials, and high-risk, positive feedback trials with high-risk negative feedback trials; the ~6% trials on which subjects selected the wrong type of cake were removed from the analysis. The results are as follows.]

Both children and adults picked the most likely cake flavor over 90% of the time, and both groups also made more errors on high-risk than low-risk trials (though children were more prone to this mistake than adults). The risk contrast revealed more DLPFC and OFC activity in high-risk trials relative to low-risk trials, but there was no interaction with age, suggesting that children may engage these areas during risk estimation in a similar fashion as adults. Only in children was medial PFC & ACC more engaged during high-risk than low-risk trials, suggesting that age related differences in risk estimation may have to do with the amount of perceived response conflict. Midbrain ROI analyses revealed nothing significant.

In contrast, medial and ventrolateral PFC was more active for negative feedback than positive feedback across ages, suggesting that these regions are not responsible for age-related differences in feedback processing. However, children more fully engage lateral OFC during negative feedback relative to positive feedback trial than adults do. To rule out that this activity reflects that negative feedback was unexpected (since, statistically speaking, the subject actually made the most likely choice, and was wrong only by chance), the authors contrasted positive & negative feedback trials among low-risk trials alone. Consistent with their original interpretation, this contrast was not significantly different from the contrast performed only on high-risk trials. [The authors also report whole-brain analyses, which are inferior to ROI analyses in terms of statistical power, and are thus not discussed here.]

In summary, children and adults seem to differ both in risk processing (such that children more fully engage medial PFC & ACC more under conditions of high risk), as well as in their processing of feedback (children more fully engage lateral OFC during negative feedback relative to positive feedback), but also share many similarities (both groups utilize DLPFC and OFC more in high-risk than low-risk trials, as well as engage mPFC & vlPFC more during negative than positive feedback). The behavioral results indicate that children are more likely to make risky decisions in high-risk situations than adults. Based on the observation that error-related negativity increases with age during adolescence, children may be activating ACC more highly because of increased response conflict relative to adults.

At the end of the article, the authors discuss other evidence suggesting that children may have difficulty in processing negative feedback because they tend to assume that even irrelevant negative feedback is relevant to their behavior, in contrast to other explanations that suggest children are merely less able to adjust their behavior based on negative feedback.

Related Posts:
Risk Taking and Intelligence
The Rules in the Brain (and the development of oPFC, dlPFC, rlPFC & vlPFC)
Softmax rule for exploration-exploitation (Neurodudes)

2 Comments:

Blogger crapcakes said...

Aren't we reverse engineering the mind if we start at the brain? It would make more sense if we took specs from the other side of the transformer.

Cheerios,
Michel

http://meta-dna.blogspot.com

8/05/2006 04:45:00 PM  
Anonymous Anonymous said...

weird, i dont get this 1 bit

8/05/2006 08:22:00 PM  

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