alt Hacker NewsThe specific counterintuitive result is mentioned toward the end of the article, and I'm having some trouble understanding it:
> when analyzing average trends in groups of children, slower reaction times to the “Go” signal were linked to increased activity in many brain regions, including the default mode network
> However, when an individual had a slower reaction time to the “Go” signal, activity decreased in the default mode network — the opposite of the group-level pattern.
Replies
Acting on your first impulse is fast (default mode).
Denying that first impulse, thinking about it, and then acting is slow.
One way to think of it — I didn't read the article in depth so this is just an example — is in terms of overall individual differences in speed and activity level. Then, you could have slower persons having increased activity relative to faster persons, but it still be true that when a slower person had an even slower signal reaction, their activity went down, and when a faster person had a slower signal reaction, their activity went down as well.
It's a classic psychological phenomenon, where individual differences are obscuring time course patterns and vice versa.
Of course, this sidesteps the question of why (in the hypothetical example) the overall individual differences exist. Assuming those general individual differences are reliable and "real", you still have to explain why they are there, and if they predict significant outcomes, why they do, and so forth.
The message of the paper is good, although I think the press release (not surprisingly) overstates the significance of the paper. I think these kinds of issues have received a lot more attention in the literature in the last decade or so in neuroscience. It also sort of sidesteps a lot of the more thorny questions about truly person-specific patterns and how to determine when they're meaningful.
I think the plot here explains it well
Hmm I think all these replies are overcomplicating things.
At a group level, some kids are slower at this Stop/Go task than others. The group difference appears to be this increased broad-scale brain activity: the slow group is overall more prone to distraction and daydreaming.
However, at an individual level, slowing down on the task means increasing your focus (and decreasing brain activity in irrelevant regions), regardless of whether you were in the slow group or the fast group. So the group-level difference is not necessarily as profound as it might appear, and applying "slow group" with too broad a brush means you're going to sweep up some kids who are naturally cautious and focused.
Now, I don't know anything about neuroscience or brain development, but hopefully I can explain the statistics in a way useful to you.
Imagine there are two groups A and B. One group, A, has slower reactions on average and high average activity The other, Group B, has higher reactions and lower than the Group A's activity. Yet inside both groups the general trend is that if someone is slower than the average reaction of their group then they're also below the average activity for their group.
If we look at the overall means without distinguishing groups, slower reaction is correlated positively with higher activity (kids from group A have higher activity and slower reaction in general, which pushes the correlation upwards. As long as the relationship in Group B isn't too strong the upward trend from Group A can easily dominate overall correlation) but inside each group the trend is actually the opposite.
This applies pretty much every time you're comparing samples. If I understood your quote correctly, they're studying a child's reaction time vs activity level by comparing the same kid in different times. The same logic applies, a person can exhibit the opposite trend to the populational average due to the same mechanism above. This can be even more dramatic, because once you start looking at averages you start losing time dependency information.
More broadly (and more formally), multivariate covariance splits in within-group and between-group terms, so if the signs of the terms are different the magnitude of one can dominate the overall sum and flip the sign.