Evidence Suggesting that Specialized Visual Regions Are Formed by Pruning in Early Childhood
Monday, May 24, 2010
There are quite a few specialized visual regions in the brain. For example, the fusiform face area (FFA) activates for faces, and the visual word form area (VWFA) in the left fusiform is consistently active for words.
How do these specialized cortical regions develop? Is it experience dependent? Do regions have a preexisting preference for certain visual features? (For example, perhaps the visual word form region prefers high contrast stimuli with sharp borders). Do these regions form by increasing activation to preferred stimuli, or a decreasing activation to nonpreferred stimuli? Cantlon and colleagues investigated these questions in a recent study.
They tested prereading five year olds and adults in an fMRI experiment. Participants saw faces, letters, numbers, shoes and scrambled images and pressed a button if a green border appeared around the picture. There were two interesting findings.
The first concerned the visual word form area. Both adults and children had a specialized brain region in the left fusiform that activated more for letters than objects. However, while adults activated that region more for letters than for numbers, children had equally high activation for letters and numbers.*
These results support a role for both experience and low level visual features in the development of the visual word form area. Note that these children are nonreaders, but they already activate the left fusiform for letters and numbers. So perhaps there’s something hardwired in the left fusiform that prefers symbol-like, high contrast, visual stimuli. But only adults, who have had extensive experience with letters, show differential activation for words and numbers.
The authors then investigated the relationship between activation level and behavior. They tested children on a face matching task and a letter naming task. Contrary to what you might expect, activation in the fusiform face area did not correlate with face matching skill, and activation in the visual word form area did not correlate with letter naming skill.
Rather, skill was negatively correlated with activation to the nonpreferred category. Face matching performance was inversely correlated with FFA activation to shoes. And letter naming was inversely correlated with VWFA activation to faces. This suggests that that increased skill in face and letter recognition is associated not with enhancing activation to preferred stimuli, but with pruning back activation to unrelated stimuli. **
*Methodological note: ROI selection, 10 strongest voxels within a sphere 10mm radius around peaks of All>scrambled.
**Note that not all nonpreferred stimuli show this inverse correlation. In the face area, there was no correlation between face skill and symbols, and in the VWFA, there is no correlation between letter naming skill and shoe activation. Perhaps these nonpreferred stimuli too far from the preferred stimulus, so no pruning is needed?
Cantlon JF, Pinel P, Dehaene S, & Pelphrey KA (2010). Cortical Representations of Symbols, Objects, and Faces Are Pruned Back during Early Childhood. Cerebral cortex (New York, N.Y. : 1991) PMID: 20457691
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5 comments:
> So perhaps there’s something hardwired in the left fusiform that prefers symbol-like, high contrast, visual stimuli.
Perhaps. Or perhaps the children in the study all grew up in places where symbol-like, high contrast visual stimuli were a normal part of the environment.
Perhaps they need to do the study with some pre-reading children from the middle of the Amazon rain forest, or from the Congo--someplace where words and numbers aren't a part of the general environment.
Western kids are exposed to a myriad of symbols before they even leave the hospital: from labels on the maternity ward doors, to street signs and business names on buildings, to the brand names on their toys, magazines and newspapers lying around the house, all manner of stuff on TV, et cetera. Is it any surprise their brains have begun to specialize for this stuff, even before they fully understand what it means?
Jason - Yes, that's also a definite possibility. Scanning younger or, as you suggested, getting children from an illiterate culture would speak to that.
Although if that were true, we still have the quesiton of why is it this specific region, rather than another region, that ends up being specialized in words and symbols.
That's a good question too. Do you know if this symbol-specific region is near or co-located with regions that process lines, edges, and other high-contrast parts of the visual input space?
I remember reading a description in Lise Eliot's wonderfully layman-friendly "What's Going on in There" that the visual cortex of kids from western culture was more highly adapted to detect horizontal and vertical lines, while kids from some other cultures were better at diagonals.
The hypothesis was that western architecture makes a pretty big deal about horizontal and vertical lines (our floors are flat and our walls are at right angles to them) while cultures that make more use of tents, teepees, and other pole-and-cloth type structures naturally have a lot more diagonals in them.
This, to me, seems at least vaguely related to the question of symbol processing: could a symbol-processing region have emerged culturally (rather than evolutionarily) as the result of hyper-specialization of some sub-portion of the visual cortex which really did evolve to detect edges?
Jason -- the lines and edges are generally in lower level regions, but the word region is close to the face region, and there are some similarities between how the two are processed (foveally).
So to some extent, symbol-processing regions can't have evolved -- there just wasn't enough time evolutionarily for that to happen. The only way evolution could have come in is to create an area the was suited to processing word-like stimuli, which then got coopted for reading. I believe Stanislas Dehaene has a book out on reading which covers this.
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