Monday, June 2, 2008

Cohen et al. (2008) in Neuroimage

In this fMRI experiment, the authors presented words that were progressively degraded, under three manipulations:
  • Shifting the word into the LVF
  • Increasing the spacing between letters
  • Rotating the string
Within each manipulation, there were 5 levels of degradation, where level 1 was normal presentation, and level 5 was maximally degraded. For levels 4 and 5, the authors found a behavioral length effect under all three degradations. Parietal activity increased from level 1 to level 5. The authors conclude that words are normally processed in parallel, while degradation causes attention-driven, serial processing.

I wrote a commentary on this article, but Neuroimage would not publish it. Briefly, the article points out that there are 3 main problems with their analysis.
  • If there's an abrupt shift in processing mode at the onset of the length effect (between levels 3 and 4), parietal activity should show a large jump between levels 3 and 4. However within each manipulation, parietal activity was similar for levels 3 and 4.
  • Attention-driven processing cannot explain the time scale of the length effect, which was ~20 ms/letter, as it's been shown that serial covert shifts of attention take at least 300 ms per shift.
  • The authors cannot explain the results of Whitney & Lavidor (2004), who showed that the LVF length effect can be abolished via a contrast manipulation.
Furthermore, it is straightforward to explain their results under the SERIOL model. As I previously proposed in email to Andy Ellis, degradation would interfere with automatic bottom-up formation of the locational gradient. Therefore, top-down attention is recruited to form the activation gradient. This yields a less finely tuned (steeper) gradient than normal, and a length effect emerges from the usual serial processing. See the commentary for details.


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