Thursday, June 5, 2008

Dubois et al. (2007) in Cognitive Psychology

The authors look at perceptual patterns for a seventh-grade French dyslexic, MT, who has no phonological deficit. In particular, they look at trigram identification across a range of retinal locations (centered from -7 to 7), for MT versus seven age-matched controls. The authors fit curves to the trigram data, and did not find any difference between MT and the controls.

However, the SERIOL model makes quite specific predictions of how perceptual patterns should differ between dyslexics and controls, which the authors did not evaluate. The model predicts that a letter's position within the string should have a much stronger influence in the LVF than the RVF. This is due to the proposal of learned left-to-right inhibition in the LVF/RH. For younger readers, this effect should be strongest near fixation, where perceptual learning is the strongest. For example, accuracy for a letter at retinal location -2 should be much better when it is the 1st letter in the string than when it is the 3rd letter. In contrast, accuracy for a letter at retinal location 2 should be minimally affected by its position within the string. This asymmetry should be a signature of normal visual/orthographic processing, and it should be absent for dyslexics, under the assumption that they are not performing normal visual processing.

Indeed, inspection of the data in Figure 6, shown above, supports this prediction. In this figure, a filled circle represents the 1st letter in the LVF and the 3rd letter in the RVF. Conversely, an open circle represents the 3rd letter in the LVF and 1st letter in the RVF. For controls for eccentricities of 1 to 3 letter widths, it is evident that string position had a strong effect in the LVF, but not the RVF, while the pattern was symmetric for MT. Examination of the individual data shows that the asymmetric pattern held at the individual level.

Of course, this is a very small sample size. I would suggest that it is important to try this experiment on a large group of school-age controls and dyslexics to see how diagnostic this asymmetric vs. symmetric pattern truly is. If it is highly diagnostic, this would be quite informative as to the nature of core deficits in developmental dyslexia.


MatthieuDubois said...

Hello Carol,

I’m particularly happy to see that our work was used to test hypotheses made by the SERIOL model, a really interesting and influential framework to think about visual perception in reading. Following your suggestion, I plotted some data from our paper in a more convenient way. Unfortunately, I was not able to upload it in this comment, but you could find it on my website, at the following adress:

On the average, control subjects behave like the predictions. That is, a strong linear positional effect (first > second > third letter of the trigram) in the LVF is observed for the various trigrams eccentricities. Also, the RVF is marked by no obvious effect of letter position near fixation and a marked last letter primacy at larger eccentricities. Letter position effect was similar on RVF in MT, our dyslexic participant. However, despite differences between MT and controls in the LVF, visual inspection of the plots does not fully support the
left-right visual fields symmetry, hypothesized in MT. For example, a linear letter positional effect was sometimes observed in MT’s LVF (for the trigram centered at position -3).

I also would like to emphasize the huge inter-individual variability within the control group, which was not obvious in the original plotting. This variability is indexed by large SD (up to 22% for some visual eccentricities) and it is clearly shown in the individual control subjects’ panels (in the replotted figures). Some control subjects display letter recognition patterns similar in some way to MT’s performance.

Unfortunately, these results cannot be straightforwardly interpreted, and did not fully support/exclude unequivocally the hypotheses you proposed. This experiment was not designed to specifically test this hypothesis. Particular methodological aspects might have added some noise to the data, yielding to this highly variable pattern of results. Especially, the difference in exposure durations might be problematic (MT: 166 ms, controls: 66 ms), as the first/last letter primacy asymmetry between the visual hemifields varies with exposure duration in normal readers (Whitney & Cornelissen, 2005). Note however that these conclusions only rely on visual inspection, and not formal assessment, of the differences. I would be happy to realise new analyses to better test your hypotheses.

As you suggests, it would be interesting to try this experiment on a large group of school-age controls and dyslexics.


Matthieu Dubois, Ph.D.
Psychology and NeuroCognition Lab
Grenoble, France

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