Tuesday, June 3, 2008

Martin et al. (2007) in Brain Research

In this study, the subjects performed the Reicher-Wheeler task on five-letter words versus unpronounceable nonwords, for exposure durations of 50 and 66 ms. The stimuli were presented so that the target letter always occured at fixation. So for example, when the target was the second letter, the second letter appeared at fixation, putting the first letter in the LVF and the third to fifth letters in the RVF. Thus the retinal location and visual acuity of the target letter did not vary with its position within the string. The task was performed by adult unimpaired readers and dyslexics.

This provides an opportunity to look at how accuracy interacts with string position and exposure duration. First we consider unimpaired readers. Under the assumption of serial processing, some letters may be read out before the mask occurs, and others will be read out after the mask occurs. The latter letters should be at a disadvantage. In general, the SERIOL model predicts that an increase in exposure duration should have the strongest effect at string positions in the transition zone (i.e., letters that were formerly read out after the mask occurred, but now are read out before the mask.) In this experiment, the change in exposure duration was 16ms, which is on the time scale proposed for per-letter processing. So at first glance, this suggests that early string positions should not be affected by an increase in exposure (because they are read out before the mask in any case) and later string positions should also not be affected (because they are read out after the mask in any case), while a transitional position should be affected. Here are the results from the experiment:

The nonword results for control subjects show an asymmetric effect of increased exposure, with the largest improvement for position 1, and no improvement at positions 4 and 5. This pattern is difficult to explain under parallel processing, but does not exactly match the SERIOL intuition that the improvement should be localized at the position that was not read out at 50 ms, but was read out at 66ms.

However, let's consider the mechanics in more detail. Due to strong bottom-up activation in the LVF/RH, an increase in string position at fixation will not necessarily cause that letter to be read out (at the letter level) a full "time slot" (~15 ms) later, because the additional LVF/RH letters in earlier positions can "fill in" earlier time periods. That is, at the feature level, an initial letter at -1 reaches a higher activation than an initial letter at 0 (fixation). Hence, for a letter at fixation, activations (at the feature level) are similar for position 1 versus position 2. Therefore the timing of activation at the letter level does not vary much either. This is illustrated in the following figure, which shows the proposed time that a letter starts firing at the letter level, based on its retinal location and string position. It shows how each increase in string position from 1 to 3 at fixation could delay firing by ~5 ms, rather than ~15 ms. There is a much larger difference going from position 3 to 4 under the assumption that an initial letter at -3 is too far from fixation to reach maximal activation at the feature level; the reduced activation level then percolates through the string, due to RH-left-to-right and cross-hemispheric lateral inhibition.

Under this account, for the 50 ms exposure, the letter at fixation does not fire before the mask appears. For a 67 ms exposure, the letter at fixation can start to fire before the mask when it is in positions 1, 2, or 3. This explains the observed interaction of increased exposure with string position. (However, this doesn't explain why having the third letter at fixation yields the poorest results overall. This may be due to greater positional uncertainty about the middle letter.)

It is also interesting that there was no or a very weak initial-letter advantage in the nonword data. This is consistent with the idea that the initial-letter advantage is essentially a LVF non-initial-letter disadvantage. That is, when a second letter falls in the LVF, it receives much more additional lateral inhibition (at the feature level) than when it is the first letter in the LVF. In contrast, when a second-letter falls at fixation, it only receives slightly more lateral inhibition than when it is the first letter. Thus the advantage for being the first letter is much reduced at fixation, compared to retinal locations in the LVF. In contrast, Tydat and Grainger (in press, JEP:HPP) claim that the initial-letter advantage is due to reduced receptive-field sizes for letters, such that a letter receives considerably less inhibition with 1 immediate flanker than with 2 flankers. This account incorrectly predicts that an initial-letter advantage should be present at fixation.

Note that the best overall firing patterns are obtained when fixation falls on the second or third letter. That is, these conditions allow the earliest completion of letter readout. This explains the OVP effect observed in the word conditions. Thus the word and nonword conditions yield different patterns, with fixation on the third letter yielding the poorest results for nonwords, but the best results for words. This is because accuracy in the word condition is influenced by the processing of the entire string (to yield lexical activation), which is best at positions 2 and 3, while accuracy in the nonword condition is influenced primarily by the processing and localization of the target letter.

It is also interesting to see that the dyslexics showed a different pattern. First, there was no position X exposure-duration interaction. This is consistent with my proposal that dyslexics process letters in parallel, while unimpaired readers process them serially. Secondly, there was no word-superiority effect, except when fixation fell on the third letter. This may indicate that these dyslexics use a retinotopic method to encode letter position, which is keyed to having two letters in the LVF. When the presentation condition matches this requirement, lexical representations are well activated; otherwise they are not. This is consistent with the case study of a single French dyslexic (Dubois et al., 2007, Cognitive Neuropsychology), which showed that lexical recognition was best when fixation fell on the third letter, independently of string length.


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