Vinckier et al. (2007) in Neuron

In this fMRI study, the authors varied the word-likeness of string stimuli at six levels - false fonts, rare letters forming rare (contiguous) bigrams, frequent letters forming rare bigrams, frequent letters forming frequent bigrams but rare quadrigrams, frequent bigrams forming frequent quadrigrams, words - and investigated the sensitivity of brain regions around the VWFA to this manipulation. In their Discussion, the authors state:

"Our results demonstrate effects of letter and quadrigram frequency above and beyond those of bigram frequency, suggesting that all of these levels (Dehaene et al., 2005), not just bigrams (Grainger & Whitney, 2004; Whitney, 2001), may be useful subcomponents of visual word recognition."

However the claim that their data provides evidence for quadrigram detectors is tenuous, at best. The claim comes from the comparison of frequent bigrams forming rare quadrigrams vs. frequent bigrams forming frequent quadrigrams. However, this comparison is confounded with the pronounceability of the stimuli. The rare quadrigram stimuli were not pronounceable, whereas the frequent quadrigram stimuli were pronounceable. Thus frequent-quadrigram stimuli were much more likely to yield partial activation of lexical representations, and therefore any difference between the two may reflect different levels of lexical activation, rather than quadrigram activation. This is supported by their finding that only words and frequent quadrigrams yielded significant activation of posterior middle temporal gyrus, a region associated with lexico-semantic processing.

On the other hand, the contrast between frequent letters forming rare bigrams vs frequent letters forming frequent bigrams provides does not suffer this confound, as both types of strings were not pronounceable. Differences between between these types of stimuli were found in middle/anterior left fusiform. Binder et al. (2006, Neuroimage) also found sensitivity to bigram frequency in this area in another fMRI study. These results support the claim of multi-letter units, such as open-bigrams, and are difficult to explain under models that do not include them, such as Davis's SOLAR model and Gomez et al.'s Overlap model.

SOLAR Model - Davis & Bowers (2006) in JEP:HPP

For five-letter targets in lexical decision, the authors compare primes of the form 1d345 vs. 13d45. The SOLAR model predicts stronger priming for 1d345. In the original specification of the SERIOL model, open-bigram activation decreased with increasing string position. Under this assumption, the SERIOL model predicts stronger priming for 13d45. Davis and Bowers found stronger priming for 1d345 than 13d45, and concluded that the SERIOL model is inconsistent with their experimental results.

However, the conclusions of Davis and Bowers (2006) are based on an obsolete parameterization of the SERIOL model. In my 2004 dissertation, the assumption that bigram activation depends on string position was dropped; bigram activations now depend only on the separation between the constituent letters. As a result, the SERIOL model, like the SOLAR model, predicts stronger priming for 1d345 than 13d45. Thus the priming results do not actually differentiate between the two models. (For a discussion of why the positional variation was originally included, why it was dropped, and how the model now accounts for the phenomenon originally explained by the positional variation, see Whitney (in press, LC&P) ).

Despite the fact that a primary focus of Davis and Bowers (2006) was to compare the SERIOL and SOLAR models, JEP:HPP did not invite me to review their article. Hence, their article includes claims that are no longer accurate. (It also includes a claim that is outright incorrect - that even if the positional variation were dropped, the SERIOL model would still be inconsistent with their results.) Nor would JEP:HPP allow me to publish a reply. Fortunately, I found a fairer venue, and my reply to Davis and Bowers (2006) is now in press at Brain and Language.

Briefly, the main points of the response are:

• The SERIOL model is consistent with the results of Davis & Bowers (2006).
• SOLAR's lexical activation function is not biologically plausible.
• A recent re-parameterization of the SOLAR model leads a contradiction of Davis and Bower's core claim that contiguity matters.
• Experimental comparisons between models should test inherent aspects of the models and not depend on particular parameterizations.
• A critical comparison is presented. That is, I specify a pair of primes for which the two models inherently make different predictions. (Colin Davis agrees with this analysis.)