Thursday, July 17, 2008

Greek, RT patterns, and the final-letter advantage.

Nikki Pitchford, her student Maria Ktori, Tim Ledgeway and Jackie Masterson have been looking at reaction-time patterns for letter search. In this task, a target letter is presented, and then a random string of 5 letters is displayed, and the subject specifies whether the target letter is in the string. The string remains displayed until the response. Looking at reaction times for positive trials as a function of the position of the target, English readers show initial- and final-letter advantages; that is, a target in the first position is detected more quickly than one in the second position and a target in the fifth position is detected more quickly than one in the fourth position. However Greek readers presented with Greek stimuli show an initial-letter advantage, but not a final-letter advantage. This is true for children and adults.

Now the final-letter advantage is one of my favorite topics, and I've noted that the final-letter advantage is not present in English for exposure durations < 100 ms. My explanation has been that the final letter is not reached for exposures of < 100 ms (due to seriality); at longer exposures, the final letter is activated and can fire for an extended period because it is not inhibited by a subsequent letter, creating a final-letter advantage. So this advantage is taken to occur at the letter level.

However, this explanation is inconsistent with the Greek data. If the final-letter advantage occurs at the letter level, it should be present for Greek, but it is not. This caused me to think that perhaps the initial- and final-letter advantages actually arise an the open-bigram level. Since 2004, the SERIOL model has included edge bigrams, which encode the first and last letters. Recall that open-bigrams are taken to be specific to the ventral/visual route. Greek is a transparent language and there is evidence that transparent languages weight the dorsal/phonological route relatively more heavily than English. Thus ventral-route orthographic representations (i.e., open-bigrams) may play less of a role in Greek string processing than English string processing. If the final-letter advantage actually reflects activation of the final edge-bigram, this would explain why it is absent for Greek.

Note, however, that the original argument on temporal dependency still holds for English. For very brief exposures, the final letter is activated weakly or not at all, and so the final edge-bigram is weakly activated, so there is no final-letter advantage. At longer exposures, the final letter and the final edge-bigram are activated, and so there is a final-letter advantage.

The proposal that these letter effects occur at the bigram level also explains another aspect of their data. For English, they found that positional letter frequency influenced RTs at the first and final positions (i.e., faster RTs for letters more likely to occur at a given position), but there was no effect of positional frequency at the internal positions. Now, under the SERIOL model, the only position-specific representations are edge bigrams: letter units and non-edge open-bigrams are not position-specific. Thus the only positions at which position-specific letter effects could possibly occur are at the edges. If one assumes that frequency affects excitability of bigram units, and bigram excitability affects RTs, this then explains the effect of positional letter frequency at the exterior letters.

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