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The First Letter Position Effect in Visual Word Recognition: The Role of Spatial Attention
Introduction:
Visual processing of letters within a word has historically been proposed to occur in parallel; that is, all letters are processed simultaneously. This notion has been challenged by recent research suggesting the occurrence of serial position effects during visual word recognition, with preferential processing of the middle and the first letter. While the middle letter advantage can be explained by foveal acuity at central fixation, mechanisms underlying the first letter advantage are less well-understood. One proposed mechanism known as the Modified Receptive Field Hypothesis suggests that the first letter advantage is attributable to a leftward elongation of receptive fields that evolves during the acquisition of skilled reading in languages read from left to right. We designed a paradigm to directly test this hypothesis.
Methods:
Our paradigm involved a two-word forced-choice task in which a target word was briefly presented followed by two alternatives, one which directly matched the target word and the other which differed by one letter. The word length and the position of letter mismatch varied across trials. Importantly, stimuli were randomly presented in either a horizontal orientation or a vertical orientation. If the Modified Receptive Field Hypothesis is true, one would expect only to observe a first letter advantage in horizontally-oriented trials but not vertically-oriented trials, as leftward elongated receptive fields would not bias processing toward a particular letter when all letters are presented in the same location along the horizontal meridian.
Results:
Results indicated a significant main effect of letter position on accuracy and response latency across all word lengths, which was largely driven by an increased accuracy and decreased response latency for a first-letter mismatch. Critically, there was not a significant interaction between letter position and orientation for either accuracy or response latency; the first letter advantage persisted regardless of stimulus orientation.
Conclusions:
1) Our consistent observation of a first letter advantage, as well as some indication of a stepwise decrease in accuracy and increase in response latency across letter positions, provides evidence against the parallel processing theory of visual word recognition.
2) The lack of an observed advantage for the last letter suggests that the first letter advantage is likely not attributable to reduced perceptual crowding, as has previously been proposed as a potential mechanism.
3) The persistence of the first letter advantage independent of stimulus orientation suggests that the first letter advantage is likely not attributable to a leftward elongation of receptive fields, as suggested by the Modified Receptive Field Hypothesis.
4) We suggest that the first letter advantage may be due to a highly flexible spatial attention system that involves rapid reallocation of attention to the letter providing the most lexical information (i.e., the first letter), thus increasing the efficiency of word processing.