How orthographic-specific characteristics shape letter position coding: The case of Thai script

A central question for any model of visual word identification is the representation of the position at which letters are encoded (e.g., calm vs. clam). In this article, we examine whether the orthographic-specific characteristics of a writing system-namely, Thai-shape the process of letter position coding. Thai is an alphabetic script that lacks interword spaces and has an orthographic order that does not necessarily correspond to the phonological order for initial vowels. This implies that the initial letter position coding in Thai needs to be flexible enough that readers can successfully encode the letter positions of words. To compare letter position coding in Thai to that in English, we conducted an experiment that paralleled Experiment 3 in Gomez, Ratcliff, and Perea (Psychological Review, 115, 577-600, 2008), including 23 conditions (single-letter replacements, letter transpositions, letter migrations, and a corresponding control). We obtained fits from Gomez et al.'s overlap model, which is a model that has been shown to account for letter position coding in the Roman alphabet across this variety of letter manipulations. The overlap model was found to successfully fit the Thai data. Our results revealed that the position encoding was better for the first letter than for the rest of the positions in both languages; however, in English the position uncertainty grows as a function of letter order quite abruptly, whereas in Thai it grows gradually. Thus, the orthographic-specific characteristics of the Thai writing system do play a role in shaping the process of letter position coding.