Tone languages represent a large proportion of the spoken languages of the world and yet lexical tone is understudied. Thai offers a unique opportunity to investigate the role of lexical tone processing during visual-word recognition, as tone is explicitly expressed in its script. We used colour words and their orthographic neighbours as stimuli to investigate facilitation (Experiment 1) and interference (Experiment 2) Stroop effects. Five experimental conditions were created: (a) the colour word (e.g., ขาว /khã:w/ [white]), (b) tone different word (e.g., ข่าว /khà:w/[news]), (c) initial consonant phonologically same word (e.g., คาว /kha:w/ [fishy]), where the initial consonant of the word was phonologically the same but orthographically different, (d) initial consonant different, tone same word (e.g., หาว /hã:w/ yawn), where the initial consonant was orthographically different but the tone of the word was the same, and (e) initial consonant different, tone different word (e.g., กาว /ka:w/ glue), where the initial consonant was orthographically different, and the tone was different. In order to examine whether tone information per se had a facilitative effect, we also included a colour congruent word condition where the segmental (S) information was different but the tone (T) matched the colour word (S-T+) in Experiment 2. Facilitation/interference effects were found for all five conditions when compared with a neutral control word. Results of the critical comparisons revealed that tone information comes into play at a later stage in lexical processing, and orthographic information contributes more than phonological information.
Keywords: Lexical tone; Reading; Stroop effect; Thai; Visual-word recognition.