Cortical evidence of the perceptual backward masking effect on /l/ and /r/ sounds from a following vowel in japanese speakers

Abstract

We examined the influence of stimulus duration of foreign consonant vowel stimuli on the MMNm (magnetic counter part of mismatch negativity). In Experiment 1, /ra/ and /la/ stimuli were synthesized and subjects were native Japanese speakers who are known to have difficulty discriminating the stimuli. “Short” duration stimuli were terminated in the middle of the consonant-to-vowel transition (110 ms). They were nevertheless clearly identifiable by English speakers. A clear MMNm was observed only for short-duration stimuli but not for untruncated long-duration (150-ms) stimuli. We suggest that the diminished MMNm for longer duration stimuli result from more effective masking by the longer vowel part. In Experiment 2 we examined this hypothesis by presenting only the third formant (F3) component of the original stimuli, since the acoustic difference between /la/ and /ra/ is most evident in the third formant, whereas F1 and F2 play a major role in vowel perception. If the MMNm effect depends on the acoustic property of F3, a stimulus duration effect comparable to that found with the original /la/ and /ra/ stimuli might be expected. However, if the effect is attributable to the masking effect from the vowel, no influence of stimulus duration would be expected, since neither stimulus contains F1 and F2 components. In fact, the results showed that the “F3 only” stimuli did not show a duration effect; MMNm was always elicited independent of stimulus duration. The MMN stimulus duration effect is thus suggested to come from the backward masking of foreign consonants by subsequent vowels.