The impact of tone systems on the categorical perception of lexical tones: An event-related potentials study

This study investigates the categorical perception (CP) of pitch contours (level and rising) by native listeners of two tone languages, Mandarin and Cantonese, for both speech and nonspeech. Language background was found to modulate participants' behavioural and electrophysiological responses to stimuli presented in an active oddball paradigm, comprising a standard and two equally spaced deviants (within- and across-category). The stimuli were divided into two sets according to the results of a two-alternative forced-choice identification test: a rising set, using a standard that listeners identified as high rising tone, and a level set, using a standard that listeners identified as high level tone. For the rising set, both groups of listeners exhibited CP in terms of their behavioural response. However, only Cantonese listeners exhibited a significant CP effect in terms of P300 amplitude. For the level set, the behavioural data revealed a shift in category boundary due, in part, to the range-frequency effect. According to the d' scores, the CP effect elicited from Mandarin listeners was greater for nonspeech stimuli than for speech, suggesting the presence of a psychophysical boundary. There was no such behavioural contrast for Cantonese listeners. However, Cantonese listeners exhibited a significant CP effect in P300 amplitude that was influenced by the range-frequency effect, as well as a possible secondary phonological boundary. P300 amplitude is believed to index the ease of discrimination of speech stimuli by phonological information. We conclude that Cantonese listeners engaged phonological processing in order to discriminate speech stimuli more efficiently than Mandarin listeners. These findings may be due to the different tonal inventories of Mandarin and Cantonese, with Cantonese listeners required to make finer distinctions in perception of pitch height and slope than Mandarin listeners in order to discriminate the denser tone system of Cantonese.