Vowel normalization and the perception of speaker changes: an exploration of the contextual tuning hypothesis

Many experiments have reported a perceptual advantage for vowels presented in blocked-versus mixed-voice conditions. Nusbaum and colleagues [Nusbaum and Morin (1992). in Speech Perception, Speech Production, and Linguistic Structure, edited by Y. Tohkura, Y. Sagisaka, and E. Vatikiotis-Bateson (OHM, Tokyo), pp. 113-134; Magnuson and Nusbaum (2007). J. Exp. Psychol. Hum. Percept. Perform. 33(2), 391-409] present results which suggest that the size of this advantage may be related to the facility with which listeners can detect speaker changes, so that combinations of less similar voices can result in better performance than combinations of more similar voices. To test this, a series of synthetic voices (differing in their source characteristics and/or formant-spaces) was used in a speeded-monitoring task. Vowels were presented in blocks made up of tokens from one or two synthetic voices. Results indicate that formant-space differences, in the absence of source differences between voices in a block, were unlikely to result in the perception of multiple voices, leading to lower accuracy and relatively faster reaction times. Source differences between voices in a block resulted in the perception of multiple voices, increased reaction times, and a decreased negative effect of formant-space differences between voices on identification accuracy. These results are consistent with a process in which the detection of speaker changes guides the appropriate or inappropriate use of extrinsic information in normalization.