Exploration of the perceptual magnet effect using the mismatch negativity auditory evoked potential

The goals of this study were (i) to assess the replicability of the "perceptual magnet effect" [Iverson and Kuhl, J. Acoust. Soc. Am. 97(1), 553-561 (1995)] and (ii) to investigate neurophysiologic processes underlying the perceptual magnet effect by using the mismatch negativity (MMN) auditory evoked potential. A stimulus continuum from /i/ to /e/ was synthesized by varying F1 and F2 in equal mel steps. Ten adult subjects identified and rated the goodness of the stimuli. Results revealed that the prototype was the stimulus with the lowest F1 and highest F2 values and the nonprototype stimulus was close to the category boundary. Subjects discriminated stimulus pairs differing in equal mel steps. The results indicated that discrimination accuracy was not significantly different in the prototype and the nonprototype condition. That is, no perceptual magnet effect was observed. The MMN evoked potential (a preattentive, neurophysiologic index of auditory discrimination) revealed that despite equal mel differences between the stimulus pairs the MMN was largest for the prototype pair (i.e., the pair that had the lowest F1 and highest F2 values). Therefore the MMN appears to be sensitive to within category acoustic differences. Taken together, the behavioral and electrophysiologic results indicate that discrimination of stimulus pairs near a prototype is based on the auditory structure of the stimulus pairs.