Investigating the perceptual magnet effect in speech perception: An event-related potential study

The “Perceptual Magnet Effect” (PME) is characterized by perceptual warping in favor of stronger assimilation near the prototypical sound against other nonprotypical variants. This ERP study adopted a modified oddball paradigm to examine the PME predictions. The participants were 18 normal adults. The stimuli were computer-synthesized /a/ sounds and their acoustic controls by replacing the formant frequencies with sinewaves. Prior to the ERP sessions, the prototype (P) and nonprototype (NP) were chosen based on behavioral identification and goodness rating. Then 4 variants were created with ±40 mels apart in reference to P or NP, respectively, in either the first formant (F1) or second formant (F2). Mismatch negativity (MMN) responses were derived with the P (or NP) as the standard and the 4 variants as the deviant. The same ERP procedure was used for the nonspeech stimuli. Behavioral results showed that individual subjects consistently rated vowel category goodness based on the F2/F1 ratio. MMN amplitudes in the P and NP conditions showed a pattern consistent with the PME model. This pattern was observed for both speech and nonspeech stimuli. These results provide neural evidence in support of the PME that may extend to general auditory processing of spectral patterns.