Formant rise time (FRT) and amplitude rise time (ART) are acoustic cues that inform phonetic identity. FRT represents the rate of transition of the formant(s) to a steady state, while ART represents the rate at which the sound reaches its peak amplitude. Normal-hearing (NH) native English speakers weight FRT more than ART during the perceptual labeling of the /ba/–/wa/ contrast. This weighting strategy is reflected neurophysiologically in the magnitude of the mismatch negativity (MMN)—MMN is larger during the FRT than the ART distinction. The present study examined the neurophysiological basis of acoustic cue weighting in adult cochlear implant (CI) listeners using the MMN design. It was hypothesized that individuals with CIs who weight ART more in behavioral labeling (ART users) would show larger MMNs during the ART than the FRT contrast, and the opposite would be seen for FRT users.
Electroencephalography was recorded while 20 adults with CIs listened passively to combinations of 3 synthetic speech stimuli: a /ba/ with /ba/-like FRT and ART; a /wa/ with /wa/-like FRT and ART; and a /ba/wa stimulus with /ba/-like FRT and /wa/-like ART. The MMN response was elicited during the FRT contrast by having participants passively listen to a train of /wa/ stimuli interrupted occasionally by /ba/wa stimuli, and vice versa. For the ART contrast, the same procedure was implemented using the /ba/ and /ba/wa stimuli.
Both ART and FRT users with CIs elicited MMNs that were equal in magnitudes during FRT and ART contrasts, with the exception that FRT users exhibited MMNs for ART and FRT contrasts that were temporally segregated. That is, their MMNs occurred significantly earlier during the ART contrast (~100 msec following sound onset) than during the FRT contrast (~200 msec). In contrast, the MMNs for ART users of both contrasts occurred later and were not significantly separable in time (~230 msec). Interestingly, this temporal segregation observed in FRT users is consistent with the MMN behavior in NH listeners.
Results suggest that listeners with CIs who learn to classify phonemes based on formant dynamics, consistent with NH listeners, develop a strategy similar to NH listeners, in which the organization of the amplitude and spectral representations of phonemes in auditory memory are temporally segregated.
Native English speakers with normal hearing (NH) weight some acoustic cues more strongly than others during speech perception, and these weighting strategies are reflected neurophysiologically in the magnitude of the mismatch negativity (MMN). The present study examined acoustic cue-weighting in adult listeners with cochlear implants (CIs) using the MMN design. Results suggest that for listeners with CIs who learn to classify phonemes in a manner similar to listeners with NH, neurophysiological representations of phonemes in auditory memory show MMN features that are also consistent with listeners with NH, namely temporal segregation for amplitude and spectral cues.
1Department of Otolaryngology-Head & Neck Surgery, The Ohio State University Wexner Medical Center, Columbus, Ohio, USA; and 2University of California Davis Center for Mind and Brain, Davis, California, USA.
This work was supported by NIH/NIDCD Grants (AJS, R03-DC011168 & R01-DC013543).
The authors have no conflicts of interest to disclose.
Received December 31, 2014; accepted October 29, 2015.
Address for correspondence: Aaron C. Moberly, Department of Otolaryngology, The Ohio State University, 915 Olentangy River Rd., Suite 4000, Columbus, OH 43212, USA. E-mail: Aaron.Moberly@osumc.edu