Electrophysiological Responses to Spectral Ripple Envelope Phase Inversion in 3-Month-Olds and Adults

A critical problem in the clinical management of infants is the limited methods available to assess whether hearing aids or cochlear implants are providing sufficient input for speech sound discrimination. Spectral ripple discrimination is one nonlinguistic, behavioral measure that has been shown to correlate with speech perception in both adults and children. Electrophysiological responses to spectral ripple stimuli have been proposed as a potentially clinically feasible tool. In this study, a behavioral spectral ripple envelope inversion discrimination paradigm was adapted to investigate whether an electrophysiological response can be obtained using electroencephalography (EEG) in infants. Specifically, we investigate whether the detection of envelope inversion via the acoustic change complex (ACC) would match previously reported behavioral discrimination of the same stimuli.

Spectral ripple stimuli included two ripple densities selected based on prior behavioral results: 1 ripple-per-octave (RPO), which can be discriminated by both infants and adults, and 8 RPO, which was not discriminated by either age group. Stimuli were constructed from two concatenated 1-sec noise carriers, with the spectral envelope sinusoidally modulated at either 1 RPO or 8 RPO with 10 dB depth. To elicit an ACC, a spectral envelope inversion was imposed mid-stimulus at 1 sec for both RPOs. Adult and 3-month-old infant participants were presented with stimuli from both conditions while undergoing a 32-channel EEG recording.

At the group level, an ACC to the spectral envelope inversion was detected only in the 1 RPO stimulus condition but not the 8 RPO condition for both infant and adult groups. In contrast, an Onset response to the stimuli was detected in both conditions, demonstrating that the stimuli themselves were detected. In the 1 RPO condition, the ACC detection rate was 81% for individual infants and 73% for individual adults, while in the 8 RPO condition, an ACC was detected in one infant and no adult participants. Individual participant detection rates for the Onset response were 94% for infants and 100% for adults in both conditions.

These findings show a relationship between EEG ACC detection and previously reported behavioral discrimination abilities, with the ACC detected in the 1 RPO but not the 8 RPO condition. These results demonstrate the ACC's potential for infant discrimination threshold estimation because it was elicited by a suprathreshold acoustic change (1 RPO) but not a subthreshold acoustic change (8 RPO). Critically, both 1 RPO and 8 RPO stimuli contained an envelope inversion at 1 sec, suggesting the ACC could serve as an objective measure of stimulus discriminability in infants as young as 3 months. Future studies with more stimulus conditions to fully establish a threshold, online monitoring of responses to increase the success rate of infant testing, and comparison of testing infants awake versus asleep are needed to further develop this approach.