Brainstem correlates of cochlear nonlinearity measured via the scalp-recorded frequency-following response

Perceptual warping exposes categorical representations for speech in human brainstem responses

The brain transforms continuous acoustic events into discrete category representations to downsample the speech signal for our perceptual-cognitive systems. Such phonetic categories are highly malleable, and their percepts can change depending on surrounding stimulus context. Previous work suggests these acoustic-phonetic mapping and perceptual warping of speech emerge in the brain no earlier than auditory cortex. Here, we examined whether these auditory-category phenomena inherent to speech perception occur even earlier in the human brain, at the level of auditory brainstem. We recorded speech-evoked frequency following responses (FFRs) during a task designed to induce more/less warping of listeners' perceptual categories depending on stimulus presentation order of a speech continuum (random, forward, backward directions). We used a novel clustered stimulus paradigm to rapidly record the high trial counts needed for FFRs concurrent with active behavioral tasks. We found serial stimulus order caused perceptual shifts (hysteresis) near listeners' category boundary confirming identical speech tokens are perceived differentially depending on stimulus context. Critically, we further show neural FFRs during active (but not passive) listening are enhanced for prototypical vs. category-ambiguous tokens and are biased in the direction of listeners' phonetic label even for acoustically-identical speech stimuli. These findings were not observed in the stimulus acoustics nor model FFR responses generated via a computational model of cochlear and auditory nerve transduction, confirming a central origin to the effects. Our data reveal FFRs carry category-level information and suggest top-down processing actively shapes the neural encoding and categorization of speech at subcortical levels. These findings suggest the acoustic-phonetic mapping and perceptual warping in speech perception occur surprisingly early along the auditory neuroaxis, which might aid understanding by reducing ambiguity inherent to the speech signal.

Temporal Modulation Transfer Functions of Amplitude-Modulated Cervical Vestibular-Evoked Myogenic Potentials in Young Adults

OBJECTIVES

Cervical vestibular-evoked myogenic potentials (cVEMPs) are widely used to evaluate saccular function in clinical and research applications. Typically, transient tonebursts are used to elicit cVEMPs. In this study, we used bone-conducted amplitude-modulated (AM) tones to elicit AMcVEMPs. This new approach allows the examination of phase-locked vestibular responses across a range of modulation frequencies. Currently, cVEMP temporal modulation transfer functions (TMTFs) are not well defined. The purposes of the present study were (1) to characterize the AMcVEMP TMTF in young, healthy individuals, (2) to compare AMcVEMP TMTFs across different analysis approaches, and (3) to determine the upper frequency limit of the AMcVEMP TMTF.

DESIGN

Young adults (ages 21 to 25) with no history of vestibular lesions or middle ear pathologies participated in this study. Stimuli were amplitude-modulated tones with a carrier frequency of 500 Hz and modulation frequencies ranging from 7 to 403 Hz. Stimuli were presented at 65 dB HL via a B81 bone-oscillator.

RESULTS

AMcVEMP waveforms consisted of transient onset responses, steady-state responses, and transient offset responses; the behavior of these different types of responses varied with modulation frequency. Differences in the TMTF shape were noted across different measures. The amplitude TMTF had a sharp peak, while signal-to-noise ratio and phase coherence TMTFs had broader shapes with plateaus across a range of modulation frequencies. Amplitude was maximal at modulation frequencies of 29 and 37 Hz. Signal-to-noise ratio maintained its peak value at modulation frequencies between 17 Hz and 127 Hz. Phase coherence and modulation gain maintained their peak values at modulation frequencies between 17 Hz and 143 Hz.

CONCLUSIONS

AMcVEMPs reflect transient onset and offset responses, as well as a sustained response with the periodicity of an amplitude-modulation frequency. AMcVEMP TMTFs had variable shapes depending on the analysis being applied to the response; amplitude had a narrow shape while others were broader. Average upper frequency limits of the AMcVEMP TMTF were as high as approximately 300 Hz in young, healthy adults.

Nonlinearity in bone-conducted amplitude-modulated cervical vestibular evoked myogenic potentials: Harmonic distortion products

Otolith organs of the balance system, the saccule and utricle, encode linear acceleration. Integrity of the saccule is commonly assessed using cervical vestibular evoked myogenic potentials (cVEMPs) arising from an inhibitory reflex along the vestibulospinal pathway. Conventional approaches to eliciting these responses use brief, transient sounds to elicit onset responses. Here we used long-duration amplitude-modulated (AM) tones to elicit cVEMPs (AMcVEMPs) and analyzed their spectral content for evidence of nonlinear processing consistent with known characteristics of vestibular hair cells. Twelve young adults (ages 21-25) with no hearing or vestibular pathologies participated in this study. AMcVEMPs were elicited by bone-conducted AM tones with a 500 Hz carrier frequency. Eighteen modulation frequencies were used between 7 and 403 Hz. All participants had robust distortion products at harmonics of the modulation frequency. Total harmonic distortion ranged from approximately 10 to 80%. AMcVEMPs contain harmonic distortion products consistent with vestibular hair cell nonlinearities, and this new approach to studying the otolith organs may provide a non-invasive, in vivo method to study nonlinearity of vestibular hair cells in humans.