Frequency-Following Responses to Speech Sounds Are Highly Conserved across Species and Contain Cortical Contributions.

Time-varying pitch is a vital cue for human speech perception. Neural processing of time-varying pitch has been extensively assayed using scalp-recorded frequency-following responses (FFRs), an electrophysiological signal thought to reflect integrated phase-locked neural ensemble activity from subcortical auditory areas. Emerging evidence increasingly points to a putative contribution of auditory cortical ensembles to the scalp-recorded FFRs. However, the properties of cortical FFRs and precise characterization of laminar sources are still unclear. Here we used direct human intracortical recordings as well as extracranial and intracranial recordings from macaques and guinea pigs to characterize the properties of cortical sources of FFRs to time-varying pitch patterns. We found robust FFRs in the auditory cortex across all species. We leveraged representational similarity analysis as a translational bridge to characterize similarities between the human and animal models. Laminar recordings in animal models showed FFRs emerging primarily from the thalamorecipient layers of the auditory cortex. FFRs arising from these cortical sources significantly contributed to the scalp-recorded FFRs via volume conduction. Our research paves the way for a wide array of studies to investigate the role of cortical FFRs in auditory perception and plasticity.

Dichotic phase effects on frequency following responses reveal phase variant and invariant harmonic distortion products.

The dichotic frequency following responses (FFR) have been used in studies to infer about dichotic auditory processing. In the present study, we hypothesize that the proximity of the binaural neural generators of the FFR would result in interference of the volume-conducted electrical fields. This might lead to contamination of the scalp-recorded dichotic FFRs due to which it might be difficult to infer about true dichotic processing in the putative neural generators. We investigated this by recording FFRs to binaurally presented 200 Hz pure tone with graded dichotic phase offsets (0°, 90°, 180° and 270°) in normal hearing young adults. Spectral analysis of the FFRs was performed for the estimation of the magnitude and phase at the component frequencies. FFR spectra were compared using non-parametric paired randomizations within the subjects. We found that the brainstem responses to a 200 Hz pure tone consisted of prominent peaks at 200 Hz, and at frequencies corresponding to the harmonics of 200 Hz. The FFR spectral magnitude at 200 Hz diminished with a phase offset of 180°. Phase offsets of 90° and 270° showed reduced spectral magnitudes at 200 Hz than those in the 0° condition. Our findings, in line with the hypothesis, show that the dichotic FFRs do not reflect true dichotic processing and that they are contaminated during volume conduction. Additionally, we found harmonic distortion products (HDP) in the FFRs. We found that the response at 200 Hz and the 3rd HDP systematically varied with a change in phase of the stimulus, while the even HDPs (2nd and 4th) were phase-invariant. Based on our findings, and modeling FFRs using auditory models, we propose a rectification process as the contributors for the generation of HDPs. We also discuss the implications of this HDP generating mechanism in understanding the pitch represented in FFRs.

Novel Paradigm to Record Bilateral Click-Evoked Auditory Brainstem Responses Simultaneously (BiSi-ABR).

OBJECTIVES: The current study proposes a new and fast technique to record the auditory brainstem responses (ABRs) simultaneously (BiSi-ABR) from two ears. The BiSi-ABR technique can be used to record the ABRs two times faster than with a conventional ABR recording method. The objective of the study was to show the proof of concept and to compare the BiSi-ABR technique with that of a conventional ABRs recording method to test its clinical feasibility. MATERIALS AND METHODS: A repeated-measures design was used, wherein ABRs recorded in the BiSi-ABR were compared with that of conventional ABRs recordings. Twenty-five normal-hearing adults participated in the study. ABRs were recorded using the BiSi-ABR technique, as well as the conventional method. The peak latencies (in ms) of waves III and V between the new technique and conventional method were compared. The minimum intensity at which the wave V was present was tracked using both the methods. RESULTS: The wave latencies and thresholds of ABR using the BiSi-ABR technique were remarkably similar to those recorded in the conventional ABR technique. The ABR wave latencies and thresholds did not differ significantly between the new technique and the conventional method. CONCLUSION: ABRs recorded with the BiSi-ABR technique can be used to estimate ear-specific hearing thresholds with the same reliability as that of conventional ABRs, in half the recording time. The results of the study have strong implications for screening, diagnostic, and research purposes as they aid in cutting down the ABR testing time.

Gender-bias in the sensory representation of infant cry.

The auditory neural pathway in females appears to be more sensitive to the cry of an infant (De Pisapia et al., 2013; Messina et al., 2016). Cortical responses in females have shown a distinct advantage compared to males in the auditory processing of infant cry. Such gender-bias in the cortical responses might emanate either at higher levels of processing such as cognitive and emotional processing or at the lower level representation of stimulus features. We assessed for a difference if any, between the two genders, in the sensory representation of an infant's cry. We used frequency following responses (FFR) to assess the sensory representation of an infant cry. This was done in sixteen male and fifteen female non-parent adults. The FFR closely mimics the stimulus acoustics with fine temporal precision and is the measure of choice to assess the sensory encoding of sounds in the auditory system. We performed spectral analysis of the FFRs and compared the spectral magnitudes between males and females. We found significantly higher FFR spectral magnitudes in females compared to males. The gender differences found were not related to the confounding variables such as head size or differences in the volume-conducting media. By systematically controlling other influencing variables, we show that the bias in neural processing of infant cry in females emerges right at the sensory representation levels.

Acoustic basis of context dependent brainstem encoding of speech.

The newfound context dependent brainstem encoding of speech is evidence of online regularity detection and modulation of the sub-cortical responses. We studied the influence of spectral structure of the contextual stimulus on context dependent encoding of speech at the brainstem, in an attempt to understand the acoustic basis for this effect. Fourteen normal hearing adults participated in a randomized true experimental design in whom brainstem responses were recorded. Brainstem responses for a high pass filtered /da/ in the context of syllables, that either had same or different spectral structure were compared with each other. The findings suggest that spectral structure is one of the parameters which cue the context dependent sub-cortical encoding of speech. Interestingly, the results also revealed that, brainstem can encode pitch even with negligible acoustic information below the second formant frequency.