Envelope Periodicity Research Articles

Temporal fine structure sensitivity measured with pulse-spreading harmonic complexes.

Two experiments investigated sensitivity to temporal fine structure (TFS) in a group of normal hearing participants. The stimuli were bandpass filtered pulse-spreading harmonic complexes (PSHCs) with a regular envelope repetition rate and a phase adjusted so that the TFS peaks were progressively shifted across envelope periods. For up-PSHCs, the TFS peaks were advanced, yielding a rising pitch percept, while for down-PSHCs, the peaks were delayed, yielding a falling pitch percept. Experiment 1 showed that in a fixed frequency region, there was a range of rates for which the direction of the pitch change could be identified. Cochlear model simulations suggested that participants may use either place-of-excitation and/or temporal cues to perform this task. Experiment 2 showed that there was an envelope rate below which down-PSHCs and up-PSHCs could not be discriminated. This lower envelope rate limit of TFS sensitivity significantly increased with increases in frequency region and was similar to the lower rate limit of melodic pitch. The results in high frequency regions suggest that TFS cues are available up to 10 kHz when the rank of the lowest component present in the passband is 18, and all harmonics are presumably unresolved.

Using vocoded speech to study temporal weighting in spatial release from masking

Several aspects of binaural and spatial hearing appear to be dominated by moments of rising envelope fluctuations (“onsets”), although those phenomena are limited to events occurring less than a few hundred times per second. Thus, for example, the localization of a brief tone is dominated by the binaural information available in the overall sound onset. Whereas for a modulated sound each envelope period may contribute equally, assuming the modulation rate is not too high. For speech signals, syllabic modulations provide multiple onset-like events which may occur synchronously or asynchronously across frequency bands. Although previous studies have shown that rising envelope fluctuations play an outsized role in localization and lateralization, it is unknown whether other aspects of spatial hearing–such as spatial release from masking (SRM)–are similarly dominated by rising envelopes. Here, we used a Gabor-click-train vocoder to transform speech sounds and manipulate the spatial content of rising- versus falling-envelope segments of the speech in each frequency band. Introducing binaural differences between target and masker speech allows the assessment of localization and/or SRM across conditions in which the cues are limited to rising- or falling-envelope segments, or available throughout the signal. [Work supported by NIH R01DC016643, T32000013.]

Kinematic error model and non-uniform calibration method for heavy load multi-DOF envelope forming press

Multi-DOF envelope forming is a new continuous and local plastic forming process for manufacturing high performance complex components with thin wall and high rib used in the aerospace industry. To achieve the new process, this paper develops a new heavy load multi-DOF envelope forming press with parallel kinematic mechanism (PKM), which can achieve the multi-DOF motion of envelope die. However, owing to the large scale and heavy load characteristics of the press, the kinematic error (fabrication and installation error) of the press is relatively large, resulting in the increased motion error of envelope die. Therefore, it is necessary to establish a reasonable kinematic error model and a calibration method to provide an essential basis for the precision design and control of the new press. Firstly, the kinematic error model, in which the 114 kinematic errors and multi-DOF motion error of envelope die are unitedly expressed and calculated based on the small displacement screw theory, and error Jacobian matrix for the press are established. Then the kinematic error mechanism for the press is disclosed. It is found that the motion error of envelope die is mainly attributed to 10 positional errors among 114 kinematic errors. Moreover, all the six error components of motion error of envelope die are time-varying with position changing of envelope die, and the position distributions of the maximum six error components are non-uniform during one motion period of envelope die. Based on the above results, a new non-uniform kinematic error calibration method for the press is proposed. Finally, the kinematic error calibration experiments are conducted. It is shown that the prediction accuracy of motion error of envelope die with the proposed non-uniform kinematic error calibration method is significantly improved from 89.06% to 99.67% compared with the traditional uniform kinematic error calibration method, which verifies that the established kinematic error model and non-uniform calibration method for heavy load multi-DOF envelope forming press are reliable.

The influence of phoneme contexts on adaptation in vowel‐evoked envelope following responses

Repeated stimulus presentation leads to neural adaptation and consequent amplitude reduction in vowel‐evoked envelope following responses (EFRs)—a response that reflects neural activity phase‐locked to envelope periodicity. EFRs are elicited by vowels presented in isolation or in the context of other phonemes such as consonants in syllables. While context phonemes could exert some forward influence on vowel‐evoked EFRs, they may reduce the degree of adaptation. Here, we evaluated whether the properties of context phonemes between consecutive vowel stimuli influence adaptation. EFRs were elicited by the low‐frequency first formant (resolved harmonics) and middle‐to‐high‐frequency second and higher formants (unresolved harmonics) of a male‐spoken /i/ when the presence, number and predictability of context phonemes (/s/, /a/, /∫/ and /u/) between vowel repetitions varied. Monitored over four iterations of /i/, adaptation was evident only for EFRs elicited by the unresolved harmonics. EFRs elicited by the unresolved harmonics decreased in amplitude by ~16–20 nV (10%–17%) after the first presentation of /i/ and remained stable thereafter. EFR adaptation was reduced by the presence of a context phoneme, but the reduction did not change with their number or predictability. The presence of a context phoneme, however, attenuated EFRs by a degree similar to that caused by adaptation (~21–23 nV). Such a trade‐off in the short‐ and long‐term influence of context phonemes suggests that the benefit of interleaving EFR‐eliciting vowels with other context phonemes depends on whether the use of consonant‐vowel syllables is critical to improve the validity of EFR applications.

Frequency-Following Response to Steady-State Vowel in Quiet and Background Noise Among Marching Band Participants With Normal Hearing

Human studies enrolling individuals at high risk for cochlear synaptopathy (CS) have reported difficulties in speech perception in adverse listening conditions. The aim of this study is to determine if these individuals show a degradation in the neural encoding of speech in quiet and in the presence of background noise as reflected in neural phase-locking to both envelope periodicity and temporal fine structure (TFS). To our knowledge, there are no published reports that have specifically examined the neural encoding of both envelope periodicity and TFS of speech stimuli (in quiet and in adverse listening conditions) among a sample with loud-sound exposure history who are at risk for CS. Using scalp-recorded frequency-following response (FFR), the authors evaluated the neural encoding of envelope periodicity (FFRENV) and TFS (FFRTFS) for a steady-state vowel (English back vowel /u/) in quiet and in the presence of speech-shaped noise presented at +5- and 0 dB SNR. Participants were young individuals with normal hearing who participated in the marching band for at least 5 years (high-risk group) and non-marching band group with low-noise exposure history (low-risk group). The results showed no group differences in the neural encoding of either the FFRENV or the first formant (F1) in the FFRTFS in quiet and in noise. Paradoxically, the high-risk group demonstrated enhanced representation of F2 harmonics across all stimulus conditions. These results appear to be in line with a music experience-dependent enhancement of F2 harmonics. However, due to sound overexposure in the high-risk group, the role of homeostatic central compensation cannot be ruled out. A larger scale data set with different noise exposure background, longitudinal measurements with an array of behavioral and electrophysiological tests is needed to disentangle the nature of the complex interaction between the effects of central compensatory gain and experience-dependent enhancement.

Comparing frequency-chirp sensitivity in the inferior colliculus to basic response properties using novel stimuli

Most neurons in the central nucleus of the inferior colliculus (ICC) are sensitive to the direction of phase-curvature of Schroeder-phase (SCHR) stimuli. This phase-curvature controls the direction and velocity of frequency chirps within the fundamental SCHR period. Differences in responses to different SCHR stimuli may originate from an underlying sensitivity to frequency chirp direction or velocity. However, known ICC sensitivities, such as those to frequency or envelope periodicity and duty cycle, may also contribute to the observed effect. To parse these confounding SCHR stimulus features, we designed novel frequency-chirping stimuli to isolate the influences of periodicity, duty cycle, and frequency-chirp direction or velocity on ICC neuron responses. Extracellular, single-unit recordings of chirp responses were made in awake rabbit ICC. Simultaneously, basic response properties were characterized using methods such as frequency response maps, modulation transfer functions, and spectro-temporal receptive fields. By comparing basic response properties across chirp-sensitive neurons (comprising 80% of ICC neurons), we can outline the shared characteristics among them. Our goal is to determine whether frequency-chirp sensitivity in ICC neurons is independent of other established feature sensitivities. Frequency-chirp-sensitive neurons are interesting, because they may play a unique role in encoding sounds with complex phase, including speech. [Work supported by NIDCD-R01-001641.]

Cascaded Sagnac Loops Embedded With Two Polarization Maintaining Photonic Crystal Fibers for Highly Sensitive Strain Measurement

A strain sensor based on cascaded Sagnac loops with two polarization-maintaining photonic crystal fibers (PMPCFs) has been experimentally demonstrated. Air holes distributed on the cross section of PMPCF go through the whole fiber, which makes the special fiber more sensitive to ambient force compared with conventional optical fiber. The lengths of the two PMPCFs are close, but not the same, to form envelopes on the total output spectrum and then obtain high resolution to the detected parameter by the Vernier effect. The influences of the difference in length of the two PMPCFS on spectral envelopes are discussed. As the length of the PMPCF in reference loop increases, free spectrum range of the cascaded Sagnac loops (or envelope period) and magnification decrease. The sensitivities of this strain sensor are calculated by tracking upper and lower envelopes. Results reveal that its average sensitivity is up to 45.15 pm/με and the corresponding resolution is 0.44 με as strain varies from 0 to 2000 με. Compared with a single Sagnac loop, the sensitivity magnification is about 28.94. The sensing characteristics of a conventional polarization-maintaining fiber (PMF) are studied to compare with the PMPCF. Moreover, reversibility has also been proved to possess low error rate. In short, the fiber strain sensor possesses high sensitivity and great reversibility.

Potential Destructive Binaural Interaction Effects in Auditory Steady-State Response Measurements.

An aided sound-field auditory steady-state response (ASSR) has the potential to be used to objectively validate hearing-aid (HA) fittings in clinics. Each aided ear should ideally be tested independently, but it is suspected that binaural testing may be used by clinics to reduce test time. This study simulates dichotic ASSR sound-field conditions to examine the risk of making false judgments due to unchecked binaural effects. Unaided ASSRs were recorded with a clinical two-channel electroencephalography (EEG) system for 15 normal hearing subjects using a three-band CE-Chirp® stimulus. It was found that the noise corrected power of a response harmonic can be suppressed by up to 10 dB by introducing large interaural time differences equal to half the time period of the stimulus envelope, which may occur in unilateral HA users. These large interaural time differences also changed the expression of ASSR power across the scalp, resulting in dramatically altered topographies. This would lead to considerably lower measured response power and possibly nondetections, evidencing that even well fit HAs are fit poorly (false referral), whereas monaural ASSR tests would pass. No effect was found for simulated lateralizations of the stimulus, which is beneficial for a proposed aided ASSR approach. Full-scalp ASSR recordings match previously found 40 Hz topographies but demonstrate suppression of cortical ASSR sources when using stimuli in interaural envelope antiphase.

LCC-based framework for building envelope and structure co-design considering energy efficiency and natural hazard performance

This paper presents a life-cycle cost (LCC) informed co-design framework for building structures and envelope systems, holistically considering the influences of energy and natural hazard performance. The proposed method is consisted of a two-stage design and decision-making process, aiming to provide a quantitative guideline for building's structural and envelope co-design based on the its geographic locations. First, the building's structural configuration and envelope type are selected based on the life cycle cost. Then, the long-term cost effectiveness of various energy-saving building envelope options (e.g., high-performance glazing and insulation) is evaluated to refine the envelope design. The proposed co-design framework was demonstrated through the case study of a medium-size office building archetype in three locations with distinct climate conditions and seismic activities (i.e., Los Angeles, Memphis, and Boston). The results highlighted the interplay between building's structural (seismic) performance and the cost-effectiveness of energy-saving design options – e.g., for buildings located in high-seismic regions, seismic enhancing designs greatly reduce the paybak period of high performance building envelope by reducing the seismic loss; whereas for buildings located in regions with cold climate and low seismic risk such as Boston, spatial frame with high insulation building envelope shows the lowest LCC.

On Vivid Manifestations of Perigean Spring Tides in the Atmosphere

Perigean spring tides in the atmosphere are discussed. In 2016–2017, the smoothed anomalies of air temperature over European Russia were found to repeat the evolution of the distance of the perigee (i.e., the distance between the Moon and the Earth at times when the former is at the perigee), which varies as a sine function with a period of about 206 days. The origin of the 206-day lunar cycle is described. Atmospheric spring tides are studied. They are exhibited as atmospheric pressure oscillations similar to perigean spring oscillations of the sea level. The basic dependence of perigean spring tides in the form of beats of atmospheric pressure anomalies at new and full moons is found. The pressure anomalies at full and new moons can be approximated by sinusoids with beat envelope periods of about 412 days and with opposite phases. The range of oscillations at antinodes reaches 40 mb, which is comparable, in order of magnitude, with actual mesoscale fluctuations of atmospheric pressure. The violations in seasonal weather variation caused by the 206-day lunar cycle are described.

Theoretical research on the annular intensive relativistic electron motion focused by coaxial cusped periodic permanent magnetic field

This paper theoretically investigates the motion characteristics of an annular intensive relativistic electron beam (IREB) in a coaxial, smooth waveguide under a cusped periodic permanent magnetic (CPPM) field. The motion of the electron beam clearly exhibits a periodic oscillation in a CPPM. For the effective transmission of an IREB, the structural parameters of the CPPM must be chosen carefully. Based on theoretical analysis, stability conditions are suggested. The influence of the magnetic field phase, strength, and period on the oscillation amplitude of the beam envelope is analyzed, and the criteria for minimal oscillation amplitude are proposed accordingly. Notably, the theoretical formula, used to predict the oscillation period of the beam envelope, is proposed and verified by single-particle and particle-in-cell simulation results.

The First Ultracompact Roche Lobe–Filling Hot Subdwarf Binary

Abstract We report the discovery of the first short-period binary in which a hot subdwarf star (sdOB) filled its Roche lobe and started mass transfer to its companion. The object was discovered as part of a dedicated high-cadence survey of the Galactic plane named the Zwicky Transient Facility and exhibits a period of P = 39.3401(1) minutes, making it the most compact hot subdwarf binary currently known. Spectroscopic observations are consistent with an intermediate He-sdOB star with an effective temperature of = 42,400 ± 300 K and a surface gravity of = 5.77 ± 0.05. A high signal-to-noise ratio GTC+HiPERCAM light curve is dominated by the ellipsoidal deformation of the sdOB star and an eclipse of the sdOB by an accretion disk. We infer a low-mass hot subdwarf donor with a mass M sdOB = 0.337 ± 0.015 and a white dwarf accretor with a mass M WD = 0.545 ± 0.020 . Theoretical binary modeling indicates the hot subdwarf formed during a common envelope phase when a 2.5–2.8 star lost its envelope when crossing the Hertzsprung gap. To match its current , , , and masses, we estimate a post–common envelope period of ≈ 150 minutes and find that the sdOB star is currently undergoing hydrogen shell burning. We estimate that the hot subdwarf will become a white dwarf with a thick helium layer of ≈0.1 , merge with its carbon/oxygen white dwarf companion after ≈17 Myr, and presumably explode as a thermonuclear supernova or form an R CrB star.

Human Frequency Following Responses to Vocoded Speech: Amplitude Modulation Versus Amplitude Plus Frequency Modulation.

The most commonly employed speech processing strategies in cochlear implants (CIs) only extract and encode amplitude modulation (AM) in a limited number of frequency channels. proposed a novel speech processing strategy that encodes both frequency modulation (FM) and AM to improve CI performance. Using behavioral tests, they reported better speech, speaker, and tone recognition with this novel strategy than with the AM-alone strategy. Here, we used the scalp-recorded human frequency following responses (FFRs) to examine the differences in the neural representation of vocoded speech sounds with AM alone and AM + FM as the spectral and temporal cues were varied. Specifically, we were interested in determining whether the addition of FM to AM improved the neural representation of envelope periodicity (FFRENV) and temporal fine structure (FFRTFS), as reflected in the temporal pattern of the phase-locked neural activity generating the FFR. FFRs were recorded from 13 normal-hearing, adult listeners in response to the original unprocessed stimulus (a synthetic diphthong /au/ with a 110-Hz fundamental frequency or F0 and a 250-msec duration) and the 2-, 4-, 8- and 16-channel sine vocoded versions of /au/ with AM alone and AM + FM. Temporal waveforms, autocorrelation analyses, fast Fourier Transform, and stimulus-response spectral correlations were used to analyze both the strength and fidelity of the neural representation of envelope periodicity (F0) and TFS (formant structure). The periodicity strength in the FFRENV decreased more for the AM stimuli than for the relatively resilient AM + FM stimuli as the number of channels was increased. Regardless of the number of channels, a clear spectral peak of FFRENV was consistently observed at the stimulus F0 for all the AM + FM stimuli but not for the AM stimuli. Neural representation as revealed by the spectral correlation of FFRTFS was better for the AM + FM stimuli when compared to the AM stimuli. Neural representation of the time-varying formant-related harmonics as revealed by the spectral correlation was also better for the AM + FM stimuli as compared to the AM stimuli. These results are consistent with previously reported behavioral results and suggest that the AM + FM processing strategy elicited brainstem neural activity that better preserved periodicity, temporal fine structure, and time-varying spectral information than the AM processing strategy. The relatively more robust neural representation of AM + FM stimuli observed here likely contributes to the superior performance on speech, speaker, and tone recognition with the AM + FM processing strategy. Taken together, these results suggest that neural information preserved in the FFR may be used to evaluate signal processing strategies considered for CIs.

Consonance perception beyond the traditional existence region of pitch.

Some theories posit that the perception of consonance is based on neural periodicity detection, which is dependent on accurate phase locking of auditory nerve fibers to features of the stimulus waveform. In the current study, 15 listeners were asked to rate the pleasantness of complex tone dyads (2 note chords) forming various harmonic intervals and bandpass filtered in a high-frequency region (all components >5.8 kHz), where phase locking to the rapid stimulus fine structure is thought to be severely degraded or absent. The two notes were presented to opposite ears. Consonant intervals (minor third and perfect fifth) received higher ratings than dissonant intervals (minor second and tritone). The results could not be explained in terms of phase locking to the slower waveform envelope because the preference for consonant intervals was higher when the stimuli were harmonic, compared to a condition in which they were made inharmonic by shifting their component frequencies by a constant offset, so as to preserve their envelope periodicity. Overall the results indicate that, if phase locking is indeed absent at frequencies greater than ∼5 kHz, neural periodicity detection is not necessary for the perception of consonance.

Sustained envelope periodicity representations are associated with speech-in-noise performance in difficult listening conditions for younger and older adults.

Temporal modulations are an important part of speech signals. An accurate perception of these time-varying qualities of sound is necessary for successful communication. The current study investigates the relationship between sustained envelope encoding and speech-in-noise perception in a cohort of normal-hearing younger (ages 18-30 yr, n = 22) and older adults (ages 55-90+ yr, n = 35) using the subcortical auditory steady-state response (ASSR). ASSRs were measured in response to the envelope of 400-ms amplitude-modulated (AM) tones with 3,000-Hz carrier frequencies and 80-Hz modulation frequencies. AM tones had modulation depths of 0, -4, and -8 dB relative to m = 1 (m = 1, 0.631, and 0.398, respectively). The robustness, strength at modulation frequency, and synchrony of subcortical envelope encoding were quantified via time-domain correlations, spectral amplitude, and phase-locking value, respectively. Speech-in-noise ability was quantified via the QuickSIN test in the 0- and 5-dB signal-to-noise (SNR) conditions. All ASSR metrics increased with increasing modulation depth and there were no effects of age group. ASSR metrics in response to shallow modulation depths predicted 0-dB speech scores. Results demonstrate that sustained amplitude envelope processing in the brainstem relates to speech-in-noise abilities, but primarily in difficult listening conditions at low SNRs. These findings furthermore highlight the utility of shallow modulation depths for studying temporal processing. The absence of age effects in these data demonstrate that individual differences in the robustness, strength, and specificity of subcortical envelope processing, and not age, predict speech-in-noise performance in the most difficult listening conditions.NEW & NOTEWORTHY Failure to correctly understand speech in the presence of background noise is a significant problem for many normal-hearing adults and may impede healthy communication. The relationship between sustained envelope encoding in the brainstem and speech-in-noise perception remains to be clarified. The present study demonstrates that the strength, specificity, and robustness of the brainstem's representations of sustained stimulus periodicity relates to speech-in-noise perception in older and younger normal-hearing adults, but only in highly challenging listening environments.

The Medieval Climate Anomaly in Oceania

Temperatures in Oceania have risen by 0.5–1 °C over the past 100 years, resulting in significant retreat of New Zealand’s glaciers as an example. To better understand natural and anthropogenic contributions to this warming process, the observed climatic change must be placed in a longer-term palaeotemperature context. Of interest is the Medieval Climate Anomaly (MCA, 1000–1200 AD), a recognized period of natural pre-industrial climate change, associated with marked temperature and hydroclimatic variability that is best known from the Northern Hemisphere. Temperature reconstructions for Oceania were traditionally based on two classical tree ring series. Here, we enlarge the Oceania reference data set with another 13 published temperature reconstructions from SE Australia, New Zealand, and West Papua. These are based on a variety of proxy types and help to geographically and methodologically augment the regional palaeoclimate database. The proxy series have been thoroughly compared and the MCA trends palaeoclimatologically mapped. Ten out of the 15 sites show a relatively warm MCA, compared to the last 1500 years, with warming generally occurring in the envelope period 900–1500 AD. In some sites of SE Australia and at the west coast of New Zealand’s South Island, warming appears to be delayed by 200–300 years. The end of the medieval warming at around 1500 AD occurred about two centuries later than on most other continents, suggesting a possible interhemispheric climate lag mechanism possibly involving deep-water circulation. Likely drivers for the medieval warming in Oceania are atmospheric-ocean cycles such as the Southern Annular Mode and El Niño-Southern Oscillation, in combination with solar activity changes. MCA palaeotemperature data are still lacking for large parts of Oceania, namely the arid and tropical parts of Australia, Micronesia, central and northern Polynesia, as well as central and eastern Melanesia, highlighting the need for future research.

Phase-locked responses to the vowel envelope vary in scalp-recorded amplitude due to across-frequency response interactions.

Neural encoding of the envelope of sounds like vowels is essential to access temporal information useful for speech recognition. Subcortical responses to envelope periodicity of vowels can be assessed using scalp-recorded envelope following responses (EFRs); however, the amplitude of EFRs vary by vowel spectra and the causal relationship is not well understood. One cause for spectral dependency could be interactions between responses with different phases, initiated by multiple stimulus frequencies. Phase differences can arise from earlier initiation of processing high frequencies relative to low frequencies in the cochlea. This study investigated the presence of such phase interactions by measuring EFRs to two naturally spoken vowels (/ε/ and /u/), while delaying the envelope phase of the second formant band (F2+) relative to the first formant (F1) band in 45° increments. At 0° F2+ phase delay, EFRs elicited by the vowel /ε/ were lower in amplitude than the EFRs elicited by /u/. Using vector computations, we found that the lower amplitude of /ε/-EFRs was caused by linear superposition of F1- and F2+-contributions with larger F1-F2+ phase differences (166°) compared to /u/ (19°). While the variation in amplitude across F2+ phase delays could be modeled with two dominant EFR sources for both vowels, the degree of variation was dependent on F1 and F2+ EFR characteristics. Together, we demonstrate that (a) broadband sounds like vowels elicit independent responses from different stimulus frequencies that may be out-of-phase and affect scalp-based measurements, and (b) delaying higher frequency formants can maximize EFR amplitudes for some vowels.

The role of envelope periodicity in the perception of masked speech with simulated and real cochlear implants.

In normal hearing, complex tones with pitch-related periodic envelope modulations are far less effective maskers of speech than aperiodic noise. Here, it is shown that this masker-periodicity benefit is diminished in noise-vocoder simulations of cochlear implants (CIs) and further reduced with real CIs. Nevertheless, both listener groups still benefitted significantly from masker periodicity, despite the lack of salient spectral pitch cues. The main reason for the smaller effect observed in CI users is thought to be an even stronger channel interaction than in the CI simulations, which smears out the random envelope modulations that are characteristic for aperiodic sounds. In contrast, neither interferers that were amplitude-modulated at a rate of 10 Hz nor maskers with envelopes specifically designed to reveal the target speech enabled a masking release in CI users. Hence, even at the high signal-to-noise ratios at which they were tested, CI users can still exploit pitch cues transmitted by the temporal envelope of a non-speech masker, whereas slow amplitude modulations of the masker envelope are no longer helpful.

Phase delays between tone pairs reveal interactions in scalp-recorded envelope following responses

Evoked potentials to envelope periodicity in sounds, such as vowels, are dependent on the stimulus spectrum. We hypothesize that phase differences between responses elicited by multiple frequencies spread tonotopically across the cochlear partition may contribute to variation in scalp-recorded amplitude. The present study evaluated this hypothesis by measuring envelope following responses (EFRs) to two concurrent tone pairs, p1 and p2, that approximated the first and second formant frequencies of a vowel, while controlling their relative envelope phase. We found that the scalp-recorded amplitude of EFRs changed significantly in phase and amplitude when the envelope phase of p2, the higher frequency tone pair, was delayed. The maximum EFR amplitude occurred at the p2 envelope phase delay of 90°, likely because the stimulus delay compensated for the average phase lead of 73.57° exhibited by p2-contributed EFRs relative to p1-contributed EFRs, owing to earlier cochlear processing of higher frequencies. Findings suggest a linear superimposition of independently generated EFRs from tonotopically separated pathways. This suggests that introducing frequency-specific delays may help to optimize EFRs to broadband stimuli like vowels.

Human Frequency Following Responses to Vocoded Speech.

Vocoders offer an effective platform to simulate the effects of cochlear implant speech processing strategies in normal-hearing listeners. Several behavioral studies have examined the effects of varying spectral and temporal cues on vocoded speech perception; however, little is known about the neural indices of vocoded speech perception. Here, the scalp-recorded frequency following response (FFR) was used to study the effects of varying spectral and temporal cues on brainstem neural representation of specific acoustic cues, the temporal envelope periodicity related to fundamental frequency (F0) and temporal fine structure (TFS) related to formant and formant-related frequencies, as reflected in the phase-locked neural activity in response to vocoded speech. In experiment 1, FFRs were measured in 12 normal-hearing, adult listeners in response to a steady state English back vowel /u/ presented in an unaltered, unprocessed condition and six sine-vocoder conditions with varying numbers of channels (1, 2, 4, 8, 16, and 32), while the temporal envelope cutoff frequency was fixed at 500 Hz. In experiment 2, FFRs were obtained from 14 normal-hearing, adult listeners in response to the same English vowel /u/, presented in an unprocessed condition and four vocoded conditions where both the temporal envelope cutoff frequency (50 versus 500 Hz) and carrier type (sine wave versus noise band) were varied separately with the number of channels fixed at 8. Fast Fourier Transform was applied to the time waveforms of FFR to analyze the strength of brainstem neural representation of temporal envelope periodicity (F0) and TFS-related peaks (formant structure). Brainstem neural representation of both temporal envelope and TFS cues improved when the number of channels increased from 1 to 4, followed by a plateau with 8 and 16 channels, and a reduction in phase-locking strength with 32 channels. For the sine vocoders, peaks in the FFRTFS spectra corresponded with the low-frequency sine-wave carriers and side band frequencies in the stimulus spectra. When the temporal envelope cutoff frequency increased from 50 to 500 Hz, an improvement was observed in brainstem F0 representation with no change in brainstem representation of spectral peaks proximal to the first formant frequency (F1). There was no significant effect of carrier type (sine- versus noise-vocoder) on brainstem neural representation of F0 cues when the temporal envelope cutoff frequency was 500 Hz. While the improvement in neural representation of temporal envelope and TFS cues with up to 4 vocoder channels is consistent with the behavioral literature, the reduced neural phase-locking strength noted with even more channels may be because of the narrow bandwidth of each channel as the number of channels increases. Stronger neural representation of temporal envelope cues with higher temporal envelope cutoff frequencies is likely a reflection of brainstem neural phase-locking to F0-related periodicity fluctuations preserved in the 500-Hz temporal envelopes, which are unavailable in the 50-Hz temporal envelopes. No effect of temporal envelope cutoff frequency was seen for neural representation of TFS cues, suggesting that spectral side band frequencies created by the 500-Hz temporal envelopes did not improve neural representation of F1 cues over the 50-Hz temporal envelopes. Finally, brainstem F0 representation was not significantly affected by carrier type with a temporal envelope cutoff frequency of 500 Hz, which is inconsistent with previous results of behavioral studies examining pitch perception of vocoded stimuli.