On musical interval perception for complex tones at very high frequencies.

Listeners appear able to extract a residue pitch from high-frequency harmonics for which phase locking to the temporal fine structure is weak or absent. The present study investigated musical interval perception for high-frequency harmonic complex tones using the same stimuli as Lau, Mehta, and Oxenham [J. Neurosci. 37, 9013-9021 (2017)]. Nine young musically trained listeners with especially good high-frequency hearing adjusted various musical intervals using harmonic complex tones containing harmonics 6-10. The reference notes had fundamental frequencies (F0s) of 280 or 1400 Hz. Interval matches were possible, albeit markedly worse, even when all harmonic frequencies were above the presumed limit of phase locking. Matches showed significantly larger systematic errors and higher variability, and subjects required more trials to finish a match for the high than for the low F0. Additional absolute pitch judgments from one subject with absolute pitch, for complex tones containing harmonics 1-5 or 6-10 with a wide range of F0s, were perfect when the lowest frequency component was below about 7 kHz, but at least 50% of responses were incorrect when it was 8 kHz or higher. The results are discussed in terms of the possible effects of phase-locking information and familiarity with high-frequency stimuli on pitch

A Re-examination of the Effect of Masker Phase Curvature on Non-simultaneous Masking.

Forward masking of a sinusoidal signal is determined not only by the masker's power spectrum but also by its phase spectrum. Specifically, when the phase spectrum is such that the output of an auditory filter centred on the signal has a highly modulated ("peaked") envelope, there is less masking than when that envelope is flat. This finding has been attributed to non-linearities, such as compression, reducing the average neural response to maskers that produce more peaked auditory filter outputs (Carlyon and Datta, J Acoust Soc Am 101:3636-3647, 1997). Here we evaluate an alternative explanation proposed by Wotcjzak and Oxenham (Wojtczak and Oxenham, J Assoc Res Otolaryngol 10:595-607, 2009). They reported a masker phase effect for 6-kHz signals when the masker components were at least an octave below the signal frequency. Wotcjzak and Oxenham argued that this effect was inconsistent with cochlear compression, and, because it did not occur at lower signal frequencies, was also inconsistent with more central compression. It was instead attributed to activation of the efferent system reducing the response to the subsequent probe. Here, experiment 1 replicated their main findings. Experiment 2 showed that the phase effect on off-frequency forward masking is similar at signal frequencies of 2 and 6 kHz, provided that one equates the number of components likely to interact within an auditory filter centred on the signal, thereby roughly equating the effect of masker phase on the peakiness of that filter output. Experiment 3 showed that for some subjects, masker phase also had a strong influence on off-frequency backward masking of the signal, and that the size of this effect correlated across subjects with that observed in forward masking. We conclude that the masker phase effect is mediated mainly by cochlear non-linearities, with a possible additional effect of more central compression. The data are not consistent with a role for the efferent system

Temporal Regularity Detection and Rate Discrimination in Cochlear-Implant Listeners

Cochlear implants (CIs) convey fundamental-frequency information using primarily temporal cues. However, temporal pitch perception in CI users is weak and, when measured using rate discrimination tasks, deteriorates markedly as the rate increases beyond 300 pulses-per-second. Rate pitch may be weak because the electrical stimulation of the surviving neural population of the implant recipient may not allow accurate coding of inter-pulse time intervals. If so, this phenomenon should prevent listeners from detecting when a pulse train is physically temporally jittered. Performance in a jitter detection task was compared to that in a rate-pitch discrimination task. Stimuli were delivered using direct stimulation in cochlear implants, on a mid-array and an apical electrode, and at two different rates (100 and 300 pps). Average performance on both tasks was worse at the higher pulse rate and did not depend on electrode. However, there was a large variability across and within listeners that did not correlate between the two tasks, suggesting that rate-pitch judgement and regularity detection are to some extent limited by task-specific processes. Simulations with filtered pulse trains presented to NH listeners yielded broadly similar results, except that, for the rate discrimination task, the difference between performance with 100- and 300-pps base rates was smaller than observed for CI users.</p

3MRC Cognition and Brain Sciences Unit

Acoustic sequences such as speech and music are generally perceived as coherent auditory “streams,” which can be individually attended to and followed over time. Although the psychophysical stimulus parameters governing this “auditory streaming ” are well established, the brain mechanisms underlying the formation of auditory streams remain largely unknown. In particular, an essential feature of the phenomenon, which corresponds to the fact that the segregation of sounds into streams typically takes several seconds to build up, remains unexplained. Here, we show that this and other major features of auditorystream formation measured in humans using alternating-tone sequences can be quantitatively accounted for based on single-unit responses recorded in the primary auditory cortex (A1) of awake rhesus monkeys listening to the same sound sequences

The effect of a coding strategy that removes temporally masked pulses on speech perception by cochlear implant users.

Speech recognition in noisy environments remains a challenge for cochlear implant (CI) recipients. Unwanted charge interactions between current pulses, both within and between electrode channels, are likely to impair performance. Here we investigate the effect of reducing the number of current pulses on speech perception. This was achieved by implementing a psychoacoustic temporal-masking model where current pulses in each channel were passed through a temporal integrator to identify and remove pulses that were less likely to be perceived by the recipient. The decision criterion of the temporal integrator was varied to control the percentage of pulses removed in each condition. In experiment 1, speech in quiet was processed with a standard Continuous Interleaved Sampling (CIS) strategy and with 25, 50 and 75% of pulses removed. In experiment 2, performance was measured for speech in noise with the CIS reference and with 50 and 75% of pulses removed. Speech intelligibility in quiet revealed no significant difference between reference and test conditions. For speech in noise, results showed a significant improvement of 2.4 dB when removing 50% of pulses and performance was not significantly different between the reference and when 75% of pulses were removed. Further, by reducing the overall amount of current pulses by 25, 50, and 75% but accounting for the increase in charge necessary to compensate for the decrease in loudness, estimated average power savings of 21.15, 40.95, and 63.45%, respectively, could be possible for this set of listeners. In conclusion, removing temporally masked pulses may improve speech perception in noise and result in substantial power savings

Pitch perception at very high frequencies: On psychometric functions and integration of frequency information.

Lau et al. [J. Neurosci. 37, 9013-9021 (2017)] showed that discrimination of the fundamental frequency (F0) of complex tones with components in a high-frequency region was better than predicted from the optimal combination of information from the individual harmonics. The predictions depend on the assumption that psychometric functions for frequency discrimination have a slope of 1 at high frequencies. This was tested by measuring psychometric functions for F0 discrimination and frequency discrimination. Difference limens for F0 (F0DLs) and difference limens for frequency for each frequency component were also measured. Complex tones contained harmonics 6-10 and had F0s of 280 or 1400 Hz. Thresholds were measured using 210-ms tones presented diotically in diotic threshold-equalizing noise (TEN), and 1000-ms tones presented diotically in dichotic TEN. The slopes of the psychometric functions were close to 1 for all frequencies and F0s. The ratio of predicted to observed F0DLs was around 1 or smaller for both F0s, i.e., not super-optimal, and was significantly smaller for the low than for the high F0. The results are consistent with the idea that place information alone can convey pitch, but pitch is more salient when phase-locking information is available

Polarity Sensitivity as a Potential Correlate of Neural Degeneration in Cochlear Implant Users.

Cochlear implant (CI) performance varies dramatically between subjects. Although the causes of this variability remain unclear, the electrode-neuron interface is thought to play an important role. Here we evaluate the contribution of two parameters of this interface on the perception of CI listeners: the electrode-to-modiolar wall distance (EMD), estimated from cone-beam computed tomography (CT) scans, and a measure of neural health. Since there is no objective way to quantify neural health in CI users, we measure stimulus polarity sensitivity, which is assumed to be related to neural degeneration, and investigate whether it also correlates with subjects' performance in speech recognition and spectro-temporal modulation detection tasks. Detection thresholds were measured in fifteen CI users (sixteen ears) for partial-tripolar triphasic pulses having an anodic or a cathodic central phase. The polarity effect was defined as the difference in threshold between cathodic and anodic stimuli. Our results show that both the EMD and the polarity effect correlate with detection thresholds, both across and within subjects, although the within-subject correlations were weak. Furthermore, the mean polarity effect, averaged across all electrodes for each subject, was negatively correlated with performance on a spectro-temporal modulation detection task. In other words, lower cathodic thresholds were associated with better spectro-temporal modulation detection performance, which is also consistent with polarity sensitivity being a marker of neural degeneration. Implications for the design of future subject-specific fitting strategies are discussed

Cochlear Implant Research and Development in the Twenty-first Century: A Critical Update

Abstract: Cochlear implants (CIs) are the world’s most successful sensory prosthesis and have been the subject of intense research and development in recent decades. We critically review the progress in CI research, and its success in improving patient outcomes, from the turn of the century to the present day. The review focuses on the processing, stimulation, and audiological methods that have been used to try to improve speech perception by human CI listeners, and on fundamental new insights in the response of the auditory system to electrical stimulation. The introduction of directional microphones and of new noise reduction and pre-processing algorithms has produced robust and sometimes substantial improvements. Novel speech-processing algorithms, the use of current-focusing methods, and individualised (patient-by-patient) deactivation of subsets of electrodes have produced more modest improvements. We argue that incremental advances have and will continue to be made, that collectively these may substantially improve patient outcomes, but that the modest size of each individual advance will require greater attention to experimental design and power. We also briefly discuss the potential and limitations of promising technologies that are currently being developed in animal models, and suggest strategies for researchers to collectively maximise the potential of CIs to improve hearing in a wide range of listening situations

Effect of Context on the Contribution of Individual Harmonics to Residue Pitch.

There is evidence that the contribution of a given harmonic in a complex tone to residue pitch is influenced by the accuracy with which the frequency of that harmonic is encoded. The present study investigated whether listeners adjust the weights assigned to individual harmonics based on acquired knowledge of the reliability of the frequency estimates of those harmonics. In a two-interval forced-choice task, seven listeners indicated which of two 12-harmonic complex tones had the higher overall pitch. In context trials (60 % of all trials), the fundamental frequency (F0) was 200 Hz in one interval and 200 + ΔF0 Hz in the other. In different (blocked) conditions, either the 3rd or the 4th harmonic (plus the 7th, 9th, and 12th harmonics), were replaced by narrowband noises that were identical in the two intervals. Feedback was provided. In randomly interspersed test trials (40 % of all trials), the fundamental frequency was 200 + ΔF0/2 Hz in both intervals; in the second interval, either the third or the fourth harmonic was shifted slightly up or down in frequency with equal probability. There were no narrowband noises. Feedback was not provided. The results showed that substitution of a harmonic by noise in context trials reduced the contribution of that harmonic to pitch judgements in the test trials by a small but significant amount. This is consistent with the notion that listeners give smaller weight to a harmonic or frequency region when they have learned that this frequency region does not provide reliable information for a given task

Specificity of the human frequency following response for carrier and modulation frequency assessed using adaptation

The frequency following response (FFR) is a scalp-recorded measure of phase-locked brainstem activity to stimulus-related periodicities. Three experiments investigated the specificity of the FFR for carrier and modulation frequency using adaptation. FFR waveforms evoked by alternating-polarity stimuli were averaged for each polarity and added, to enhance envelope, or subtracted, to enhance temporal fine structure information. The first experiment investigated peristimulus adaptation of the FFR for pure and complex tones as a function of stimulus frequency and fundamental frequency (F0). It showed more adaptation of the FFR in response to sounds with higher frequencies or F0s than to sounds with lower frequency or F0s. The second experiment investigated tuning to modulation rate in the FFR. The FFR to a complex tone with a modulation rate of 213 Hz was not reduced more by an adaptor that had the same modulation rate than by an adaptor with a different modulation rate (90 or 504 Hz), thus providing no evidence that the FFR originates mainly from neurons that respond selectively to the modulation rate of the stimulus. The third experiment investigated tuning to audio frequency in the FFR using pure tones. An adaptor that had the same frequency as the target (213 or 504 Hz) did not generally reduce the FFR to the target more than an adaptor that differed in frequency (by 1.24 octaves). Thus, there was no evidence that the FFR originated mainly from neurons tuned to the frequency of the target. Instead, the results are consistent with the suggestion that the FFR for low-frequency pure tones at medium to high levels mainly originates from neurons tuned to higher frequencies. Implications for the use and interpretation of the FFR are discussed