Pitch matches between unresolved complex tones differing by a single interpulse interval

Temporal Pitch Sensitivity in an Animal Model: Psychophysics and Scalp Recordings : Temporal Pitch Sensitivity in Cat

Cochlear implant (CI) users show limited sensitivity to the temporal pitch conveyed by electric stimulation, contributing to impaired perception of music and of speech in noise. Neurophysiological studies in cats suggest that this limitation is due, in part, to poor transmission of the temporal fine structure (TFS) by the brainstem pathways that are activated by electrical cochlear stimulation. It remains unknown, however, how that neural limit might influence perception in the same animal model. For that reason, we developed non-invasive psychophysical and electrophysiological measures of temporal (i.e., non-spectral) pitch processing in the cat. Normal-hearing (NH) cats were presented with acoustic pulse trains consisting of band-limited harmonic complexes that simulated CI stimulation of the basal cochlea while removing cochlear place-of-excitation cues. In the psychophysical procedure, trained cats detected changes from a base pulse rate to a higher pulse rate. In the scalp-recording procedure, the cortical-evoked acoustic change complex (ACC) and brainstem-generated frequency following response (FFR) were recorded simultaneously in sedated cats for pulse trains that alternated between the base and higher rates. The range of perceptual sensitivity to temporal pitch broadly resembled that of humans but was shifted to somewhat higher rates. The ACC largely paralleled these perceptual patterns, validating its use as an objective measure of temporal pitch sensitivity. The phase-locked FFR, in contrast, showed strong brainstem encoding for all tested pulse rates. These measures demonstrate the cat's perceptual sensitivity to pitch in the absence of cochlear-place cues and may be valuable for evaluating neural mechanisms of temporal pitch perception in the feline animal model of stimulation by a CI or novel auditory prostheses.

Combined neural and behavioural measures of temporal pitch perception in cochlear implant users

Four experiments measured the perceptual and neural correlates of the temporal pattern of electrical stimulation applied to one cochlear-implant (CI) electrode, for several subjects. Neural effects were estimated from the electrically evoked compound action potential (ECAP) to each pulse. Experiment 1 attenuated every second pulse of a 200-pps pulse train. Increasing attenuation caused pitch to drop and the ECAP to become amplitude modulated, thereby providing an estimate of the relationship between neural modulation and pitch. Experiment 2 showed that the pitch of a 200-pps pulse train can be reduced by delaying every second pulse, so that the inter-pulse-intervals alternate between longer and shorter intervals. This caused the ECAP to become amplitude modulated, but not by enough to account for the change in pitch. Experiment 3 replicated the finding that rate discrimination deteriorates with increases in baseline rate. This was accompanied by an increase in ECAP modulation, but by an amount that produced only a small effect on pitch in experiment 1. Experiment 4 showed that preceding a pulse train with a carefully selected "pre-pulse" could reduce ECAP modulation, but did not improve rate discrimination. Implications for theories of pitch and for limitations of pitch perception in CI users are discussed.

Distortion products and their influence on representation of pitch-relevant information in the human brainstem for unresolved harmonic complex tones

Pitch experiments aimed at evaluating temporal pitch mechanism(s) often utilize complex sounds with only unresolved harmonic components, and a low-pass noise masker to eliminate the potential contribution of audible distortion products to the pitch percept. Herein we examine how: (i) masker induced reduction of neural distortion products (difference tone: DT; and cubic difference tone: CDT) alters the representation of pitch relevant information in the brainstem; and (ii) the pitch salience is altered when distortion products are reduced and/or eliminated. Scalp recorded brainstem frequency following responses (FFR) were recorded in normal hearing individuals using a complex tone with only unresolved harmonics presented in quiet, and in the presence of a low-pass masker at SNRs of +15, +5, and -5 dB. Difference limen for F0 discrimination (F0 DL) was obtained in quiet and in the presence of low-pass noise. Magnitude of DT components (with the exception of components at F0 and 2F0), and the CDT components decreased with increasing masker level. Neural pitch strength decreased with increasing masker level for both the envelope-related (FFR(ENV)) and spectral-related (FFR(SPEC)) phase-locked activity. Finally, F0 DLs increased with decreasing SNRs suggesting poorer F0 discrimination with reduction of the distortion products. Collectively, these findings support the notion that both DT and CDT, as reflected in the FFR(ENV) and FFR(SPEC), respectively, influence both the brainstem representation of pitch relevant information and the pitch salience of the complex sounds.

Perceived continuity and pitch shifts for complex tones with unresolved harmonics

Brief complex tone bursts with fundamental frequencies (F0s) of 100, 125, 166.7, and 250 Hz were bandpass filtered between the 22nd and 30th harmonics, to produce waveforms with five regularly occurring envelope peaks ("pitch pulses") that evoked pitches associated with their repetition period. Two such tone bursts were presented sequentially and separated by a silent interval of two periods (2/F0). When the relative phases of the two bursts were varied, such that the interpulse interval (IPI) between the last pulse of the first burst and the first pulse of the second burst was varied, the pitch of the whole sequence was little affected. This is consistent with previous results suggesting that the pitch integration window may be "reset" by a discontinuity. However, when the interval between the two bursts was filled with a noise with the same spectral envelope as the complex, variations in IPI had substantial effects on the pitch of the sequence. It is suggested that the presence of the noise causes the two tones bursts to appear continuous, hence, resetting does not occur, and the pitch mechanism is sensitive to the phase discontinuity across the silent interval.

Observer weighting of level and timing cues in bilateral cochlear implant users

The contribution of binaural level and timing cues available from each pulse in brief electrical pulse trains was determined in a lateralization task using an observer weighting paradigm. Four bilateral cochlear implant users were tested with randomized interaural time delays (ITDs) or, in a separate experimental condition, interaural level differences (ILDs) applied to each pulse at 100, 300, and 600 pulsess. To examine the effect of cue randomization, weights were also determined for stimuli with a common ITD applied to all postonset pulses. Listeners were further tested for the ability to restart binaural ITD sensitivity at a rate that produced onset dominance by reducing a single interpulse interval in the stimulus. Results showed that the onset always exerted a strong influence. At 100 pulsess, postonset ITDs and ILDs also contributed strongly. At 300 and 600 pulsess, contributions from postonset pulses remained substantial for ILDs but were much reduced for ITDs, particularly at 600 pulsess. Weights were similar with either independent or common ITD cues applied to postonset pulses, indicating that the cue randomization process itself did not affect weights. Restarting of ITD sensitivity at 300 pulsess by introducing a single reduced interpulse interval was not observed.

Behavioral and physiological correlates of temporal pitch perception in electric and acoustic hearing

In the "4-6" condition of experiment 1, normal-hearing (NH) listeners compared the pitch of a bandpass-filtered pulse train, whose inter-pulse intervals (IPIs) alternated between 4 and 6 ms, to that of isochronous pulse trains. Consistent with previous results obtained at a lower signal level, the pitch of the 4-6 stimulus corresponded to that of an isochronous pulse train having a period of 5.7 ms-longer than the mean IPI of 5 ms. In other conditions the IPI alternated between 3.5-5.5 and 4.5-6.5 ms. Experiment 2 was similar but presented electric pulse trains to one channel of a cochlear implant. In both cases, as overall IPI increased, the pitch of the alternating-interval stimulus approached that of an isochronous train having a period equal to the mean IPI. Experiment 3 measured compound action potentials (CAPs) to alternating-interval stimuli in guinea pigs and in NH listeners. The CAPs to pulses occurring after 4-ms intervals were smaller than responses to pulses occurring after 6-ms intervals, resulting in a modulated pattern that was independent of overall level. The results are compared to the predictions of a simple model incorporating auditory-nerve (AN) refractoriness, and where pitch is estimated from first-order intervals in the AN response.

Virtual pitch in a computational physiological model

A computational model of nervous activity in the auditory nerve, cochlear nucleus, and inferior colliculus is presented and evaluated in terms of its ability to simulate psychophysically-measured pitch perception. The model has a similar architecture to previous autocorrelation models except that the mathematical operations of autocorrelation are replaced by the combined action of thousands of physiologically plausible neuronal components. The evaluation employs pitch stimuli including complex tones with a missing fundamental frequency, tones with alternating phase, inharmonic tones with equally spaced frequencies and iterated rippled noise. Particular attention is paid to differences in response to resolved and unresolved component harmonics. The results indicate that the model is able to simulate qualitatively the related pitch-perceptions. This physiological model is similar in many respects to autocorrelation models of pitch and the success of the evaluations suggests that autocorrelation models may, after all, be physiologically plausible.

Reduced contribution of a nonsimultaneous mistuned harmonic to residue pitch

Ciocca and Darwin [V. Ciocca and C. J. Darwin, J. Acoust. Soc. Am. 105, 2421-2430 (1999)] reported that the shift in residue pitch caused by mistuning a single harmonic (the fourth out of the first 12) was the same when the mistuned harmonic was presented after the remainder of the complex as when it was simultaneous, even though subjects were asked to ignore the pure-tone percept. The present study tried to replicate this result, and investigated the role of the presence of the nominally mistuned harmonic in the matching sound. Subjects adjusted a "matching" sound so that its pitch equaled that of a subsequent 90-ms complex tone (12 harmonics of a 155-Hz F0), whose mistuned (+/-3%) third harmonic was presented either simultaneously with or after the remaining harmonics. In experiment 1, the matching sound was a harmonic complex whose third harmonic was either present or absent. In experiments 2A and 2B, the target and matching sound had nonoverlapping spectra. Pitch shifts were reduced both when the mistuned component was nonsimultaneous, and when the third harmonic was absent in the matching sound. The results indicate a shorter than originally estimated time window for obligatory integration of nonsimultaneous components into a virtual pitch.

Stimulus Induced Desynchronization of Human Auditory 40-Hz Steady-State Responses

The hypothesis that gamma-band oscillations are related to the representation of an environmental scene in the cerebral cortex after binding of corresponding perceptual elements is currently under discussion. One question is how the sensory system reacts to a fast change in the scene if perceptual elements are rigidly bound together. A reset of the gamma-band oscillation forced by a change in sensory input may dissolve the binding, which then would be re-established for the new sensation. We studied the reset of gamma-band oscillations on the 40-Hz auditory steady-state responses (ASSR) by means of whole-head magnetoencephalography (MEG). The rhythm of 40-Hz AM of a 500-Hz tone evoked the ASSR, and a short noise burst served as a concurrent stimulus. Possible direct interactions of the auditory stimuli were excluded by presenting the noise impulse in a different frequency channel (2,000–3,000 Hz) to the contralateral ear. The concurrent stimulus induced a considerable decrement in the amplitude of ASSR, which was localized in primary auditory cortices. This decrement lasted 250 ms and was significantly longer than the duration of the transient gamma-band response evoked by the noise burst. Thus it could not be explained by any linear superimposition of the responses. The time courses of ASSR amplitude and phase during recovery from the decrement resembled those after stimulus onset, indicating that a new ASSR was built up after the resetting stimulus. The results are discussed as reset of oscillations in human thalamocortical networks.

Pitch shifts for complex tones with unresolved harmonics and the implications for models of pitch perception

Complex tone bursts were bandpass filtered, 22nd-30th harmonic, to produce waveforms with five regularly occurring envelope peaks ("pitch pulses") that evoked pitches associated with their repetition period. Two such tone bursts were presented sequentially and separated by an interpulse interval (IPI). When the IPI was varied, the pitch of the whole sequence was shifted by between +2% and -5%. When the IPI was greater than one period, little effect was seen. This is consistent with a pitch mechanism employing a long integration time for continuous stimuli that resets in response to temporal discontinuities of greater than about one period of the waveform. Similar pitch shifts were observed for fundamental frequencies from 100 to 250 Hz. The pitch shifts depended on the IPI duration relative to the period of the complex, not on the absolute IPI duration. The pitch shifts are inconsistent with the autocorrelation model of Meddis and O'Mard [J. Acoust. Soc. Am. 102, 1811-1820 (1997)], although a modified version of the weighted mean-interval model of Carlyon et al. [J. Acoust. Soc. Am. 112, 621-633 (2002)] was successful. The pitch shifts suggest that, when two pulses occur close together, one of the pulses is ignored on a probabilistic basis.

Across-frequency interference effects in fundamental frequency discrimination: questioning evidence for two pitch mechanisms

Carlyon and Shackleton [J. Acoust. Soc. Am. 95, 3541-3554 (1994)] presented an influential study supporting the existence of two pitch mechanisms, one for complex tones containing resolved and one for complex tones containing only unresolved components. The current experiments provide an alternative explanation for their finding, namely the existence of across-frequency interference in fundamental frequency (F0) discrimination. Sensitivity (d') was measured for F0 discrimination between two sequentially presented 400 ms complex (target) tones containing only unresolved components. In experiment 1, the target was filtered between 1375 and 15,000 Hz, had a nominal F0 of 88 Hz, and was presented either alone or with an additional complex tone ("interferer"). The interferer was filtered between 125-625 Hz, and its F0 varied between 88 and 114.4 Hz across blocks. Sensitivity was significantly reduced in the presence of the interferer, and this effect decreased as its F0 was moved progressively further from that of the target. Experiment 2 showed that increasing the level of a synchronously gated lowpass noise that spectrally overlapped with the interferer reduced this "pitch discrimination interference (PDI)". In experiment 3A, the target was filtered between 3900 and 5400 Hz and had an F0 of either 88 or 250 Hz. It was presented either alone or with an interferer, filtered between 1375 and 1875 Hz with an F0 corresponding to the nominal target F0. PDI was larger in the presence of the resolved (250 Hz F0) than in the presence of the unresolved (88 Hz F0) interferer, presumably because the pitch of the former was more salient than that of the latter. Experiments 4A and 4B showed that PDI was reduced but not eliminated when the interferer was gated on 200 ms before and off 200 ms after the target, and that some PDI was observed with a continuous interferer. The current findings provide an alternative interpretation of a study supposedly providing strong evidence for the existence of two pitch mechanisms.

Temporal pitch mechanisms in acoustic and electric hearing

Two experiments investigated pitch perception for stimuli where the place of excitation was held constant. Experiment 1 used pulse trains in which the interpulse interval alternated between 4 and 6 ms. In experiment 1a these "4-6" pulse trains were bandpass filtered between 3900 and 5300 Hz and presented acoustically against a noise background to normal listeners. The rate of an isochronous pulse train (in which all the interpulse intervals were equal) was adjusted so that its pitch matched that of the "4-6" stimulus. The pitch matches were distributed unimodally, had a mean of 5.7 ms, and never corresponded to either 4 or to 10 ms (the period of the stimulus). In experiment 1b the pulse trains were presented both acoustically to normal listeners and electrically to users of the LAURA cochlear implant, via a single channel of their device. A forced-choice procedure was used to measure psychometric functions, in which subjects judged whether the 4-6 stimulus was higher or lower in pitch than isochronous pulse trains having periods of 3, 4, 5, 6, or 7 ms. For both groups of listeners, the point of subjective equality corresponded to a period of 5.6 to 5.7 ms. Experiment 1c confirmed that these psychometric functions were monotonic over the range 4-12 ms. In experiment 2, normal listeners adjusted the rate of an isochronous filtered pulse train to match the pitch of mixtures of pulse trains having rates of F1 and F2 Hz, passed through the same bandpass filter (3900-5400 Hz). The ratio F2/F1 was 1.29 and F1 was either 70, 92, 109, or 124 Hz. Matches were always close to F2 Hz. It is concluded that the results of both experiments are inconsistent with models of pitch perception which rely on higher-order intervals. Together with those of other published data on purely temporal pitch perception, the data are consistent with a model in which only first-order interpulse intervals contribute to pitch, and in which, over the range 0-12 ms, longer intervals receive higher weights than short intervals.

Searching for the time constant of neural pitch extraction

Multichannel, auditory models have been repeatedly used to explain many aspects of human pitch perception. Among the most successful ones are models where pitch is estimated based on an analysis of periodicity in the simulated auditory-nerve firing. This periodicity analysis is typically implemented as a running autocorrelation, i.e., the autocorrelation is calculated within a temporal window which is shifted along the time axis. The window was suggested to have an exponential decay with time-constant estimates between 1.5 and 100 ms. The window length determines the minimal integration time of pitch extraction. The present experiments are designed to quantify the temporal window of pitch extraction using regular-interval noises (RINs). RINs were generated by concatenating equal-duration noise samples which produce a pitch corresponding to the reciprocal of the sample duration when the samples are identical (periodic noise). When the samples are independent, the stimulus is Gaussian noise and produces no pitch. Using RIN stimuli where periodic portions interchange with aperiodic portions, it is shown that the temporal window of pitch extraction cannot be modeled using a single time constant but that the size of the temporal window depends on the pitch itself.

Perceived continuity and pitch perception

Three experiments investigated the importance of perceived stimulus continuity for the perception of the fundamental frequency (F0) of an unresolved complex tone. The F0 of the complex was 250 Hz and the harmonics were bandpass filtered between 5500 and 7500 Hz. In the first experiment, F0 discrimination was measured for single-burst tones with durations of 20, 40, and 80 ms, and for stimuli containing two 20- or 40-ms tone bursts separated by an 8- or 16-ms gap. For the single-burst conditions, there was a large decrease in threshold as the duration was increased from 20 to 40 ms. However, performance in the gapped conditions was much worse than that for the single-burst condition with the same cumulative duration (e.g., two 20-ms bursts separated by 8 ms produced higher thresholds than one 40-ms burst). Adding a bandpass noise (with the same spectral envelope as the tone) in the gap between the two tone bursts improved performance to the level of the single-burst condition. When the noise was added, the two discrete tone bursts were perceived as one single tone burst interrupted by the noise, and this seemed to facilitate discrimination. In a second experiment, the effects on pitch of an envelope delay (phase shift) of 0.75 periods between two tone bursts separated by an 8-ms gap were investigated. If the gap was silent, the pitch of the pair was unaffected by the phase shift. However, if the gap contained the bandpass noise, the phase shift between the bursts did produce a significant downward shift in the pitch of the pair. Finally, the third experiment showed that presenting a noise before a single 20-ms burst may improve discrimination performance in some listeners, but not sufficiently to account for the results of the first experiment purely in terms of an improvement in the discriminability of the second tone burst in the pair. The experiments suggest that a level decrease between two tone bursts may disrupt or reset a long integration mechanism, decreasing performance. When there is no level decrease between the bursts, the auditory system may assume that the two bursts belong to the same single tone and analyze them together in order to derive F0.