Discrimination of complex tones with unresolved components using temporal fine structure information

Exploiting individual differences to assess the role of place and phase locking cues in auditory frequency discrimination at 2 kHz

The relative role of place and temporal mechanisms in auditory frequency discrimination was assessed for a centre frequency of 2 kHz. Four measures of frequency discrimination were obtained for 63 normal-hearing participants: detection of frequency modulation using modulation rates of 2 Hz (FM2) and 20 Hz (FM20); detection of a change in frequency across successive pure tones (difference limen for frequency, DLF); and detection of changes in the temporal fine structure of bandpass filtered complex tones centred at 2 kHz (TFS). Previous work has suggested that: FM2 depends on the use of both temporal and place cues; FM20 depends primarily on the use of place cues because the temporal mechanism cannot track rapid changes in frequency; DLF depends primarily on temporal cues; TFS depends exclusively on temporal cues. This led to the following predicted patterns of the correlations of scores across participants: DLF and TFS should be highly correlated; FM2 should be correlated with DLF and TFS; FM20 should not be correlated with DLF or TFS. The results were broadly consistent with these predictions and with the idea that frequency discrimination at 2 kHz depends partly or primarily on temporal cues except for frequency modulation detection at a high rate.

Assessing mechanisms of frequency discrimination by comparison of different measures over a wide frequency range

It has been hypothesized that auditory detection of frequency modulation (FM) for low FM rates depends on the use of both temporal (phase locking) and place cues, depending on the carrier frequency, while detection of FM at high rates depends primarily on the use of place cues. To test this, FM detection for 2 and 20 Hz rates was measured over a wide frequency range, 1-10 kHz, including high frequencies for which temporal cues are assumed to be very weak. Performance was measured over the same frequency range for a task involving detection of changes in the temporal fine structure (TFS) of bandpass filtered complex tones, for which performance is assumed to depend primarily on the use of temporal cues. FM thresholds were better for the 2- than for the 20-Hz rate for center frequencies up to 4 kHz, while the reverse was true for higher center frequencies. For both FM rates, the thresholds, expressed as a proportion of the center frequency, were roughly constant for center frequencies from 6 to 10 Hz, consistent with the use of place cues. For the TFS task, thresholds worsened progressively with increasing frequency above 4 kHz, consistent with the weakening of temporal cues.

Speech in noise perception improved by training fine auditory discrimination: far and applicable transfer of perceptual learning

A longstanding focus of perceptual learning research is learning specificity, the difficulty for learning to transfer to tasks and situations beyond the training setting. Previous studies have focused on promoting transfer across stimuli, such as from one sound frequency to another. Here we examined whether learning could transfer across tasks, particularly from fine discrimination of sound features to speech perception in noise, one of the most frequently encountered perceptual challenges in real life. Separate groups of normal-hearing listeners were trained on auditory interaural level difference (ILD) discrimination, interaural time difference (ITD) discrimination, and fundamental frequency (F₀) discrimination with non-speech stimuli delivered through headphones. While ITD training led to no improvement, both ILD and F₀ training produced learning as well as transfer to speech-in-noise perception when noise differed from speech in the trained feature. These training benefits did not require similarity of task or stimuli between training and application settings, construing far and wide transfer. Thus, notwithstanding task specificity among basic perceptual skills such as discrimination of different sound features, auditory learning appears readily transferable between these skills and their “upstream” tasks utilizing them, providing an effective approach to improving performance in challenging situations or challenged populations.

The effect of musicianship, contralateral noise, and ear of presentation on the detection of changes in temporal fine structure

Musicians are better than non-musicians at discriminating changes in the fundamental frequency (F0) of harmonic complex tones. Such discrimination may be based on place cues derived from low resolved harmonics, envelope cues derived from high harmonics, and temporal fine structure (TFS) cues derived from both low and high harmonics. The present study compared the ability of highly trained violinists and non-musicians to discriminate changes in complex sounds that differed primarily in their TFS. The task was to discriminate harmonic (H) and frequency-shifted inharmonic (I) tones that were bandpass filtered such that the components were largely or completely unresolved. The effect of contralateral noise and ear of presentation was also investigated. It was hypothesized that contralateral noise would activate the efferent system, helping to preserve the neural representation of envelope fluctuations in the H and I stimuli, thereby improving their discrimination. Violinists were significantly better than non-musicians at discriminating the H and I tones. However, contralateral noise and ear of presentation had no effect. It is concluded that, compared to non-musicians, violinists have a superior ability to discriminate complex sounds based on their TFS, and this ability is unaffected by contralateral stimulation or ear of presentation.

How aging impacts the encoding of binaural cues and the perception of auditory space

Over the years, the effect of aging on auditory function has been investigated in animal models and humans in an effort to characterize age-related changes in both perception and physiology. Here, we review how aging may impact neural encoding and processing of binaural and spatial cues in human listeners with a focus on recent work by the authors as well as others. Age-related declines in monaural temporal processing, as estimated from measures of gap detection and temporal fine structure discrimination, have been associated with poorer performance on binaural tasks that require precise temporal processing. In lateralization and localization tasks, as well as in the detection of signals in noise, marked age-related changes have been demonstrated in both behavioral and electrophysiological measures and have been attributed to declines in neural synchrony and reduced central inhibition with advancing age. Evidence for such mechanisms, however, are influenced by the task (passive vs. attending) and the stimulus paradigm (e.g., static vs. continuous with dynamic change). That is, cortical auditory evoked potentials (CAEP) measured in response to static interaural time differences (ITDs) are larger in older versus younger listeners, consistent with reduced inhibition, while continuous stimuli with dynamic ITD changes lead to smaller responses in older compared to younger adults, suggestive of poorer neural synchrony. Additionally, the distribution of cortical activity is broader and less asymmetric in older than younger adults, consistent with the hemispheric asymmetry reduction in older adults model of cognitive aging. When older listeners attend to selected target locations in the free field, their CAEP components (N1, P2, P3) are again consistently smaller relative to younger listeners, and the reduced asymmetry in the distribution of cortical activity is maintained. As this research matures, proper neural biomarkers for changes in spatial hearing can provide objective evidence of impairment and targets for remediation. Future research should focus on the development and evaluation of effective approaches for remediating these spatial processing deficits associated with aging and hearing loss.

The role of excitation-pattern cues in the detection of frequency shifts in bandpass-filtered complex tones

One task intended to measure sensitivity to temporal fine structure (TFS) involves the discrimination of a harmonic complex tone from a tone in which all harmonics are shifted upwards by the same amount in hertz. Both tones are passed through a fixed bandpass filter centered on the high harmonics to reduce the availability of excitation-pattern cues and a background noise is used to mask combination tones. The role of frequency selectivity in this "TFS1" task was investigated by varying level. Experiment 1 showed that listeners performed more poorly at a high level than at a low level. Experiment 2 included intermediate levels and showed that performance deteriorated for levels above about 57 dB sound pressure level. Experiment 3 estimated the magnitude of excitation-pattern cues from the variation in forward masking of a pure tone as a function of frequency shift in the complex tones. There was negligible variation, except for the lowest level used. The results indicate that the changes in excitation level at threshold for the TFS1 task would be too small to be usable. The results are consistent with the TFS1 task being performed using TFS cues, and with frequency selectivity having an indirect effect on performance via its influence on TFS cues.

Electrophysiological and behavioural processing of complex acoustic cues

OBJECTIVES

To examine behavioural and neural processing of pitch cues in adults with normal hearing (NH) and adults with sensorineural hearing loss (SNHL).

METHODS

All participants completed a test of behavioural sensitivity to pitch cues using the TFS1 test (Moore and Sek, 2009a). Cortical potentials (N1, P2 and acoustic change complex) were recorded in response to frequency shifted (deltaF) tone complexes in an 'ABA' pattern.

RESULTS

The SNHL group performed more poorly than the NH group for the TFS1 test. P2 was more reflective of pitch differences between the complexes than N1. The presence of acoustic change complex in response to the TFS transitions in the ABA stimulus varied with deltaF. Acoustic change complex amplitudes were reduced for the group with SNHL compared to controls.

CONCLUSION

Behavioural performance and cortical responses reflect pitch processing depending on the salience of pitch cues.

SIGNIFICANCE

These data support the use of cortical potentials and behavioural sensitivity tests to measure processing of complex acoustic cues in people with hearing loss. This approach has potential for evaluation of benefit from auditory training and hearing instrument digital signal processing strategies.

Implications of within-fiber temporal coding for perceptual studies of F0 discrimination and discrimination of harmonic and inharmonic tone complexes

Recent psychophysical studies suggest that normal-hearing (NH) listeners can use acoustic temporal-fine-structure (TFS) cues for accurately discriminating shifts in the fundamental frequency (F0) of complex tones, or equal shifts in all component frequencies, even when the components are peripherally unresolved. The present study quantified both envelope (ENV) and TFS cues in single auditory-nerve (AN) fiber responses (henceforth referred to as neural ENV and TFS cues) from NH chinchillas in response to harmonic and inharmonic complex tones similar to those used in recent psychophysical studies. The lowest component in the tone complex (i.e., harmonic rank N) was systematically varied from 2 to 20 to produce various resolvability conditions in chinchillas (partially resolved to completely unresolved). Neural responses to different pairs of TEST (F0 or frequency shifted) and standard or reference (REF) stimuli were used to compute shuffled cross-correlograms, from which cross-correlation coefficients representing the degree of similarity between responses were derived separately for TFS and ENV. For a given F0 shift, the dissimilarity (TEST vs. REF) was greater for neural TFS than ENV. However, this difference was stimulus-based; the sensitivities of the neural TFS and ENV metrics were equivalent for equal absolute shifts of their relevant frequencies (center component and F0, respectively). For the F0-discrimination task, both ENV and TFS cues were available and could in principle be used for task performance. However, in contrast to human performance, neural TFS cues quantified with our cross-correlation coefficients were unaffected by phase randomization, suggesting that F0 discrimination for unresolved harmonics does not depend solely on TFS cues. For the frequency-shift (harmonic-versus-inharmonic) discrimination task, neural ENV cues were not available. Neural TFS cues were available and could in principle support performance in this task; however, in contrast to human-listeners' performance, these TFS cues showed no dependence on N. We conclude that while AN-fiber responses contain TFS-related cues, which can in principle be used to discriminate changes in F0 or equal shifts in component frequencies of peripherally unresolved harmonics, performance in these two psychophysical tasks appears to be limited by other factors (e.g., central processing noise).

Influence of musical training on sensitivity to temporal fine structure

OBJECTIVE

The objective of this study was to extend the findings that temporal fine structure encoding is altered in musicians by examining sensitivity to temporal fine structure (TFS) in an alternative (non-Western) musician model that is rarely adopted--Indian classical music.

DESIGN

The sensitivity to TFS was measured by the ability to discriminate two complex tones that differed in TFS but not in envelope repetition rate.

STUDY SAMPLE

Sixteen South Indian classical (Carnatic) musicians and 28 non-musicians with normal hearing participated in this study.

RESULTS

Musicians have significantly lower relative frequency shift at threshold in the TFS task compared to non-musicians. A significant negative correlation was observed between years of musical experience and relative frequency shift at threshold in the TFS task. Test-retest repeatability of thresholds in the TFS tasks was similar for both musicians and non-musicians.

CONCLUSIONS

The enhanced performance of the Carnatic-trained musicians suggests that the musician advantage for frequency and harmonicity discrimination is not restricted to training in Western classical music, on which much of the previous research on musical training has narrowly focused. The perceptual judgments obtained from non-musicians were as reliable as those of musicians.

The role of excitation-pattern, temporal-fine-structure, and envelope cues in the discrimination of complex tones

The discrimination of bandpass-filtered harmonic (H) from inharmonic (I) tones (produced by shifting all components of the H tones upwards by a fixed amount in Hz) could be based on shifts in the pattern of ripples in the excitation pattern (EP) or on changes in the temporal fine structure evoked by the tones. The predictions of two computational EP models were compared with measured performance. One model used auditory filters with bandwidth values specified by Glasberg and Moore [(1990). Hear. Res. 47, 103-138] and one used filters that were twice as sharp. Stimulus variables were passband width, fundamental frequency, harmonic rank (N) of the lowest component within the passband, component phase (cosine or random), signal-to-noise ratio (SNR), and random perturbation in level of each component in the tones. While the EP models correctly predicted the lack of an effect of phase and some of the trends in the data as a function of fundamental frequency and N, neither model predicted the worsening in performance with increasing passband width or the lack of effect of SNR and level perturbation. It is concluded that discrimination of the H and I tones is not based solely on the use of EP cues.

Effects of sensorineural hearing loss on temporal coding of harmonic and inharmonic tone complexes in the auditory nerve

Listeners with sensorineural hearing loss (SNHL) often show poorer thresholds for fundamental-frequency (F0) discrimination and poorer discrimination between harmonic and frequency-shifted (inharmonic) complex tones, than normal-hearing (NH) listeners-especially when these tones contain resolved or partially resolved components. It has been suggested that these perceptual deficits reflect reduced access to temporal-fine-structure (TFS) information and could be due to degraded phase locking in the auditory nerve (AN) with SNHL. In the present study, TFS and temporal-envelope (ENV) cues in single AN-fiber responses to band-pass-filtered harmonic and inharmonic complex tones were -measured in chinchillas with either normal-hearing or noise-induced SNHL. The stimuli were comparable to those used in recent psychophysical studies of F0 and harmonic/inharmonic discrimination. As in those studies, the rank of the center component was manipulated to produce -different resolvability conditions, different phase relationships (cosine and random phase) were tested, and background noise was present. Neural TFS and ENV cues were quantified using cross-correlation coefficients computed using shuffled cross correlograms between neural responses to REF (harmonic) and TEST (F0- or frequency-shifted) stimuli. In animals with SNHL, AN-fiber tuning curves showed elevated thresholds, broadened tuning, best-frequency shifts, and downward shifts in the dominant TFS response component; however, no significant degradation in the ability of AN fibers to encode TFS or ENV cues was found. Consistent with optimal-observer analyses, the results indicate that TFS and ENV cues depended only on the relevant frequency shift in Hz and thus were not degraded because phase locking remained intact. These results suggest that perceptual "TFS-processing" deficits do not simply reflect degraded phase locking at the level of the AN. To the extent that performance in F0- and harmonic/inharmonic discrimination tasks depend on TFS cues, it is likely through a more complicated (suboptimal) decoding mechanism, which may involve "spatiotemporal" (place-time) neural representations.

The dominant region for the pitch of complex tones with low fundamental frequencies

The dominant region for pitch for complex tones with low fundamental frequency (F0) was investigated. Thresholds for detection of a change in F0 (F0DLs) were measured for a group of harmonics (group B) embedded in a group of fixed non-overlapping harmonics (group A) with the same mean F0. It was assumed that F0DLs would be smallest when the harmonics in group B fell in the dominant region. The rank of the lowest harmonic in group B, N, was varied from 1 to 15. When all components had the same level, F0DLs increased with increasing N, but the increase started at a lower value of N for F0 = 200 Hz than for F0 = 50 or 100 Hz, the opposite of what would be expected if the dominant region corresponds to resolved harmonics. When the component levels followed an equal-loudness contour, F0DLs for F0 = 50 Hz were lowest for N = 1, but overall performance was much worse than for equal-level components, suggesting that the lowest harmonics were masking information from the higher harmonics.

Contribution of temporal fine structure information and fundamental frequency separation to intelligibility in a competing-speaker paradigm

The speech reception threshold (SRT) for identifying a target speaker in a background speaker was measured as a function of the difference (F0sep) in fundamental frequency (F0) between the two speakers. The amount of original temporal fine structure (TFS) information in the mixed signals was manipulated by tone vocoding channels above a certain cutoff channel (CO). When the natural variations in F0 of both speakers were preserved, the SRT did not decrease with increasing F0sep, indicating that short-term differences in F0 can allow perceptual segregation of two speakers even when their F0s cross. When F0 variations were removed from both speakers, increasing F0sep led to decreased (better) SRTs. The decrease was greater for unprocessed signals than for fully tone-vocoded signals. However, the decrease was similar for unprocessed signals and for signals with original TFS below 1600 Hz, suggesting that most of the benefit from increasing F0 difference depends on the use of TFS information at lower frequencies. Adding original TFS information to channels centered above 1600 Hz produced roughly the same decrease in SRT as adding original TFS information to channels centered below 1600 Hz, suggesting a benefit from original TFS information apart from that related to differences in F0.

On the possibility of a place code for the low pitch of high-frequency complex tones

Harmonics are considered unresolved when they interact with neighboring harmonics and cannot be heard out separately. Several studies have suggested that the pitch derived from unresolved harmonics is coded via temporal fine-structure cues emerging from their peripheral interactions. Such conclusions rely on the assumption that the components of complex tones with harmonic ranks down to at least 9 were indeed unresolved. The present study tested this assumption via three different measures: (1) the effects of relative component phase on pitch matches, (2) the effects of dichotic presentation on pitch matches, and (3) listeners' ability to hear out the individual components. No effects of relative component phase or dichotic presentation on pitch matches were found in the tested conditions. Large individual differences were found in listeners' ability to hear out individual components. Overall, the results are consistent with the coding of individual harmonic frequencies, based on the tonotopic activity pattern or phase locking to individual harmonics, rather than with temporal coding of single-channel interactions. However, they are also consistent with more general temporal theories of pitch involving the across-channel summation of information from resolved and/or unresolved harmonics. Simulations of auditory-nerve responses to the stimuli suggest potential benefits to a spatiotemporal mechanism.

Pitch perception of concurrent harmonic tones with overlapping spectra

Fundamental frequency difference limens (F0DLs) were measured for a target harmonic complex tone with nominal fundamental frequency (F0) of 200 Hz, in the presence and absence of a harmonic masker with overlapping spectrum. The F0 of the masker was 0, ± 3, or ± 6 semitones relative to 200 Hz. The stimuli were bandpass filtered into three regions: 0-1000 Hz (low, L), 1600-2400 Hz (medium, M), and 2800-3600 Hz (high, H), and a background noise was used to mask combination tones and to limit the audibility of components falling on the filter skirts. The components of the target or masker started either in cosine or random phase. Generally, the effect of F0 difference between target and masker was small. For the target alone, F0DLs were larger for random than cosine phase for region H. For the target plus masker, F0DLs were larger when the target had random phase than cosine phase for regions M and H. F0DLs increased with increasing center frequency of the bandpass filter. Modeling using excitation patterns and "summary autocorrelation" and "stabilized auditory image" models suggested that use of temporal fine structure information can account for the small F0DLs obtained when harmonics are barely, if at all, resolved.

Implementation of two tests for measuring sensitivity to temporal fine structure

OBJECTIVE

To implement two methods for measuring sensitivity to temporal fine structure (TFS) for use in assessing effects of hearing loss and age that may not be apparent from the audiogram.

DESIGN

The TFS1 test was described by Moore and Sek (2009). The task is to discriminate a harmonic complex tone from a tone in which all frequency components are shifted upwards by the same amount in Hz. The TFSLF test was described by Hopkins and Moore (2010a). The task is to detect changes in lateral position of a binaurally presented tone based on interaural phase difference (IPD). Both tests have been implemented in software that can be run on a PC with a good-quality sound card. The software includes a routine for measuring the absolute threshold at the test frequency.

RESULTS

For each test, an experimental run at a single frequency takes about three minutes. Practice tasks (frequency discrimination of pure tones for TFS1 and discrimination of changes in lateral position based on interaural level difference for TFSLF) are also implemented that are similar to the main task, but easier.

CONCLUSIONS

The software implementation allows sensitivity to TFS to be measured quickly without a requirement for specialized equipment.

Psychometric functions for pure-tone frequency discrimination

The form of the psychometric function (PF) for auditory frequency discrimination is of theoretical interest and practical importance. In this study, PFs for pure-tone frequency discrimination were measured for several standard frequencies (200-8000 Hz) and levels [35-85 dB sound pressure level (SPL)] in normal-hearing listeners. The proportion-correct data were fitted using a cumulative-Gaussian function of the sensitivity index, d', computed as a power transformation of the frequency difference, Δf. The exponent of the power function corresponded to the slope of the PF on log(d')-log(Δf) coordinates. The influence of attentional lapses on PF-slope estimates was investigated. When attentional lapses were not taken into account, the estimated PF slopes on log(d')-log(Δf) coordinates were found to be significantly lower than 1, suggesting a nonlinear relationship between d' and Δf. However, when lapse rate was included as a free parameter in the fits, PF slopes were found not to differ significantly from 1, consistent with a linear relationship between d' and Δf. This was the case across the wide ranges of frequencies and levels tested in this study. Therefore, spectral and temporal models of frequency discrimination must account for a linear relationship between d' and Δf across a wide range of frequencies and levels.

The effects of age and cochlear hearing loss on temporal fine structure sensitivity, frequency selectivity, and speech reception in noise

Temporal fine structure (TFS) sensitivity, frequency selectivity, and speech reception in noise were measured for young normal-hearing (NHY), old normal-hearing (NHO), and hearing-impaired (HI) subjects. Two measures of TFS sensitivity were used: the "TFS-LF test" (interaural phase difference discrimination) and the "TFS2 test" (discrimination of harmonic and frequency-shifted tones). These measures were not significantly correlated with frequency selectivity (after partialing out the effect of audiometric threshold), suggesting that insensitivity to TFS cannot be wholly explained by a broadening of auditory filters. The results of the two tests of TFS sensitivity were significantly but modestly correlated, suggesting that performance of the tests may be partly influenced by different factors. The NHO group performed significantly more poorly than the NHY group for both measures of TFS sensitivity, but not frequency selectivity, suggesting that TFS sensitivity declines with age in the absence of elevated audiometric thresholds or broadened auditory filters. When the effect of mean audiometric threshold was partialed out, speech reception thresholds in modulated noise were correlated with TFS2 scores, but not measures of frequency selectivity or TFS-LF test scores, suggesting that a reduction in sensitivity to TFS can partly account for the speech perception difficulties experienced by hearing-impaired subjects.

Effect of level on the discrimination of harmonic and frequency-shifted complex tones at high frequencies

Moore and Sęk [J. Acoust. Soc. Am. 125, 3186-3193 (2009)] measured discrimination of a harmonic complex tone and a tone in which all harmonics were shifted upwards by the same amount in Hertz. Both tones were passed through a fixed bandpass filter and a background noise was used to mask combination tones. Performance was well above chance when the fundamental frequency was 800 Hz, and all audible components were above 8000 Hz. Moore and Sęk argued that this suggested the use of temporal fine structure information at high frequencies. However, the task may have been performed using excitation-pattern cues. To test this idea, performance on a similar task was measured as a function of level. The auditory filters broaden with increasing level, so performance based on excitation-pattern cues would be expected to worsen as level increases. The results did not show such an effect, suggesting that the task was not performed using excitation-pattern cues.

Processing of temporal fine structure as a function of age

OBJECTIVES

The purpose of this study was to determine whether the processing of temporal fine structure diminishes with age, even in the presence of relatively normal audiometric hearing. Temporal fine structure processing was assessed by measuring the discrimination of interaural phase differences (IPDs). The hypothesis was that IPD discrimination is more acute in middle-aged observers than in older observers but that acuity in middle-aged observers is nevertheless poorer than in young adults.

DESIGN

Two experiments were undertaken. The first measured discrimination of 0- and π-radian interaural phases as a function of carrier frequency. The stimulus was a 5-Hz sinusoidally amplitude-modulated tone in which, in the signal waveform, the interaural phase of the carrier was inverted during alternate modulation periods. The second experiment measured IPD discrimination at fixed frequencies. The stimulus was a pair of tone pulses in which, in the signal, the trailing pulse contained an IPD. A total of 39 adults with normal audiograms ≤2000 Hz participated in this study, of which 15 were younger, 12 middle aged, and 12 older.

RESULTS

Experiment 1 showed that the highest carrier frequency at which a π-radian IPD could be discriminated from the diotic, 0-radian standard was significantly lower in middle-aged listeners than young adults, and still lower in older listeners. Experiment 2 indicated that middle-aged listeners were less sensitive to IPDs than young adults at all but the lowest frequencies tested. Older listeners, as a group, had the poorest thresholds.

CONCLUSIONS

These results suggest that deficits in temporal fine structure processing are evident in the presenescent auditory system. This adds to the accumulating evidence that deficiencies in some aspects of auditory temporal processing emerge relatively early in the aging process. It is possible that early-emerging temporal processing deficits manifest themselves in challenging speech in noise environments.

Resolvability of components in complex tones and implications for theories of pitch perception

This paper reviews methods that have been used to estimate the resolvability of individual partials in harmonic and inharmonic complex tones and considers the implications of the results for theories of pitch perception. The methods include: requiring comparisons of the pitch of an isolated pure tone and a partial within a complex tone as a measure of the ability to "hear out" that partial; considering the magnitude of ripples in the calculated excitation pattern of a complex tone; using a complex tone as a forward masker and using ripples in the masking pattern to estimate resolvability; measuring sensitivity to the relative phase of the components within complex tones. The measures are broadly consistent in indicating that harmonics with numbers up to about five are well resolved, but that resolution decreases for higher harmonics. Most measures suggest that harmonics with numbers above eight are poorly, if at all, resolved. However, there are uncertainties associated with each method that make the exact upper limit of resolvability uncertain. Evidence is presented suggesting a partial dissociation between resolution in the excitation pattern and the ability to hear out a partial. It is proposed that the latter requires information from temporal fine structure (phase locking).

The role of temporal fine structure information for the low pitch of high-frequency complex tones

The fused low pitch evoked by complex tones containing only unresolved high-frequency components demonstrates the ability of the human auditory system to extract pitch using a temporal mechanism in the absence of spectral cues. However, the temporal features used by such a mechanism have been a matter of debate. For stimuli with components lying exclusively in high-frequency spectral regions, the slowly varying temporal envelope of sounds is often assumed to be the only information contained in auditory temporal representations, and it has remained controversial to what extent the fast amplitude fluctuations, or temporal fine structure (TFS), of the conveyed signal can be processed. Using a pitch matching paradigm, the present study found that the low pitch of inharmonic transposed tones with unresolved components was consistent with the timing between the most prominent TFS maxima in their waveforms, rather than envelope maxima. Moreover, envelope cues did not take over as the absolute frequency or rank of the lowest component was raised and TFS cues thus became less effective. Instead, the low pitch became less salient. This suggests that complex pitch perception does not rely on envelope coding as such, and that TFS representation might persist at higher frequencies than previously thought.

Perceptual learning of fundamental frequency discrimination: effects of fundamental frequency, harmonic number, and component phase

Thresholds (F0DLs) were measured for discrimination of the fundamental frequency (F0) of a group of harmonics (group B) embedded in harmonics with a fixed F0. Miyazono and Moore [(2009). Acoust. Sci. & Tech. 30, 383386] found a large training effect for tones with high harmonics in group B, when the harmonics were added in cosine phase. It is shown here that this effect was due to use of a cue related to pitch pulse asynchrony (PPA). When PPA cues were disrupted by introducing a temporal offset between the envelope peaks of the harmonics in group B and the remaining harmonics, F0DLs increased markedly. Perceptual learning was examined using a training stimulus with cosine-phase harmonics, F0 = 50 Hz, and high harmonics in group B, under conditions where PPA was not useful. Learning occurred, and it transferred to other cosine-phase tones, but not to random-phase tones. A similar experiment with F0 = 100 Hz showed a learning effect which transferred to a cosine-phase tone with mainly high unresolved harmonics, but not to cosine-phase tones with low harmonics, and not to random-phase tones. The learning found here appears to be specific to tones for which F0 discrimination is based on distinct peaks in the temporal envelope.

The role of temporal fine structure in harmonic segregation through mistuning

Bernstein and Oxenham [(2008). J. Acoust. Soc. Am. 124, 1653-1667] measured thresholds for discriminating the fundamental frequency, F0, of a complex tone that was passed through a fixed bandpass filter. They found that performance worsened when the F0 was decreased so that only harmonics above the tenth were audible. However, performance in this case was improved by mistuning the odd harmonics by 3%. Bernstein and Oxenham considered whether the results could be explained in terms of temporal fine structure information available at the output of a single auditory filter and concluded that their results did not appear to be consistent with such an explanation. Here, it is argued that such cues could have led to the improvement in performance produced by mistuning the odd harmonics.

Effects of pulsing of the target tone on the audibility of partials in inharmonic complex tones

The audibility of partials was measured for complex tones with partials uniformly spaced on an ERB(N)-number scale. On each trial, subjects heard a sinusoidal "probe" followed by a complex tone. The probe was mistuned downwards or upwards (at random) by 3% or 4.5% from the frequency of one randomly selected partial in the complex (the "target"). The subject indicated whether the target was higher or lower in frequency than the probe. The probe and the target were pulsed on and off and the ramp times and inter-pulse intervals were systematically varied. Performance was better for longer ramp times and longer inter-pulse intervals. In a second experiment, the ability to detect which of two complex tones contained a pulsed partial was measured. The pattern of results was similar to that for experiment 1. A model of auditory processing including an adaptation stage was able to account for the general pattern of the results of experiment 2. The results suggest that the improvement in ability to hear out a partial in a complex tone produced by pulsing that partial is partly mediated by a release from adaptation produced by the pulsing, and does not result solely from reduction of perceptual confusion.