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

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.

The upper frequency limit for the use of phase locking to code temporal fine structure in humans: A compilation of viewpoints

The relative importance of neural temporal and place coding in auditory perception is still a matter of much debate. The current article is a compilation of viewpoints from leading auditory psychophysicists and physiologists regarding the upper frequency limit for the use of neural phase locking to code temporal fine structure in humans. While phase locking is used for binaural processing up to about 1500 Hz, there is disagreement regarding the use of monaural phase-locking information at higher frequencies. Estimates of the general upper limit proposed by the contributors range from 1500 to 10000 Hz. The arguments depend on whether or not phase locking is needed to explain psychophysical discrimination performance at frequencies above 1500 Hz, and whether or not the phase-locked neural representation is sufficiently robust at these frequencies to provide useable information. The contributors suggest key experiments that may help to resolve this issue, and experimental findings that may cause them to change their minds. This issue is of crucial importance to our understanding of the neural basis of auditory perception in general, and of pitch perception in particular.

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.

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 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.

On the controversy about the sharpness of human cochlear tuning

In signal processing terms, the operation of the mammalian cochlea in the inner ear may be likened to a bank of filters. Based on otoacoustic emission evidence, it has been recently claimed that cochlear tuning is sharper for human than for other mammals. The claim was corroborated with a behavioral method that involves the masking of pure tones with forward notched noises (NN). Using this method, it has been further claimed that human cochlear tuning is sharper than suggested by earlier behavioral studies. These claims are controversial. Here, we contribute to the controversy by theoretically assessing the accuracy of the NN method at inferring the bandwidth (BW) of nonlinear cochlear filters. Behavioral forward masking was mimicked using a computer model of the squared basilar membrane response followed by a temporal integrator. Isoresponse and isolevel versions of the forward masking NN method were applied to infer the already known BW of the cochlear filter used in the model. We show that isolevel methods were overall more accurate than isoresponse methods. We also show that BWs for NNs and sinusoids equate only for isolevel methods and when the levels of the two stimuli are appropriately scaled. Lastly, we show that the inferred BW depends on the method version (isolevel BW was twice as broad as isoresponse BW at 40 dB SPL) and on the stimulus level (isoresponse and isolevel BW decreased and increased, respectively, with increasing level over the level range where cochlear responses went from linear to compressive). We suggest that the latter may contribute to explaining the reported differences in cochlear tuning across behavioral studies and species. We further suggest that given the well-established nonlinear nature of cochlear responses, even greater care must be exercised when using a single BW value to describe and compare cochlear tuning.

Frequency difference limens at high frequencies: evidence for a transition from a temporal to a place code

It is commonly believed that difference limens for frequency (DLFs) for pure tones depend on a temporal mechanism (phase locking) for frequencies up to 4-5 kHz and a place mechanism at higher frequencies. The DLFs predicted from a place mechanism, expressed as a proportion of center frequency (Δf/f), should be approximately invariant with frequency at medium to high frequencies. If there is a transition from a temporal to a place mechanism, Δf/f should increase with increasing center frequency until the transition occurs, and then reach a plateau. Published data do not show such an effect. In this study, DLFs were measured for center frequencies from 2 to 14 kHz, using earphones designed to produce a flat response at the eardrum. The level of every tone was varied over a range of ±4 dB, to reduce loudness cues. The value of Δf/f increased progressively from 2 to 8 kHz, but did not change significantly for frequencies from 8 to 14 kHz. The results are consistent with the idea that there is a transition from a temporal to a place mechanism at about 8 kHz, rather than at 4-5 kHz, as is commonly assumed.

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.