Fundamental frequency differences (ΔF0) between competing talkers aid in the perceptual segregation of the talkers (ΔF0 benefit), but the underlying mechanisms remain incompletely understood. A model of ΔF0 benefit based on harmonic cancellation proposes that a masker's periodicity can be used to cancel (i.e., filter out) its neural representation. Earlier work suggested that an octave ΔF0 provided little benefit, an effect predicted by harmonic cancellation due to the shared periodicity of masker and target. Alternatively, this effect can be explained by spectral overlap between the harmonic components of the target and masker. To assess these competing explanations, speech intelligibility of a monotonized target talker, masked by a speech-shaped harmonic complex tone, was measured as a function of ΔF0, masker spectrum (all harmonics or odd harmonics only), and masker temporal envelope (amplitude modulated or unmodulated). Removal of the masker's even harmonics when the target was one octave above the masker improved speech reception thresholds by about 5 dB. Because this manipulation eliminated spectral overlap between target and masker components but preserved shared periodicity, the finding is consistent with the explanation for the lack of ΔF0 benefit at the octave based on spectral overlap, but not with the explanation based on harmonic cancellation.

Accurate pitch perception of harmonic complex tones is widely believed to rely on temporal fine structure information conveyed by the precise phase-locked responses of auditory-nerve fibers. However, accurate pitch perception remains possible even when spectrally resolved harmonics are presented at frequencies beyond the putative limits of neural phase locking, and it is unclear whether residual temporal information, or a coarser rate-place code, underlies this ability. We addressed this question by measuring human pitch discrimination at low and high frequencies for harmonic complex tones, presented either in isolation or in the presence of concurrent complex-tone maskers. We found that concurrent complex-tone maskers impaired performance at both low and high frequencies, although the impairment introduced by adding maskers at high frequencies relative to low frequencies differed between the tested masker types. We then combined simulated auditory-nerve responses to our stimuli with ideal-observer analysis to quantify the extent to which performance was limited by peripheral factors. We found that the worsening of both frequency discrimination and F0 discrimination at high frequencies could be well accounted for (in relative terms) by optimal decoding of all available information at the level of the auditory nerve. A Python package is provided to reproduce these results, and to simulate responses to acoustic stimuli from the three previously published models of the human auditory nerve used in our analyses.

The present study examined the effect of frequency shifts on perceived talker recognition in foreign-accented speech compared to native-accented speech. Sentences were processed using the STRAIGHT vocoder. The spectral envelope and the fundamental frequency were shifted up or down in seven steps (3 up, 3 down plus unshifted) using scale factors of 8% and 30%, respectively, at each step. Listeners heard pairs of sentences and were asked to judge whether the identity of the talker was the same or different. Frequency shifts had similar effects for native- and foreign-accent conditions, in that listeners perceived the shifted versions as different talkers when, in fact, the talkers were the same. However, listeners were more likely to judge native-accented sentence pairs as the same talker regardless of whether or not they were the same; foreign-accented sentence pairs were more likely to be heard as different talkers. Overall, these results indicate that patterns of frequency-shifted foreign-accented speech are similar to previously reported patterns for frequency-shifted native speech; however, the small differences in patterns between the accent conditions might be attributed to listeners being less familiar with non-native speech patterns.

Profile analysis tests the ability to discriminate sounds based on patterns in amplitude spectra. Prior work has largely interpreted profile-analysis data using the power-spectrum model of masking. Under this model, performance relies on analyzing the output of a peripheral bandpass filterbank, and thresholds reflect limits on performance due to frequency selectivity and neural noise. Although this model successfully captures some basic trends in profile-analysis data, it has difficulty explaining others, such as poorer performance at high frequencies. We hypothesize that these trends can instead be explained by midbrain sensitivity to neural fluctuations. Profile-analysis stimuli contain rich temporal modulations, which elicit fluctuations in neural responses that are shaped by the auditory periphery and encoded by average discharge rates in the midbrain. We used physiologically realistic models to simulate midbrain responses to profile-analysis stimuli over a wide range of frequencies, sound levels, and component numbers/spacing. Some features of profile analysis that are difficult to explain with the power-spectrum model, such as frequency dependence and the effects of hearing loss, were readily accounted for by midbrain tuning to neural fluctuations. These results inform the role of fluctuations and effects of hearing loss on discrimination of complex sounds. [Work supported by NIH R01 DC010813.]