Speech intelligibility for audio-alone and audiovisual (AV) sentences was estimated as a function of signal-to-noise ratio (SNR) for a female target talker presented in a stationary noise, an interfering male talker, or a speech-modulated noise background, for eight hearing-impaired (HI) and five normal-hearing (NH) listeners. At the 50% keywords-correct performance level, HI listeners showed 7-12 dB less fluctuating-masker benefit (FMB) than NH listeners, consistent with previous results. Both groups showed significantly more FMB under AV than audio-alone conditions. When compared at the same stationary-noise SNR, FMB differences between listener groups and modalities were substantially smaller, suggesting that most of the FMB differences at the 50% performance level may reflect a SNR dependence of the FMB. Still, 1-5 dB of the FMB difference between listener groups remained, indicating a possible role for reduced audibility, limited spectral or temporal resolution, or an inability to use auditory source-segregation cues, in directly limiting the ability to listen in the dips of a fluctuating masker. A modified version of the extended speech-intelligibility index that predicts a larger FMB at less favorable SNRs accounted for most of the FMB differences between listener groups and modalities. Overall, these data suggest that HI listeners retain more of an ability to listen in the dips of a fluctuating masker than previously thought. Instead, the fluctuating-masker difficulties exhibited by HI listeners may derive from the reduced FMB associated with the more favorable SNRs they require to identify a reasonable proportion of the target speech.

Three experiments investigated the relationship between harmonic number, harmonic resolvability, and the perception of harmonic complexes. Complexes with successive equal-amplitude sine- or random-phase harmonic components of a 100- or 200-Hz fundamental frequency (f0) were presented dichotically, with even and odd components to opposite ears, or diotically, with all harmonics presented to both ears. Experiment 1 measured performance in discriminating a 3.5%-5% frequency difference between a component of a harmonic complex and a pure tone in isolation. Listeners achieved at least 75% correct for approximately the first 10 and 20 individual harmonics in the diotic and dichotic conditions, respectively, verifying that only processes before the binaural combination of information limit frequency selectivity. Experiment 2 measured fundamental frequency difference limens (f0 DLs) as a function of the average lowest harmonic number. Similar results at both f0's provide further evidence that harmonic number, not absolute frequency, underlies the order-of-magnitude increase observed in f0 DLs when only harmonics above about the 10th are presented. Similar results under diotic and dichotic conditions indicate that the auditory system, in performing f0 discrimination, is unable to utilize the additional peripherally resolved harmonics in the dichotic case. In experiment 3, dichotic complexes containing harmonics below the 12th, or only above the 15th, elicited pitches of the f0 and twice the f0, respectively. Together, experiments 2 and 3 suggest that harmonic number, regardless of peripheral resolvability, governs the transition between two different pitch percepts, one based on the frequencies of individual resolved harmonics and the other based on the periodicity of the temporal envelope.

The ability to extract a pitch from complex harmonic sounds, such as human speech, animal vocalizations, and musical instruments, is a fundamental attribute of hearing. Some theories of pitch rely on the frequency-to-place mapping, or tonotopy, in the inner ear (cochlea), but most current models are based solely on the relative timing of spikes in the auditory nerve. So far, it has proved to be difficult to distinguish between these two possible representations, primarily because temporal and place information usually covary in the cochlea. In this study, ''transposed stimuli'' were used to dissociate temporal from place information. By presenting the temporal information of low-frequency sinusoids to locations in the cochlea tuned to high frequencies, we found that human subjects displayed poor pitch perception for single tones. More importantly, none of the subjects was able to extract the fundamental frequency from multiple low-frequency harmonics presented to high-frequency regions of the cochlea. The experiments demonstrate that tonotopic representation is crucial to complex pitch perception and provide a new tool in the search for the neural basis of pitch. P itch is one of the primary attributes of auditory sensation, playing a crucial role in music and speech perception, and in analyzing complex auditory scenes (1, 2). For most sounds, pitch is an emergent perceptual property, formed by the integration of many harmonically related components into a single pitch, usually corresponding to the sound's fundamental frequency (F0). The ability to extract the F0 from a complex tone, even in the absence of energy at the F0 itself, is shared by a wide variety of species (3, 4) and is present from an early developmental stage in humans (5).The question of how pitch is encoded by the auditory system has a long and distinguished history, with lively debates on the subject going back to the time of Ohm (6) and Helmholtz (7). Although the debate has evolved considerably since its inception, one of the basic questions, whether timing (8, 9) or place information (10-12) (or both) from the cochlea is used to derive pitch, remains basically unanswered. In recent years, the weight of opinion and investigation has favored temporal codes (13-17). With few exceptions (18), recent models of pitch perception have been based solely on the timing information available in the interspike intervals represented in the simulated (17,(19)(20)(21) or actual (22, 23) auditory nerve. Such temporal models derive a pitch estimate by pooling timing information across auditorynerve fibers without regard to the frequency-to-place mapping (tonotopic organization) of the peripheral auditory system.Temporal models of pitch perception are attractive for at least two reasons. First, they provide a unified and parsimonious way of dealing with a diverse range of pitch phenomena (20). Second, the postulated neural mechanisms are essentially identical to those required of the binaural system when performing interaural timing comparisons for spatial loc...