Effect of stimulus type and pitch salience on pitch-sequence processing.

Using a same-different discrimination task, it has been shown that discrimination performance for sequences of complex tones varying just detectably in pitch is less dependent on sequence length (1, 2, or 4 elements) when the tones contain resolved harmonics than when they do not [Cousineau, Demany, and Pessnitzer (2009). J. Acoust. Soc. Am. 126, 3179-3187]. This effect had been attributed to the activation of automatic frequency-shift detectors (FSDs) by the shifts in resolved harmonics. The present study provides evidence against this hypothesis by showing that the sequence-processing advantage found for complex tones with resolved harmonics is not found for pure tones or other sounds supposed to activate FSDs (narrow bands of noise and wide-band noises eliciting pitch sensations due to interaural phase shifts). The present results also indicate that for pitch sequences, processing performance is largely unrelated to pitch salience per se: for a fixed level of discriminability between sequence elements, sequences of elements with salient pitches are not necessarily better processed than sequences of elements with less salient pitches. An ideal-observer model for the same-different binary-sequence discrimination task is also developed in the present study. The model allows the computation of d' for this task using numerical methods.

Frequency tagging to track the neural processing of contrast in fast, continuous sound sequences.

The human auditory system presents a remarkable ability to detect rapid changes in fast, continuous acoustic sequences, as best illustrated in speech and music. However, the neural processing of rapid auditory contrast remains largely unclear, probably due to the lack of methods to objectively dissociate the response components specifically related to the contrast from the other components in response to the sequence of fast continuous sounds. To overcome this issue, we tested a novel use of the frequency-tagging approach allowing contrast-specific neural responses to be tracked based on their expected frequencies. The EEG was recorded while participants listened to 40-s sequences of sounds presented at 8Hz. A tone or interaural time contrast was embedded every fifth sound (AAAAB), such that a response observed in the EEG at exactly 8 Hz/5 (1.6 Hz) or harmonics should be the signature of contrast processing by neural populations. Contrast-related responses were successfully identified, even in the case of very fine contrasts. Moreover, analysis of the time course of the responses revealed a stable amplitude over repetitions of the AAAAB patterns in the sequence, except for the response to perceptually salient contrasts that showed a buildup and decay across repetitions of the sounds. Overall, this new combination of frequency-tagging with an oddball design provides a valuable complement to the classic, transient, evoked potentials approach, especially in the context of rapid auditory information. Specifically, we provide objective evidence on the neural processing of contrast embedded in fast, continuous sound sequences.NEW & NOTEWORTHY Recent theories suggest that the basis of neurodevelopmental auditory disorders such as dyslexia might be an impaired processing of fast auditory changes, highlighting how the encoding of rapid acoustic information is critical for auditory communication. Here, we present a novel electrophysiological approach to capture in humans neural markers of contrasts in fast continuous tone sequences. Contrast-specific responses were successfully identified, even for very fine contrasts, providing direct insight on the encoding of rapid auditory information.

Impaired encoding of rapid pitch information underlies perception and memory deficits in congenital amusia.

Recent theories suggest that the basis of neurodevelopmental auditory disorders such as dyslexia or specific language impairment might be a low-level sensory dysfunction. In the present study we test this hypothesis in congenital amusia, a neurodevelopmental disorder characterized by severe deficits in the processing of pitch-based material. We manipulated the temporal characteristics of auditory stimuli and investigated the influence of the time given to encode pitch information on participants' performance in discrimination and short-term memory. Our results show that amusics' performance in such tasks scales with the duration available to encode acoustic information. This suggests that in auditory neuro-developmental disorders, abnormalities in early steps of the auditory processing can underlie the high-level deficits (here musical disabilities). Observing that the slowing down of temporal dynamics improves amusics' pitch abilities allows considering this approach as a potential tool for remediation in developmental auditory disorders.

Congenital amusia: a cognitive disorder limited to resolved harmonics and with no peripheral basis.

Pitch plays a fundamental role in audition, from speech and music perception to auditory scene analysis. Congenital amusia is a neurogenetic disorder that appears to affect primarily pitch and melody perception. Pitch is normally conveyed by the spectro-temporal fine structure of low harmonics, but some pitch information is available in the temporal envelope produced by the interactions of higher harmonics. Using 10 amusic subjects and 10 matched controls, we tested the hypothesis that amusics suffer exclusively from impaired processing of spectro-temporal fine structure. We also tested whether the inability of amusics to process acoustic temporal fine structure extends beyond pitch by measuring sensitivity to interaural time differences, which also rely on temporal fine structure. Further tests were carried out on basic intensity and spectral resolution. As expected, pitch perception based on spectro-temporal fine structure was impaired in amusics; however, no significant deficits were observed in amusics' ability to perceive the pitch conveyed via temporal-envelope cues. Sensitivity to interaural time differences was also not significantly different between the amusic and control groups, ruling out deficits in the peripheral coding of temporal fine structure. Finally, no significant differences in intensity or spectral resolution were found between the amusic and control groups. The results demonstrate a pitch-specific deficit in fine spectro-temporal information processing in amusia that seems unrelated to temporal or spectral coding in the auditory periphery. These results are consistent with the view that there are distinct mechanisms dedicated to processing resolved and unresolved harmonics in the general population, the former being altered in congenital amusia while the latter is spared.

On the Relevance of Natural Stimuli for the Study of Brainstem Correlates: The Example of Consonance Perception.

Some combinations of musical tones sound pleasing to Western listeners, and are termed consonant, while others sound discordant, and are termed dissonant. The perceptual phenomenon of consonance has been traced to the acoustic property of harmonicity. It has been repeatedly shown that neural correlates of consonance can be found as early as the auditory brainstem as reflected in the harmonicity of the scalp-recorded frequency-following response (FFR). "Neural Pitch Salience" (NPS) measured from FFRs-essentially a time-domain equivalent of the classic pattern recognition models of pitch-has been found to correlate with behavioral judgments of consonance for synthetic stimuli. Following the idea that the auditory system has evolved to process behaviorally relevant natural sounds, and in order to test the generalizability of this finding made with synthetic tones, we recorded FFRs for consonant and dissonant intervals composed of synthetic and natural stimuli. We found that NPS correlated with behavioral judgments of consonance and dissonance for synthetic but not for naturalistic sounds. These results suggest that while some form of harmonicity can be computed from the auditory brainstem response, the general percept of consonance and dissonance is not captured by this measure. It might either be represented in the brainstem in a different code (such as place code) or arise at higher levels of the auditory pathway. Our findings further illustrate the importance of using natural sounds, as a complementary tool to fully-controlled synthetic sounds, when probing auditory perception.

What is a melody? On the relationship between pitch and brightness of timbre.

Previous studies showed that the perceptual processing of sound sequences is more efficient when the sounds vary in pitch than when they vary in loudness. We show here that sequences of sounds varying in brightness of timbre are processed with the same efficiency as pitch sequences. The sounds used consisted of two simultaneous pure tones one octave apart, and the listeners' task was to make same/different judgments on pairs of sequences varying in length (one, two, or four sounds). In one condition, brightness of timbre was varied within the sequences by changing the relative level of the two pure tones. In other conditions, pitch was varied by changing fundamental frequency, or loudness was varied by changing the overall level. In all conditions, only two possible sounds could be used in a given sequence, and these two sounds were equally discriminable. When sequence length increased from one to four, discrimination performance decreased substantially for loudness sequences, but to a smaller extent for brightness sequences and pitch sequences. In the latter two conditions, sequence length had a similar effect on performance. These results suggest that the processes dedicated to pitch and brightness analysis, when probed with a sequence-discrimination task, share unexpected similarities.

The basis of musical consonance as revealed by congenital amusia.

Some combinations of musical notes sound pleasing and are termed "consonant," but others sound unpleasant and are termed "dissonant." The distinction between consonance and dissonance plays a central role in Western music, and its origins have posed one of the oldest and most debated problems in perception. In modern times, dissonance has been widely believed to be the product of "beating": interference between frequency components in the cochlea that has been believed to be more pronounced in dissonant than consonant sounds. However, harmonic frequency relations, a higher-order sound attribute closely related to pitch perception, has also been proposed to account for consonance. To tease apart theories of musical consonance, we tested sound preferences in individuals with congenital amusia, a neurogenetic disorder characterized by abnormal pitch perception. We assessed amusics' preferences for musical chords as well as for the isolated acoustic properties of beating and harmonicity. In contrast to control subjects, amusic listeners showed no preference for consonance, rating the pleasantness of consonant chords no higher than that of dissonant chords. Amusics also failed to exhibit the normally observed preference for harmonic over inharmonic tones, nor could they discriminate such tones from each other. Despite these abnormalities, amusics exhibited normal preferences and discrimination for stimuli with and without beating. This dissociation indicates that, contrary to classic theories, beating is unlikely to underlie consonance. Our results instead suggest the need to integrate harmonicity as a foundation of music preferences, and illustrate how amusia may be used to investigate normal auditory function.

The role of peripheral resolvability in pitch-sequence processing.

The authors previously reported that same/different judgments on pitch sequences were more accurate for tones with resolved (low-rank) harmonics compared to unresolved (high-rank) harmonics, even when discriminability between tones was equated [Cousineau et al. (2009). J. Acoust. Soc. Am. 126, 3179-3187]. Here, peripheral resolvability, defined by the number of harmonics per cochlear filter, was contrasted with harmonic number. Tones were presented either diotically or dichotically. In the latter case, even and odd harmonics were presented to different ears, thus halving the number of harmonics per cochlear filter. Performance was better for dichotic than for diotic presentations. This indicates that peripheral resolvability is necessary and sufficient for efficient pitch-sequence processing.

What breaks a melody: perceiving F0 and intensity sequences with a cochlear implant.

Pitch perception has been extensively studied using discrimination tasks on pairs of single sounds. When comparing pitch discrimination performance for normal-hearing (NH) and cochlear implant (CI) listeners, it usually appears that CI users have relatively poor pitch discrimination. Tasks involving pitch sequences, such as melody perception or auditory scene analysis, are also usually difficult for CI users. However, it is unclear whether the issue with pitch sequences is a consequence of sound discriminability, or if an impairment exists for sequence processing per se. Here, we compared sequence processing abilities across stimulus dimensions (fundamental frequency and intensity) and listener groups (NH, CI, and NH listeners presented with noise-vocoded sequences). The sequence elements were firstly matched in discriminability, for each listener and dimension. Participants were then presented with pairs of sequences, constituted by up to four elements varying on a single dimension, and they performed a same/different task. In agreement with a previous study (Cousineau et al., 2009) fundamental frequency sequences were processed more accurately than intensity sequences by NH listeners. However, this was not the case for CI listeners, nor for NH listeners presented with noise-vocoded sequences. Intensity sequence processing was, nonetheless, equally accurate in the three groups. These results show that the reduced pitch cues received by CI listeners do not only elevate thresholds, as previously documented, but also affect pitch sequence processing above threshold. We suggest that efficient sequence processing for pitch requires the resolution of individual harmonics in the auditory periphery, which is not achieved with the current generation of implants.

What makes a melody: The perceptual singularity of pitch sequences.

This study investigated the ability of normal-hearing listeners to process random sequences of tones varying in either pitch or loudness. Same/different judgments were collected for pairs of sequences with a variable length (up to eight elements) and built from only two different elements, which were 200-ms harmonic complex tones. The two possible elements of all sequences had a fixed level of discriminability, corresponding to a d(') value of about 2, irrespective of the auditory dimension (pitch or loudness) along which they differed. This made it possible to assess sequence processing per se, independent of the accuracy of sound encoding. Pitch sequences were found to be processed more effectively than loudness sequences. However, that was the case only when the sequence elements included low-rank harmonics, which could be at least partially resolved in the auditory periphery. The effect of roving and transposition was also investigated. These manipulations reduced overall performance, especially transposition, but an advantage for pitch sequences was still observed. These results suggest that automatic frequency-shift detectors, available for pitch sequences but not loudness sequences, participate in the effective encoding of melodies.