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.

The Effect of Cochlear Damage on the Sensitivity to Harmonicity.

OBJECTIVES: A sum of simultaneous pure tones with harmonic relationships (i.e., simple frequency ratios) is normally heard as a single sound, with a single pitch, even when its components are fully resolved in the auditory periphery. This perceptual phenomenon called "harmonic fusion" is thought to play an important role in auditory scene analysis as listeners often have to segregate simultaneous harmonic sounds with different fundamental frequencies. The present study explored the consequences of mild or moderate cochlear hearing loss for the sensitivity to harmonicity and the detection of inharmonicity. DESIGN: The subjects were 12 normal-hearing (NH) listeners and 8 listeners with cochlear hearing loss amounting to 30 to 50 dB (mean: 42 dB) from 0.25 to 3 kHz. In each subject, thresholds for the detection of a change in the frequency ratio of simultaneous pure tones were measured with an adaptive forced-choice procedure. The standard frequency ratio was either harmonic (2:1, i.e., one octave) or inharmonic (0.8 or 1.2 octaves). The tones were presented at a low sensation level (at most 15 dB) within broadband noise, to minimize their cochlear interactions. In the main experimental conditions, the frequency register of the stimuli varied randomly within each trial, so that subjects were forced to process frequency ratios to achieve good performance; frequency discrimination was not sufficient. In other conditions, by contrast, frequency discrimination was sufficient to perform the task optimally. RESULTS: For both groups of subjects, thresholds in the main experimental conditions were lower (i.e., better) when the standard frequency ratio was harmonic than when it was inharmonic. This effect, revealing sensitivity to harmonicity, was weak for some members of the hearing-impaired group, but could be observed even in subjects showing a very poor frequency discrimination ability. The two groups, however, differed from each other with respect to the detection of inharmonicity: for the NH group, in agreement with previous results, negative deviations from one octave (i.e., compressions of this frequency ratio) were better detected than positive deviations (stretchings); for the hearing-impaired group, on the other hand, the sign of the deviations had no effect on performance. CONCLUSIONS: Sensitivity to harmonicity appears to be remarkably robust. However, it can be reduced in some listeners with mild or moderate cochlear damage. Moreover, as inharmonicity detection is asymmetric for NH listeners but apparently becomes symmetric in case of cochlear damage, it may be that listeners with cochlear damage do not detect inharmonicity in the same manner as NH listeners do. In some circumstances, inharmonicity can be detected on the basis of "beat" cues available in single frequency channels; however, the subjects tested here were unlikely to use cues of this type.

Pitch priming in sequences of two sounds.

Frequency discrimination limens (FDLs) were measured for pairs of stimuli differing from each other with respect to pitch salience. One of the two stimuli to be compared within a trial was a pure tone of at least 100 ms, evoking a salient pitch, while the other stimulus consisted of only eight sinusoidal cycles (experiment 1), or was a noise band with a Gaussian spectral envelope, evoking a weak pitch corresponding to the peak frequency (experiment 2). From trial to trial, frequency was varied randomly and widely. In both experiments, the FDLs were lower, by an average factor of about 3, when the stimulus with the more salient pitch preceded the other stimulus than vice versa. Evidence is presented against an interpretation of this temporal asymmetry in terms of memory limitations. It is suggested that the asymmetry reflects a pitch-priming effect. In two additional experiments, both of the stimuli to be compared within a trial were very short tone bursts or noise bands; perceptually, they differed only with respect to pitch height. Performance was markedly better than in experiments 1 and 2, and was not improved when the two stimuli were preceded by a 300-ms tone intended to produce pitch priming.

Enhancement of increments in spectral amplitude: further evidence for a mechanism based on central adaptation.

The threshold for detecting a tone in a multitone masker is lower when the masker-plus-signal stimulus is preceded by a copy of the masker. One potential explanation of this "enhancement" phenomenon is that the -precursor stimulus acts as a "template" of the subsequent masker, thus helping listeners to segregate the signal from the masker. To assess this idea, we measured enhancement for precursors that were perceptually similar to the masker and for precursors that were made dissimilar to the masker by gating their components asynchronously. We found that the two types of precursor produced similar amounts of enhancement. This was true not only when the precursor and the subsequent test stimulus were presented to the same ear but also when they were presented to opposite ears. In a second experiment, we checked that the precursors with asynchronously gated components were perceptually poor templates of the subsequent maskers. Listeners now had to discriminate between test stimuli -containing the same components as the precursor and test stimuli containing all but one of the precursor components. We found that in this experimental situation, where enhancement could play no role, gating the precursor components asynchronously disrupted performance. Overall, our results are inconsistent with the hypothesis that precursors producing enhancement are beneficial because they are used as perceptual templates of the masker. Our results are instead consistent with an -explanation of enhancement based on selective neural adaptation taking place at a central locus of the auditory system.

Assessing the possible role of frequency-shift detectors in the ability to hear out partials in complex tones.

The possible role of frequency-shift detectors (FSDs) was assessed for a task measuring the ability to hear out individual "inner" partials in a chord with seven partials uniformly spaced on the ERBN-number (Cam) scale. In each of the two intervals in a trial, a pure-tone probe was followed by a chord. In one randomly selected interval, the frequency of the probe was the same as that of a partial in the chord. In the other interval, the probe was mistuned upwards or downwards from the "target" partial. The task was to indicate the interval in which the probe coincided with the target. In the "symmetric" condition, the frequency of the mistuned probe was midway in Cams between that of two partials in the chord. This should have led to approximately symmetric activation of the up-FSDs and down-FSDs, such that differential activation provided a minimal cue. In the "asymmetric" condition, the mistuned probe was much closer in frequency to one partial in the chord than to the next closest partial. This should have led to differential activation of the up-FSDs and down-FSDs, providing a strong discrimination cue. Performance was predicted to be better in the asymmetric than in the symmetric condition. The results were consistent with this prediction except when the probe was mistuned above the sixth (second highest) partial in the chord. To explain this, it is argued that activation of FSDs depends both on the size of the frequency shift between successive components and on the pitch strength of each component.

Controlling audibility with noise for online experiments using sound.

Online auditory experiments use the sound delivery equipment of each participant, with no practical way to calibrate sound level or frequency response. Here, a method is proposed to control sensation level across frequencies: embedding stimuli in threshold-equalizing noise. In a cohort of 100 online participants, noise could equate detection thresholds from 125 to 4000 Hz. Equalization was successful even for participants with atypical thresholds in quiet, due either to poor quality equipment or unreported hearing loss. Moreover, audibility in quiet was highly variable, as overall level was uncalibrated, but variability was much reduced with noise. Use cases are discussed.

Enhancing a tone by shifting its frequency or intensity.

When a test sound consisting of pure tones with equal intensities is preceded by a precursor sound identical to the test sound except for a reduction in the intensity of one tone, an auditory "enhancement" phenomenon occurs: In the test sound, the tone which was previously softer stands out perceptually. Here, enhancement was investigated using inharmonic sounds made up of five pure tones well resolved in the auditory periphery. It was found that enhancement can be elicited not only by increases in intensity but also by shifts in frequency. In both cases, when the precursor and test sounds are separated by a 500-ms delay, inserting a burst of pink noise during the delay has little effect on enhancement. Presenting the precursor and test sounds to opposite ears rather than to the same ear significantly reduces the enhancement resulting from increases in intensity, but not the enhancement resulting from shifts in frequency. This difference suggests that the mechanisms of enhancement are not identical for the two types of change. For frequency shifts, enhancement may be partly based on the existence of automatic "frequency-shift detectors" [Demany and Ramos, J. Acoust. Soc. Am. 117, 833-841 (2005)].

A note about insensitivity to pitch-change direction.

Some listeners are insensitive to the direction of pure-tone frequency changes when the standard frequency is roved widely over trials, but less so when the standard frequency is fixed and trial-by-trial feedback is provided. The present experiment tested the hypothesis that fixing the standard frequency and providing feedback is advantageous for direction-impaired listeners because under these conditions the listeners can learn to respond correctly without genuinely perceiving frequency-change direction. This hypothesis was ruled out by the experiment. It appears instead that direction-impaired listeners find it difficult to ignore the irrelevant frequency changes introduced by roving.

The perception of octave pitch affinity and harmonic fusion have a common origin.

Musicians say that the pitches of tones with a frequency ratio of 2:1 (one octave) have a distinctive affinity, even if the tones do not have common spectral components. It has been suggested, however, that this affinity judgment has no biological basis and originates instead from an acculturation process ‒ the learning of musical rules unrelated to auditory physiology. We measured, in young amateur musicians, the perceptual detectability of octave mistunings for tones presented alternately (melodic condition) or simultaneously (harmonic condition). In the melodic condition, mistuning was detectable only by means of explicit pitch comparisons. In the harmonic condition, listeners could use a different and more efficient perceptual cue: in the absence of mistuning, the tones fused into a single sound percept; mistunings decreased fusion. Performance was globally better in the harmonic condition, in line with the hypothesis that listeners used a fusion cue in this condition; this hypothesis was also supported by results showing that an illusory simultaneity of the tones was much less advantageous than a real simultaneity. In the two conditions, mistuning detection was generally better for octave compressions than for octave stretchings. This asymmetry varied across listeners, but crucially the listener-specific asymmetries observed in the two conditions were highly correlated. Thus, the perception of the melodic octave appeared to be closely linked to the phenomenon of harmonic fusion. As harmonic fusion is thought to be determined by biological factors rather than factors related to musical culture or training, we argue that octave pitch affinity also has, at least in part, a biological basis.

Fundamental differences in change detection between vision and audition.

We compared auditory change detection to visual change detection using closely matched stimuli and tasks in the two modalities. On each trial, participants were presented with a test stimulus consisting of ten elements: pure tones with various frequencies for audition, or dots with various spatial positions for vision. The test stimulus was preceded or followed by a probe stimulus consisting of a single element, and two change-detection tasks were performed. In the "present/absent" task, the probe either matched one randomly selected element of the test stimulus or none of them; participants reported present or absent. In the "direction-judgment" task, the probe was always slightly shifted relative to one randomly selected element of the test stimulus; participants reported the direction of the shift. Whereas visual performance was systematically better in the present/absent task than in the direction-judgment task, the opposite was true for auditory performance. Moreover, whereas visual performance was strongly dependent on selective attention and on the time interval separating the probe from the test stimulus, this was not the case for auditory performance. Our results show that small auditory changes can be detected automatically across relatively long temporal gaps, using an implicit memory system that seems to have no similar counterpart in the visual domain.

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 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.

Tuning properties of the auditory frequency-shift detectors.

Demany and Ramos [(2005). J. Acoust. Soc. Am. 117, 833-841] found that it is possible to hear an upward or downward pitch change between two successive pure tones differing in frequency even when the first tone is informationally masked by other tones, preventing a conscious perception of its pitch. This provides evidence for the existence of automatic frequency-shift detectors (FSDs) in the auditory system. The present study was intended to estimate the magnitude of the frequency shifts optimally detected by the FSDs. Listeners were presented with sound sequences consisting of (1) a 300-ms or 100-ms random "chord" of synchronous pure tones, separated by constant intervals of either 650 cents or 1000 cents; (2) an interstimulus interval (ISI) varying from 100 to 900 ms; (3) a single pure tone at a variable frequency distance (Delta) from a randomly selected component of the chord. The task was to indicate if the final pure tone was higher or lower than the nearest component of the chord. Irrespective of the chord's properties and of the ISI, performance was best when Delta was equal to about 120 cents (1/10 octave). Therefore, this interval seems to be the frequency shift optimally detected by the FSDs.

Continuous versus discrete frequency changes: different detection mechanisms?

Sek and Moore [J. Acoust. Soc. Am. 106, 351-359 (1999)] and Lyzenga et al. [J. Acoust. Soc. Am. 116, 491-501 (2004)] found that the just-noticeable frequency difference between two pure tones relatively close in time is smaller when these tones are smoothly connected by a frequency glide than when they are separated by a silent interval. This "glide effect" was interpreted as evidence that frequency glides can be detected by a specific auditory mechanism, not involved in the detection of discrete, time-delayed frequency changes. Lyzenga et al. argued in addition that the glide-detection mechanism provides little information on the direction of frequency changes near their detection threshold. The first experiment reported here confirms the existence of the glide effect, but also shows that it disappears when the glide is not connected smoothly to the neighboring steady tones. A second experiment demonstrates that the direction of a 750 ms frequency glide can be perceptually identified as soon as the glide is detectable. These results, and some other observations, lead to a new interpretation of the glide effect, and to the conclusion that continuous frequency changes may be detected in the same manner as discrete frequency changes.

Auditory Perception: Relative Universals for Musical Pitch.

Do members of a remote Amazonian tribe and Boston-trained musicians share similarities in their mental representations of auditory pitch? According to an impressive new set of psychoacoustic evidence they do, a finding which highlights the universal importance of relative pitch patterns.

Auditory temporal processing in Parkinson's disease.

Previous research has suggested that Parkinson's disease (PD) impairs perceptual acuity in the temporal domain. In the present study, psychophysical tests assessing several aspects of auditory temporal processing were administered to a group of PD patients treated with bilateral subthalamic nucleus (STN) stimulation and to a normal control group. Each patient was tested in three clinical conditions: without treatment, with levodopa therapy, and during STN stimulation. In all three conditions, the patients showed a significant deficit in the detection of very short temporal gaps within noise bursts and in the discrimination between the durations of two well-detectable time intervals (circa 50ms) bounded by two temporally non-contiguous pairs of clicks. However, the patients showed no deficit in the detection of a temporal break produced by a local interval change in an otherwise isochronous sequence of 10 clicks spaced by 50-ms intervals. The latter result contradicts previous suggestions that PD slows down an internal clock or pacemaker involved in the perception of short durations. In this regard, we reinterpret previous evidence. Remarkably, the patients' deficits were not diminished by levodopa therapy; in contrast, STN stimulation slightly improved performance, overall. We tentatively ascribe the deficit observed in the gap-detection test to a dysfunctioning of the auditory cortex, impairing its ability to track rapid fluctuations in sound intensity. We argue that the deficit in the duration-discrimination test is the consequence of an impairment in memory and/or attention rather than in the perception of time per se.

An evaluation of psychophysical models of auditory change perception.

In many psychophysical experiments, the participant's task is to detect small changes along a given stimulus dimension or to identify the direction (e.g., upward vs. downward) of such changes. The results of these experiments are traditionally analyzed with a constant-variance Gaussian (CVG) model or a high-threshold (HT) model. Here, the authors demonstrate that for changes along three basic sound dimensions (frequency, intensity, and amplitude-modulation rate), such models cannot account for the observed relationship between detection thresholds and direction-identification thresholds. It is shown that two alternative models can account for this relationship. One of them is based on the idea of sensory quanta; the other assumes that small changes are detected on the basis of Poisson processes with low means. The predictions of these two models are then compared against receiver operating characteristics (ROCs) for the detection of changes in sound intensity. It is concluded that human listeners' perception of small and unidimensional acoustic changes is better described by a discrete-state Poisson model than by the more commonly used CVG model or by the less favored HT and quantum models.

Enhancement, adaptation, and the binaural system.

In a test sound consisting of a burst of pink noise, an arbitrarily selected target frequency band can be "enhanced" by the previous presentation of a similar noise with a spectral notch in the target frequency region. As a result of the enhancement, the test sound evokes a pitch sensation corresponding to the pitch of the target band. Here, a pitch comparison task was used to assess enhancement. In the first experiment, a stronger enhancement effect was found when the test sound and its precursor had the same interaural time difference (ITD) than when they had opposite ITDs. Two subsequent experiments were concerned with the audibility of an instance of dichotic pitch in binaural test sounds preceded by precursors. They showed that it is possible to enhance a frequency region on the sole basis of ITD manipulations, using spectrally identical test sounds and precursors. However, the observed effects were small. A major goal of this study was to test the hypothesis that enhancement originates at least in part from neural adaptation processes taking place at a central level of the auditory system. The data failed to provide strong support for this hypothesis.

Automatic Frequency-Shift Detection in the Auditory System: A Review of Psychophysical Findings.

The human brain has the task of binding successive sounds produced by the same acoustic source into a coherent perceptual stream, and binding must be selective when several sources are concurrently active. Binding appears to obey a principle of spectral proximity: pure tones close in frequency are more likely to be bound than pure tones with remote frequencies. It has been hypothesized that the binding process is realized by automatic "frequency-shift detectors" (FSDs), comparable to the detectors of spatial motion in the visual system. In 2005, this hypothesis was supported by a psychophysical study showing that human listeners are able to identify the direction of a frequency shift between two successive pure tones while the first of these tones is not audible individually due to informational masking by other tones presented synchronously. A number of variants of this study have been performed since 2005, in order to confirm the existence of FSDs, to characterize their properties, and to clarify as far as possible their neural underpinnings. The results obtained up to now suggest that the working of the FSDs exploits an implicit sensory memory which is powerful with respect to both capacity and retention time. Tones within chords can be perceptually enhanced by small frequency shifts, in a manner suggesting that the FSDs can serve in auditory scene analysis not only as binding tools but also, to a limited extent, as segregation tools.

Detecting temporal changes in acoustic scenes: The variable benefit of selective attention.

Four experiments investigated change detection in acoustic scenes consisting of a sum of five amplitude-modulated pure tones. As the tones were about 0.7 octave apart and were amplitude-modulated with different frequencies (in the range 2-32 Hz), they were perceived as separate streams. Listeners had to detect a change in the frequency (experiments 1 and 2) or the shape (experiments 3 and 4) of the modulation of one of the five tones, in the presence of an informative cue orienting selective attention either before the scene (pre-cue) or after it (post-cue). The changes left intensity unchanged and were not detectable in the spectral (tonotopic) domain. Performance was much better with pre-cues than with post-cues. Thus, change deafness was manifest in the absence of an appropriate focusing of attention when the change occurred, even though the streams and the changes to be detected were acoustically very simple (in contrast to the conditions used in previous demonstrations of change deafness). In one case, the results were consistent with a model based on the assumption that change detection was possible if and only if attention was endogenously focused on a single tone. However, it was also found that changes resulting in a steepening of amplitude rises were to some extent able to draw attention exogenously. Change detection was not markedly facilitated when the change produced a discontinuity in the modulation domain, contrary to what could be expected from the perspective of predictive coding.