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The sounds of music play with many perceptual dimensions. We devised a set of psychophysical procedures to better understand how recipients of cochlear implants perceive basic sound attributes involved in music listening.
The acceptance of cochlear implantation as an effective and safe treatment for deafness has increased steadily over the past quarter century. The earliest devices were the first implanted prostheses found to be successful in compensating partially for lost sensory function by direct electrical stimulation of nerves. Initially, the main intention was to provide limited auditory sensations to people with profound or total sensorineural hearing impairment in both ears. Although the first cochlear implants aimed to provide patients with little more than awareness of environmental sounds and some cues to assist visual speech-reading, the technology has advanced rapidly. Currently, most people with modern cochlear implant systems can understand speech using the device alone, at least in favorable listening conditions. In recent years, an increasing research effort has been directed towards implant users' perception of nonspeech sounds, especially music. This paper reviews that research, discusses the published experimental results in terms of both psychophysical observations and device function, and concludes with some practical suggestions about how perception of music might be enhanced for implant recipients in the future. The most significant findings of past research are: (1) On average, implant users perceive rhythm about as well as listeners with normal hearing; (2) Even with technically sophisticated multiple-channel sound processors, recognition of melodies, especially without rhythmic or verbal cues, is poor, with performance at little better than chance levels for many implant users; (3) Perception of timbre, which is usually evaluated by experimental procedures that require subjects to identify musical instrument sounds, is generally unsatisfactory; (4) Implant users tend to rate the quality of musical sounds as less pleasant than listeners with normal hearing; (5) Auditory training programs that have been devised specifically to provide implant users with structured musical listening experience may improve the subjective acceptability of music that is heard through a prosthesis; (6) Pitch perception might be improved by designing innovative sound processors that use both temporal and spatial patterns of electric stimulation more effectively and precisely to overcome the inherent limitations of signal coding in existing implant systems; (7) For the growing population of implant recipients who have usable acoustic hearing, at least for low-frequency sounds, perception of music is likely to be much better with combined acoustic and electric stimulation than is typical for deaf people who rely solely on the hearing provided by their prostheses.
A procedure for the schematic and automatic extraction of 'fundamental pitch' from complex tonal signals, such as voiced speech and music, has been developed. While the auditively relevant 'fundamental' of a complex signal cannot be defined in purely mathematical terms, an existent model of virtual-pitch perception turns out to provide a suitable basis. The procedure comprises the formation of determinant spectral pitches (or 'fundamental frequency') from those spectral pitches. The latter deduction is accomplished by a principle of subharmonic matching, for whose realization a simple, universal and efficient algorithm was found. While the calculation may be confined to the determination of 'nominal' virtual pitch, certain typical auditory phenomena, such as the influence of SPL, partial masking and interval stretch, may be accounted for as well, in which case 'true' virtual pitch is obtained. The procedure operates on the frequencies and amplitudes of the signal's spectral components, is suitable for implementation on readily available programmable calculators and other arithmetic computers, and may be used in real-time 'fundamental-pitch' extraction as well. The procedure's performance and its applicability to the research and engineering of auditory communication are illustrated by some examples.
Simultaneous or near-simultaneous activation of adjacent cochlear implant electrodes can produce pitch percepts intermediate to those produced by each electrode separately, thereby increasing the number of place-pitch steps available to cochlear implant listeners. To estimate how many distinct pitches could be generated with simultaneous dual-electrode stimulation, the present study measured place-pitch discrimination thresholds for single- versus dual-electrode stimuli in users of the Clarion CII device. Discrimination thresholds were expressed as the proportion of current directed to the secondary electrode of the dual-electrode pair. For 16 of 17 electrode pairs tested in six subjects, thresholds ranged from 0.11 to 0.64, suggesting that dual-electrode stimuli can produce 2-9 discriminable pitches between the pitches of single electrodes. Some subjects demonstrated a level effect, with better place-pitch discrimination at higher stimulus levels. Equal loudness was achieved with dual-electrode stimuli at net current levels that were similar to or slightly higher than those for single-electrode stimuli.
The hypothesis that temporal code is mainly responsible for speech recognition is discussed. Data from cochlear implant research demonstrate that single-channel speech processors, supplying only temporal code, enable understanding of spoken language. Physiological findings supporting the observation are presented. A temporal theory of tone pitch perception is discussed.
Periodicity pitch is the most salient and important of all pitch percepts. Psychoacoustical models of this percept have long postulated the existence of internalized harmonic templates against which incoming resolved spectra can be compared, and pitch determined according to the best matching templates [J. Goldstein, J. Acoust. Soc. Am. 54, 1496-1516 (1973)]. However, it has been a mystery where and how such harmonic templates can come about. We present here a biologically plausible model for how such templates can form in the early stages of the auditory system. The model demonstrates that any broadband stimulus, including noise and random click trains, suffices for generating the templates, and that there is no need for any delay lines, oscillators, or other neural temporal structures. The model consists of two key stages: cochlear filtering followed by coincidence detection. The cochlear stage provides responses analogous to those recorded in the auditory nerve and cochlear nucleus. Specifically, it performs moderately sharp frequency analysis via a filterbank with tonotopically ordered center frequencies (CFs); the rectified and phase-locked filter responses are further enhanced temporally to resemble the synchronized responses of cells in the cochlear nucleus. The second stage is a matrix of coincidence detectors that compute the average pairwise instantaneous correlation (or product) between responses from all CFs across the channels. Model simulations show that for any broadband stimulus, a degree of high coincidence occurs among cochlear channels that are spaced precisely at harmonic intervals. Accumulating coincidences over time results in the formation of harmonic templates for all fundamental frequencies in the phase-locking frequency range. The model accounts for the critical role played by three subtle but important factors in cochlear function: the nonlinear transformations following the filtering stage, the rapid phase shifts of the traveling wave near its resonance, and the spectral resolution of the cochlear filters. Finally, we discuss the physiological correlates and location of such a process and its resulting templates.
OBJECTIVE: To compare the depth of electrode insertion in two types of cochlear implants, and to assess the ability of the implantees in each group to place-pitch during random electrical stimulation. STUDY DESIGN: This was a prospective clinical study. SETTING: This study was performed at an implant program within a university teaching hospital. PATIENTS: Five consecutive patients with the Clarion (Advanced Bionics, Symlar, CA, U.S.A.) device and 5 with the Nucleus-22 (Cochlear Corporation, Sydney, Australia) implants were enrolled. All 10 implantees had fully active and functioning electrodes. INTERVENTIONS: The depth of insertion was determined using plain anteroposterior skull film and high resolution computed tomography (CT). The quality of electrode insertion was assessed by pitch scaling; electrodes were randomly stimulated to generate subjective pitch responses. OUTCOME MEASURES: The depth of electrode insertion was measured radiographically as degrees of angular rotation within the cochlea. For pitch scaling, the averaged responses to electrical stimulation was plotted against the "place" of the electrodes along the array. Pitch range, plateauing, and reversal of pitches were also noted. Insertion depth was correlated with the result of pitch scaling and open-set speech discrimination at 3 months. RESULTS: The mean insertion depth was 406 degrees for the Clarion device and 254 degrees for the Nucleus device. CT was used to confirm the intracochlear placement of the electrodes and their relationships to the cochleostomy site. It did not confer more information than the plain films unless kinking had occurred. Pitch perception was consistent with the tonotopic organization of the cochlea. The Nucleus-22 recipients displayed a broader range of pitches with less plateaus and reversals than the Clarion implants. The depth of insertion did not compare well with the outcome of pitch scaling or with open-set speech discrimination scores in either group of implantees. CONCLUSION: The preformed spiral array of the Clarion device allowed deeper electrode insertion compared to the Nucleus-22 device. However, depth of insertion did not translate into better pitch placement.
Neural processes underlying pitch perception at the level of the cerebral cortex are influenced by language experience. We investigated whether early, pre-attentive stages of pitch processing at the level of the human brainstem may also be influenced by language experience. The human frequency following response (FFR), reflecting sustained phase-locked activity in a population of neural elements, was used to measure activity within the rostral brainstem. FFRs elicited by four Mandarin tones were recorded from native speakers of Mandarin Chinese and English. Pitch strength (reflecting robustness of neural phase-locking at the pitch periods) and accuracy of pitch tracking were extracted from the FFRs using autocorrelation algorithms. These measures revealed that the Chinese group exhibits stronger pitch representation and smoother pitch tracking than the English group. Consistent with the pitch data, FFR spectral data showed that the Chinese group exhibits stronger representation of the second harmonic relative to the English group across all four tones. These results cannot be explained by a temporal pitch encoding scheme which simply extracts the dominant interspike interval. Rather, these results support the possibility of neural plasticity at the brainstem level that is induced by language experience that may be enhancing or priming linguistically relevant features of the speech input.
Cerebral blood flow (CBF) was measured with PET during rudimentary singing of a single pitch and vowel, contrasted to passive listening to complex tones. CBF increases in cortical areas related to motor control were seen in the supplementary motor area, anterior cingulate cortex, precentral gyri, anterior insula (and the adjacent inner face of the precentral operculum) and cerebellum, replicating most previously seen during speech. Increases in auditory cortex were seen within right Heschl's gyrus, and in the posterior superior temporal plane (and the immediately overlying parietal cortex). Since cortex near right Heschl's has been linked to complex pitch perception, its asymmetric activation here may be related to analyzing the fundamental frequency of one's own voice for feedback-guided modulation.
Click trains were generated with first- and second-order statistics following Kaernbach and Demany [J. Acoust. Soc. Am. 104, 2298-2306 (1998)]. First-order intervals are between successive clicks, while second-order intervals are those between every other click. Click trains were generated with a repeating alternation of fixed and random intervals which produce a pitch at the reciprocal of the duration of the fixed interval. The intervals were then randomly shuffled and compared to the unshuffled, alternating click trains in pitch-strength comparison experiments. In almost all comparisons for the first-order interval stimuli, the shuffled-interval click trains had a stronger pitch strength than the unshuffled-interval click trains. The shuffled-interval click trains only produced stronger pitches for second-order interval stimuli when the click trains were unfiltered. Several experimental conditions and an analysis of runs of regular and random intervals in these click trains suggest that the auditory system is sensitive to runs of regular intervals in a stimulus that contains a mix of regular and random intervals. These results indicate that fine-structure regularity plays a more important role in pitch perception than randomness, and that the long-term autocorrelation function or spectra of these click trains are not good predictors of pitch strength.
Highly proficient alaryngeal speakers are known to convey prosody successfully. The present study investigated whether alaryngeal speakers not selected on grounds of proficiency were able to convey pitch accent (a pitch accent is realized on the word that is in focus, cf. Bolinger, 1958). The participating speakers (10 tracheoesophageal, 9 esophageal, and 10 laryngeal [control] speakers) produced sentences in which accent was cued by the preceding context. For each utterance, a group of listeners identified which word conveyed accent. All speakers were able to convey accent. Acoustic analyses showed that some alaryngeal speakers had little or no control over fundamental frequency. Contrary to expectation, these speakers did not compensate by using nonmelodic cues, whereas speakers using F0 did use nonmelodic cues. Thus, temporal and intensity cues are concomitant with the use of F0; if F0 is affected, these nonmelodic cues will be as well. A pitch perception experiment confirmed that alaryngeal speakers who had no control over F0 and who did not use nonmelodic cues were nevertheless able to produce pitch movements. Speakers with no control over F0 apparently relied on an alternative pitch system to convey accents and other pitch movements.
The pitch-onset response (POR) is a negative component of the auditory evoked field which is elicited when the temporal fine structure of a continuous noise is regularized to produce a pitch perception without altering the gross spectral characteristics of the sound. Previously, we showed that the latency of the POR is inversely related to the pitch value and its amplitude is correlated with the salience of the pitch, suggesting that the underlying generators are part of a pitch-processing network [Krumbholz, K., Patterson, R.D., Seither-Preisler, A., Lammertmann, C. & Lütkenhöner, B. (2003) Cereb. Cortex,13, 765-772]. The source of the POR was located near the medial part of Heschl's gyrus. The present study was designed to determine whether the POR originates from the same generators as the energy-onset response (EOR) represented by the N100m/P200m complex. The EOR to the onset of a noise, and the POR to a subsequent transition from noise to pitch, were recorded as the time interval between the noise onset and the transition varied from 500 to 4000 ms. The mean amplitude of the POR increased by approximately 5.9 nA.m with each doubling of the time between noise onset and transition. This suggests an interaction between the POR and the EOR, which may be based on common neural generators.
In order to account for the phenomenon of virtual pitch, various theories assume implicitly or explicitly that each spectral component introduces a series of subharmonics. The spectral-compression method for pitch determination can be viewed as a direct implementation of this principle. The widespread application of this principle in pitch determination is, however, impeded by numerical problems with respect to accuracy and computational efficiency. A modified algorithm is described that solves these problems. Its performance is tested for normal speech and "telephone" speech, i.e., speech high-pass filtered at 300 Hz. The algorithm out-performs the harmonic-sieve method for pitch determination, while its computational requirements are about the same. The algorithm is described in terms of nonlinear system theory, i.c., subharmonic summation. It is argued that the favorable performance of the subharmonic-summation algorithm stems from its corresponding more closely with current pitch-perception theories than does the harmonic sieve.
We used a twin study to investigate the genetic and environmental contributions to differences in musical pitch perception abilities in humans. We administered a Distorted Tunes Test (DTT), which requires subjects to judge whether simple popular melodies contain notes with incorrect pitch, to 136 monozygotic twin pairs and 148 dizygotic twin pairs. The correlation of DTT scores between twins was estimated at 0.67 for monozygotic pairs and 0.44 for dizygotic pairs. Genetic model-fitting techniques supported an additive genetic model, with heritability estimated at 0.71 to 0.80, depending on how subjects were categorized, and with no effect of shared environment. DTT scores were only weakly correlated with measures of peripheral hearing. This suggests that variation in musical pitch recognition is primarily due to highly heritable differences in auditory functions not tested by conventional audiologic methods.
Progress in the knowledge of auditory processing of complex sounds has been made through coordinated psychophysical, physiological and theoretical studies of periodicity pitch and combination tones. Periodicity pitch is the basis for human perception of musical notes and pitch of voiced speech. The mechanism of perception involves harmonic pattern recognition on the complex Fourier frequency spectra generated by auditory frequency analysis. Combination tones are perceptible distortion tones generated within the cochlea by nonlinear interaction of component stimulus tones. Perception of periodicity pitch is quantitatively accounted for by a two-stage process of frequency analysis subject to random errors and significant nonlinearities, followed by a pattern recognizer that operates very efficiently to measure the period of musical and speech sounds. The basic characteristic of the first stage is a Gaussian standard error function that quantifies the randomness in aural estimation of frequencies of component tones in a complex tone stimulus. Efficient aural measurement of neural spike intervals from the eighth nerve provides a physiological account for the psychophysical characteristic of aural frequency analysis with complex sounds. Although cochlear filtering is an essential stage in auditory frequency analysis, neural time following, rather than details of the filter characteristics, is the decisive factor in determining the precision of aural frequency measurement. It is likely that peripheral auditory coding is similar for sounds in periodicity pitch and in speech perception, although the 'second stage' representing central processing would differ.
The optimum processor theory of Goldstein can, in principle, account for pitch perception phenomena involving simultaneous dichotic complex tones. The frequency-coding noise function, which is the only free parameter of the model, was estimated with pitch identification data of two simultaneous two-tone complexes presented to different ears. This "sigma" function was found to have a shape similar to that of the function derived from data on identification performance for single pitches. The sigmas in the simultaneous pitch identification experiment are larger by an amount that differs from subject to subject. By using different methods of data analysis it was found that the pitch estimation processes for the two tones are independent for most subjects. This allows a simple extension of Goldstein's optimum processor theory.
Most studies of the musical pitch of harmonic tone complexes have utilized signals comparing two or more successive harmonics. The present study provides systematic data on melodic interval recognition by three musically experienced subjects with sounds whose missing fundamentals were represented by two nonsuccessive harmonics nf0,(n + m)f0, delivered to separate ears. Data were obtained in the ranges 1 less than or equal to n less than or equal to 9, 2 less than or equal to m less than or equal to 4, and 200 Hz less than or equal to f0 less than or equal to 1000 Hz. The data are interpreted in the light of three theories, the "optimum processor theory," the "virtual pitch theory," and the "pattern transformation theory." For each theory, a constraint on preformance is proposed based on interference between the "analytic" and "synthetic" pitch perception modes. The former is obtained with large spacings between harmonics, where listeners are more likely to perceive harmonics as individual tones, each having their own pitch. This degrades the listener's ability to hear the fundamental pitch.
The phenomenology of pitch has been difficult to rationalize and remains the subject of much debate. Here we test the hypothesis that audition generates pitch percepts by relating inherently ambiguous sound stimuli to their probable sources in the human auditory environment. A database of speech sounds, the principal source of periodic sound energy for human listeners, was compiled and the dominant periodicity of each speech sound determined. A set of synthetic test stimuli were used to assess whether the major pitch phenomena described in the literature could be explained by the probabilistic relationship between the stimuli and their probable sources (i.e., speech sounds). The phenomena tested included the perception of the missing fundamental, the pitch-shift of the residue, spectral dominance and the perception of pitch strength. In each case, the conditional probability distribution of speech sound periodicities accurately predicted the pitches normally heard in response to the test stimuli. We conclude from these findings that pitch entails an auditory process that relates inevitably ambiguous sound stimuli to their probable natural sources.