Binaural Pitch Research Articles

The cochlear implant electrode–pitch function

The cochlear frequency-place function in normal hearing ears has been found to be an exponential relationship in a wide variety of species [D.D. Greenwood, J. Acoust. Soc. Am. 87 (1990) 2592-2605]. Although it seems reasonable to assume a similar function for electrical stimulation by means of an intra-cochlear electrode array, the exact frequency-place function for this special type of stimulation needs to be investigated. Six users of the MED-EL COMBI 40+ cochlear implant device with moderate to profound hearing loss between 125 and 1000 Hz in the non-implanted ear took part in a binaural pitch adjustment experiment. The COMBI 40+ electrode array provides a deep insertion into the scala tympani and a wide spatial separation between the stimulating electrodes. Insertion depth was controlled by Stenver's view plain radiographs and the insertion angle was estimated. The task of the subjects was to adjust the frequency of a sinusoid presented in the non-implanted ear by means of an adjusting knob until they perceived the same pitch as was elicited by a reference stimulus in the implanted ear. The results show adjustments corresponding to electrode positions along the cochlea, with the exception of the two most apical electrodes for most of the subjects. Pitch increased in an orderly fashion with an average of 98 Hz per electrode separation (40 Hz/mm). In contrast to the exponential predictions according to [D.D. Greenwood, J. Acoust. Soc. Am. 87 (1990) 2592-2605] for normal hearing, the average electrode-pitch function shows a linear relationship.

Neural mechanism for binaural pitch perception via ghost stochastic resonance

We present a physiologically plausible binaural mechanism for the perception of the pitch of complex sounds via ghost stochastic resonance. In this scheme, two neurons are driven by noise and a different periodic signal each (with frequencies f(1)=kf(0) and f(2)=(k+1)f(0), where k>1), and their outputs (plus noise) are applied synaptically to a third neuron. Our numerical results, using the Morris-Lecar neuron model with chemical synapses explicitly considered, show that intermediate noise levels enhance the response of the third neuron at frequencies close to f(0), as in the cases previously described of ghost resonance. For the case of an inharmonic combination of inputs (f(1)=kf(0)+Deltaf and f(2)=(k+1)f(0)+Deltaf) noise is also seen to enhance the rates of most probable spiking for the third neuron at a frequency f(r)=f(0)+[Deltaf(k+12)]. In addition, we show that similar resonances can be observed as a function of the synaptic time constant. The suggested ghost-resonance-based stochastic mechanism can thus arise either at the peripheral level or at a higher level of neural processing in the perception of pitch.

How do listeners detect interaural incoherence?

Listeners are remarkably sensitive to interaural incoherence. They can easily distinguish between a coherence of 1.00 and a coherence of 0.99. Interaural incoherence leads to the sensation of apparent source width; it is also held to be responsible for the masking level difference and for the creation of binaural pitch. However, incoherence per se is only a statistical description of signals; it is not indicative of any particular auditory binaural property. To discover the relevant binaural attribute(s), three-interval oddity experiments were performed using narrow-band noises that were perfectly coherent or slightly incoherent. The listener’s task was to identify the incoherent noise. First, it was shown that different frozen noises with identical values of incoherence could often differ greatly in detectability. Subsequent experiments studied the detectability of incoherent frozen noises that were were particularly strong or particularly weak according to selected binaural difference functions. These functions tested different rules for combining interaural phase differences (IPD) and interaural level differences (ILD)—either weighted sums of squares or rules that combine IPD and ILD to form a fluctuating lateralized image. The goal was to discover the best predictor of incoherence detectability. [Work supported by the NIDCD Grant DC 00181.]

A low-frequency paradox in binaural pitch

Binaural pitches, such as the Huggins pitch and binaural coherence edge pitch (BICEP), can be approached in terms of a model network of binaural neurons that are tuned in frequency and in interaural delay [e.g., Raatgever and Bilsen, J. Acoust. Soc. Am. 80, 429–441 (1986)]. The basic rule of binaural combination resembles addition. Binaural pitches appear as peaks or edge features in the frequency-delay plane. The characteristic frequency of a feature determines the pitch, and the internal delay of the feature determines the lateralization of the pitch image. Because delay lines become sparser with increasing delay, the model predicts that binaural pitches of very low frequency should be more difficult to hear the more they are lateralized. However, our Huggins pitch and BICEP discrimination and detection experiments show that the reverse is true. These experiments favor a model in which the basic rule of binaural combination resembles subtraction. However, the predictions of the subtractive model disagree with the observed lateralization. Furthermore, the low-frequency discrimination and detection experiments do not agree with a model in which additive and subtractive binaural combinations are alternative options. This is the low-frequency paradox. [Work supported by NIDCD.]

Binaural coherence edge pitch—Manipulations and models

Binaural coherence edge pitch (BICEP) is a dichotic broadband noise pitch effect. The stimulus is made by summing spectrally dense sine components with random phases. The interaural phase angle is a constant (e.g., 0 or π) for components with frequencies below (or above) a chosen edge frequency, and it is a random variable for the remaining components. Pitch-matching experiments show that the BICEP exists for coherence edge frequencies between 300 and 1000 Hz. It is matched by a pure tone frequency shifted from the edge frequency by 5% to 10%. The matching frequency falls on the incoherent side of the edge. The BICEP persists if the noise to one of the ears is delayed (<3 ms). Some BICEP exists even when the component amplitudes are entirely independent in the two ears, so long as the phase coherence conditions of the stimulus are maintained. The existence of the BICEP is a challenge for current models of dichotic pitch, though the pitch shift is consistent with the equalization-cancellation model as previously applied to dichotic pitch effects by M. A. Klein and W. M. Hartmann [J. Acoust. Soc. Am. 70, 51–61 (1981)]. [Work supported by the NIDCD of the NIH and by the NSF REU Program.]

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Detection of interaural phase shifts and level differences in portions of broadband noise

Versions of Cramer‐Huggins pitch stimuli were generated such that a narrow band (bandwidths ranged from 1–100 Hz) of a wideband, diotic noise (low‐passed filtered at 8000 Hz) was presented dichotically with an interaural phase shift or an interaural level difference. The threshold value for detecting a phase shift or a level difference in the dichotic, narrowband portion of the broadband noise was determined as a function of the center frequency of the dichotic, narrow band of noise and as a function of the bandwidth of the dichotic, narrow band of noise. A single‐interval procedure without feedback was used to estimate three point psychometric functions relating d′ to interaural phase or d′ to interauralevel difference. A random variation in the interaural level difference of the entire noise waveform was also used to force the listeners to use pitch as the cue for detection rather than some aspect of lateral position. The data will be discussed in terms of models of binaural processing and binaural pitch. [Work supported by NINCDS.]

Binaural coherence edge pitch

The binaural coherence edge pitch (BICEP) is a dichotic broadband noise pitch effect similar to the binaural edge pitch (BEP). The BICEP stimulus is made by summing spectrally dense sine wave components with random phases. The interaural phase angle is a constant (0 or pi) for components with frequencies below (or above) a chosen edge frequency, and it is a random variable for the remaining components. The chosen edge frequency is a coherence edge because the noises to the two ears are mutually coherent within any band of frequencies on one side of the edge and they are mutually incoherent in any band on the other side. Pitch-matching experiments show that the BICEP exists for coherence edge frequencies between about 300 and 1000 Hz. It is matched by a pure-tone frequency that differs from the edge frequency by 5% to 10%. The matching frequency lies on the incoherent side of the edge, an important result that is consistent with the way that the equalization-cancellation model has been applied to binaural pitch effects, especially the BEP. The results of BICEP experiments depend upon whether the coherent components are presented in 0 or pi interaural phase for some listeners but not for all. The BICEP persists if the noise to one of the ears is delayed, but it becomes weaker and less well matched as the delay increases beyond 2 ms. The BICEP does not depend on whether the component amplitudes are all created equal or are given a Rayleigh distribution. Some reliable pitch sensation exists even when the component amplitudes are entirely independent in the two ears, so long as the phase coherence conditions of the BICEP stimulus are maintained. The existence of the BICEP is a challenge for current models of dichotic pitch because none of them predicts all its features.

A search for central lateral inhibition.

A central spectrum explanation of the existence of the binaural edge pitch [Klein and Hartmann, J. Acoust. Soc. Am. 70, 51-61 (1981)] appears to require central lateral inhibition in the human auditory system. We have looked for this effect in central masking experiments. Using the binaural-edge noise, which creates the binaural edge pitch, as a masker (in standard notation: No below and N pi above a phase boundary frequency), we measured pulsation thresholds for sine tones in two frequency ranges where the binaural edge pitch exists. We also obtained reference data, using the same pulsation threshold technique, for standard binaural conditions NoSo, NoS pi, N pi So, and N pi S pi. These data revealed masking level differences. Theoretically the difference between the binaural-edge thresholds and the reference data should show the peak and valley signature of lateral inhibition. No such structure was found. We suggest that this negative result does not exclude the possibility of central lateral inhibition, but that the time course of central lateral inhibition makes the pulsation threshold technique an inappropriate means for observing the effect.

Diplacusis and the pitch of the cubic difference tone

Extensive monaural and binaural pure tone pitch matches to the cubic difference tone (CDT) 2f2−f2 were obtained for various CDT frequencies, with several primary tone (f1, f2) frequency Configurations for each CDT frequency. Two types of conditions were of special interest: (1) those for which moderate diplacusis existed at the frequency of the CDT but negligible diplacusis existed at the frequencies of the primary tones; and (2) those for which negligible diplacusis was present at the CDT frequency but moderate diplacusis existed at one or both of the primary tone frequencies. The results are discussed in regard to assumptions concerning the origin of monaural and binaural pitch anomalies (musical paracusis, binaural diplacusis) and in regard to the question of whether the CDT is coupled back to the basilar membrane at the characteristic place of the CDT frequency.

Effect of unilateral bandwidth increase on ear dominance of dichotic chords

When a 1650-Hz tone is presented to one ear while a 1750-Hz tone of equal intensity is presented to the other ear, most subjects report that there is a significant difference in the relative salience of the pitch of these two tones. This has been called “ear dominance” in the perception of dichotic chords [R. Efron and E. W. Yund, J. Acoust. Soc. Am. 59, 889–898 (1976)]. The present experiments were designed to neutralize or even reverse a given subject's ear dominance by increasing the bandwidth of the tone presented to his dominant ear while continuing to present a pure tone in the nondominant ear. This increase in bandwidth was accomplished by modulating, in brief, abrupt steps, the frequency of the dominant ear tone. The duration of each of the brief steady-state portions of the FM tone was random between 1.4 and 2.0 msec; its frequency was Gaussian-distributed around the selected center frequency. To make the salience of the two pitch components of a dichotic chord equal, a larger bandwidth increase was required in subjects having a stronger ear dominance. Lowering the sound pressure level of the stimuli in both ears increased the amount of bandwidth increase required. These results can be interpreted in terms of a model of binaural pitch mixing. [Work supported by the Veterans Administration.]

Pronounced binaural pitch phenomenon

Using a special multiple-phase-shift filter with a flat amplitude response, a rather strong sensation of pitch was evoked by dichotic presentation of white Gaussian noise. The existence of this pitch, which resembles a periodicity pitch, can be understood on the basis of the ’’central spectrum’’ model of pitch perception [Bilsen and Goldstein, J. Acoust. Soc. Am. 55, 292–296 (1974)]. Subject Classification: [43]65.54, [43]65.62.

Pitch of dichotically delayed noise and its possible spectral basis.

Pitch can be signalled by interaural differences in timing, but it is unknown how the intercochlear timing mediates pitch. Pitch of complex tones can be well understood in terms of a central pitch mechanism that reads across the frequencies of a centrally presented spectrum. We consider whether pitch phenomena signalled by binaural timing are also compatible with auditory processing through tonotopic organization. Known binaural pitch phenomena produced by stimuli comprising two wideband Gaussian noise sources, each with different interaural delay were examined. The major findings are that (i) a single noise source with interaural delay τ is adequate to produce a pitch corresponding to 1/τ, (ii) the medially lateralized pitch image is little affected by the addition of a second independent noise source presented in antiphase, (iii) the dichotic pitch is similar to monaural repetition pitch, for which the stimulus to the ear comprises a noise source added to its delay, and (iv) the image of dichotic repetition pitch is much more faint and compact than that of monaural repetition pitch. Durlach's [J. Acoust. Soc. Am. 34, 1096–1099 (1962)] equalization and cancellation model of binaural signal detection is adapted to describe the compatibility between the binaural pitch phenomena and auditory mediation of pitch by central spectral patterns.

Binaural Pitch Phenomena

The possibility of having a sensation of pitch associated only with a bilateral presentation of acoustic stimuli to the listener was first discovered by Huggins [Cramer and Huggins, J. Acoust. Soc. Am. 30, 413–417 (1958)]. Huggins employed a single, smooth-frequency-spectrum noise generator, and fed its output directly to one ear, but via an all-pass phase-changing network to the other. In the present work, the possibility that this effect is a by-product of a process of auditory localization has been considered. This has led to the experimental use of a family of new binaural pitch-producing stimuli obtained from a multiplicity of independent, smooth-spectrum noise generators which operate in conjunction with continuously variable all-pass delay lines. Each stimulus configuration may be associated with a simple localization pattern. The pitch associated with a particular pattern depends both on its form and on the magnitudes of the delays involved. Pitch-matching measurements with these stimuli and a comparison with the more complex pattern of localization which might be associated with the Huggins stimuli support the view that these binaural pitch phenomena are mediated by a process akin to localization rather than by a central frequency-analysis mechanism.