Changes In Interaural Phase Research Articles

Temporal model of edge pitch effects

Musical pitches can be perceived in broadband noise stimuli near frequencies corresponding to parameter changes along the frequency dimension. For example, monaural edge pitches (MEPs) are produced by noise stimuli with sharp spectral edges. Binaural edge pitches (BEPs) are produced by dichotic noise with interaural-phase changes at frequency boundaries (e.g., 0-pi), and binaural-coherence edge pitches (BICEPs) arise from boundaries between frequency regions with different interaural coherence (such as changes from correlated to uncorrelated noises). Perceived pitches are shifted slightly in frequency away from spectral edges: MEPs are shifted into noise bands, BICEPs are shifted into the incoherent region, and BEPs are shifted bimodally. This presentation proposes a temporal model for these edge pitches based on population-wide all-order interspike interval distributions (summary autocorrelations, SACFs), computed using the Zilany-Bruce-Carney 2014 model of the auditory nerve. Pitches were estimated from mean SACF values among lags at all F0-subharmonics (0–30 ms). Binaural pitches were estimated from computations based on corresponding left and right PSTs after a binaural delay and cancellation process. Model predictions agreed well with pitch judgments in both monaural and binaural cases. [Work supported by NIDCD (R01 DC00100 and P30 DC004663) and by AFOSR FA9550-11-1-0101.]

Aging in Binaural Hearing Begins in Mid-Life: Evidence from Cortical Auditory-Evoked Responses to Changes in Interaural Phase

Older adults often have difficulty understanding speech in a noisy environment or with multiple speakers. In such situations, binaural hearing improves the signal-to-noise ratio. How does this binaural advantage change with increasing age? Using magnetoencephalography, we recorded cortical activity evoked by changes in interaural phase differences of amplitude-modulated tones. These responses occurred for frequencies up to 1225 Hz in young subjects but only up to 940 Hz in middle-aged and 760 Hz in older adults. Behavioral thresholds also decreased with increasing age but were more variable, likely because some older adults make effective use of compensatory mechanisms. The reduced frequency range for binaural hearing became significant in middle age, before decline in hearing sensation and the morphology of cortical responses, which became apparent only in the older subjects. This study provides evidence from human physiological data for the early onset of biological aging in binaural hearing.

Auditory evoked MEG responses to interaural phase changes: Effects of aging on response latencies

Abstract The ability to perceive time-varying acoustic changes deteriorates with advancing age. To learn more about the mechanisms underlying such perceptual deficits, we used magnetoencephalography (MEG) to examine the physiological response to interaural phase differences (IPD) in young, middle-aged, and older adults. Stimulus onset evoked P1m–N1m–P2m cortical responses, as did the change in interaural phase. There were no significant age-related latency differences in the detection of sound “onset”; however, the ability to detect “changes” within the stimulus deteriorated with increased age. Peak latencies were prolonged for the “change” response when compared to the “onset” response, suggesting that the physiological detection of IPDs requires more time than detecting sound “onset”. These results provide physiological evidence of impaired temporal processing in the elderly that is likely unrelated to hearing loss or cognitive ability because brain activity was measured passively in response to audible low frequency stimuli.

Sensitivity to simulated directional sound motion in the rat primary auditory cortex.

Sensitivity to simulated directional sound motion in the rat primary auditory cortex. This paper examines neuron responses in rat primary auditory cortex (AI) during sound stimulation of the two ears designed to simulate sound motion in the horizontal plane. The simulated sound motion was synthesized from mathematical equations that generated dynamic changes in interaural phase, intensity, and Doppler shifts at the two ears. The simulated sounds were based on moving sources in the right frontal horizontal quadrant. Stimuli consisted of three circumferential segments between 0 and 30 degrees, 30 and 60 degrees, and 60 and 90 degrees and four radial segments at 0, 30, 60, and 90 degrees. The constant velocity portion of each segment was 0.84 m long. The circumferential segments and center of the radial segments were calculated to simulate a distance of 2 m from the head. Each segment had two trajectories that simulated motion in both directions, and each trajectory was presented at two velocities. Young adult rats were anesthetized, the left primary auditory cortex was exposed, and microelectrode recordings were obtained from sound responsive cells in AI. All testing took place at a tonal frequency that most closely approximated the best frequency of the unit at a level 20 dB above the tuning curve threshold. The results were presented on polar plots that emphasized the two directions of simulated motion for each segment rather than the location of sound in space. The trajectory exhibiting a "maximum motion response" could be identified from these plots. "Neuron discharge profiles" within these trajectories were used to demonstrate neuron activity for the two motion directions. Cells were identified that clearly responded to simulated uni- or multidirectional sound motion (39%), that were sensitive to sound location only (19%), or that were sound driven but insensitive to our location or sound motion stimuli (42%). The results demonstrated the capacity of neurons in rat auditory cortex to selectively process dynamic stimulus conditions representing simulated motion on the horizontal plane. Our data further show that some cells were responsive to location along the horizontal plane but not sensitive to motion. Cells sensitive to motion, however, also responded best to the moving sound at a particular location within the trajectory. It would seem that the mechanisms underlying sensitivity to sound location as well as direction of motion converge on the same cell.

The profile of interaural decorrelation across frequency: A binaural representation of pitch and timbre

Tones can be detected in noise at lower signal-to-noise ratios (SNRs) when they possess different interaural timing to the noise. One account, equalization-cancellation (E-C), suggests that the left- and right-ear signals are adjusted in time and amplitude to maximize their similarity, and are then cancelled by subtraction. At low SNRs, E-C improves the SNR of the tone by cancelling the noise. E-C is also a method for measuring interaural decorrelation, because the remainder after subtraction is a measure of the extent to which the tone reduces the interaural correlation of the noise. Applying E-C independently in each frequency channel reveals the profile of interaural decorrelation across frequency induced by a complex signal in noise. The profile is a place representation akin to an excitation pattern. It has three properties. (i) It displays the spectrum of speech at low SNRs. (ii) Listeners can integrate it perceptually with the conventional profile of spectral amplitude. (iii) Its spectral structure is compatible with a subset of the dichotic pitches phenomena in which tonal percepts are induced by introducing interaural phase changes into broadband noises. Finally, the profile can be recast in a form compatible with temporal accounts of pitch and timbre based on autocorrelation.

Interaural time sensitivity in medial superior olive of cat

1. We studied the sensitivity of cells in the medial superior olive (MSO) of the anesthetized cat to variations in interaural phase differences (IPDs) of low-frequency tones and in interaural time differences (ITDs) of tones and broad-band noise signals. Our sample consisted of 39 cells histologically localized to the MSO. 2. All but one of the cells had characteristic frequencies less than 3 kHz, and 79% were sensitive to ITDs and IPDs. More than one-half (56%) of the cells responded to monaural stimulation of either ear, and both the binaural and monaural responses were highly phase locked. All of the cells that were sensitive to IPDs and monaurally driven by either ear responded in accord with that predicted by the coincidence model of Jeffress, as judged by comparisons of the phases at which the monaural and binaural responses occurred. The optimal IPDs were tightly clustered between 0.0 and 0.2 cycles. Most cells exhibited facilitation of the response at favorable ITDs and inhibition at unfavorable ITDs compared with the monaural responses. 3. Cells in the MSO exhibited characteristic delay, as judged by a linear relationship between the mean interaural phase and stimulating frequency. Characteristic phases were clustered near 0 indicating the most cells responded maximally when the two input tones were in phase. With the use of the binaural beat stimulus we found no differential selectivity for either the direction or speed of interaural phase changes. 4. The cells were also sensitive to ITDs of broad-band noise signals. The ITD curve in response to broad-band noise was similar to that predicted by the composite curve, which was calculated by linearly summating the tonal responses over the frequencies in the response area of the cell. Most (93%) of the peaks of the composite curves were between 0 and +400 microseconds, corresponding to locations in the contralateral sound field. Moreover, computer cross correlations of the monaural spike trains were similar to the ITD curve generated binaurally for both correlated and uncorrelated noise signals to the two ears. Thus our data suggest that the cells in the MSO behave much like cross-correlators. 5. By combining data from different animals and lcoating each cell on a standard MSO, we found evidence for a spatial map of ITDs across the anterior-posterior (A-P) axis of the MSO.(ABSTRACT TRUNCATED AT 400 WORDS)

Interaural phase and level effects on binaural interaction component (BIC) of the brain‐stem auditory evoked potential

Electrophysiological studies have elegantly demonstrated that neurons in lower brain‐stem nuclei are exquisitely sensitive to the parameters of sound that mediate localization and lateralization phenomena. In the present study, the binaural interaction component (BIC), derived from brain‐stem auditory evoked potentials, was obtained using low‐frequency stimuli with various combinations of time and level differences delivered through earphones. The aim has been to determine whether these volume‐conducted multineuronal responses reveal any of the neural mechanisms of lateralization. The findings show that characteristics of the binaural interaction component, e.g., latency, amplitude, and waveform morphology, evoked by low‐frequency tone bursts, are differentially altered with changes in interaural phase. The effects on the binaural interaction component with interaural level differences were also substantial but not similar to the changes produced with phase. From this research, it has become apparent that, given appropriate recording and stimulating parameters, many nuances of binaural hearing may be neurophysiologically addressed in studies of volume‐conducted evoked and derived potentials.

Interaural phase-sensitive units in the inferior colliculus of the unanesthetized rabbit: effects of changing frequency.

We studied the interaural phase sensitivity of 85 units in the inferior colliculus (IC) of the unanesthetized rabbit. We assessed this sensitivity at several frequencies within each unit's responsive range. The interaural phase disparity was varied by delivering tones that differed by 1 Hz to the two ears, resulting in a 1-Hz binaural beat. We analyzed each unit's response to different frequencies by calculating four measures: characteristic delay (CD), characteristic phase (CP), composite peak delay, and mean peak delay. We estimated the CD and CP from the slope and phase intercept, respectively, of the regression line fitted to a plot of the mean interaural phase against stimulating frequency. The composite peak delay was estimated from the peak of a composite delay curve. This was generated by replotting the response to changes in interaural phase, as a function of the equivalent interaural delay and averaging the resultant interaural delay curves. The composite delay curve reflects the unit's average response to interaural delays across frequencies. Last, we calculated a mean peak delay, derived by converting the mean interaural phase of the response at each frequency to an equivalent delay and then averaging these delays. Interaural phase sensitivity was observed to frequencies as high as 2,150 Hz. However, the majority of units showed such sensitivity below 1,500 Hz. For most units, the interaural delay curves measured at several frequencies coincided near the peak discharge. This result is consistent with a neural model, where excitatory inputs from each ear converge upon a binaural cell, evoking maximum discharge only when the two inputs arrive simultaneously. As a first approximation, our data fit this model, indicating that IC neurons can act like coincidence detectors or cross-correlators. The distributions of CD, composite peak delay, and mean peak delay showed that most units preferred ipsilateral stimulus delays, which in the natural situation corresponds to sounds emanating from the contralateral field. Moreover, most units preferred delays that were within the estimated physiological range of the rabbit. These results support the viewpoint that neurons in the IC participate in sound localization. The distributions of CP and CD differ substantially from those found in the IC of the anesthetized cat. These differences may reflect species differences, the effects of anesthesia, or a difference in the population of units sampled. For each unit, we assessed the linearity of the plot of mean interaural phase against frequency of stimulation using a chi 2 method. For most units the plots were significantly nonlinear.(ABSTRACT TRUNCATED AT 400 WORDS)

Binaural interaction in low-frequency neurons in inferior colliculus of the cat. II. Effects of changing rate and direction of interaural phase

We used the binaural beat stimulus to study the interaural phase sensitivity of inferior colliculus (IC) neurons in the cat. The binaural beat, produced by delivering tones of slightly different frequencies to the two ears, generates continuous and graded changes in interaural phase. Over 90% of the cells that exhibit a sensitivity to changes in the interaural delay also show a sensitivity to interaural phase disparities with the binaural beat. Cells respond with a burst of impulses with each complete cycle of the beat frequency. The period histogram obtained by binning the poststimulus time histogram on the beat frequency gives a measure of the interaural phase sensitivity of the cell. In general, there is good correspondence in the shapes of the period histograms generated from binaural beats and the interaural phase curves derived from interaural delays and in the mean interaural phase angle calculated from them. The magnitude of the beat frequency determines the rate of change of interaural phase and the sign determines the direction of phase change. While most cells respond in a phase-locked manner up to beat frequencies of 10 Hz, there are some cells tht will phase lock up to 80 Hz. Beat frequency and mean interaural phase angle are linearly related for most cells. Most cells respond equally in the two directions of phase change and with different rates of change, at least up to 10 Hz. However, some IC cells exhibit marked sensitivity to the speed of phase change, either responding more vigorously at low beat frequencies or at high beat frequencies. In addition, other cells demonstrate a clear directional sensitivity. The cells that show sensitivity to the direction and speed of phase changes would be expected to demonstrate a sensitivity to moving sound sources in the free field. Changes in the mean interaural phase of the binaural beat period histograms are used to determine the effects of changes in average and interaural intensity on the phase sensitivity of the cells. The effects of both forms of intensity variation are continuously distributed. The binaural beat offers a number of advantages for studying the interaural phase sensitivity of binaural cells. The dynamic characteristics of the interaural phase can be varied so that the speed and direction of phase change are under direct control. The data can be obtained in a much more efficient manner, as the binaural beat is about 10 times faster in terms of data collection than the interaural delay.

Binaural interaction in low-frequency neurons in inferior colliculus of the cat. I. Effects of long interaural delays, intensity, and repetition rate on interaural delay function.

Detailed, quantitative studies were made of the interaural phase sensitivity of 197 neurons with low best frequency in the inferior colliculus (IC) of the barbiturate-anesthetized cat. We analyzed the responses of single cells to interaural delays in which tone bursts were delivered to the two ears via sealed earphones and the onset of the tone to one ear with respect to the other was varied. For most (80%) cells the discharge rate is a cyclic function of interaural delay at a period corresponding to that of the stimulating frequency. The cyclic nature of the interaural delay curve indicates that these cells are sensitive to the interaural phase difference. These cells are distributed throughout the low-frequency zone of the IC, but they are less numerous in the medial and caudal zones. Cells with a wide variety of response patterns will exhibit interaural phase sensitivities at stimulating frequencies up to 3,100 Hz, although above 2,500 Hz the number of such cells decrease markedly. Using dichotic stimuli we could study the cell's sensitivity to the onset delay and interaural phase independently. The large majority of IC cells respond only to changes in interaural phase, with no sensitivity to the onset delay. However, a small number (7%) of cells exhibit a sensitivity to the onset delay as well as to the interaural phase disparity, and most of these cells show an onset response. The effects of changing the stimulus intensity equally to both ears or of changing the interaural intensity difference on the mean interaural phase were studied. While some neurons are not affected by level changes, others exhibit systematic phase shifts for both average and interaural intensity variations, and there is a continuous distribution of sensitivities between these extremes. A few cells also showed systematic changes in the shape of the interaural delay curves as a function of interaural intensity difference, especially at very long delays. These shifts can be interpreted as a form of time-intensity trading. A few cells demonstrated orderly changes in the interaural delay curve as the repetition rate of the stimulus was varied. Some of these changes are consonant with an inhibitory effect that occurs at stimulus offset. The responses of the neurons show a strong bias for stimuli that would originate from he contralateral sound field; 77% of the responses display mean interaural phase angles that are less than 0.5 of a cycle, which are delays to the ipsilateral tone.(ABSTRACT TRUNCATED AT 400 WORDS)

Interaurally phase/time sensitive cells in the cat inferior colliculus

Using dichotically presented sinusoids, the response of many low‐frequency cells in the inferior colliculus (IC) will vary cyclically as a function of the interaural delay. Furthermore, the interval between peaks of this cyclic pattern equals the period of the stimulating frequency, indicating that these neurons are sensitive to changes in interaural phase created by the delays. This is further demonstrated by the use of binaural beat stimuli which yield results similar to those obtained by the interaural delay method. Most IC neurons show an equal amplitude symmetrical cyclic response independent of the magnitude or sign of the interaural delay and rate or direction of interaural phase change. However, we have encountered a sizeable number of cells that are sensitive to the above parameters as well as to the stimulus features such as off‐time and interaural intensity differences. These results have been helpful in developing a computer simulated model of a binaural cell. Recently, we have extended our studies to high‐frequency IC neurons and have found that under appropriate stimulus conditions they are sensitive to changes in interaural delay in a manner similar to their low‐frequency counterparts.

More on the binaural edge pitch

The binaural edge pitch is created by dichotic, broadband noise with an interaural phase spectrum which changes abruptly. [M. A. Klein and W. M. Hartmann, J. Acoust. Sec. Am. Suppl. 1 67, S20 (1980)]. We employed dichotic noise with Fourier components in phase below frequency f0 and out of phase above f0. The pitch of this noise corresponds to a frequency which is about 4% above or below f0 for 250 < f0 < 1000 Hz. For f0 < 250 Hz the deviation f0 increases rapidly with decreasing f0. We performed auxiliary experiments to find the pitches of high-pass and low-pass noise bands with sharp cutoffs at f0. The pitches deviate from f0 in the same way as for the binaural edge pitch. The correspondance supports the Equalization Cancellation model of binaural processing; it provides stronger support than does the Huggins pitch effect. Further experiments, with interaural phase changes from −π/2 to +π2, or changes superimposed upon a slowly varying phase shift show that the binaural processing must be a flexible system capable of enhancing local spectral features. Generally any abrupt change in interaural phase provides some pitch cue. [Work supported by NSF Grant BNS 79-14155.]

Lateralization of tonal signals in noise

When listeners attempt to localize a tone in noise, the “signal” for lateralization is the change in interaural phase between tone plus noise and the interaural phase of the noise alone. Data are presented showing how lateralization performance varies as a function of the interaural correlation of the noise, signal-to-noise ratio, and tone frequency. The relationship between the results of this study and some phenomena of binaural signal detection is discussed.

Binaural interaction with an aural harmonic: Cancellation and augmentation

In the monaural tone‐on‐tone masking procedure, the audibility of a maskee an octave above the masker depends on masker:maskee phase. If vector summation between the maskee and an aural harmonic is assumed, the amplitude and phase of the aural harmonic can be estimated. An alternative interpretation of the phase effects is two‐tone interaction between the masker and maskee. To decide between these explanations, a binaural technique has been developed for the study of octave phase effects. Using a forced‐choice procedure, listeners are able to detect a change in interaural phase between a 400‐Hz continuous masker presented to one ear (65–75 dB SPL) and an 800‐Hz pulsed probe tone presented contralaterally (10 dB SL). When a subthreshold 800‐Hz tone is added to the masker ear at a phase and level appropriate to cancel the estimated second aural harmonic, performance deteriorates significantly. Addition of this same tone at augmentation phase tends to improve performance. These findings fail to confirm the two‐tone interaction interpretation, but they support the aural‐harmonic explanation for monaural tone‐on‐tone masking results. Furthermore, these inaudible “subjective” products must be represented in the neural input to the brainstem since, like inaudible acoustic tones, they are capable of interacting binaurally with audible tones.

Signal detection under dynamic changes in interaural phase

A brief tonal signal (500 Hz, 20 msec) was presented against a random noise background. The interaural phase of the background noise was switched periodically. Signal thresholds are determined not only by the ongoing interaural phase of the signal and of the noise, but also by the temporal proximity of the signal to a transition in interaural phase. [Research assisted by a grant of the National Science Foundation.]

Interaural phase discrimination for combination tone stimuli.

A binaural interaction between the combination tone 2f1—f2 generated by two primary tones and a pure tone of the same frequency was demonstrated in an investigation of interaural phase discrimination. Sensitivity to changes in interaural phase was comparable to that obtained with pure tone stimuli. In a second experiment, combination tones of the same frequency, but different order, generated by independent pairs of primary tones presented to the two ears, were also shown to be easily lateralized. Finally, observers were able to discriminate changes in the phase of a low‐frequency combination tone generated by high‐frequency primaries, which themselves could not produce a binaural interaction based on phase. The results are in accord with the prevailing conception of combination tones as being functionally identical to spectral components in the stimulus.Subject Classification: [43]65.56, [43]65.62, [43]65.60.

Discriminations of interaural phase differences.

Listeners were asked to discriminate between two pulsed sinusoids, one presented with an interaural phase delay θ and the other presented with a longer delay θ + Δθ. Frequency of the sinusoids ranged from 250 to 4000 Hz. The difference threshold for Δθ was defined as the dependent variable. This threshold increased as θ increased from zero degrees to half the period of the sinusoid, then decreased as θ was further increased to a full period of the sinusoid. As θ was increased to half the period of the sinusoid, the lateral image appeared to shift away from midline toward the ear leading in time; but as θ was increased beyond half a period of the sinusoid, the lateral image appeared to shift toward the midline from the ear lagging in time. These results were obtained at 250, 500, and 900 Hz. At 2000 Hz, the value of Δθ remained a constant as θ was varied, while at 4000 Hz the lateral image was not moved by any interaural delay. The results imply that (1) for frequencies less than 2000 Hz, lateral discriminations based on interaural phase differences are more acute toward midline than at either side, and (2) for frequencies greater than 2000 Hz, changes in interaural phase are unable to shift the lateral image.

The Effect of Interaural Phase at Different Frequencies on the Discharges of Cells in the Posterior Colliculus of the Chinchilla

Some cells in the posterior colliculus with discharge latencies between 7 and 10 msec (presumably in the central nucleus of the colliculus) show variation in their discharge rates with changes in interaural phase of the acoustic stimuli. These cells may show a characteristic interaural delay since their response rates can reach a maximum (or minimum) at the same interaural time difference for different acoustic frequencies. However, for frequencies about Z oct below an optimum characteristic delay frequency, optimal delays may produce opposite effects. That is, delays that were inhibitory at the higher frequencies may produce excitation at the lower frequencies, and delays that produced excitation at the higher frequencies may produce inhibition at the lower frequencies. Although our data support the idea of a temporal regularity in changes of discharge rate with interaural phase changes, the label, characteristic delay, may be too limited a descriptor for cells that respond to interaural time or phase differences.

Responses of Inferior Collicular Units in the Chinchilla to Binaural Stimuli

Extracellularly recorded discharges of units in the inferior colliculus of the Flaxedilized chinchila will be described. The two ears were stimulated independently so that interaural intensity and phase could be manipulated. Of those units encountered whose activity could controlled by acoustic stimuli below about 2 kHz, a large majority were found to be differentially responsive to alterations of interaural phase and intensity. The time patterns (histograms) in response to binaural stimulation showed characteristic alterations of excitation and inhibition similar to those which have been reported for neurons in the cat. The effect of interaural phase changes could be maximized by appropriate choice of interaural intensity differences.

Binaural Effects in Remote Masking

A first experiment has confirmed the findings of Hirsh and of Jeffress with respect to the effects of changes in the interaural phase difference for both noise and tone on the masked binaural threshold. The threshold for a low-frequency tone is some 12 db higher when both tone and noise are in phase at the two ears, than when either one is 180°. A second experiment utilized a high-frequency band of noise at a high level to produce “remote masking” for the same low tones in order to determine whether or not this other kind of masking would be similarly affected by changes in interaural phase. With the noise band in phase, a 180° change in the phase of the low-frequency tone produces a change in threshold of only slightly less than 12 db. But a similar change in the phase of the noise band produces no change in masking, except for a slight shift that can be predicted from the time shift at low frequency that would correspond to a half-wave in the high-frequency band.