Binaural Phase Difference Research Articles

Psychoacoustics in the United Kingdom in the period 1900 to 1950

Much research in psychoacoustics during this period was conducted in the “Psychological Laboratory” in Cambridge (H. Bannister, A. F. Rawdon-Smith) and in Manchester (T. S. Littler, A. W. Ewing). Although Lord Rayleigh had published his influential duplex theory of sound localization in 1907, lingering doubts remained about whether interaural phase could truly be discriminated—rather, it had been proposed that somehow interaural phase differences were converted into interaural intensity differences. Bannister conducted a series of experiments to assess this idea and concluded that “binaural phase differences are appreciated in some manner which is distinct from the appreciation of binaural intensity differences.” Bannister also reported experiments on time-intensity trading in sound localization and reported that sometimes two images were perceived. Rawdon Smith and independently Littler and Ewing studied the phenomenon of auditory fatigue (temporary threshold shift, TTS). They showed that the maximum TTS produced by an intense fatiguing tone occurred for test tones with frequencies well above that of the fatiguing tone, a phenomenon that was repeatedly confirmed in later studies and that has only recently been explained. A common feature of the papers describing this work was a detailed description of the (often ingenious) apparatus that was developed to conduct the experiments.

Roles for Coincidence Detection in Coding Amplitude-Modulated Sounds.

Many sensory neurons encode temporal information by detecting coincident arrivals of synaptic inputs. In the mammalian auditory brainstem, binaural neurons of the medial superior olive (MSO) are known to act as coincidence detectors, whereas in the lateral superior olive (LSO) roles of coincidence detection have remained unclear. LSO neurons receive excitatory and inhibitory inputs driven by ipsilateral and contralateral acoustic stimuli, respectively, and vary their output spike rates according to interaural level differences. In addition, LSO neurons are also sensitive to binaural phase differences of low-frequency tones and envelopes of amplitude-modulated (AM) sounds. Previous physiological recordings in vivo found considerable variations in monaural AM-tuning across neurons. To investigate the underlying mechanisms of the observed temporal tuning properties of LSO and their sources of variability, we used a simple coincidence counting model and examined how specific parameters of coincidence detection affect monaural and binaural AM coding. Spike rates and phase-locking of evoked excitatory and spontaneous inhibitory inputs had only minor effects on LSO output to monaural AM inputs. In contrast, the coincidence threshold of the model neuron affected both the overall spike rates and the half-peak positions of the AM-tuning curve, whereas the width of the coincidence window merely influenced the output spike rates. The duration of the refractory period affected only the low-frequency portion of the monaural AM-tuning curve. Unlike monaural AM coding, temporal factors, such as the coincidence window and the effective duration of inhibition, played a major role in determining the trough positions of simulated binaural phase-response curves. In addition, empirically-observed level-dependence of binaural phase-coding was reproduced in the framework of our minimalistic coincidence counting model. These modeling results suggest that coincidence detection of excitatory and inhibitory synaptic inputs is essential for LSO neurons to encode both monaural and binaural AM sounds.

Laboratory rats do not use binaural time cues to localize sound

The use of binaural time and intensity cues to localize sound can be investigated by determining the ability of a subject to localize pure tones in a free-field. Specifically, the ability to localize low-frequency tones, which do not produce an intensity difference at the two ears, demonstrates the use of the binaural phase (time) cue whereas the ability to localize high-frequency tones, to which the auditory system cannot phase lock, demonstrates the use of the binaural intensity-difference cue. Because previous studies of the laboratory rat (Rattus norvegicus) disagreed on the highest frequency that could be localized using binaural phase difference, the ability of rats to localize pure tones was reexamined. The results indicated that, contrary to previous studies, laboratory rats are not able to localize low-frequency tones even when they are amplitude modulated. Thus, it appears that laboratory rats are unable to use the binaural time-difference cue to localize sound. Because the previous studies were conducted before the widespread availability of spectrum analyzers, it is possible that their results were due to the presence of high-frequency harmonics in their low-frequency tones. These results have relevance for the anatomical and physiological study of binaural processing in laboratory rats. a)Presently at Disney’s Animal Kingdom, Lake Buena Vista, FL.

Neuromagnetic evaluation of binaural unmasking

Binaural unmasking refers to the improvement in intelligibility under conditions of masking when a tone is presented out of phase rather than in phase. In the present study, binaural unmasking was evaluated using auditory-evoked magnetoencephalography (MEG) in eight healthy right-handed volunteers (7 males and 1 female, mean age 25.9 years). Peak latency and amplitude of the N1m response to tone bursts of 250 Hz ( n = 8), 1000 Hz ( n = 3), and 4000 Hz ( n = 3) were measured under S 0N 0 (binaural phase difference was zero radian (in phase) for both stimulus sound and masker noise) and S piN 0 (binaural phase difference was pi radian (out of phase) for stimulus sound and zero radian for masker noise) conditions. The level of tone bursts was swept by 5 or 10 dB steps from the level of 20 dB above the psychophysical threshold under the S 0N 0 condition until no significant auditory-evoked field could be observed. Identical background noise was presented to both ears continuously at 50 dB SPL. N1m responses to stimuli at or above the psychophysical threshold were found bilaterally in all subjects except one who had only right hemispheric N1m. N1m response for the S piN 0 stimulus had larger amplitude and shorter latency than that for the S 0N 0 stimulus in each hemisphere and at each sound level. Neuromagnetic binaural unmasking was greatest around the threshold level, corresponding to psychophysical binaural unmasking; became smaller with greater stimuli, indicating the suprathreshold unmasking effect; and disappeared at around 15–20 dB above the threshold. Psychophysical binaural unmasking can be quantitatively evaluated by MEG in the auditory cortex level of the bilateral hemispheres.

Dichotic sound interference products, sound localization cues, and wave interaction at a neural level

The possibility of sound interference products arising from neural wave interaction is reexamined through five dichotic experiments. It is argued that interference percepts do not arise unless sound waves interact physically. Experiment 1 examines the possibility of beating for diotic sine stimuli. In this case, diplacusis would introduce beating if neural wave interaction could result in interference products. No beating was observed. Experiments 2a, 2b, and 2c examine the effect of dichotic phase changes on sine stimuli in terms of loudness, direction, and stereo-image-width. It is shown that such phase changes lead to perceptions incompatible with the interference principle and consistent with claims of binaural phase differences as sound-localization cues. Experiment 3 uses two-component dichotic stimuli. The results indicate that, when two sines with slightly different frequencies are presented dichotically, the constantly shifting phase between them affects a constantly shifting localization of the two-component complex stimulus. Depending on the frequency difference, this shift may be interpreted as a perceived rotation of the sound and/or a timbral fluctuation, often confused with loudness fluctuation. The findings do not support the claim of sound interference products arising from neural wave interaction. They are consistent with sound localization studies and indicate that the dichotic beats reported previously must have been a misidentified rotating sensation.

Sound localization in an Old-World fruit bat (Rousettus aegyptiacus): acuity, use of binaural cues, and relationship to vision.

The passive sound-localization acuity of Egyptian fruit bats (Rousettus aegyptiacus) was determined using a conditioned-avoidance procedure. The mean minimum audible angle for left-right discrimination for 3 bats was 11.6 degrees--very near the mean for terrestrial mammals. The bats also were able to localize low- and high-frequency pure tones, indicating that they can use both binaural phase-difference and binaural intensity-difference cues to localize sound. Moreover, they were able to use the binaural phase-difference cue up to at least 5.6 kHz, which is higher than other mammals yet tested. The width of the Egyptian fruit bats' field of best vision was 27 degrees. This value is consistent with the hypothesis that the role of passive sound localization is to direct the eyes for visual scrutiny of sound sources. Thus, the passive localization abilities of these echolocating megachiropteran fruit bats do not deviate from the patterns established for nonecholocating mammals.

Localization of noise, use of binaural cues, and a description of the superior olivary complex in the smallest carnivore, the least weasel (Mustela nivalis).

Cats and dogs have relatively good sound-localization acuity, and the question arises as to whether this trait is a characteristic of all carnivores or whether it is due to the fact that they have large heads and correspondingly large binaural localization cues available to them. The localization acuity of the least weasel, the smallest extant carnivore, was found to be less accurate than larger carnivores but more accurate than other small mammals. This suggests that carnivores may be under strong selective pressure to localize accurately but that interaural distance may be a limiting factor. The least weasel is capable of using both binaural phase differences and intensity differences to localize, but has a relatively broad mid-frequency range for which neither cue is optimal. Finally, the superior olivary complex of the least weasel is well developed and resembles that of larger carnivores more than that of small rodents.

Binaural phase differences and binaural auditory adaptation.

In order to determine the relationship between binaural phase localization and auditory adaptation, subjects were asked to perform a series of phase-based localization tasks, as well as an SDLB adaptation task. A binaurally presented 400 Hz tone was used for midplane localization tasks, and adaptation balances. The tone was presented monaurally during adaptation after binaural localization and the SDLB pre-test. Following 7 min exposure, subjects readjusted phase to centre the binaural stimulus in the midline, and then adjusted intensity to determine dB of adaptation. No consistent or statistically significant change in phase settings occurred. There was a significant decline in the balance intensity as a function of adaptation. A further re-establishment of the midline by phase manipulation also showed no localization effect. For the stimulus values used in the present study, there was no inter-effect of binaural phase localization, and SDLB auditory adaptation.

Brain-Stem Neuronal Coding of Binaural and Monaural Tones

Within the auditory medulla, there exists a class of neurons that, when stimulated by binaural tones, demonstrates a large latency shift. The range of this shift beyond the monaural latency varies from several milliseconds to 60 msec, depending upon the intensity of sound in the ear opposite the driving ear. Also, units in this part of the brain are responsive to changes in the binaural phase difference of low-frequency sinusoids. These units are inhibited by in-phase binaural tones; however, as the phase angle is increased in small steps to 180°, this inhibition is decreased. Beyond 180°, where now the opposite ear leads in phase, inhibition is effective again, and at 330° the response grouping of the unit is identical to its response grouping at 30°. Monitoring of the cochlear-microphonic responses at the two ears throughout these phase studies shows that no peripheral cancellations or distortions are taking place. Evidence for introfiber volleying is shown by medullary units. These units discharge, during stimulation by monaural tones, with interspike intervals that are equal to and multiples of the period of the stimulating tone up to approximately 2.0 kc/sec. This intra-fiber volleying is maintained down to the unit's threshold at each of the effective frequencies.

Binaural Phase Difference and Pitch Variation

Binaural Phase Difference and Pitch Variation

Experiments in Binaural Phase Difference Effect with Pure Tones

Experiments in Binaural Phase Difference Effect with Pure Tones