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The aim of this study was to investigate spatial masking of noise signals in the delayed motion paradigm. Spatial effects were created by interaural level differences (ILDs). Stationary maskers were located laterally or near the head midline, while test signals moved at different velocities from the head midline towards the ears, or in the opposite direction. The masking effect was measured by shifts in the perceived azimuthal positions of the starting and final points of signal trajectories, compared to their positions in silence. The perceived trajectories of all test signals shifted in the opposite direction from the masker. The masking effect was most pronounced in the spatial regions closest to the maskers and was stronger when the signal moved towards the masker, compared to moving away from it. The final points were perceptually shifted further than the starting points. Signal velocity and masker presentation side (left or right) did not change the degree of masking. [ABSTRACT FROM AUTHOR]

The interhemispheric asymmetry of electrical brain activity was investigated in the conditions of spatial auditory masking. Moving test signals were presented either in silence or against the background of stationary maskers of various spatial positions. The spatial properties of the stimuli were defined by interaural level differences (ILDs). Onset-energy responses (ON-responses), motion-onset responses (MORs), and OFF-responses were analyzed. To compute the topograms and to analyze asymmetry, the amplitudes of each component were averaged over the symmetric electrode clusters in the left and right hemispheres. The ON-responses showed a contralateral dominance of the N1 component in silence, and the degree of contralateral bias increased in masking conditions. Interhemispheric asymmetry of the P2 component was absent in silence. However, the P2 amplitude was higher in the right hemisphere in all combinations of masker and signal. The asymmetry of both deflections was maximal when the masker and the initial portion of the signal were separated by 180 deg. On the contrary, the interhemispheric asymmetry of the ON-response was found only in silence: the cN1 deflection was biased to the side contralateral to the signal. The topography of the OFF-response was symmetrical under all experimental conditions. [ABSTRACT FROM AUTHOR]

Current study is focused on human event-related potentials (ERPs) elicited in the auditory spatial masking paradigm. ERPs of the test subjects (n = 18) were recorded in the passive conditions using uncorrelated low-frequency noise signals. Spatial positions of the stimuli were determined by interaural level differences (ILDs). Test signal and masker were colocated or separated by 90 or 180 deg of azimuth and were presented simultaneously. The onset of a test signal was delayed from masker onset in order to separate the onset responses. We estimated the ERPs dependence on angular distance between signal and masker. When the test signal was presented against the background of a masker, the N1 and P2 components decreased in magnitude and increased in latency, as compared to presentation in silence. Spatial release from masking resulted in ERP magnitude increase and latency decrease as the angular distance between test signal and masker changed from 0 to 90 deg. A further spatial separation of the lateralized signal and masker did not result in further release from masking. ERPs magnitude was a more reliable indicator of the angular separation of signals than its latency. These findings suggest that binaural unmasking mechanisms operate at the preconscious stage of complex auditory scene analysis. [ABSTRACT FROM AUTHOR]

The characteristics of a specific event-related potential elicited by sound motion onset (motion-onset response, MOR) were estimated. Moving sound stimuli with delayed motion onset were presented to right-handed subjects in dichotic conditions. Sound motion was produced by linear changes of interaural time differences. The amplitude and latency of cN1 and cP2 components of evoked potentials were measured in a wide range of velocities. The latency of cN1 increased linearly with the increase in time of 1° azimuthal shift of a stimulus, whereas the cP2 latency remained nearly constant. The time interval of motion integration was about 140–180 ms. Thus, the latency of cN1 deflection may be considered as an electrophysiological correlate of integrative neuronal processes activated by sound motion presented as a separate auditory event. [ABSTRACT FROM AUTHOR]

We studied the effects of the acoustic context on active and passive discrimination of moving sound signals. Different contexts were created by reversing the role of standard and deviant stimuli in the oddball blocks, while their acoustical features were kept the same. Three types of sounds were used as standard or deviant stimuli in different blocks: stationary midline noises and two (smooth and abrupt) moving sounds moving to the left or right of the midline. Auditory event-related potentials (ERPs) were recorded during passive listening (the sound stimulation ignored), and mismatch negativity potentials (MMNs) were obtained. Active discrimination of sound movements was measured by the hit rate (percent correct responses) and false alarm rate, as well as the reaction time. The influence of the stimulus context on active and passive discrimination of the moving sound stimuli was reflected in the phenomenon known as the effect of deviance direction. The hit rate and MMN amplitude were higher when the deviant moved faster than the standard. The MMN amplitude was more responsive to the velocity of sound stimuli than the hit rate and false alarm rate. The psychophysical measurements in the reversed contexts suggest that smooth and abrupt sound movements may belong to the same perceptual category (moving sounds), while the stationary stimuli form another perceptual category. [ABSTRACT FROM AUTHOR]

During localization of a moving sound source, a shift of the perceived position relative to the actual one of the starting point is an expression of the perception of sluggishness of the auditory system. In this study, the human ability to localize starting points during a gradual or abrupt movement of fused auditory images (FAIs) was compared with the ability to localize the position of a stationary sound image. Sound images moved from the midline of the head in the direction of each of the ears. The subject’s responses were recorded using a graphics table. There was a tendency to shift the starting point of the trajectory in the direction of the movement. This tendency was stronger for gradual rather than for abrupt FAI movement and for shorter stimuli (100 ms) than for long ones (200 ms). The value of the starting point’s displacement depended on the final interaural time delay. The results obtained are discussed in terms of the “snapshots” and “movement detector” theories, as well as in terms of the sluggish and anticipatory ability of auditory perception. [ABSTRACT FROM AUTHOR]

The study focused on the left–right asymmetry in discriminating moving sound stimuli in dichotic conditions. Healthy adult right-handed participants discriminated the left and right directions of sound motion. A sound stimulus consisted of two parts, a stationary sound at the head midline was followed by a moving sound of a variable duration, which shifted leftward or rightward from the midline. Seven velocities of motion (from 80 to 480 deg/s) were produced by linear changes of interaural time differences (ITDs). The shift was varied from well distinguishable to vanishing values, and the duration of the moving part was decreased in proportion. Hit rates and mean reaction times were measured separately in each of the conditions. As the task complexity increased due to decreasing sound shifts in shorter time, the percentage of rightward responses became higher than that of leftward responses. However, the total amount of leftward responses contained a larger portion of hits. This asymmetry in performance showed no correlation with the consistency of hand preference. [ABSTRACT FROM AUTHOR]

Processing of acoustic space in the human auditory system implicates the interaction of the left and right hemispheres of the brain. In this study, we investigated the influence of the position and motion direction of sound stimuli on the amplitude and latency of N1, P2, MMN, and P3a components of auditory evoked potentials in the left and right brain hemispheres. The data suggest that functional asymmetry of the N1P2 complex and difference waveforms can be described as weak contralateral dominance. Deviants moving from the lateral positions evoked later P3a components than those moving from the midline, which is consistent with the concept of higher localization accuracy in the frontal part of the acoustic space. The motion direction affected the MMN and P3a latency: the centrifugal deviants produced earlier MMN and P3a responses than the centripetal deviants. This could result from a stronger orienting reaction caused by stimuli moving from the center towards the periphery as compared with the opposite direction. [ABSTRACT FROM AUTHOR]

The review presents the current understanding of interhemispheric asymmetry of the human brain activity, recorded by electrophysiological and hemodynamic methods using various sound features. Along with the anatomical differences which may exist between the auditory cortices of the left and right brain hemispheres, the authors consider how lateralization of auditory responses depends on the parameters of sound stimulation and what patterns of interhemispheric asymmetry can be found in auditory evoked potentials (AEP) elicited during spatial processing of sound stimuli. [ABSTRACT FROM AUTHOR]

The spatial resolution of the human auditory system was studied under conditions, where the location of the sound source was changed according to different temporal patterns of interaural time delay. Two experimental procedures were run in the same group of subjects: a psychophysical procedure (the transformed staircase method) and an electrophysiological one (which requires recording of mismatch negativity, the auditory evoked response component). It was established that (1) the value of the mismatch negativity reflected the degree of spatial deviation of the sound source; (2) the mismatch negativity was elicited even at minimum (20μs) interaural time delays under both temporal patterns (abrupt azimuth change and gradual sound movement at different velocities); (3) an abrupt change of the sound source azimuth resulted in a greater mismatch negativity than gradual sound movement did if the interaural time delay exceeded 40 μs; (4) the discrimination threshold values of the interaural delay obtained in the psychophysical procedure were greater than the minimum interaural delays that elicited mismatch negativity, with the exception of the expert listeners, who exhibited no significant difference. [ABSTRACT FROM AUTHOR]