Lateralization of brain responses to auditory motion: A study using single-trial analysis.

The present study investigates hemispheric asymmetry of the ERPs and low-frequency oscillatory responses evoked in both hemispheres of the brain by the sound stimuli with delayed onset of motion. EEG was recorded for three patterns of sound motion produced by changes in interaural time differences. Event-related spectral perturbation (ERSP) and inter-trial phase coherence (ITC) were computed from the time-frequency decomposition of EEG signals. The participants either read books of their choice (passive listening) or indicated the sound trajectories perceived using a graphic tablet (active listening). Our goal was to find out whether the lateralization of the motion-onset response (MOR) and oscillatory responses to sound motion were more consistent with the right-hemispheric dominance, contralateral or neglect model of interhemispheric asymmetry. Apparent dominance of the right hemisphere was found only in the ERSP responses. Stronger contralaterality of the left hemisphere corresponding to the "neglect model" of asymmetry was shown by the MOR components and by the phase coherence of the delta-alpha oscillations. Velocity and attention did not change consistently the interhemispheric asymmetry of both the MOR and the oscillatory responses. Our findings demonstrate how the lateralization pattern shown by the MOR potential was interrelated with that of the motion-related single-trial measures.

Hemispheric asymmetry of ERPs and MMNs evoked by slow, fast and abrupt auditory motion.

The current MMN study investigates whether brain lateralization during automatic discrimination of sound stimuli moving at different velocities is consistent with one of the three models of asymmetry: the right-hemispheric dominance model, the contralateral dominance model, or the neglect model. Auditory event-related potentials (ERPs) were recorded for three patterns of sound motion produced by linear or abrupt changes of interaural time differences. The slow motion (450deg/s) was used as standard, and the fast motion (620deg/s) and the abrupt sound shift served as deviants in the oddball blocks. All stimuli had the same onset/offset spatial positions. We compared the effects of the recording side (left, right) and of the direction of sound displacement (ipsi- or contralateral with reference to the side of recording) on the ERPs and mismatch negativity (MMN). Our results indicated different patterns of asymmetry for the ERPs and MMN responses. The ERPs showed a velocity-independent right-hemispheric dominance that emerged at the descending limb of N1 wave (at around 120-160ms) and could be related to overall context of the preattentive spatial perception. The MMNs elicited in the left hemisphere (at around 230-270ms) exhibited a contralateral dominance, whereas the right-hemispheric MMNs were insensitive to the direction of sound displacement. These differences in contralaterality between MMN responses produced by the left and the right hemisphere favour the neglect model of the preattentive motion processing indexed by MMN.

[Motion-Onset Responses During Active and Passive Listening to the Moving Sound Stimuli].

This study investigated the energy-onset and motion-onset responses (N1, P2, cN 1 and cP2 components of the auditory evoked potential) elicited by moving sound stimuli in the passive and active listening conditions. In the passive conditions the subjects were distracted from auditory information; in active conditions they lo- calized the starting and final points of the stimulus trajectory. The sound movement to the left/right from the head midline was produced by linear-changes of the interaural time delay (ITD). The onset of motion was preceded by stationary sound located near the head midline. In the active conditions, the NI component was higher and the P2 component was higher and peaked later as compared to the passive listening. The early and later parts of the motion-onset response (cN 1 and cP2) also were larger in magnitude and peaked later during active listening. Both in active and passive conditions, cNI and cP2 amplitude exhibited increase and latency showed decrease when the stimulus velocity increased. Contralateral asymmetry was found only in the mo- tion-onset responses recorded from the left hemisphere.

Contextual effects on preattentive processing of sound motion as revealed by spatial MMN.

The magnitude of spatial distance between sound stimuli is critically important for their preattentive discrimination, yet the effect of stimulus context on auditory motion processing is not clear. This study investigated the effects of acoustical change and stimulus context on preattentive spatial change detection. Auditory event-related potentials (ERPs) were recorded for stationary midline noises and two patterns of sound motion produced by linear or abrupt changes of interaural time differences. Each of the three types of stimuli was used as standard or deviant in different blocks. Context effects on mismatch negativity (MMN) elicited by stationary and moving sound stimuli were investigated by reversing the role of standard and deviant stimuli, while the acoustical stimulus parameters were kept the same. That is, MMN amplitudes were calculated by subtracting ERPs to identical stimuli presented as standard in one block and deviant in another block. In contrast, effects of acoustical change on MMN amplitudes were calculated by subtracting ERPs of standards and deviants presented within the same block. Preattentive discrimination of moving and stationary sounds indexed by MMN was strongly dependent on the stimulus context. Higher MMNs were produced in oddball configurations where deviance represented increments of the sound velocity, as compared to configurations with velocity decrements. The effect of standard-deviant reversal was more pronounced with the abrupt sound displacement than with gradual sound motion.

[Topography of the Event-Related Brain Responses during Discrimination of Auditory Motion in Humans].

The present study investigates the hemispheric asymmetry of auditory event-related potentials (ERPs) and mismatch negativity (MMN) during passive discrimination of the moving sound stimuli presented according to the oddball paradigm. The sound movement to the left/right from the head midline was produced by linear changes of the interaural time delay (ITD). It was found that the right-hemispheric N1 and P2 responses were more prominent than the left-hemispheric ones, especially in the fronto-lateral region. On the contrary, N250 and MMN responses demonstrated contralateral dominance in the fronto-lateral and fronto-medial regions. Direction of sound motion had no significant effect on the ERP or MMN topography. The right-hemispheric asymmetry of N1 increased with sound velocity. Maximal asymmetry of P2 was obtained with short stimulus trajectories. The contralateral bias of N250 and MMN increased with the spatial difference between standard and deviant stimuli. The results showed different type of hemispheric asymmetry for the early and late ERP components which could reflect the activity of distinct neural populations involved in the sensory and cognitive processing of the auditory input.

[Objective and subjective indexes of auditory motion processing].

The study focused at the objective and subjective indexes of human hearing system sensitivity towards different types of moving sound stimuli. The experiment employed two methods: electrophysiological (MMN recording) and psychophysical method (two-alternative forced choice). Two types of spatial sound stimuli simulated gradual and abrupt sound motion from the head midline. MMN as an objective index of spatial discrimination has been obtained in response to the subthreshold and the suprathreshold stimuli. An increase of trajectory length of the moving stimuli resulted in an increase of the MMN amplitude and of subjective discrimination as well, although their correlation remained below the significance level. The results obtained are discussed from the point of view of preconscious perception of auditory spatial information.

Discrimination of auditory motion patterns: the mismatch negativity study.

The aim of the present study is to test whether mismatch negativity (MMN) response can be elicited by changes in auditory motion dynamics. The discrimination of auditory motion patterns was investigated using psychophysical and electrophysiological methods in the same group of subjects. Auditory event-related potentials (ERP) were recorded for stationary midline noises and moving noises shifting to the left/right from the head midline. Two patterns of auditory motion were used with gradual (Motion) and stepwise (Step) movements which started and ended at the same loci. Auditory motion was produced by linear and abrupt changes of interaural time differences (ITD) in binaurally presented stimuli. In Experiment 1, ERPs were recorded for stationary midline standards and for Motion and Step deviants. It was found that Step deviants result in larger MMN amplitudes than Motion deviants with the same distance travelled, which implies that information contained in the stimulus midportion could be involved in the processing of the auditory motion. The threshold ITD values for the detection of Step and Motion stimuli displacement obtained during psychoacoustic tests were greater than the minimal ITD changes which elicited significant MMN. Experiment 2 demonstrated that Step deviants elicited significant MMNs in the context of Motion standards, although these stimuli could not be discriminated behaviourally. MMNs elicited by Step deviants in different acoustic contexts are discussed from the viewpoint of different brain processes underlying the discrimination of the abrupt ITD change. These results suggest that the early cortical mechanism of auditory motion processing reflected by MMN could not be considered as a spatial discriminator of the onset/offset stimulus positions, that is, a simple onset-offset detector. Combining psychoacoustic data with MMN results we may conclude that motion discrimination in auditory system might be better at the preattentive level.

[Event-related potentials of human brain in spatial hearing].

The review presents the data concerning auditory event-related potentials and their "mismatch negativity" component under conditions of stationary and moving sound source localization. Both free-field and dichotic experimental conditions are considered. The interhemispheric asymmetry of the brain responses elicited by the sound sources of various spatial properties is also discussed.

How does mismatch negativity reflect auditory motion?

Recent studies have shown that the mismatch negativity (MMN), a change-specific component of the auditory event-related potential (ERP), is accurately tracking the spatial location of the stationary sound source. The aim of the present study was to estimate the parameters of MMNs evoked by auditory motion and to compare the motion discrimination measured by MMN in normally hearing subjects with the psychophysical data obtained in the same group of subjects. The auditory motion was simulated by introducing variable interaural time differences (ITDs) into the deviant stimuli. The ERPs were recorded for frequently occurring stationary midline standards and for infrequent deviant sounds moving horizontally at different velocities. It was established that all the deviant stimuli elicited significant MMNs. The MMN increased monotonically in amplitude with growing angular distances travelled by the deviant stimuli. The deviants that travelled over the same angular distances at different velocities caused MMNs that agreed in magnitude but differed in latency. These results indicated that the angular distance rather than sound image velocity was the most essential cue involved in the MMN generation. To test the psychophysical performance, a two-interval forced-choice task was employed, in which the ITD was the main dependent variable. The deviants that evoked significant MMNs at the minimal ITDs were not discriminated behaviorally, indicating that the motion discrimination of the hearing system may be better at a preattentive level.

[Mismatch negativity in the auditory event-related potentials in response to abrupt and smooth displacements of virtual sound source].

The work investigated event-related potentials, mismatch negativity (MMN), and P3a component under dichotic stimulation with deviant stimuli simulating abrupt or smooth displacement of auditory images to the left or to the right from the head midline by means of interaural time delay introduced into the deviant stimuli. Repetitive standard stimuli were localized near the head midline. All deviant stimuli elicited mismatch negativity and P3a component. It was shown the MMN for smooth deviant motion was lower than that for the abrupt deviant displacement. MMN amplitude for both deviant types obviously depended on interaural time delay, which confirms that MMN might be considered as a measure of the auditory system spatial discriminative ability. The P3a component demonstrated the same amplitude dependences as the MMN. The results obtained are discussed in respect to manifestation of the processes underlying the auditory motion detection in the event-related potentials.

Effects of the azimuthal position of stationary and moving sound images on the mismatch negativity phenomenon.

This report presents results obtained from studies of the phenomenon of mismatch negativity in conditions of dichotic stimulation with presentation of deviant stimuli modeling movement of a sound image towards or away from a standard stimulus and on presentation of stationary deviants located at an angle of 90 degrees to the standard. Standard stimuli were located close to the left or right ear or in the midline of the head. All deviant stimuli induced mismatch negativity. Movement of the deviant stimulus from the standard was found to induce mismatch negativity with the longest latency and smallest amplitude for all azimuthal positions of the standard stimulus. In addition, it was only in this direction of movement that there was a relationship between measures of mismatch negativity and the azimuth of the standard. It was suggested that the process of the recognition of differences between interaural delay times is significantly dependent on the nature of changes in this parameter at the moment at which the deviant stimulus is presented.

Changes in evoked potentials during the action of sound signals with different localizing characteristics.

This report presents results of studies of the phenomenon of mismatch negativity (MMN) during exposure to four blocks of sound stimuli each containing identical standards creating an immobile sound image located along the midline of the head and one of a set of deviants, creating a sound image located either by the left ear or moving from the midline of the head towards the left ear or in the opposite direction. All deviants induced mismatch negativity; the minimal amplitude and longest latent period were seen in the mismatch negativity produced by the deviant modeling movement of the sound image from the midline of the head to the left ear. The question of the appearance of mismatch negativity as a criterion for the accurate discrimination of signals with different localizing characteristics is discussed.

Comparison of binaural release from forward masking in animals and humans. Electrophysiological studies.

Evoked potentials in the inferior colliculus and auditory areas of the cortex were studied in anesthetized guinea pigs and long-latency auditory evoked potentials (LAEP) were studied in waking humans using sequential binaural presentation of pairs of clicks--the masker and the masked signal--with a variable interval between them, to provide the conditions needed for the psychophysical phenomenon of direct forward masking. Introduction of phase differences between the masker and the masked signal led to decreases in suppression of responses to the masked signal and to faster recovery of the reaction types recorded. The greatest relative differences between response magnitudes to antiphase and synphase masked signals were seen at the beginning of the recovery process, and were 1.6, 1.5, and 1.4 respectively for responses from the inferior colliculus, auditory area of the cortex, and LAEP at stimulus intensities of 50-65 dB sound pressure level, differences subsequently decreasing to zero. There was a positive correlation between this measure and the stimulus intensity. The greatest differences between the time at which the recovery process ended for responses to antiphase and synphase masked signals were 4, 250, and about 2000 msec respectively for the inferior colliculus, auditory area of the cortex, and LAEP.