Sluggishness of auditory perception during localization of short moving sound images

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.     

A mismatch negativity elicited with stationary and moving auditory images of different azimuth positions

Characteristics of mismatch negativity elicited by dichotic stimulation were examined using deviant stimuli simulating movement of fused auditory images towards the standard stimuli or in the reverse direction. The effect of stationary deviants localized at 90 degrees in respect to standards was also measured. The standard stimuli were localized near either of ears or along the head midline. The spatial locations were produced by introducing interaural time differences into the click trains. All deviant stimuli evoked the mismatch negativity. The deviants moving from standards seem to evoke the lowest mismatch negativity with the longest latency at all azimuthal locations of standard stimuli. Besides, the deviant shift from standards proved to be the only direction at which the characteristics of mismatch negativity depended upon the standard's azimuth. It is seems that the discrimination of interaural time delay is essentially dependent on the pattern of interaural delay changes at the moment when the deviant occurs.     

Changes of evoked potentials caused by sound signals with different localization characteristics

Characteristics of the mismatch negativity (MMN) were studied by presenting the subjects with four blocks of stimuli containing standard series of clicks (90%) simulating a stationery sound image located in the head midline, and one of three different deviant series of clicks (10%) simulating either a stationary sound image located near the left ear or a moving sound image which shifted from the head midline to the left ear or in the opposite direction. All the deviant stimuli elicited the MMN with the minimal peak amplitude and the greatest latency evoked by the deviant series of clicks simulating the sound image moving from the head midline to the left ear. These findings suggest that the MMN may be considered as a pre-perceptual physiological measure of the discrimination accuracy for the sound signals with various spatial locations.     

Human Assessment of Signals Modeling Different Directions of the Motion of Sound Images

The characteristics of a subjective sound field formed under conditions of dichotic stimulation were studied in healthy subjects (six females and seven males) with normal auditory sensitivity upon movement of a sound image (SI) in different directions. The character and the trajectory values of the emerging subjective sound image (SSI) were determined depending on the direction of its motion and the initial interaural delay (700, 400, and 200 s). Certain differences in the assessment of the parameters of moving sound images between the groups of male and female subjects were revealed. In female subjects, the averaged trajectory values in the right and left hemispheres were the same when the SSI moved in both directions and shortened uniformly with a decrease in the initial interaural delay. With a 700-s delay, the trajectory values of the male subjects for all directions of motion of the SSI were the same as those of female subjects. With initial 400- and 200-s delays, the trajectory values were significantly greater in the group of male subjects if a SI moved from the right or left ear to the median line of the head. With the method used, no interhemispheric asymmetry was revealed in the process of lateralization of moving sound images, which, under certain conditions, may be of importance for increasing the accuracy of localization of sound sources in the environment.     

Perceptual categories for auditory motion as reflected in mismatch negativity

Auditory evoked response and mismatch negativity potential have been studied using the reversed oddball paradigm of standard and deviant stimulus presentation. In the experiments, three types of spatial sound stimuli (stationary and moving gradually or abruptly from the head midline) were presented in three configurations. In each configuration, one stimulus type served as the standard one, and the other two, as deviants. The reversal of the configuration of the presentation of the standard and deviant stimuli was shown to significantly influence the evoked response and mismatch negativity. The results are discussed as possible evidence of the categorical perception of auditory motion at early stages of sound processing in the hearing system.     

Estimation by humans of signals simulating different sound movement directions and specificity of the perception of these signals by patients with temporal epilepsy

The perception of moving sound stimuli that imitate directional sound source movement was studied in healthy subjects and in patients with temporal lobe lesions, as well as in a group of patients with simultaneous lesions of the temporal cortex and hippocampus. Under the conditions of dichotic stimulation of patients with the rightor left-side foci of convulsive activity, the nature and length of the trajectories of the emerging subjective sound images (SSI) were estimated depending on the direction of movement and interaural time difference (700, 400, 200 μs). The audiograms of all patients did not differ from those of healthy subjects, suggesting that the auditory sensitivity of patients remained unimpaired. However, in the patients, the trajectories were shorter than the trajectories in healthy subjects at all the values of the initial time delay and at all the directions of SSI movements. In patients with the cortical temporal epilepsy, changes of the subjective sound field were the most significant in the case of the right-side localization of foci of the convulsive activity. In patients with simultaneous lesions of the temporal cortex and hippocampus, the averaged trajectories of SSI movement differed significantly from those in the group of healthy subjects (p < 0.01) and in patients with a relatively isolated lesion of the temporal cortex (p < 0.05); these trajectories were independent of the initial delay. The mediobasal structures of the temporal lobe that are involved in the epileptic process proved to play a significant role in the perception and estimation of the moving sound stimuli, although they do not belong to the auditory system proper. The possible mechanisms underlying disorders in patients with temporal epilepsy are discussed.     

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.     

Avian interaural canal enhances interaural delay

Summary Using a simple model of the birds' binaural pressure difference acoustic receivers it was predicted that the interaural delay achieved by birds at low frequencies is far greater than that of mammals with a similar head-size. This was upheld when interaural delay was recorded, between the cochlear microphonics, for six species. Stimulus positions were varied over the azimuthal range from the frontal midline to the interaural axis. Predicted delays were frequency dependent (higher frequency, smaller delay), as were the actual delays, and the magnitude of the measured delays were comparable with predictions. Delays measured at high frequencies were close to those expected from pathlength around the head, but delays measured at low frequencies could be more than three times this expectation. This finding raises the possibility that interaural delay may be a useful localization cue in birds, even for those species with very small heads, since the large delays at low frequencies are sufficient to provide a physiological cue to azimuth.     

Directionality of sound perception in the external ear of the dog

Sound pressure level of tone was measured using a probe tube microphone at entrance to the dog's external meatus as a function of the azimuth of the sound source. It was demonstrated that directionality of the dog's external ear and corresponding values of interaural intensity differences (delta I) were gradually increased as the tone frequency raised from 0.5 to 40 kHz. Transfer in pinnae locations from lateral to frontal positions (one of the components of orientation reaction to an unexpected sound) resulted in some narrowing of directionality diagrams and in a displacement of their maxima towards the head midline. It was calculated that owing to this effects the extent of monotonic part of the function relating delta I and azimuth of a source were enlarged. The lateral pinnae position was suggested to be optimal for sound detection and the frontal one for localization of the moving sound source.     

Evoked potentials of the cat inferior colliculus to acoustic stimuli simulating sound source movement with different velocities in opposite directions

Amplitude changes of inferior colliculus evoked potentials (EPs) in anaesthetized adult cats were studied under presentation of acoustic stimuli simulating both azimuth-moving and stationary sound source. The movement was simulated with gradual changes of interaural time delay between binaurally presented click trains. It was shown that the amplitude of EPs elicited by "moving" signals depended on the velocity of movement. Amplitude differences between EPs to "moving" and stationary stimuli were observed under motion velocities up to 320 deg./s. The greatest response amplitudes in different experiments took place under velocities within the range of 67-320 deg./s with most of them recorded under velocities of 170 and 125 deg./s. Amplitude of the responses to lateral-medial movement with any velocity were always greater than those to opposite direction of movement with the same velocity.     

Moving sound source discrimination in humans: Mismatch negativity and psychophysics

The ability to discriminate moving sounds sources with different dynamic properties was studied in humans. Mismatch negativity was studied in an experiment on dichotic stimulation, with deviant stimuli simulating the instantaneous movement of the auditory image to the right or left of the head midline in the horizontal plane. Standard stimuli simulated continuous movement of the sound source to the right or to the left to the same angular distances. It was also established that both deviant stimuli caused mismatch negativity, its parameters being independent on the direction of sound movement. Psychophysical testing of the same group of subjects showed that discrimination between the stimuli was below the psychophysical threshold. The results obtained are discussed from the point of view of current theories of moving sound localization. The correlation between the objective and subjective levels of discrimination of moving auditory images are discussed.     

Effects of Acoustic Context on the Perceptual Differences between Spatial Sound Stimuli

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.     

Discrimination of the dynamic properties of sound source spatial location in humans: Electrophysiology and psychophysics

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.     

Specific features of lateralization of a stationary auditory image by a human under condition of stimulation without interaural differences: a neurophysiological analysis

Analysis of the lateralization phenomena of a fused auditory image (FAI) was performed on the basis of the previously developed model of the binaural directional hearing. It was found earlier that, under conditions of auditory stimulation without interaural differences, the FAI was localized at the head midline only in about a quarter of subjects. In a greater part of the listeners, the FAI was lateralized within the range of -4.6 ... +11.2 degrees from the midline. It was shown that FAI localization with reference to the head midline may be determined by the extent of asymmetry and spatial contrast between the "active" neural zones in the left and right halves of the subjective auditory space. In turn, the asymmetry (or its absence) of these "active" zones fully depends on a distribution of neurons by characteristic time delays in the left and right halves of the subjective auditory field. The model also explains the fact of a decrease in localization precision with the FAI position just at the midline.     

AEPs at the onset and offset of repetitive sound modulation,due to mismatch with the contents of an auditory sensory store

Long latency auditory evoked potentials (AEPs), chiefly consisting of a negative peak at about 150 msec and a positivity at 250 msec, were recorded at the beginning and end of periods during which the interaural time difference of binaural noise was switched between 0.0 and 0.8 msec at a fast rate (ISI = 50 or 25 msec) or the frequency of continuous binaural clicks was switched between 167 and 200 Hz every 80, 50 or 25 msec. In the latter case the offset responses occurred later than onset by a mean of 89, 47 and 27 msec respectively, suggesting they were probably generated at the moment the next switch was expected but failed to occur.The offset responses must be non-specific with respect to the interaural delay or the frequency of clicks, since neurones which respond to particular delays or frequencies and are made refractory by a rapid rate of stimulation should not suddenly become less so at the last in a series of identical stimuli, or be activated by the absence of a further event. It is proposed that the potentials are due to a higher order of neurone which automatically responds to the occurrence of a “mismatch” between the immediate sound and an image of that which was previously present, encoded in a short-term sensory store. In addition to frequency content and interaural delay, the image must contain information about the temporal modulation pattern of the sound over the previous few seconds.     

Coding interaural time differences at low best frequencies in the barn owl.

In birds and mammals, precisely timed spikes encode the timing of acoustic stimuli, and interaural acoustic disparities propagate to binaural processing centers. The Jeffress model proposes that these projections act as delay lines to innervate an array of coincidence detectors, every element of which has a different relative delay between its ipsilateral and contralateral excitatory inputs. Thus, interaural time difference (ITD) is encoded into the position of the coincidence detector whose delay lines best cancel out the acoustic ITD. Neurons of the avian nucleus laminaris and mammalian MSO phase-lock to both monaural and binaural stimuli but respond maximally when phase-locked spikes from each side arrive simultaneously, i.e. when the difference in the conduction delays compensates for the ITD. McAlpine et al. [Nat. Neurosci. 4 (2001) 396] identified an apparent difference between avian and mammalian ITD coding. In the barn owl, the maximum firing rate appears to encode ITD. This may not be the case for the guinea pig, where the steepest region of the function relating discharge rate to interaural time delay (ITD) is close to midline for all neurons, irrespective of best frequency (BF). These data suggest that low BF ITD sensitivity in the guinea pig is mediated by detection of a change in slope of the ITD function, and not by maximum rate. We review coding of low best frequency ITDs in barn owls and mammals and discuss whether there may be differences in the code used to signal ITD in mammals and birds.     

Mismatch Negativity during Exposure to Acoustic Stimuli Simulating Fused Auditory Images

The authors consider the results of study of the phenomenon of mismatch negativity (MMN) during exposure to acoustic stimuli simulating fused auditory images with different spatial localization: along the head midline (a standard stimulus used in all series), near either of the ears (lateralized), and moving from the midline to or from an ear. All deviant stimuli evoked the mismatch negativity; the minimum MMN amplitude with the longest latency was observed when the stimulus simulated motion of the auditory image from the midline to either ear. When the deviant auditory images were localized on the left of the midline, the contralateral MMN dominance was more pronounced and responses to various deviant stimuli differed more than when the images were localized on the right. The mismatch negativity as a criterion of discrimination accuracy for signals with different localization features is discussed.     

Subjective Acoustic Field of Patients with Cortical Temporal Lobe Epilepsy as Revealed Using Signals Simulating Various Directions of Sound Movement

Perception of signals simulating directional movement of a sound source was studied in two groups of patients with cortical temporal lobe epilepsy and epileptic activity foci in the right or left temporal area of the cortex. On dichotic stimulation, the character and length of the trajectories of subjective auditory images (SAIs) were determined as dependent on the direction of SAI movement and the initial interaural delay (700, 400, and 200 s). For any delay or direction examined, SAI trajectories were shorter in the patients of both groups than in healthy subjects. Regardless of the side of an epileptic focus, the shortest trajectories were detected in the hemisphere where SAI movement ended, especially at an interaural delay of 200 s. The narrowest subjective acoustic field was observed in patients with epileptic foci in the right temporal cortex. Possible mechanisms of the changes in spatial hearing are discussed. The changes in SAI perception are assumed to result from distorted binaural interactions, which manifest themselves in functional asymmetry of the two auditory centers and may be caused by a convulsive activity focus present in one temporal lobe.     

Responses of neurons in the primary auditory cortex of the cat to the auditory motion stimuli with variable interaural delay

Unit responses in the primary auditory cortex of anesthetized cats to stationary and apparently moving stimuli resulted from a static and dynamically varying interaural delay (ITD) were recorded. The static stimuli consisted of binaurally presented tones and clicks. The dynamic stimuli were produced by in-phase and out-of-phase binaurally presented click trains with time-varying ITD. Sensitivity to ITDs was mostly seen in responses of the neurons with low characteristic frequency (below 2.8 kHz). All cells sampled with static stimuli responded to simulated motion. A motion effect could take the form of a difference in response magnitude depending on the direction of stimulus motion and a shift in the ITD-function opposite the direction of motion. The magnitude of motion effects was influenced by the position of motion trajectory relative to the ITD-function. The greatest motion effect was produced by motion crossing the ITD-function slopes.     

The Opponent Channel Population Code of Sound Location Is an Efficient Representation of Natural Binaural Sounds

In mammalian auditory cortex, sound source position is represented by a population of broadly tuned neurons whose firing is modulated by sounds located at all positions surrounding the animal. Peaks of their tuning curves are concentrated at lateral position, while their slopes are steepest at the interaural midline, allowing for the maximum localization accuracy in that area. These experimental observations contradict initial assumptions that the auditory space is represented as a topographic cortical map. It has been suggested that a “panoramic” code has evolved to match specific demands of the sound localization task. This work provides evidence suggesting that properties of spatial auditory neurons identified experimentally follow from a general design principle- learning a sparse, efficient representation of natural stimuli. Natural binaural sounds were recorded and served as input to a hierarchical sparse-coding model. In the first layer, left and right ear sounds were separately encoded by a population of complex-valued basis functions which separated phase and amplitude. Both parameters are known to carry information relevant for spatial hearing. Monaural input converged in the second layer, which learned a joint representation of amplitude and interaural phase difference. Spatial selectivity of each second-layer unit was measured by exposing the model to natural sound sources recorded at different positions. Obtained tuning curves match well tuning characteristics of neurons in the mammalian auditory cortex. This study connects neuronal coding of the auditory space with natural stimulus statistics and generates new experimental predictions. Moreover, results presented here suggest that cortical regions with seemingly different functions may implement the same computational strategy-efficient coding.