Values Of Interaural Time Difference Research Articles

Electrophysiological characteristic of the cerebral cortex of the gerbil

The effect of interaural time difference (ITD) and interaural level difference (ILD) on wave 4 of the binaural and summed monaural auditory brainstem responses (ABRs) as well as on the DN1 component of the binaural interaction component (BIC) of the ABR in young and old Mongolian gerbils (Meriones unguiculatus) was investigated. Measurements were made at a fixed sound pressure level (SPL) and a fixed level above visually detected ABR threshold to compensate for individual hearing threshold differences. In both stimulation modes (fixed SPL and fixed level above visually detected ABR threshold) an effect of ITD on the latency and the amplitude of wave 4 as well as of the BIC was observed. With increasing absolute ITD values BIC latencies were increased and amplitudes were decreased. ILD had a much smaller effect on these measures. Old animals showed a reduced amplitude of the DN1 component. This difference was due to a smaller wave 4 in the summed monaural ABRs of old animals compared to young animals whereas wave 4 in the binaural-evoked ABR showed no age-related difference. In old animals the small amplitude of the DN1 component was correlated with small binaural-evoked wave 1 and wave 3 amplitudes. This suggests that the reduced peripheral input affects central binaural processing which is reflected in the BIC.

Effects of Reference Interaural Time and Intensity Differences on Binaural Performance in Listeners with Normal and Impaired Hearing

The purpose of this research was to measure the effects of reference interaural time and intensity differences on binaural performance in listeners with normal hearing and impaired hearing for a number of different binaural tests. Experiment 1 measures the dependence of binaural detection and discrimination performance on reference interaural intensity differences (IID) in the range of +/- 12 dB for listeners with normal hearing. Experiment 2 extends these measures to include reference IIDs and interaural time differences (ITD) for two groups of listeners with normal hearing with offsets in the range of +/- 12 dB and +/- 300 microseconds (group 1) and +/-24 dB and +/- 600 microseconds (group 2). Experiment 3 includes the same tests and conditions as experiment 2 for listeners with various hearing impairments. A set of psychophysical measurements was completed on 11 listeners with sensorineural hearing losses and 9 listeners with clinically normal hearing. The primary measurements were a set of four binaural detection and interaural discrimination thresholds measured for two 1/3-octave bands of Gaussian noise, one centered at 500 Hz and the other at 4000 Hz. Specifically, we measured binaural (antiphasic) detection thresholds for tones centered in the masking noise as well as the just-noticeable differences (JNDs) in IID, ITD, and interaural cross-correlation (ICC) for each of the noise-band stimuli. All measurements were done for a number of combinations of reference IID and ITD. In addition to these primary measurements, several other measurements were made on each subject, including monaural absolute thresholds, monaural intensity discrimination, monaural masked thresholds, and intensity levels required for interaurally balanced loudness and for a centered image. All measurements were made using a relatively quick, adaptive procedure. For the subjects with normal hearing, measured dependencies of the IID and ITD JNDs using noise stimuli on reference ITD and IID are different from those previously reported for tonal stimuli. Binaural performance of the listeners with impaired hearing varies widely across subjects and tests and is generally poorer than that of listeners with normal hearing. On the basis of the results for subjects with hearing impairments, we have reached several conclusions. First, the results for the binaural measurements cannot be explained in terms of available monaural audiometric and psychophysical measurements on these subjects. Second, the subjects' binaural abilities show no significant improvement with any combinations in the reference values of ITD and IID, providing negative evidence for the hypothesis that degraded performance for some subjects may be due to internal interaural offsets in ITD or IID. Third, the hypothesis that binaural detection and ICC discrimination are related, suggested by Durlach et al (1986), is generally supported. Fourth, binaural detection performance is not simply explained in terms of sensitivities to ITD and IID.

Binaural tuning of auditory units in the forebrain archistriatal gaze fields of the barn owl: local organization but no space map

We identified a region in the archistriatum of the barn owl forebrain that contains neurons sensitive to auditory stimuli. Nearly all of these neurons are tuned for binaural localization cues. The archistriatum is known to be the primary source of motor-related output from the avian forebrain and, in barn owls, contributes to the control of gaze, much like the frontal eye fields in monkeys. The auditory region is located in the medial portion of the archistriatum, at the level of the anterior commissure, and is within the region of the archistriatum from which head saccades can be elicited by electrical microstimulation (see preceding companion article, Knudsen et al., 1995). Free-field measurements revealed that auditory sites have large, spatial receptive fields. However, within these large receptive fields, responses are tuned sharply for sound source location. Dichotic measurements showed that auditory sites are tuned broadly for frequency and that the majority are tuned to particular values of interaural time differences and interaural level differences, the principal cues used by barn owls for sound localization. The tuning of sites to these binaural cues is essentially independent of sound level. The auditory properties of units in the medial archistriatum are similar to those of units in the optic tectum, a structure that also contributes to gaze control. Unlike the optic tectum, however, the auditory region of the archistriatum does not contain a single, continuous auditory map of space. Instead, it is organized into dorsoventral clusters of sites with similar binaural (spatial) tuning. The different representations of auditory space in closely related structures in the forebrain (archistriatum) and midbrain (optic tectum) probably reflect the fact that the forebrain contributes to a wide variety of sensorimotor tasks more complicated than gaze control.

Dynamics of visually guided auditory plasticity in the optic tectum of the barn owl

1. In the optic tectum of normal barn owls, bimodal (auditory-visual) neurons are tuned to the values of interaural time difference (ITD) that are produced by sounds at the locations of their visual receptive fields (VRFs). The auditory tuning of tectal neurons is actively guided by visual experience during development: in the tectum of adult owls reared with an optically displaced visual field, neurons are tuned to abnormal values of ITD that are close to the values produced by sounds at the locations of their optically displaced VRFs. In this study we investigated the dynamics of this experience-dependent plasticity. 2. Owls were raised from shortly after eye-opening (14-22 days of age) with prismatic spectacles that displaced the visual field to the right or left. Starting at approximately 60 days of age, multiunit recordings were made to assess the tuning of tectal neurons to ITD presented via earphones. In the earliest recording sessions (ages 60-80 days), ITD tuning was often close to normal, even though the majority of the owls' previous experience was with an altered correspondence between ITD values and VRF locations. Subsequently, over a period of weeks, responses to the normal range of ITDs were gradually eliminated while responses to values of ITD corresponding with the optically displaced VRF were acquired. 3. At intermediate stages in this process, the ITD tuning at many sites became abnormally broad, so that responses were simultaneously present to both normal values of ITD and to values corresponding with the optically displaced VRF. At this stage the latencies and durations of newly acquired responses systematically exceeded the latencies and durations of the responses to normal values of ITD. 4. Dynamic changes in ITD tuning similar to those recorded in the optic tectum also occurred in the external nucleus of the inferior colliculus (ICX), which provides the major source of ascending auditory input to the tectum. 5. These results suggest the hypothesis that the neural selectivity for ITD in the barn owl's tectum is first established by vision-independent mechanisms and only subsequently calibrated by visual experience. This calibration involves both the elimination of responses to normal values of ITD and the visually guided acquisition of responses to novel values and can be accounted for by plasticity at the level of the ICX.

Experience-dependent plasticity in the inferior colliculus: a site for visual calibration of the neural representation of auditory space in the barn owl

The optic tectum (homolog of the superior colliculus) contains mutually aligned neural maps of auditory and visual space. During development, the organization of the auditory map is guided by spatial information provided by vision: barn owls raised wearing prismatic spectacles, which optically shift the visual field and the visual map in the optic tectum, develop an auditory map that is shifted by an approximately equivalent amount, such that alignment between the two maps is preserved (Knudsen and Brainard, 1991). In this study we investigated whether this shift in the auditory map is intrinsic to the optic tectum or whether it reflects plasticity at an earlier stage in the auditory pathway. Owls were raised wearing prismatic spectacles that displaced the visual field by 23 degrees to the left or right. This manipulation alters the normal correspondence between locations in the visual field and interaural time difference (ITD), the primary cue for the azimuth of a sound source. In normal owls and in owls with at least 150 d of prism experience, extracellular unit recordings were used to assess the representations of ITD at anatomically and physiologically defined sites in the optic tectum and in the two prior stages of the auditory pathway, the external and central nuclei of the inferior colliculus (ICx and ICc). In the optic tectum of normal owls, the values of ITD to which units responded most strongly (best ITDs) varied systematically with the azimuths of unit visual receptive fields (VRFs). In the prism-reared owls, best ITDs were shifted from normal toward the values of ITD produced by sounds at the locations of the units' optically displaced VRFs. In the ICx of prism-reared owls, the representation of ITD also was shifted from normal, by an amount and in a direction that could completely account for the shift in ITD measured in the optic tectum. At some sites in the ICx, the shift in ITD tuning was apparent within the first 7-8 msec of the response; shifted tuning at such short latencies argues that the altered representation of ITD in the ICx reflects plasticity in the ascending auditory pathway, and is not the result of descending activity from higher auditory centers. In the ICc, which immediately precedes the ICx in the ascending pathway, the representation of ITD was normal. The results indicate that the visual instruction of auditory spatial tuning of neurons in the optic tectum reflects plasticity at the level of the ICx, the site where the auditory map of space is first synthesized.

Adaptive adjustment of unit tuning to sound localization cues in response to monaural occlusion in developing owl optic tectum

Bimodal units in the barn owl's optic tectum are tuned to the location of auditory and visual stimuli, and are systematically organized according to their spatial tuning to form mutually aligned maps of auditory and visual space. Map alignment results from the fact that, normally, units are tuned to the values of interaural level difference (ILD) and interaural time difference (ITD) produced by a sound source at the location of their visual receptive fields (VRFs). Monaural occlusion alters the correspondence of ILD and ITD values with locations in space. We investigated the effect that raising owls with a chronic monaural occlusion has on the tuning of tectal units to ILD and ITD. Owls were monaurally occluded beginning at 1 month of age. The effects of monaural occlusion were assessed 2-4 months later by comparing the ILD and ITD tuning of units in monaurally occluded owls with the ILD and ITD tuning of units with equivalent VRFs in normal owls. ILD and ITD tuning was shifted substantially and in the direction of the unoccluded ear (the adaptive direction) in owls raised with a monaural occlusion. In most tecta, the mapped representations of ILD and ITD were shifted systematically. In addition, in some tecta, monaural occlusion induced a change in the topography of the ILD map such that ILD tuning remained essentially constant at values near 0 dB over abnormally large portions of the tectum. Across all recording sites, the average shift in ILD tuning was 9 dB (n = 396) and the average shift in ITD tuning was 40 microseconds (n = 414). In four of five animals, the magnitude of the effect was not equivalent on the two sides of the brain, the adjustments being significantly larger and more systematic on the side ipsilateral to the occlusion. Such differences in the altered ILD and ITD maps on the two sides of the brain in individual animals indicate that, although a component of the adaptive adjustment might be due to regulation of the gain and phase response of the monaural signals early in the auditory pathway, a major component of the adjustment must occur at or beyond the level where the encoding of ILDs and ITDs for left and right space separates.

Time-intensity trades revisited.

Two experiments were accomplished which involved time-intensity trades on a lateralization task. In the first experiment, subjects adjusted interaural time differences between binaural stimuli to compensate for interaural intensity disparities. In the second, the same listeners adjusted the interaural intensity relationship to offset interaural time differences. Three pure tones (250, 500, and 1000 Hz) were employed as binaural stimuli. The two experiments yielded substantially different time-intensity trading ratios for large values of interaural time differences and intensity disparities. This finding supplies evidence of a new type that the time-intensity trading ratio does not accurately reflect the central auditory processes that are involved with interaural time and intensity on a lateralization task.