Function Of Interaural Time Differences Research Articles

Neural maps of interaural time and intensity differences in the optic tectum of the barn owl

This report describes the binaural basis of the auditory space map in the optic tectum of the barn owl (Tyto alba). Single units were recorded extracellularly in ketamine-anesthetized birds. Unit tuning for interaural differences in timing and intensity of wideband noise was measured using digitally synthesized sound presented through earphones. Spatial receptive fields of the same units were measured with a free field sound source. Auditory units in the optic tectum are sharply tuned for both the azimuth and the elevation of a free field sound source. To determine the binaural cues that could be responsible for this spatial tuning, we measured in the ear canals the amplitude and phase spectra produced by a free field noise source and calculated from these measurements the interaural differences in time and intensity associated with each of 178 locations throughout the frontal hemisphere. For all frequencies, interaural time differences (ITDs) varied systematically and most strongly with source azimuth. The pattern of variation of interaural intensity differences (IIDs) depended on frequency. For low frequencies (below 4 kHz) IID varied primarily with source azimuth, whereas for high frequencies (above 5 kHz) IID varied primarily with source elevation. Tectal units were tuned for interaural differences in both time and intensity of dichotic stimuli. Changing either parameter away from the best value for the unit decreased the unit's response. The tuning of units to either parameter was sharp: the width of ITD tuning curves, measured at 50% of the maximum response with IID held constant (50% tuning width), ranged from 18 to 82 microsecs. The 50% tuning widths of IID tuning curves, measured with ITD held constant, ranged from 8 to 37 dB. For most units, tuning for ITD was largely independent of IID, and vice versa. A few units exhibited systematic shifts of the best ITD with changes in IID (or shifts of the best IID with changes in ITD); for these units, a change in the value of one parameter to favor one ear shifted the best value of the other parameter in favor of the same ear, i.e., in the direction opposite to that expected from "time-intensity trading." Overall sound intensity had little or no effect on ITD tuning, but did increase the best IIDs of units tuned to nonzero IIDs. The tuning of units for ITD and IID changed systematically along different dimensions of the optic tectum to create coextensive, independent neurophysiological maps of ITD and IID.(ABSTRACT TRUNCATED AT 400 WORDS)

Responses of low-frequency cells in the inferior colliculus to interaural time differences of clicks: excitatory and inhibitory components.

1. We studied extracellular responses of low-frequency cells in the central nucleus of the inferior colliculus (ICC) to interaural time differences (ITDs) of clicks and compared their responses to ITDs of noise and tones. Most cells that displayed sensitivity to ITDs of clicks responded cyclically as a function of ITD with central peaks and troughs at the same ITDs as in response to noise. The positions of these peaks and troughs also matched those predicted from tonal ITD curves. Thus over the range of physiologically relevant ITDs, the binaural cells in the ICC showed similar sensitivity to ITDs of tones, noise, and clicks. 2. The transient nature of the response to a click allowed association of individual discharges with either the ipsilateral or contralateral stimulus when the binaural stimulus included a large ITD. We studied the influence of the click presented to one side on responses to the click presented to the other side. By examining responses to clicks with large ITDs, ranging from 2 to 3 up to 200 ms, we could identify both excitatory and inhibitory components in response to binaural clicks. 3. For many cells, there was evidence for a short-lasting excitation arising from one or both inputs of the binaural stimulus. Inhibitory interactions could also be demonstrated over a large range of ITDs. Long-lasting, late inhibitory components arose from both contralateral and ipsilateral inputs. In 87% of cells that were driven by the contralateral input, a late inhibitory component originating from the ipsilateral side was detected. In all cells that were driven by the ipsilateral side, a late inhibitory contralateral component was detected. This late inhibition of the excitatory response to one side by a leading stimulus to the other side could be evoked even when the leading stimulus was not effective in evoking an excitatory response. 4. Some cells also exhibited an early inhibitory component that preceded the excitation. An early contralateral inhibition was detected in 44% of cells that were driven by the ipsilateral input, whereas an early ipsilateral component was detected in 17% of cells driven by the contralateral input. 5. We confirmed hypotheses about the laterality and time course of the inhibitory and excitatory components by introducing interaural level differences (ILDs) into the binaural clicks and thus varying the strengths of the different components. 6. Inhibitory components may play a role in shaping the sensitivity of individual cells to ITDs of stimuli other than clicks; they were also apparent in responses to noise.(ABSTRACT TRUNCATED AT 400 WORDS)

Click lateralization by multiple sclerosis patients

Lateralization of dichotic clicks can be tested with gradually increasing interaural level differences (ILD) and interaural time differences (ITD). However, the lateralization of dichotic clicks with ITD is limited to small ITDS only (< 1 ms). The ability to lateralize dichotic clicks was tested in multiple sclerosis (MS) patients with normal audiograms. Two kinds of psychoacoustical experiments were employed: (1) a matching experiment in which the subject was asked to match the perceived positions of two click trains, where one included dichotic clicks with ILD and the other included dichotic clicks with ITD; (2) a positional jnd experiment in which the subject was asked to determine the difference in position of two successive click trains. Two reference positions were tested, the head center (ITD = 0 and ILD = 0) and midway between the center and the ear (ITD = 0.8 ms and ILD = 0 or ILD = 20 dB and ITD = 0). For each reference the experiment was performed first with control on ITD and then with control on ILD. From a group of 15 MS patients, seven performed poorly in the positional jnd experiment when the control was on ITD, and normally when the control was on ILD. Those subjects also reported that the position of each dichotic click with ITD < 1 ms was perceived at the head center, and, therefore, they had difficulties in performing the matching experiment. Brainstem auditory potentials (BAP) evoked by dichotic clicks with different ILDs and ITDs were measured in all the above MS patients. Principal component analysis of the measured waveforms indicates an abnormal behavior as a function of ITD in patients who performed abnormally in the psychoacoustical experiments. The anaylsis of the waveforms of all patients as a function of ILD resembles the normal behavior.

Axonal delay lines for time measurement in the owl's brainstem.

Interaural time difference is an important cue for sound localization. In the barn owl (Tyto alba) neuronal sensitivity to this disparity originates in the brainstem nucleus laminaris. Afferents from the ipsilateral and contralateral magnocellular cochlear nuclei enter the nucleus laminaris through its dorsal and ventral surfaces, respectively, and interdigitate in the nucleus. Intracellular recordings from these afferents show orderly changes in conduction delay with depth in the nucleus. These changes are comparable to the range of interaural time differences available to the owl. Thus, these afferent axons act as delay lines and provide anatomical and physiological bases for a neuronal map of interaural time differences in the nucleus laminaris.

Effects of interaural time delays of noise stimuli on low-frequency cells in the cat's inferior colliculus. III. Evidence for cross-correlation.

1. We tested the coincidence, or cross-correlation, model of Jeffress, which proposes a neuronal mechanism for sensitivity to interaural time differences (ITDs) in low-frequency cells in the central nucleus of the inferior colliculus (ICC) of the cat. Different tokens of Gaussian noise stimuli were delivered to the two ears. We studied the neural responses to changes in ITDs of these stimuli and examined the manner in which the binaural cells responded to them. All of our results support the idea that the central binaural neurons perform an operation very similar to cross-correlation on the inputs arriving from each side. These inputs are transformed from the actual acoustic signal by the peripheral auditory system, and these transformations are reflected in the properties of the cross-correlations. 2. The responses to ITDs of identical broadband noise stimuli to the two ears varies cyclically as a function of ITD at a frequency close to the best frequency of the neuron. This cyclic response is a consequence of the narrowband filtering of the wideband acoustic signal by the auditory nerve fibers. To examine the effects of using stimuli to the two ears that were correlated to each other to different degrees, we generated pairs of noises. Each pair consisted of one standard noise, which was delivered to one ear, and a linear sum of two standard uncorrelated noises, which was delivered to the other ear. The responses of 34 neurons in the ICC to ITDs of noises with variable interaural coherence were examined. When partially correlated noises were delivered, there was a positive and approximately linear relationship between the degree of modulation of the response as a function of ITD and interaural coherence. The degree of modulation was measured by the synchronization coefficient, or vector strength, over one period of the ITD curve. 3. We examined the effects of altering the interaural phase relationships of the input noise stimuli. The phase of the noise stimuli was changed by digitally filtering the standard noise so that only a phase delay was imposed. The responses to ITDs with differing interaural phase relationships were then studied by delivering a phase-shifted noise to one ear and the standard noise to the other. The ITD curves in response to phase-shifted noise were shifted by about the same amount as the shift of the stimulus; the shift of the response was measured with respect to the case with identical noises to the two ears.(ABSTRACT TRUNCATED AT 400 WORDS)

Interaural time differences: implications regarding the neurophysiology of sound localization.

Interaural time differences (ITDs) were measured from 400--7000 Hz on cats in order to provide quantitative data for use in physiological/behavioral studies on sound localization. ITDs derived from clicks and the initial portion of tone bursts showed a pronounced roughness and frequency dependence. This frequency dependence is most evident at higher angles of incidence and indicates that a single ITD will not always represent a single position on the azimuth. Controls demonstrate that most of the roughness in these functions was due to reflections off the surface supporting the animal and that the measured ITDs corresponded to predictions made by steady-state theory. Measurements made with and without the pinnae in position indicate that they have relatively little effect on these ITD functions, particularly for frequencies below 2500 Hz and for small angles of incidence. In spite of acoustic limitations exemplified by the roughness and frequency dependence of these functions, ITDs generated by sound sources situated close to the midline provide reliable localization cues that are much better than those derived from sources well out on the azimuth. Finally, it is noted that another ITD, the group ITD, can be ascribed to an acoustic signal. Calculations based on the measured steady-state ITDs show differences between the group and steady-state ITDs over a given range of frequencies. Differences between the group and steady-state ITD can be significant, and it is argued that: (1) The group ITD can provide a localization cue to the auditory system that is distinct from the steady-state ITD; and (2) it is possible these group ITDs are used by the nervous system to localize sound sources in realistic situations.

Just Noticeable Differences for Changes of Interaural Time Differences as a Function of Interaural Time Differences

The just noticeable differences for changes of interaural time differences between two bursts of noise were investigated as a function of the interaural time difference of the first (reference) burst. Four normal hearing subjects listened through earphones to time differences between ears which were varied by electrical means. The noise went through a low-pass filter set at 1300 cps and had an intensity of 70 db SPL. The bursts were 0.85 second in duration. Interaural time differences for the reference bursts were between 0 and 600 μsec, with just noticeable differences increasing from 12 to 21 μsec, respectively, in an exponentially increasing manner. These just noticeable differences correspond to 75% correct judgements in the symmetrical two-choice tests. Group and individual thresholds and their distributions are presented. [This paper represents a report of a Master's thesis submitted to Western Reserve University, Cleveland, Ohio (January, 1958). The author is indebted to Professor Earl D. Schubert, Coordinator of Research of the Cleveland Hearing and Speech Center, for generous guidance.]