Time Difference Cues Research Articles

How the owl resolves auditory coding ambiguity.

The barn owl (Tyto alba) uses interaural time difference (ITD) cues to localize sounds in the horizontal plane. Low-order binaural auditory neurons with sharp frequency tuning act as narrow-band coincidence detectors; such neurons respond equally well to sounds with a particular ITD and its phase equivalents and are said to be phase ambiguous. Higher-order neurons with broad frequency tuning are unambiguously selective for single ITDs in response to broad-band sounds and show little or no response to phase equivalents. Selectivity for single ITDs is thought to arise from the convergence of parallel, narrow-band frequency channels that originate in the cochlea. ITD tuning to variable bandwidth stimuli was measured in higher-order neurons of the owl's inferior colliculus to examine the rules that govern the relationship between frequency channel convergence and the resolution of phase ambiguity. Ambiguity decreased as stimulus bandwidth increased, reaching a minimum at 2-3 kHz. Two independent mechanisms appear to contribute to the elimination of ambiguity: one suppressive and one facilitative. The integration of information carried by parallel, distributed processing channels is a common theme of sensory processing that spans both modality and species boundaries. The principles underlying the resolution of phase ambiguity and frequency channel convergence in the owl may have implications for other sensory systems, such as electrolocation in electric fish and the computation of binocular disparity in the avian and mammalian visual systems.

Passive sound-localization ability of the big brown bat ( Eptesicus fuscus)

The passive sound-localization ability (i.e. minimum audible angle) of the big brown bat, Eptesicus fuscus, was determined using a conditioned avoidance procedure in which the animals were trained to discriminate left sounds from right sounds. The mean threshold of three bats for a 100-ms broadband noise burst was 14°, a value that is about average for mammals. A similar threshold of 15° was obtained for one animal when it was retested with one of its own recorded echolocation calls as the stimulus. The two bats tested on pure-tone localization were able to localize high-frequency, but not low-frequency tones, even when a low-frequency tone was amplitude modulated, a result indicating that these bats are not able to use binaural time-difference cues for localization. Finally, given the width of the bat's field of best vision, as determined by a count of its ganglion-cell density, its sound-localization acuity is consistent with the hypothesis that the role of passive sound localization is to direct the eyes to the source of a sound.

Building ears for robots: Sound localization and separation

This paper describes our research on bio-mimetic robot audition. Among the many binaural and monaural sound localization cues in the human auditory system, the interaural time difference cue is selected as it can easily be obtained by omnidirectional microphones. We have used a three-microphone system to remove the anterior-posterior ambiguity which occurs in two-microphone (or ear) systems. The echo-avoidance model of the precedence effect is used to cope with the echoes and reverberations of real environments. We mimicked the cocktail party effect by perceptual grouping of continuous components according to the spatial information obtained by the sound localization method. A wheel-based mobile robot equipped with an auditory system was developed. The auditory system has two sound processing parts. One is a DSP-based realtime system; the other is an off-line system composed of remote computers. Experiments of localizing and separating multiple sound sources and robot navigation were conducted to demonstrate the system's ability and potential applications.

Structural and functional differences distinguish principal from nonprincipal cells in the guinea pig MSO slice.

1. Principal cells in the medial superior olive (MSO) receive low-frequency information from both ears via left and right cochlear nuclei. In vivo extracellular records suggest that some MSO neurons respond optimally only when the binaural acoustic signal has a precise interaural delay. Thus MSO cells, in particular principal cells, are thought to be the first stage in the processing of interaural time difference cues that provides information as to the location of a low-frequency sound in space. 2. Despite this proposed fundamental role for the MSO, certain features of this nucleus make in vivo recordings from any cell type here very difficult to obtain. Only a small number of extracellular records and no intracellular recordings are reported in the literature. Using sharp, neurobiotin-filled glass electrodes to record intracellularly from cells in an in vitro brain slice of the guinea pig superior olivary complex, I have begun to assess the anatomic and physiological features of cells in the MSO that might be relevant to such a functional role in vivo. 3. Two basic MSO cell types, designated principal and nonprincipal, could be distinguished on the basis of certain anatomic and physiological differences. 4. Labeled principal cell bodies were located at all dorsoventral location within the MSO. Labeled nonprincipal cells were located in or around the dorsal aspects of the nucleus. Principal cells typically had thick bipolar dendrites (1 directed medially, 1 laterally) that did not taper or branch significantly except at their terminations. Nonprincipal cells were multipolar with three to nine thinner primary dendrites that did not branch preferentially in a mediolateral direction. Principal cell axons gave off collaterals terminating in and around the dorsal MSO. Nonprincipal cells also had axon in and around the dorsal MSO. Nonprincipal cells also had axon collateral branches innervating dorsal MSO, but these axons could branch more extensively and project further down the dorsoventral aspect of the nucleus. 5. Principal cells typically responded to depolarizing current pulses with one or a few spikes at current onset. When bathed in saline containing 4-aminopyridine (4-AP), they fired repetitively to the same depolarizing current pulses. This would indicate a depolarization-induced nonlinearity similar to that seen in principal cell types of two other auditory brain stem nuclei, the anteroventral cochlear nucleus and medial nucleus of the trapezoid body. Nonprincipal cells normally fired repetitively to depolarizing current pulses even close to spike threshold. Both cell types could show a sag in the membrane potential to hyperpolarizing current pulses.(ABSTRACT TRUNCATED AT 400 WORDS)

The dominant role of low-frequency interaural time differences in sound localization.

Two experiments are described in which listeners judge the apparent directions of virtual sound sources-headphone-presented sounds that are processed in order to simulate free-field sounds. Previous results suggest that when the cues to sound direction are preserved by the simulation, the apparent directions of virtual sources are nearly the same as the apparent directions of real free-field sources. In the experiments reported here, the interaural phase relations in the processing algorithms are manipulated in order to produce stimuli in which the interaural time difference cues signal one direction and interaural intensity and pinna cues signal another direction. The apparent directions of these conflicting cue stimuli almost always follow the interaural time cue, as long as the wideband stimuli include low frequencies. With low frequencies removed from the stimuli, the dominance of interaural time difference disappears, and apparent direction is determined primarily by interaural intensity difference and pinna cues.

Binaural hearing after middle ear surgery. Masking-level difference for interaural time and amplitude cues.

Previous investigations have shown that binaural hearing is often abnormal in patients having conductive hearing losses, and even in patients who have had hearing thresholds corrected by middle ear surgery. The present study assessed further the masking-level difference (MLD) in patients who had previously undergone middle ear surgery. The stimuli were chosen such that the sensitivity to interaural time difference cues and to interaural amplitude difference cues could be assessed independently. Results indicated that the MLD is abnormal in more than a third of 15 patients, even after surgery has restored normal hearing at the frequency of the test. The MLD is more likely to be reduced for the cue of interaural time difference than for interaural amplitude difference. Subjects who had lesions of long duration, early onset, and large asymmetry were particularly likely to have reduced MLDs.

Localization of tones by horses: Use of binaural cues and the role of the superior olivary complex.

The ability of horses to use binaural time and intensity difference cues to localize sound was assessed in free-field localization tests by using pure tones. The animals were required to discriminate the locus of a single tone pip ranging in frequency from 250 Hz to 25 kHz emitted by loudspeakers located 30 degrees to the left and right of the animals' midline (60 degrees total separation). Three animals were tested with a two-choice procedure; 2 additional animals were tested with a conditioned avoidance procedure. All 5 animals were able to localize 250 Hz, 500 Hz, and 1 kHz but were completely unable to localize 2 kHz and above. Because the frequency of ambiguity for the binaural phase cue delta phi for horses in this test was calculated to be 1.5 kHz, these results indicate that horses can use binaural time differences in the form of delta phi but are unable to use binaural intensity differences. This finding was supported by an unconditioned orientation test involving 4 additional horses, which showed that horses correctly orient to a 500-Hz tone pip but not to an 8-kHz tone pip. Analysis of the superior olivary complex, the brain stem nucleus at which binaural interactions first take place, reveals that the lateral superior olive (LSO) is relatively small in the horse and lacks the laminar arrangement of bipolar cells characteristic of the LSO of most mammals that can use binaural delta I.

Underwater localization of pulsed pure tones by the California sea lion(Zalophus californianus).

The results of a study to determine the pulsed pure-tone sound-localization capability of a California sea lion, Zalophus californianus, in the horizontal plane are presented. The minimum audible angle (MAA) was determined as a function of frequency from 0.5 to 16 kHz in 1-octave steps. The results suggested that the animal utilized time-difference cues for the lower frequencies (0.5 to 1.0 kHz) and intensity-difference cues for the higher frequencies (4.0 to 16.0 kHz). The MAAs for the lower frequencies were smaller than for the higher frequencies, and for the transitional frequencies (2.0 to 4.0 kHz) the animal experienced extreme difficulties in localizing the sound. To further investigate the effects of sound intensity difference in the localization process, an experiment using a sea lion skull with hydrophones attached close to each bulla was conducted. Intensity differences were measured with the skull directed at different azimuths in relatioship to a sound source. The results of the measurements using the skull considered together with the behavioral data support the contention that the sea lion used both time and intensity cues for underwater localization. Subject Classification: 65.62; 80.50.

Underwater sound localization of pulsed pure tones by the California sea lion

The results of a study to determine the pulsed pure-tone sound localization capability of a California Sea Lion, Zalophus californianus in the horizontal plane are presented. The minimum audible angle (MAA) was determined as a function of frequency from 0.5 to 16 kHz in one-octave steps. The results suggested that the animal utilized time difference cues for the lower frequencies (0.5–1.0 kHz) and intensity difference cues for the higher frequencies (4.0–16.0 kHz). The MAAs for the lower frequencies were smaller than for the higher frequencies, and for the transitional frequencies (2.0–4.0 kHz) the animal experienced extreme difficulties in localizing the sound. To further investigate the effects of sound intensity difference in the localization process, an experiment using a sea lion skull with hydrophones attached close to each bulla was conducted. Intensity differences were measured with the skull directed at different azimuths in relationship to a sound source. The results of the measurements using the skull considered together with the behavioral data support the contention that the sea lion used both time and intensity cues for underwater localization.