The effect of a change in direction of resultant force on sound localization: the audiogravic illusion.
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Humans↗
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Human sound localization at near-threshold levels.
Physiological studies of spatial hearing show that the spatial receptive fields of cortical neurons typically are narrow at near-threshold levels, broadening at moderate levels. The apparent loss of neuronal spatial selectivity at increasing sound levels conflicts with the accurate performance of human subjects localizing at moderate sound levels. In the present study, human sound localization was evaluated across a wide range of sensation levels, extending down to the detection threshold. Listeners reported whether they heard each target sound and, if the target was audible, turned their heads to face the apparent source direction. Head orientation was tracked electromagnetically. At near-threshold levels, the lateral (left/right) components of responses were highly variable and slightly biased towards the midline, and front vertical components consistently exhibited a strong bias towards the horizontal plane. Stimulus levels were specified relative to the detection threshold for a front-positioned source, so low-level rear targets often were inaudible. As the sound level increased, first lateral and then vertical localization neared asymptotic levels. The improvement of localization over a range of increasing levels, in which neural spatial receptive fields presumably are broadening, indicates that sound localization does not depend on narrow spatial receptive fields of cortical neurons.
Adult↗
Effect of the temporal pattern of contralateral inhibition on sound localization cues.
We studied the temporal coding properties of identified interneurons in the auditory system of crickets, using information theory as an analytical tool. The ascending neuron 1 (AN1), which is tuned to the dominant carrier frequency (CF) of cricket songs, selectively codes the limited range of amplitude modulation (AM) frequencies that occur in these signals. AN2, which is most sensitive to the ultrasonic frequencies that occur in echolocation calls of insectivorous bats, codes a broader range of AM frequencies, as occur in bat calls. A third neuron, omega neuron 1 (ON1), which is dually tuned to both ranges of carrier frequency, was shown previously to have CF-specific coding properties, allowing it to represent accurately the differing temporal structures of both cricket songs and bat calls. ON1 is a source of contralateral inhibition to AN1 and AN2, enhancing binaural contrast and facilitating sound localization. We used dichotic stimulation to examine the importance of the temporal structure of contralateral inhibition for enhancing binaural contrast. Contralateral inhibition degrades the coding of temporal pattern by AN1 and AN2, but only if the temporal pattern of inhibitory input matches that of excitation. Firing rate is also decreased most strongly by temporally matched contralateral inhibition. This is apparent for AN1 in its mean firing rate; for AN2, high-frequency firing is selectively suppressed. Our results show that the CF-specific coding properties of ON1 allow this single neuron to enhance effectively localization cues for both cricket-like and bat-like acoustic signals.
Acoustic Stimulation↗
Pinna-based spectral cues for sound localization in cat.
The directional dependence of the transfer function from free field plane waves to a point near the tympanic membrane (TM) was measured in anesthetized domestic cats. A probe tube microphone was placed approximately 3 mm from the TM from beneath the head in order to keep the pinna intact. Transfer functions were computed as the ratio of the spectrum of a click recorded near the TM to the spectrum of the click in freefield. We analyze the transfer functions in three frequency ranges: low frequencies (less than 5 kHz) where interaural level differences vary smoothly with azimuth; midfrequencies (5-18 kHz) where a prominent spectral notch is observed; and high frequencies (greater than 18 kHz) where the transfer functions vary greatly with source location. Because no two source directions produce the same transfer function, the spectrum of a broadband sound at the TM could serve as a sound localization cue for both elevation and azimuth. In particular, we show that source direction is uniquely determined, for source directions in front of the cat, from the frequencies of the midfrequency spectral notches in the two ears. The validity of the transfer functions as measures of the acoustic input to the auditory system is considered in terms of models of sound propagation in the ear canal.
Acoustic Stimulation↗
The spatial attributes of stimulus frequency in the median sagittal plane and their role in sound localization.
Stimulus cues used to locate sounds in the median sagittal plane were investigated. Listeners first were required to report the location of noise bands 1.0 kHz wide whose center frequencies ranged from 4.0 through 14.0 kHz. It was found that the apparent locations of the stimuli depended on their frequency compositions, not their places of origin. As the center frequency was increased from 4.0 to about 12.0 kHz, the apparent location of the sound moved from in front of the subject to the rear. Then, at center frequencies of around 13.0 kHz, the sounds again appeared to come from the front. The authors infer that different frequency regions within the sound's spectrum possess specific spatial referents along the median sagittal plane. When the bandwidth was increased to 4.0 kHz, listeners could locate the stimuli with reasonable proficiency only for the sectors of the median sagittal plane that were represented by the spatial referents inherent in the frequency composition of the stimulus. Finally, a stimulus comprising all audible frequencies above 4.0 kHz could be located proficiently, and it is concluded that this sound, unlike the others, contained all spatial referents for the median sagittal plane, hence the superior performance on the localization task.
Acoustic Stimulation↗
A comparison of the contrast effects in sound localization in the horizontal and vertical planes.
The effect of a background sound on the auditory localization of a single sound source was examined. Nine loudspeakers were arranged crosswise in the horizontal and the median vertical plane. They ranged from -20 degrees to +20 degrees, with the center loudspeaker at 0 degree azimuth and elevation. Using vertical and horizontal centimeter scales, listeners verbally estimated the position of a 500-ms broadband noise stimulus being presented at the same time as a 2 s background sound, emitted by one of the four outer loudspeakers. When the background sound consisted of continuous broadband noise, listeners consistently shifted the apparent target positions away from the background sound locations. This auditory contrast effect, which is consistent with earlier findings, equally occurred in both planes. But when the background sound was changed to a pulse train of noise bursts, the contrast effect decreased in the horizontal plane and increased in the vertical plane. This discrepancy might be due to general differences in the processing of interaural and spectral localization information.
Adult↗
Sound localization in small birds: absolute localization in azimuth.
Nine small birds of 3 species (Melopsittacus undulatus, Serinus canarius, and Poephila guttata) were trained in an operant procedure to fly to sound sources for food reward. The angle between the 2 sound sources was varied on a session-by-session basis, and threshold (i.e., minimum resolvable angle) was taken as the angle that corresponded to a performance level of 75% correct. In all, thresholds were calculated for pure tones of 5 different frequencies, noise bands of 3 different spectral compositions, and species-specific contact or distance calls recorded from each of the 3 species. Thresholds for both simple and complex stimuli were larger than 25 degrees. There were statistically significant species differences for each stimulus set, but these differences were not correlated with species differences in head size. Birds with 1 ear plugged performed as well as binaural birds in this task. Birds deafened in 1 ear, however, performed at chance.
Animals↗
[Apparatus for determination of orientation of man to sounds; sound localization].
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Humans↗
Some further observations on the effects of unilateral cortical ablation on sound localization in the cat.
It has previously been shown that unilateral ablation of the whole auditory cortex in the cat disrupts the precedence effect, and also interferes with the ability of the normal animal to discriminate in the Y-maze between a single sound on one side and a double sound consisting of a signal on both left and right sides. The present work has confirmed these effects and has shown that both can be obtained with lesions confined to AI and AII. The "one-versus-two" deficit has invariably been seen in all the animals studied, but a proportion of animals do not show the precedence effect deficit. It has been confirmed that the apparent sucess of some animals can be due to the training effect of the one-versus-two paradigm, as was proposed in the earlier paper; however it has also been shown that this cannot be explanation in all cases. It has been demonstrated that cats are able to localize sound behind them with some success; turning around in the start box to reverse right and left space is therefore a possible strategy for overcoming a unilateral deficit. However, even with the head "fixed" in the forward-facing position, one animal was still able to run well above chance. The size of the lesion does not appear to be a correlate of the performance level.
Animals↗
Sound localization and early binaural experience in the deaf.
The sound localisation ability of four groups of severely deaf children is reported. It was found that those children who had some early experience of binaurally presented sound were likely to retain the ability to localise sounds, although if such experience was via binaural hearing aids, it was not certain to lead to retention of the ability. Children with early experience of sound which lacked interaural cues were likely to have lost the ability to localise, whether the experience was due to a severe asymmetric deafness or early single aid provision. The parallel with the loss of stereopsis in children with an uncorrected strabismus is drawn.
Age Factors↗
Role of spectral detail in sound-source localization.
Sounds heard over headphones are typically perceived inside the head (internalized), unlike real sound sources which are perceived outside the head (externalized). If the acoustical waveforms from a real sound source are reproduced precisely using headphones, auditory images are appropriately externalized and localized. The filtering (relative boosting, attenuation and delaying of component frequencies) of a sound by the head and outer ear provides information about the location of a sound source by means of the differences in the frequency spectra between the ears as well as the overall spectral shape. This location-dependent filtering is explicitly described by the head-related transfer function (HRTF) from sound source to ear canal. Here we present sounds to subjects through open-canal tube-phones and investigate how accurately the HRTFs must be reproduced to achieve true three-dimensional perception of auditory signals in anechoic space. Listeners attempted to discriminate between 'real' sounds presented from a loudspeaker and 'virtual' sounds presented over tube-phones. Our results show that the HRTFs can be smoothed significantly in frequency without affecting the perceived location of a sound. Listeners cannot distinguish real from virtual sources until the HRTF has lost most of its detailed variation in frequency, at which time the perceived elevation of the image is the reported cue.
Cues↗
Neuroanatomical basis of binaural phase-difference analysis for sound localization: a comparative study.
Four varieties of mammals whose medial superior olives range from large to none at all were tested for their ability to localize single, brief tone pips at various frequencies. Although each animal could localize high-frequency tone pips, their ability to localize middle- and low-frequency tone pips corresponded to the size of their medial superior olive (MSO). Since this latter range of frequencies is the one in which binaural phase-difference cues predominate, this anatomical-behavioral correspondence supports the idea that MSO is the chief binaural time-analyzing center for sound localization.
Animals↗
Excitatory action of an immature glycinergic/GABAergic sound localization pathway.
Most mammals determine the azimuthal direction of incoming sound using auditory cues arising from differences in interaural sound intensity. The first station in the ascending auditory pathway, which processes interaural intensity differences, is the lateral superior olive (LSO), a binaural nucleus in the auditory brainstem. LSO neurons encode interaural intensity differences by integrating excitatory input from the ipsilateral cochlea and inhibitory input from the contralateral cochlea. Both inputs converge on single neurons in a highly organized, frequency-specific manner. The correct development of the precise arrangement of these inputs and their physiological properties depends on neuronal activity. Previous studies have shown that inhibitory, glycinergic/GABAergic inputs to the LSO are transiently depolarizing, and it has been hypothesized that this depolarizing action enables developing inhibitory inputs to act as excitatory inputs. In support of this hypothesis, we recently demonstrated that depolarizing glycinergic/GABAergic inputs can increase the intracellular calcium concentration in immature LSO neurons and elicit action potentials. These results provide support for the notion that the influence of glycinergic/GABAergic synaptic activity on development of the LSO involves calcium-dependent signaling mechanisms.
Animals↗
Sound localization with phase audiometry.
A 500 Hz pure tone is presented binaurally with earphones. The tone is adjusted to give a midline impression at confortable loudness level. An electronic variable time delay line unit and a generator for randomizing the shift of the phase lag of the signal to the right or to the left ear, respectively, are used. By shortening the time delay the threshold of the recognition of the phase difference is reached. At every test sequence the tone is first presented in the center of the head without delay and then with delay. The patient runs the test by indicating in which ear the tone is heard. The phase difference is thus gradually reduced from 500 musec down to a threshold value of about 48 musec equal on right and left ears in normal-hearing subjects. The results of phase audiometry on subjects with normal hearing and with different types of hearing losses are in good agreement with the results of sound localization tests in free field. Phase audiometry seems to be of especial value in diagnosing retrocochlear lesions.
Acoustic Stimulation↗
Role of the caudate nucleus in performance of the conditioned reactions connected with sound localization.
The instrumental differentiation of left-right positions of sound-conditioned stimuli (series of clicks under dichotic stimulation) in 8 dogs was examined before and after the bilateral electrolytic lesions of various parts of the caudate nucleus (NCd). It was demonstrated that after damage of the NCd the correct differentiation of the side of monaural sound stimulation was preserved in all dogs. However, the differentiation of lateralization of sound produced by interaural time differences was selectively impaired. Lesions of the dorsal part of the NCd heads caused the absolute absence of the differentiation of the binaural sound image lateralization for 21-50 days after the operation. This function partially recovered but the percentage of correct responses remained significantly reduced in comparison with preoperative performance. Lesions of the ventral segment of the NCd heads of different parts of the NCd bodies had less influence on the performance of this behavioral task. In these cases there was no period of absolute absence of the differentiation, but the level of correct responses to the binaural signals was significantly reduced. It is suggested that the NCd is a part of the system of brain structures in which the parameters of the conditioned sound signals are compared with a reference system for recognition of sound location. The result of this comparison is used by the NCd for choosing a corresponding program of the motor response.
Animals↗
A neuronal population code for sound localization.
The accuracy with which listeners can locate sounds is much greater than the spatial sensitivity of single neurons. The broad spatial tuning of auditory neurons indicates that a code based on the responses of ensembles of neurons, a population code, must be used to determine the position of a sound in space. Here we show that the tuning of neurons to the most potent localization cue, the interaural time difference in low-frequency signals (< approximately 2kHz), becomes sharper as the information ascends through the auditory system. We also show that this sharper tuning increases the efficiency of the population code, in the sense that fewer neurons are required to achieve a given acuity.
Animals↗
The role of vision in sound localization.
Three experiments examined the role of vision in locating a brief sound (2-sec. speech noise) from an unseen source in the horizontal left front quadrant. The head could be freely moved. Subjects could point to the sound location more accurately with eyes open. However, since in a second study the accuracy of pointing a finger was poorer than for aiming one's eyes at the sound, the effect in the first study may reflect using vision to calibrate the hand location. A third study showed no difference in accuracy of aiming one's eyes at a sound when eyes were open versus closed during presentation of sound. More accurate auditory localization with eyes open than closed was not supported.
Adolescent↗
Sound localization with eccentric head position.
This study investigates the influence of head-to-trunk position on auditory localization in humans. Various methods of head pointing, of two-alternative forced choice, and hand pointing were employed. Head-pointing toward actual sound sources in darkness, by using only the subjective median plane of the head as a reference, resulted in systematic underestimations of target eccentricity. The deviations of the terminal head position from the target shifted with a mean slope of approximately 0.1 degrees per degree change in head position. A corresponding shift in the localization of virtual sound sources (presented via headphones during eccentric head positions) was demonstrated by requiring forced-choice (left or right) responses with respect to the subjective median plane of the head. Head pointing toward remembered auditory targets in darkness resulted in undershoots similar to those found with actual targets. However, when a visual marker of the actual median plane of the head was additionally presented to the subject during these tasks (by a laser attached to the head that projected a spot onto a screen), sound localization was fairly accurate. Localization of eccentric auditory targets by using a swivel hand pointer also showed systematic errors similar to those found with head pointing in darkness when the head was simultaneously oriented toward the sound. When the head remained in alignment with the trunk, hand pointing resulted in overshooting responses. These results may be related to neural processes, presumably in the posterior parietal cortex, that transform auditory and visual spatial coordinates into a common, trunk-centered, frame of reference.
Adult↗