Search

Measurement of localization performance will reflect errors that relate to the sensory processing of the cues to sound location and the errors associated with the method by which the subject indicates the perceived location. This study has measured the ability of human subjects to localize a short noise burst presented in the free field with the subject indicating the perceived location by pointing their nose towards the source. Subjects were first trained using a closed loop training paradigm which involved instantaneous feedback as to the accuracy of head pointing which resulted in the reduction of residual localization errors and a rapid acquisition of the task by the subjects. Once trained, 19 subjects localized between 4 and 6 blocks of 76 target locations. The data were pooled and the distribution of errors associated with each target location was examined using spherical methods. Errors in the localization estimates for about one third of the locations were rotationally symmetrical about their mean but the remaining locations were best described by an elliptical distribution (Kent distributed). For about one half of the latter locations the orientations of the directions of the greatest variance of the distributions were not aligned with the azimuth and elevation coordinates used for describing the spatial location of the targets. The accuracy (systematic errors) and the distribution of the errors (variance) in localization for our population of subjects were also examined for each test location. The size of the data set and the methods of analysis provide very reliable measures of important baseline parameters of human auditory localization.

The two principal binaural cues to sound location are interaural time differences (ITDs), which are thought to be dominant at low frequencies, and interaural level differences (ILDs), which are thought to dominate at mid to high frequencies. The outer ear also filters the sound in a location dependent manner and provides spectral cues to location. In these experiments we have examined the relative contribution of these cues to the auditory localisation performance by humans. Six subjects localised sounds by pointing their face toward the perceived location of stimuli presented in complete darkness in an anechoic chamber. Control stimuli were spectrally flat (400 Hz to 16 kHz), while the relative contribution of location cues in the low frequency channels was determined using noise high passed at 2 kHz and in the high frequency channels using stimuli low passed at 2 kHz. The removal of frequencies below 2 kHz had little effect on either the pattern of systematic errors or the distribution of localisation estimates with the exception of an increase in the size of the standard deviations associated with a few rear locations. This suggests considerable redundancy in the auditory localisation information contained within a broadband sound. In contrast, restricting the target spectrum to frequencies below 2 kHz resulted in a large increase in the cone-of-confusion errors as well as a subject dependent biasing of the front-to-back or back-to-front confusions. These biases and the reduction in localisation accuracy for high pass stimuli at some posterior locations are consistent with a contribution of spectral information at low frequencies.

Using multi-unit recordings, we compared the azimuthal spatial selectivity of auditory neurons in the deep layers of the ferret superior colliculus (SC) to broadband and tonal stimuli. Responses to noise were tuned at different sound levels to a single location, which varied topographically along the rostrocaudal axis of the nucleus to form a map of sound azimuth. Frequency response profiles tended to be multi-peaked, so the spatial tuning was examined at two or more frequencies in each case. Some of the azimuthal response profiles obtained with tonal stimuli were bilobed, as expected from the spatially ambiguous cues available at individual frequencies, although the rest were tuned to a single region of space. The preferred sound directions usually varied with the frequency used, and the range of auditory best positions at each recording site was significantly greater with tones than with noise. Comparison with the acoustical properties of the auditory periphery suggested that the near-threshold positional selectivity of many of the tonal responses may be determined by the monaural directionality of the outer ear. When the sound level was raised by 20 dB so that both ears were stimulated at all speaker locations, the range of tonal best positions obtained at each frequency increased and some of the units responded best to pure tones located in the ipsilateral hemifield. The lack of topographic order in the distribution of tonal spatial selectivity along the rostrocaudal axis of the SC indicates the need for a broadband input, incorporating the spectral localization cues provided by the outer ear, in the construction of a neural map of auditory space.

The development of the acoustics of the auditory periphery of the ferret was examined by measuring the spectral transfer functions (STFs) and the directional characteristics of the outer ears of animals ranging in age from postnatal day 32 (P32) to P54. Using an impulse response technique the STFs were obtained from up to 250 locations throughout free space. The directional responses were calculated for frequencies between 1 kHz and 30 kHz. The low frequency roll-off of the STF decreased with increasing age from around 15 kHz at P32 to an adult value of around 8 kHz by P51. The directional responses of the outer ear of the immature ferrets differed significantly from adult animals in a fashion that was consistent with the smaller size of the auditory periphery. However, by P51 the responses were generally within the normal adult range. The implications of the relatively rapid development of the acoustics of the auditory periphery are discussed in terms of the development of mechanisms subserving sound localization.

The directional sensitivity of the outer ear of the guinea pig was determined by recording changes in the amplitude of the cochlear microphonic to frequencies between 1 and 20 kHz as the location of the sound source was changed throughout 360° of horizontal auditory space. The directional responses to frequencies below 3 kHz were almost omnidirectional. The directional responses for frequencies between 3 and 12 kHz were progressively more directional toward the anterior midline. The responses for frequencies above 12 kHz were highly directional along the ipsilateral interaural axis. In contrast, the directional responses to all frequencies in animals whose pinnae had been removed were orientated along the ipsilateral interaural axis. The observations suggest that the orientation and strength of the directional response of the auditory periphery in the guinea pig are dependent on frequency and that this dependence is attributable, at least in part, to the acoustic properties of the pinna. The observations also indicate that there is a substantial change in the interaural intensity difference at various frequencies and in the spectral transfer function of the ear according to the location of the sound source in the ipsilateral hemifield. The observation that these changes are asymmetrical about the interaural axis for a substantial part of the auditory range of the animal is consistent with the hypothesis that the frequency dependent directionality of the auditory periphery provides a spectral cue for the localization of broad band sounds in the free field.

A field potential could be recorded in the deep layers of the superior colliculus (SCd) in the guinea pig following presentation of pure tone bursts at frequencies between 1 and 22 kHz, bursts of white noise, and clicks presented in the free field. The potentials evoked by these stimuli at different intensities had two negative and one positive component associated with the onset of the stimulus. The amplitude of the negative components of the potential varied according to the location of the recording electrode along the dorso-ventral and rostro-caudal axes of the SC and with the frequency of pure tone stimuli. The largest amplitude potentials were recorded in the deep grey layer of the SCd midway along the rostro-caudal axis of the nucleus. The distribution of frequency sensitivity along the rostro-caudal axis of the SCd was determined by recording the amplitude of the negative components of the field potential evoked by 10 ms pure tone bursts at a constant intensity. The distribution of frequency sensitivity across the nucleus was not cochleotopic. The caudal pole of the nucleus had largest responses to frequencies above 15 kHz, with a peak sensitivity around 20 kHz. In contrast, the rostral pole of the nucleus was most sensitive to frequencies around 10 kHz and the sensitivity to frequencies around 20 kHz was relatively low. At a point midway along the rostro-caudal axis of the SCd. the nucleus was sensitive to a broad range of frequencies from 5 to 25 kHz. The patterns of frequency sensitivity recorded in the rostral, middle and caudal SCd are qualitatively similar to the frequency transfer characteristics of the auditory periphery for sounds located in the anterior, orthogonal and posterior regions respectively of contralateral space. The correspondence between these two sets of data suggests that the pattern of frequency sensitivity along the SCd may provide a mechanism by which the nervous system can encode the spectral cues which are generated at the auditory periphery.