Alan D. Musicant

Direction‐dependent spectral properties of cat external ear: New data and cross‐species comparisons

Free-field to eardrum transfer functions were measured in anesthetized cats inside an anechoic chamber. Direction-dependent transformations were determined by measurement of sound-pressure levels using a small probe tube microphone surgically implanted in a ventral position near the tympanic membrane. Loudspeaker and probe microphone characteristics were eliminated by subtraction of the signal recorded in the free field with no animal present. Complexities of the transfer function, which include the presence of prominent spectral notches in the 8- to 18-kHz frequency region, are due primarily to the acoustical properties of the pinna. Differential amplification of frequency components within the broadband stimulus occurs as a function of source direction. Spectral features vary systematically with changes in both elevation (EL) and azimuth (AZ). The contrast between a notch and its shoulders is enhanced in the interaural spectral records. Spectral data from single source locations and spatial data for single frequencies at many locations are presented and comparisons with other species are drawn. It is suggested that spectral features in the 8- to 18-kHz region provide some of the necessary spectral information for sound localization and that the contrast in spectral energy between the frequencies at the notch and its shoulders is a potential directional cue.

The psychophysical basis of monaural localization

Listeners were required to locate, monaurally, noise bursts emanating from the horizontal plane ipsilateral to the functioning ear. Loudspeakers were positioned from 0 through 180 degrees azimuth, separated by 15 degrees. Stimulus bandwidth was 1.0 kHz, and centered at 4.0-14.0 kHz in steps of 0.5 kHz. The location judgments were governed by the frequency composition of the stimuli, not by their place of origin. With a miniature microphone positioned at the entrance of the external ear canal, the relative amplification provided by the pinna was obtained for the stimuli employed in the localization tests. For each differently centered noise burst, that loudspeaker position re other positions which was associated with the greatest amplification of the stimulus was the one most likely to have been chosen as the source of that stimulus during the localization tests.

An insert earphone system for delivery of spectrally shaped signals for physiological studies

Acoustic signals arriving at the eardrum in free-space carry directionally dependent temporal and spectral information resulting from the acoustical effects of the body, head, and external ear as well as from differences in the length of the sound path to each ear. Through analysis of the responses of single auditory neurons, the acoustical and neural mechanisms by which sounds in free-space are localized are being studied. The approach involves simulation of free-field signals at the two eardrums of a cat via earphones and a study of the neuronal responses to such a virtual acoustic space. This approach makes it possible to manipulate different stimulus parameters independently in order to examine their role in determining the spatial characteristics of neuronal response. This report describes an insert earphone system designed for the delivery of such simulated signals which are broadband transients having complex spectra that mimic the acoustic transfer function of the external ear for frequency components up to 30 kHz or more.

Monaural localization: an analysis of practice effects.

Four listeners (experimental group) received one trial a session for 60 sessions on a monaural localization task in which six loudspeakers, positioned 15 deg apart, were placed on the side of the functioning ear. Four additional listeners (control group) received 60 massed trials first and then one trial a session for 60 sessions. The stimulus consisted of a single train of five high-frequency pulses. An ANOVA conducted on the 60 trials distributed over sessions showed that the experimental group’s performance was significantly inferior to that of the control group. Members of the former continued to displace sounds toward the open ear throughout the entire test. Those of the control group exhibited this tendency, but to a much lesser extent. The experience of locating 60 massed trials clearly had a robust effect on subsequent performance. We contend that, by virtue of presenting only one trial a session, the magnitude of the massed-trial effect was little attenuated by our tests to measure it. We interpret our substantive finding to mean that during the 60 massed trials, the control group formed a perceptual representation of the auditory space which served it well when placed on the one-trial-a-session regimen. Those in the experimental group were unable to develop an adequate representation of the auditory space, presumably because their monaural localization experience was restricted to only one trial a session.

Monaural and binaural minimum audible angles for virtual sound sources

A number of researchers have employed the minimum audible angle (MAA) paradigm to investigate the precision with which listeners can discriminate differences of sound‐source location in the free field [A. W. Mills, J. Acoust. Soc. Am. 30, 237–246 (1958); W. M. Hartmann and B. Rakerd, ibid. 85, 2031–2041 (1989)]. Surprisingly, this has been done only for binaural listening conditions, although Man’s capacity for monaural localization is well documented [J. R. Angell and W. Fite, Psych. Rev. 8, 225–246 (1901); R. A. Butler, R. A. Humanski, and A. D. Musicant, Perception 19, 241–256 (1990)]. In the present study, monaural and binaural MAAs were determined using a broadband stimulus for seven virtual sound‐source positions in the horizontal plane. Listeners indicated via a three‐alternative, forced‐choice paradigm whether the stimulus presented in the second interval appeared to move clockwise, counterclockwise, or not at all, with respect to that presented in the first interval. Results will be discussed in ...

The relationship between tone frequency and perceived elevation under headphone listening conditions

The relationship between pure tone frequency and perceived in head elevation was investigated. Stimuli were pure tones ranging from 1000 to 1200 Hz in 1000 Hz steps. Insert earphones were utilized for presentation. Subjects were instructed to indicate, on two separate diagrams, the image location of the stimulus after presentation of a brief tone. Dimensions were left–right, front–back and elevation. All stimuli were presented diotically, that is with equal intensity to the two ears. Tones were presented at approximately 70 phons with a variation of ±3 dB. Perceived in head location was found to vary systematically in the elevation dimension. Tones with lower frequencies were perceived as lower in elevation than tones of higher frequency up to about 9 kHz. Stimuli with frequencies of 10 kHz and higher were perceived as lower in elevation relative to the perceived elevation of the 8‐ and 9‐kHz stimuli. Results of this experiment were compared to results from free field experiments and found to be in genera...

Distribution of sound pressure along the surface of cat tympanic membrane

We presented 10‐μs clicks free field in a specially treated sound attenuating chamber and measured the distribution of sound pressure as a function of position along the surface of the tympanic membrane. Using the fast Fourier transform, frequencies between 3 and 45 kHz were analyzed. As a probe, about 2–3 mm away from the tympanum, was advanced, sound pressure changes could be detected as a function of location for frequencies above about 12 kHz. These differences were essentially monotonic as the probe was advanced antero‐ventrally. Pressure differences reached 12 dB at some frequencies. A second probe, placed at right angles to the first and fixed in position at the canal wall, was used to monitor pressure as the first probe advanced. Detectable pressure changes were small, less than 2 dB, indicating that the advancing probe did little to disturb the sound field, at least at the wall of the canal. As the second probe was advanced perpendicularly to the malleus (dorso‐anteriorly), little detectable chan...

Effects of targeted pinna occlusion on pinna/spectral cues to localization in the median plane

Auditory localization accuracy (in humans) in the median sagittal plane has been attributed, by some authors, to an effect of “spectral notches” that occurs in the frequency region of 4–8 kHz. Another possibility for decrements in vertical plane localization accuracy has been overlooked. Roffler and Butler (1967) and Hebrank and Wright (1974) both demonstrated that removal or absence of sound frequencies above about 8 to 10 kHz led to decrements in vertical plane localization accuracy. They did this by using carefully selected types of band pass or band limited noise. A reduction in accuracy of auditory vertical localization by occluding all or part of the pinna has been known for many years [Gardner and Gardner (1973)]. We have previously reported results that demonstrate disruption in accuracy with various partial pinna occlusions [ARO (2012)] that differs from results reported by Gardner and Gardner. We now have data that seems to indicate that the reduction in localization accuracy occurs, in part, be...

Sound localization cues in the cat

The changes in spectral properties of the cat free‐field to eardrum pressure transfer function that occur with variation in the direction of a sound source have been described previously [J. A.c. oust. Soc. Am. 87, 757–781 (1990)]. In this paper, new measurements of direction‐dependent interaural time differences defined in terms of the onset or group delay to an impulsive signal are reported. Considering the geometry of the cat head and ears, the results agree surprisingly well with the classic Woodworth model, which estimates the delay between two points on the surface of a sphere by calculating the time required for the incident wave to travel around the surface of the sphere. On the basis of human psychoacoustic findings and microelectrode studies of the cat auditory system, there is speculation as to how the observed spectral and temporal attributes of a free‐field sound stimulus might be utilized in such discriminations as the determination of the minimum audible angle in both the horizontal and mid...

Localization of pure tones in the front‐back and vertical dimension under monaural and binaural conditions

Butler and Flannery (1980) proposed the spatial referent map theory. The present experiment was designed to test this theory under extreme conditions. Presentation of pure tones in the monaural condition should stimulate the spatial referent map directly. Here 500‐ms pure tone pulses were used. Frequencies ranged from 0.5 to 9.5 kHz. Speakers, circumaural earphones, or insert earphones were utilized. The method of paired comparison with a four‐alterative, forced‐choice response task was employed. Subjects were requested to respond on both front‐back and vertical dimensions. Without any possibility of interaural difference cues, tones did appear to come from different spatial locations. Despite a difference in the monaural condition the free field performances supported Butlers theory. In all conditions, responses on the vertical dimension showed a monotonic relationship between frequency and phenomenal vertical position. The data on the front‐back dimension were relatively stable across subjects in the monaural condition, however, the between‐subject variability was too great to make any kind of definitive statement.