monaural hearing and sound localization

Monaural Hearing

and

Sound Localization

Lucas Cantu

Psychology 440

2-19-99

Human hearing and the ability to perceive the location of a sound source has long been accepted as a process requiring the use of two ears (Kistler, 1997; Butler & Humanski, 1992; Carlile, 1990). This process is referred to as binaural hearing. The subjective experience of binaural hearing during the location of a sound source was thought at first to be the result of an interactive process of evaluating two auditory cues (Kistler, 1997; Butler & Humanski, 1992; Carlile, 1990; Middlebrooks & Green, 1991). A man by the name of Lord Raleigh developed a "duplex theory" (Strutt, cited by Carlile, 1990) which stated that sound localization arises out of the fact that the ears are separated by both space and an acoustically opaque mass (the head) that creates two distinctive properties to incoming sounds. First, a sound originating outside the medial vertical plane will reach one ear before it reaches the other creating a time-of-arrival difference that can be detected and used in localization. This process is referred to by Fuzessery, Wenstrup, and Pollak (1990) as an interaural time difference (ITD). Second, the mass of the head causes the incoming sound to lose intensity as it passes from one side of the head to the ear on the opposite side. Fuzessery, Wenstrup, and Pollak (1990) call this process an interaural intensity difference (IID), because the head acts as a muffler.

The duplex theory survived until neuroanatomists and neurophysiologists began to search for the biological mechanisms of which the theory attempted to predict (Butler & Humanski, 1992). The duplex theory did prove to be, at least in part, accurate. In 1936 Stevens and Newman (cited by Butler & Humanski, 1992) proved empirically the existence of IIDs and ITDs in locating a sound source. However, they neglected to consider the possibility of other auditory cues that may provide additional localization information. The duplex theory assumed there were no other ways in which the perceptual location of a sound source could be obtained. It was not until much later that the role of the external structures of the ear, namely the pinnae, were considered.

According to Butler and Humanski (1992), the role of the pinnae in localizing sound was only taken seriously when scientists began to study sound localization in situations where binaural differences were nonexistent. Some experiments were eventually performed using sound sources which lay directly on the medial vertical plane (referred to as elevation) and did not stray to either horizontal side (Butler & Humanski, 1992; Carlile, 1990; Wightman & Kistler, 1997). This component of the experiments forced the listener to interpret the sound's location through some means other than IIDs or ITDs. It was through these experiments that the pinnae were discovered to act as directionally dependent filters for determining sound location (Butler & Humanski, 1992; Middlebrooks & Green, 1991; Wightman & Kistler, 1997).

Other experiments produced perceived elevation changes without moving the auditory source (Rogers & Butler, 1992; Middlebrooks & Green, 1991). Both Middlebrooks & Green (1991) and Rogers & Butler (1992) reported that by simply changing the acoustical nature of a tone (presented on the medial vertical plane) the listener erroneously assumed it to change places in space. Interestingly, the changes reported by the listeners were along the medial vertical plane. This evidence provided additional support for the argument against the duplex theory. It also provided information on the dynamics of sound manipulation via external ear structures and how they serve to provide monaural auditory localization cues, or "pinnae cues" (Goldstein, 1999; Wightman & Kistler, 1997; Wotton et al., 1995; Butler & Humanski, 1992; Fuzessery et al., 1990).

Monaural hearing, as it relates to auditory localization, is limited to pinnae cues since IIDs and ITDs are impossible. Thus, pinnae cues are functionally different from binaural cues. They are based on the quality of sound as it enters the ear canal (Fuzessery et al., 1990; Butler & Humanski, 1992; Wightman & Kistler, 1997; Rogers & Butler, 1992; Middlebrooks & Green, 1991). The quality of a sound can be manipulated through varying means. Place a solid object in front of a sound and it will change the intensity of the sound (the same way the head acts during IIDs). Force a sound wave to act upon itself through reflection or the addition of other sound waves and the spectral frequency of which it originally was composed is altered. The convoluted structure of the pinna is such that sound waves, as they are gathered and funneled toward the ear canal, experience overlapping, cancellation, reverberation and reinforcement influences (Middlebrooks & Green, 1991; Wotton et al., 1995; Butler & Humanski, 1992). These influences produce quality, or acoustical changes in the spectral frequency make-up of the sounds, and it is these changes that provide the monaural information necessary for determining where a sound is coming from (Middlebrooks & Green, 1991; Wightman & Kistler, 1997; Wotton et al., 1995). This acoustical manipulation technique, as it relates to monaural auditory localization, presents a curious fact. If we are able to perceive the location of a sound via this technique, that means another interactive element of relativity exists other than binaural IIDs and ITDs. That is, for one to perceive a changing sound as coming from the same source and maintaining a consistent acoustical quality, one must have a baseline familiarity with it to which it can be compared. Indeed, Rogers & Butler (1992) report that our experience of a sound and our ability to locate it is contingent upon our familiarity with it. What they suggest is not a notion of the acoustical quality of a sound with respect to its location in space. Rather, they suggest that we are attuned to the acoustic receptive patterns as they are created by our own pinnae, but that we must be familiar with a sound to begin with so that we know that it is being modified (a process referred to here as referential analysis). In other words, we must have a reference pattern of the sound from which to evaluate its acoustical qualities as it is experienced in different locations in space.

Interestingly, the limitations of monaural hearing are formidable, but some researchers believe that the binaural cues specified by the duplex theory are not necessary at all for auditory localization (Carlile, 1990; Middlebrook & Green, 1991; Wightman & Kistler, 1997), although Wightman & Kistler exercise caution in their contention. They frequently mentioned in their report the risks involved in monaural experimentation. In their closing statements, they emphasize that the presently established monaural localization paradigm stands week. Problems arose during one of their experiments which produced inconclusive results. Carlile (1990), on the other hand, expressed high confidence in his results and even went as far as to say that binaural cues not only are unnecessary, but are insufficient as well for localizing sound. Other researchers accept and even support the notion that binaural hearing is not as important as it was once thought, but they also accept and support the notion of how limited monaural hearing is as it is compared to binaural hearing (Middlebrooks & Green, 1991; Morongiello, 1989; Fuzessery et al., 1990; Butler, & Humanski, 1992; Wotton et al., 1995). No matter which type of hearing is proven to be superior or inferior in the lab, though, there is a practical notion of superiority that must be considered. Therefore, speaking in a practical sense, monaural auditory localization seems to be far inferior to binaural auditory localization, but only in the practical sense. In the lab, they appear to have their own successful ways of localizing sound. However, in real life, there is one thing that serves to tip the scale of superiority in favor of binaural hearing, that being the azimuth versus elevation experience of auditory localization.

Binaural cues are used primarily for determining azimuth while Monaural cues are, for the most part, used for determining elevation (Middlebrooks, & Green, 1991; Wotton et al., 1995; Butler & Humanski, 1992; Fuzessery et al., 1990; Rogers & Butler, 1992). As was stated before, when a sound is presented on the medial vertical plane, binaural differential hearing is impossible. Therefore, the determination of where a sound is located on that plane relies upon monaural pinnae cues (Butler & Humanski, 1992; Middlebrooks & Green, 1991;). According to studies performed by Butler & Humanski (1992), Wotton et al (1995) and Fuzessery et al (1990), elevation calculations are made with respect to how the sound "sounds" as opposed to azimuth calculations which determine when the sound arrives (ITDs) and how loud the sound is (IIDs) with respect to each ear. The practical difference with humans is that we experience sounds along the horizontal plane far more frequently than we do on vertical planes. For that reason, monaural hearing could be considered inferior to binaural hearing.

Monaural and binaural hearing possess their own functional characteristics in providing localization information. Auditory localization, is one of the more vulnerable sensory experiences in that over half of the auditory image is based on the relativity of sounds as they travel through the auditory pathway, binaurally (IIDs and ITDs) or monaurally (through referential analysis) (Middlebrooks & Green, 1991; Wightman & Kistler, 1997). Monaural hearing research should exorcise caution in laboratory settings for this reason. Nevertheless monaural research should continue since it could further improve therapeutic settings, particularly with those who have suffered partial or total hearing loss in one ear. Moreover, technological advances in designing hearing aid devices may also benefit from further studies in monaural auditory localization. There is much that has been learned recently about monaural auditory localization. But the information acquired has focused mainly on the physiological dynamics of monaural hearing. Perhaps further studies could include possible surgical, technological, cognitive and/or behavioral approaches to monaural hearing and sound localization.

References

1. Butler, R. A., &Green, D. M. (1991). Sound localization by human listeners. Perception and Psychophysics, 51, 182-186.

2. Carlile, S. (1990). The auditory periphery of the ferret II: The spectral transformations of the external ear and their implications for sound localization. Journal of the Acoustical Society of America, 88, 2196-2204.

3. Fuzessery, Z. M., Wenstrup, J. J., & Pollak, G. D. (1990). Determinants of horizontal sound location selectivity of binaurally excited neurons in an isofrequency region of the mustache bat inferior colliculus. Journal of Neurophysiology, 63, 1128-1147.

4. Middlebroks, J. C., & Green, D. M. (1991). Sound localization by human listeners. Annual Review of Psychology, 42, 135-159.

5. Morongiello, B. A. (1989). Infant's monaural localization of sounds: Effects of unilateral ear infection. Journal of the Acoustical Society of America, 86, 597-602.

6. Rogers, M. E., & Butler, R. A. (1992). The linkage between stimulus frequency and covert peak areas as it relates to monaural localization. Perception and Psychophysics, 52, 536-546.

7. Wightman, F. L., & Kistler, D. J. (1997). Monaural sound localization revisited. Journal of the Acoustical Society of America, 101, 1050-1063.

8. Wotton, J. M., Haresign, T., & Simmons, J. A. (1995). Spatially dependent acoustic cues generated by the external ear of the big brown bat, Epesicus fuscus. Journal of the Acoustical Society of America, 98, 1423-1445.

9. Goldstein, B. E. (1999, 5th ed.). Sensation & Perception. Pacific Grove, CA: Brooks/Cole.