Human hearing and the ability to perceive the location of a soundsource has long been accepted as a process requiring the use of twoears (Kistler, 1997; Butler & Humanski, 1992; Carlile, 1990).This process is referred to as binaural hearing. The subjectiveexperience of binaural hearing during the location of a sound sourcewas thought at first to be the result of an interactive process ofevaluating two auditory cues (Kistler, 1997; Butler & Humanski,1992; Carlile, 1990; Middlebrooks & Green, 1991). A man by thename of Lord Raleigh developed a "duplex theory" (Strutt, cited byCarlile, 1990) which stated that sound localization arises out of thefact that the ears are separated by both space and an acousticallyopaque mass (the head) that creates two distinctive properties toincoming sounds. First, a sound originating outside the medialvertical plane will reach one ear before it reaches the othercreating a time-of-arrival difference that can be detected and usedin localization. This process is referred to by Fuzessery, Wenstrup,and Pollak (1990) as an interaural time difference (ITD). Second, themass of the head causes the incoming sound to lose intensity as itpasses from one side of the head to the ear on the opposite side.Fuzessery, Wenstrup, and Pollak (1990) call this process aninteraural intensity difference (IID), because the head acts as amuffler.
The duplex theory survived until neuroanatomists andneurophysiologists began to search for the biological mechanisms ofwhich the theory attempted to predict (Butler & Humanski, 1992).The duplex theory did prove to be, at least in part, accurate. In1936 Stevens and Newman (cited by Butler & Humanski, 1992) provedempirically the existence of IIDs and ITDs in locating a soundsource. However, they neglected to consider the possibility of otherauditory cues that may provide additional localization information.The duplex theory assumed there were no other ways in which theperceptual location of a sound source could be obtained. It was notuntil 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 inlocalizing sound was only taken seriously when scientists began tostudy sound localization in situations where binaural differenceswere nonexistent. Some experiments were eventually performed usingsound sources which lay directly on the medial vertical plane(referred to as elevation) and did not stray to either horizontalside (Butler & Humanski, 1992; Carlile, 1990; Wightman &Kistler, 1997). This component of the experiments forced the listenerto interpret the sound's location through some means other than IIDsor ITDs. It was through these experiments that the pinnae werediscovered to act as directionally dependent filters for determiningsound location (Butler & Humanski, 1992; Middlebrooks &Green, 1991; Wightman & Kistler, 1997).
Other experiments produced perceived elevation changes withoutmoving the auditory source (Rogers & Butler, 1992; Middlebrooks& Green, 1991). Both Middlebrooks & Green (1991) and Rogers& Butler (1992) reported that by simply changing the acousticalnature of a tone (presented on the medial vertical plane) thelistener erroneously assumed it to change places in space.Interestingly, the changes reported by the listeners were along themedial vertical plane. This evidence provided additional support forthe argument against the duplex theory. It also provided informationon the dynamics of sound manipulation via external ear structures andhow they serve to provide monaural auditory localization cues, or"pinnae cues" (Goldstein, 1999; Wightman & Kistler, 1997; Wottonet al., 1995; Butler & Humanski, 1992; Fuzessery et al., 1990).
Monaural hearing, as it relates to auditory localization, islimited to pinnae cues since IIDs and ITDs are impossible. Thus,pinnae cues are functionally different from binaural cues. They arebased on the quality of sound as it enters the ear canal (Fuzesseryet al., 1990; Butler & Humanski, 1992; Wightman & Kistler,1997; Rogers & Butler, 1992; Middlebrooks & Green, 1991). Thequality of a sound can be manipulated through varying means. Place asolid object in front of a sound and it will change the intensity ofthe sound (the same way the head acts during IIDs). Force a soundwave to act upon itself through reflection or the addition of othersound waves and the spectral frequency of which it originally wascomposed is altered. The convoluted structure of the pinna is suchthat sound waves, as they are gathered and funneled toward the earcanal, experience overlapping, cancellation, reverberation andreinforcement influences (Middlebrooks & Green, 1991; Wotton etal., 1995; Butler & Humanski, 1992). These influences producequality, or acoustical changes in the spectral frequency make-up ofthe sounds, and it is these changes that provide the monauralinformation necessary for determining where a sound is coming from(Middlebrooks & Green, 1991; Wightman & Kistler, 1997; Wottonet al., 1995). This acoustical manipulation technique, as it relatesto monaural auditory localization, presents a curious fact. If we areable to perceive the location of a sound via this technique, thatmeans another interactive element of relativity exists other thanbinaural IIDs and ITDs. That is, for one to perceive a changing soundas coming from the same source and maintaining a consistentacoustical quality, one must have a baseline familiarity with it towhich it can be compared. Indeed, Rogers & Butler (1992) reportthat our experience of a sound and our ability to locate it iscontingent upon our familiarity with it. What they suggest is not anotion of the acoustical quality of a sound with respect to itslocation in space. Rather, they suggest that we are attuned to theacoustic receptive patterns as they are created by our own pinnae,but that we must be familiar with a sound to begin with so that weknow that it is being modified (a process referred to here asreferential analysis). In other words, we must have a referencepattern of the sound from which to evaluate its acoustical qualitiesas 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 theduplex theory are not necessary at all for auditory localization(Carlile, 1990; Middlebrook & Green, 1991; Wightman &Kistler, 1997), although Wightman & Kistler exercise caution intheir contention. They frequently mentioned in their report the risksinvolved in monaural experimentation. In their closing statements,they emphasize that the presently established monaural localizationparadigm stands week. Problems arose during one of their experimentswhich produced inconclusive results. Carlile (1990), on the otherhand, expressed high confidence in his results and even went as faras to say that binaural cues not only are unnecessary, but areinsufficient as well for localizing sound. Other researchers acceptand even support the notion that binaural hearing is not as importantas it was once thought, but they also accept and support the notionof how limited monaural hearing is as it is compared to binauralhearing (Middlebrooks & Green, 1991; Morongiello, 1989; Fuzesseryet al., 1990; Butler, & Humanski, 1992; Wotton et al., 1995). Nomatter which type of hearing is proven to be superior or inferior inthe lab, though, there is a practical notion of superiority that mustbe considered. Therefore, speaking in a practical sense, monauralauditory localization seems to be far inferior to binaural auditorylocalization, but only in the practical sense. In the lab, theyappear to have their own successful ways of localizing sound.However, in real life, there is one thing that serves to tip thescale of superiority in favor of binaural hearing, that being theazimuth versus elevation experience of auditory localization.
Binaural cues are used primarily for determining azimuth whileMonaural 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 medialvertical plane, binaural differential hearing is impossible.Therefore, the determination of where a sound is located on thatplane relies upon monaural pinnae cues (Butler & Humanski, 1992;Middlebrooks & Green, 1991;). According to studies performed byButler & 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 thesound arrives (ITDs) and how loud the sound is (IIDs) with respect toeach ear. The practical difference with humans is that we experiencesounds along the horizontal plane far more frequently than we do onvertical planes. For that reason, monaural hearing could beconsidered inferior to binaural hearing.
Monaural and binaural hearing possess their own functionalcharacteristics in providing localization information. Auditorylocalization, is one of the more vulnerable sensory experiences inthat over half of the auditory image is based on the relativity ofsounds as they travel through the auditory pathway, binaurally (IIDsand ITDs) or monaurally (through referential analysis) (Middlebrooks& Green, 1991; Wightman & Kistler, 1997). Monaural hearingresearch should exorcise caution in laboratory settings for thisreason. Nevertheless monaural research should continue since it couldfurther improve therapeutic settings, particularly with those whohave suffered partial or total hearing loss in one ear. Moreover,technological advances in designing hearing aid devices may alsobenefit from further studies in monaural auditory localization. Thereis much that has been learned recently about monaural auditorylocalization. But the information acquired has focused mainly on thephysiological dynamics of monaural hearing. Perhaps further studiescould include possible surgical, technological, cognitive and/orbehavioral approaches to monaural hearing and sound localization.
1. Butler, R. A., &Green, D. M. (1991). Sound localization byhuman 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 theirimplications for sound localization. Journal of the AcousticalSociety of America, 88, 2196-2204.
3. Fuzessery, Z. M., Wenstrup, J. J., & Pollak, G. D. (1990).Determinants of horizontal sound location selectivity of binaurallyexcited neurons in an isofrequency region of the mustache batinferior colliculus. Journal of Neurophysiology, 63, 1128-1147.
4. Middlebroks, J. C., & Green, D. M. (1991). Soundlocalization by human listeners. Annual Review of Psychology, 42,135-159.
5. Morongiello, B. A. (1989). Infant's monaural localization ofsounds: Effects of unilateral ear infection. Journal of theAcoustical Society of America, 86, 597-602.
6. Rogers, M. E., & Butler, R. A. (1992). The linkage betweenstimulus frequency and covert peak areas as it relates to monaurallocalization. Perception and Psychophysics, 52, 536-546.
7. Wightman, F. L., & Kistler, D. J. (1997). Monaural soundlocalization 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 ofthe big brown bat, Epesicus fuscus. Journal of the Acoustical Societyof America, 98, 1423-1445.
9. Goldstein, B. E. (1999, 5th ed.). Sensation & Perception.Pacific Grove, CA: Brooks/Cole.