The monaural localization of tonal stimuli*

ROBERT A. BUTLERDepartment of Surgery (Otolaryngology), University of Chichgo,Chicago, l11inois 60637

With one ear occluded, 17 listeners were asked to locate tone bursts, .25, A, .6, .9. lA,2.0, 3.2, 4.8, and 7.2 kHz, generated by a loudspeaker concealed from view. The S'sresponse was to callout that number, from a series of numbers arranged horizontally,behind which he thought the tone bursts originated. The listeners perceived the sounds asemanating from the side of the unoccluded ear, but their judgments bore no consistentrelation to the actual location of the sound source. Rather, the listeners showed a strongtendency to locate a tone burst, within the range of .9 through 7.2 kHz, in a fixed spatialrelation to the next higher- and lower-pitched tone burst. Distorting the pinna of theunoccluded ear failed to modify the perceptual pattern. It was suggested that theperceived spatial relations among the various frequencies was a by-product of thetonotopic organization of the auditory nervous system.

"The more nearly the sounds approachpure tones, the more inaccurate thelocalization. This is true regardless of theabsolute pitch. Genuinely pure tones areessen tially unlocalizable in monauralhearing [Angell & Fite, 190I, p. 2451·" Soconcluded the authors after testingextensively with a variety of sounds thelocalizing ability of a unilaterally deafperson. Their statement should have beenqualified, since the listener did assign 72out of 90 sounds presented by tuning forksto a specific position in space. It was onlythat his choices were usually wrong.Whether he systematically positioned thosetones with respect to their pitch cannot bededuced from the description of the data.This does occur, however, when the soundsoriginate in the vertical plane, directlyahead of the listener. High-pitched tonesare perceived to originate abovelower-pitched tones (Pratt, 1930). In boththe Angell and Fite and the Pratt studies,the cues for binaural time and intensitydifferences were absent; hence, listenerswere deprived of the information necessaryfor distinguishing between the locations ofthe sound sources. The fact that Pratt'slisteners located higher-pitched sounds inthe vertical plane above lower-pitchedsounds could have been the result ofextrinsic associations. We use the words"high" and "low" when describing thepitch of a sound. In the absence of binauralcues for spatial position, these wordassociations may cause us to perceivesinusoids with brief periods as high, andthose with longer periods as low in space.On the other hand, the correctinterpretation of this phenomenon may bejust the opposite; i.e., we use the words"high" and "low" to refer to the pitch of

"This work was supported by USPHS ClinicalResearch Center Grant NS 03358-09 and aResearch Career Development Award fromNINDS.

tones because tonal stimuli of differingfrequencies do indeed appear spatiallydifferentiated in this way in the verticalplane. Some data support this suggestion.Roffler and Butler (1968), studyingvertical plane localization, reported thatchildren 4 and 5 years old perceivedhigh-pitched tones as emanating abovelow-pitched tones. During posttestconversations the children gave noindication of associating the words "high"and "low" with the stimulus frequency.

The present paper explores this problemfurther. An experiment was designed tofind out whether a systematic spatialrelation among various tonal frequenciesexists when listeners, deprived of binauralcues, are asked to locate these stimuli inthe horizontal plane. Positive results,should they occur, could not be explainedin terms of word association. The words"high" and "low" obviously are notapplicable to this type of listeningsituation; nor are any other wordsassociated with tonal frequencies likely toinfluence the perception of their relativeposition in the horizontal plane.

PILOT STUDYMethod

Five listeners participated. Theiraudiometric thresholds for frequenciesranging from .25 through 8.0 kHz werewithin IS dB of 0 re ISO standards. Thelisteners were seated comfortably at thecenter of a semicircular reinforced screenwhose radius was 5 ft. Nine KLHloudspeakers, 4-in. diam, were attached tothe screen at eye level. Each was housed ina wooden cabinet, 6.25 x 6.25 x 4 in., andwas identified by a number, 1 through 9,according to its horizontal position on thescreen. With reference to the vertex of thelistener's head, the speakers were placed10 deg apart, center to center.

Before testing, a Mine Safety Applianceear insert was fitted inside one ear canal of

the S and a Mine Safty Appliance Muff(Noisefoe Mark 11M) was placed over thesame ear. The ear chosen for occlusion wason the side of the nonpreferred hand. Afterocclusion, thresholds for the tone bursts of.25, A, .6, .9, lA, 2.0, 3.2, 4.8, and7.2 kHz were obtained. These stimuli,whose rise-fall time was 5 msec and whoseplateau was 10 rnsec, were repeatedapproximately six times per second.Following threshold measurements, Sswere instructed to callout the number ofthe loudspeaker from which the tonebursts appeared to originate. They wererequested to keep their heads firmly in theheadrest that was attached to the chair.Each S was oriented so that the array ofloudspeakers extended from directly aheadto 80 deg toward the side U i hisunoccluded ear. Only loudspeakers at 10,30, 50, and 70 deg from the S's medial:sagittal plane were used, although thelisteners were not aware of this restrictionEach of the nine frequencies was generatedthree times by each of the fourloudspeakers. Hence, every frequency waspresented 12 times and the test sessionconsisted of 108 tonal presentations, ortrials. Orders of presentation forloudspeakers and frequencies werequasirandom. Tone bursts were delivered at20 phons and continued until the listenermade a location judgment. Testing wasconducted in a sound-treated room withthe sound-generating equipment(Krohn-Hite oscillator, Grason-Stadlerelectronic switch and interval timer)located in an adjoining room. An intercomsystem served to maintain voice contactbetween Sand E.

ResultsUpon inspection of the data, there was

no indication that Ss were locating soundsources with an accuracy exceeding chance.However, a definite pattern of positioningthose frequencies of .9 kHz and above wasobserved. Specifically, the tone bursts 3.2and 4.8 kHz were perceived as nearer theSs' median sagittal plane than were .9 and2.0 kHz. The latter frequencies, in turn,were perceived to originate nearer themedian sagittal plane than were 1.4 and7.2 kHz. Since, under conditions ofbinaural listening, some frequenciesconsistently appear further displacedtoward the median sagittal plane thanothers (Butler, Roffler, & Naunton, 1967),the possibility arose that subliminal stimuliimpinging on the occluded ear may haveinfluenced the results. Certainly stimuliwhose intensities are below threshold valueobtained by standard audiometrictechniques can affect the perception ofsounds delivered to the opposite ear(Groen, 1964; Butler & Naunton, 1967).

Perception & Psychophyscis, 1971, Vol. 9 (IB) Copyright 1971, Psychonomic Journals, Inc., Austin, Texas 99

MAINSTUDY

Table 1The Number of Ss Out of 17 Who Perceivedthe Left-Listed Tonal Frequency to Originate

Nearer the Midline Than Did theRight-Listed Tonal Frequency

Accordingly, a patient totally deaf in oneear was given the same test, and he also wasunable to locate most of the sound sourcescorrectly. His placement of the tonalfrequencies .9 kHz and above followed apattern similar to that exhibited by theother five listeners. And, of course, in thecase of this patient, no possibility ofeffective binaural stimulation existed.

In view of these preliminary data, it wasconsidered worthwhile to undertake amore complete study of the apparentlocation of horizontally placed tonalstimuli heard monaurally.

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Fig. 1. Mean displacement from midline(median sagittal plane) of tone bursts as afunction of stimulus frequency. Opencircles/dashed line: S facing loudspeakerwith pinna distorted. Closed circles/solidline: S facing loudspeaker. Closedtriangles/dashed line: S facing 80 deg fromloudspeaker.

by the large standard deviations shown inFig. 2. Yet, there was a considerableamount of agreement among listeners withregard to the relative position of onefrequency with respect to another. Whenranking the judged locations of the varioustonal frequencies with respect to degreesfrom the S's median sagittal plane, thecoefficient of concordance was .40(p < .001). There was a bias similar to thatobserved in the pilot study for the relativeposition of frequencies of .9 kHz andabove. Table 1 most clearly illustrates thisfact. The third column in the tablerepresents the number of Ss of 17 whoperceived the tonal frequency listed in the

Fig. 2. Mean displacement from midline(median sagittal plane) of tone bursts as afunction of stimulus frequency for pooleddata. Vertical lines represent it SD.

lengthened to 50 msec. Interstimulusinterval was 430 msec.

A trial consisted of a train of five stimulifixed in frequency. Approximately 10secelapsed between trials, during which time Smade known his location judgment and Erecorded it. A total of 108 trials were givenduring a test session with each frequencybeing presented on 12 trials. Order offrequency presentation was haphazard. Sswere given three test sessions. A differentlistening condition was associated witheach session. This was simply aprecautionary measure. It served toconfound the influence of any standingwave pattern that might have existed at theear canal entrance due to the fact that thetest room, although sound-treated, wasnonetheless echoic. On the first testsession, nine listeners faced theloudspeaker position and eight faced apoint 80 deg removed from theloudspeaker. The situation was reversed forthe second test session. When facing awayfrom the loudspeaker, it was always on theside of the unoccluded ear. All listenersfaced the loudspeaker position on the thirdtest session. This time, however, the pinnaof the unoccluded ear was pulled tightlyforward by taping it to the listener's cheek.

ResultsAlmost without exception, Ss perceived

all tonal frequencies to emanate from theside of the unoccluded ear. One Sperceived the three lower frequencies tocome from the opposite side. If theconcept of loudness difference can beextended to a monaural listening situation,loudness was the single cue available forlocalization. But here the informationserved only to distinguish the left from theright half of the total sound field. Binauralcues necessary for more exact localizationwithin this segment of the sound field werenot present. Still, most Ss perceived moststimuli as originating from a relativelyrestricted region in the horizontal plane.When questioned later, they admitted anuncertainty associated with many of thejudgments, but there were only a fewinstances where a listener reported aninability to assign a location to a tonebecause of its diffuseness. And when thisoccured, it was confined to the lowfrequencies. Such trials were reinsertedlater in the test session and a judgment wasalways forthcoming.

The data for each test session werecomparable (see Fig. I); hence, statisticaltreatment of the results were carried outon pooled data. Listeners differedappreciably from one another with regardto where, on the side of the unoccludedear, they perceived the location of thevarious tonal frequencies. This is evident

116

1512235o

Numberof SsFrequencies

400 < 250600 < 400900 < 600

1400 < 9002000 < 14003200 < 20004800 < 32007200 < 4800

MethodSeventeen listeners not tested before

served as Ss. Their hearing acuity waswithinnormallimits, Only one loudspeakerwas used, and this was concealed bydraperies hung immediately behind thesemicircular screen. The screen was paintedwhite. Black numbers, 6 in. high, werespaced 10 deg apart, center to center, withrespect to the listener's position. Number 1was painted in the middle of the arc;Numbers 2-9 extended on each side ofcenter.

The procedure involving occlusion ofone ear and the obtaining of the thresholdsand the instructions for reporting thelocation of sound sources were the same asthose described in the pilot study. In thismain experiment, however, Ss called out anumber on the screen behind which theythought the sound originated, since noloudspeaker was visible. They were nottold that just one loudspeaker wasgenerating all the stimuli. The same tonalfrequencies as those in the pilot study wereused, and again the loudness level was20 phons. As a rapid check on the integrityof the occlusion, all Ss were asked to blockthe unoccluded ear with their fingers, andthen each frequency was presented at theoriginal 2Q-phon level. No S heard thesounds. In the present series of tests,stimulus rise-fall time was extended toto msec and the stimulus plateau was

100 Perception & Psychophysics, 1971, Vol. 9 (IB)

first column to be nearer the mediansagittal plane than the adjacent tonalfrequency listed in the second column. Forthe lower frequencies, .4 re .25 and .6 re.4 kHz, the group was dividedapproximately in half with respect towhether its members perceived one tonalfrequency as being nearer the mediansagittal plane than the just lower-pitchedfrequency. But for the remainingfrequencies, most, and in one instance, allmembers of the group perceived the spatialrelations between adjacent frequencies inthe same way. A chi-square test applied tothe data in Table 1 indicated that theobserved distribution of the listeners'perception of spatial relations betweenpairs of adjacent frequencies differedsignificantly (p < .001) from chance.

Some Ss remarked that the high-pitchedfrequencies appeared to originate above thenumbers painted on the horizontallyextended screen. Informal tests at thecompletion of the experiment indicatedthat the apparent vertical location of tonebursts with regard to their pitch was muchmore prominent under binaural listeningconditions with the loudspeaker beingpositioned directly in front of the listenerthan under the conditions that obtained inthis experiment.

DISCUSSIONIt is most improbable that evolutionary

pressure has been exerted on man or anyother mammal to develop the capacity tolocate tonal stimuli monaurally. Indeed,even binaural localization of tones isimprecise except when the sound source isdirectly ahead of the listener (Mills, 1958).That which is remarkably developed,certainly in man, is the capacity todiscriminate between small differences in

frequency. This capacity, which is utilizedfor distinguishing among complex soundsthat differ slightly in frequencycomposition, is essential for effectiveverbal communication. The fact thattonotopic organization is a prominentfeature of the mammalian auditory nervoussystem adds credence to the contentionthat frequency discrimination is basic tothe auditory repertoire of mammals. Withregard to the present study, the followingquestions arise: In the absence of relevantcues for localization, why should listenersperceive the various frequencies in areasonably consistent spatial relation toone another? Why were the locationjudgments not randomly distributed? It issuggested that the observed spatial relationbetween adjacent frequencies was aby-product of the system's tonotopicorganization. That the three lowerfrequencies bore no systematic spatialrelation to one another is not consideredem barrassing to this Viewpoint;presmuably, low frequency tones are notprocessed by the nervous system in termsof the place principle.

This explanation, although devoid offunctional significance, appears moreadequate than others that can be advanced,such as (I) peculiarities of the sound field,(2) signal distortion brought about byrelatively abrupt rise time, or (3) extrinsicassociative cues. Differences amonglistening conditions confounded anypeculiarities that the sound field mighthave had in common with the three testsessions; yet, the pattern of perceivedlocations was similar for the threeconditions. No distortion products wereaudible; no doubt, they existed because ofthe relatively rapid onset of the tonebursts. Nevertheless, since the stimuli were

presented at only 20 phons, the likelihoodthat signal distortion was responsible forthe results is minimal. And even if theywere responsible, it would be difficult toexplain the observed pattern of spatialrelations in this way. As was mentioned atthe beginning of this paper, wordsassociated with pitch do not have spatialconnotations for sound sources originatingin the horizontal plane. In view of the factthat a spatial arrangement was foundamong the different tonal frequencies inthis plane, one wonders whether the spatialarrangement found previously for tonesoriginating in the vertical plane can beattributed exclusively to associative cues.The basic organization of the auditorynervous system may playa role.

REFERENCESANGELL, J. R. & FITE, W. The monaural

localization of sound. Psychological Review,1901, 8, 225-246.

BUTLER, R. A., & NAUNTON, R. F. The effectof stimulus sensation level on the directionalhearing of unilaterally deafe;ed persons.Journal of Auditory Research, 1967,7,15-23.

BUTLER, R. A., ROFFLER, S. K., &NAUNTON, R. F. The role of stimulusfrequency in the localization of sound inspace. Journal of Auditory Research, 1967,7,169-180.

GROEN, J. J. Super- and subliminal binaural. beats. Acta Otolaryngologica, 1964, 57,

224-230.MILLS, A. W. On the minimum audible angle,

Journal of the Acoustical Society of America,1958,30,237-246.

PRATT, C. C. The spatial character of high andlow tones. Journal of ExperimentalPsychology, 1930, 13,278-285.

ROFFLER, S. K., & BUTLER, R. A.Localization of tonal stimuli in the verticalplane. Journal of the Acoustical Society ofAmerica, 1968,43,1260-1266.

(Accepted far publication May 15, 1970.)

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