Mapping lateralization of click trains in younger and older populations

Harvey Babko¡ a;1;�, Chava Muchnik b, Nofar Ben-David b, Miriam Furst c,Shmuel Even-Zohar a, Minka Hildesheimer b

a Department of Psychology, Gonda (Goldschmied) Medical Diagnostic Research Building, Bar-Ilan University, Ramat-Gan, Israelb Department of Communications Disorders, Speech, Language and Hearing, Sackler Faculty of Medicine, Tel Aviv University, Ramat-Aviv, Israel

c Department of Electrical Engineering-Systems, Faculty of Engineering, Tel Aviv University, Ramat-Aviv, Israel

Received 28 May 2001; accepted 23 October 2001

Abstract

The main purpose of this study was to describe and compare lateralization of earphone-presented stimuli in younger and olderindividuals. Lateralization functions, relating perceived location to either interaural time differences (ITDs) or interaural leveldifferences (ILDs) were determined for 78 subjects, aged 21^88 years, who responded by pressing one of nine keys to indicate theperceived location of the stimulus. All subjects were healthy, without any history of hearing loss or ear surgery and within thenormal pure tone audiometric range for their age group. Interaural pure tone and click thresholds did not differ by more than 5 dBacross ears. The ILD lateralization functions, ranging from 10 dB favoring the left ear to 10 dB favoring the right ear were linear.In contrast, the ITD lateralization functions were S-shaped with a clear linear component ranging from 750 Ws favoring one ear to750 Ws favoring the other ear and with an asymptote from 750 Ws to 1 ms. The same general shape of the ITD and ILDlateralization functions was found at all ages, but the linear slope of the ITD lateralization function became shallower with age.The ability to discriminate midline-located click trains (ITD and ILD=0) from ITD-lateralized click trains deteriorated with age,while the comparable ability to discriminate ILD-lateralized click trains did not change significantly with age. The data supporttwo general conclusions. First there seems to be an overall reduction in the range of ITD-based lateralization due to aging. Second,there is a greater reduction in sensitivity due to aging in changes from the perceived midline position (ITD and ILD=0) when ITDis manipulated than when ILD is manipulated. 9 2002 Elsevier Science B.V. All rights reserved.

Key words: Aging; Click lateralization; ITD; ILD

1. Introduction

Human listeners are capable of locating a soundsource in the environment very precisely. Free-¢eldsound localization tests the subject’s ability to identifythe direction of a sound source. For continuous sounds,the major cue to frontal horizontal (right^left) signal

location of frequencies below 1.5 kHz is the interauraltime di¡erences (ITDs) in the waveform at the two ears,while the major cues for higher frequencies are spectralcues and interaural level di¡erences (ILDs) at the twoears (Wightman and Kistler, 1992). Lateralization ofearphone-presented sounds is the laboratory-simulatedanalog of sound source localization in the frontal hor-izontal plane.Earlier studies seem to support the claim that local-

ization in the medial vertical plane depends on the pro-cessing of high frequency energy in a stimulus. Forexample, Butler (1970) reported that listeners with bi-lateral high frequency impairment were at chance levelin localizing stimuli in the medial vertical plane, butcould localize stimuli in the frontal horizontal planealmost normally. On the other hand, accurate localiza-tion of sound in the frontal horizontal plane as well as

0378-5955 / 02 / $ ^ see front matter 9 2002 Elsevier Science B.V. All rights reserved.PII: S 0 3 7 8 - 5 9 5 5 ( 0 2 ) 0 0 2 9 2 - 7

* Corresponding author.Tel. : +972 (3) 531-7968; Fax: +972 (3) 535-5317.E-mail address: babkofh@mail.biu.ac.il (H. Babko¡).

1 Dr. Babko¡ is spending the 2001^2002 academic year at theRotman Research Institute, Baycrest Geriatric Center,3560 Bathurst St., Toronto, ON, Canada M6A 2E1.

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the lateralization of sound via earphones requires intactperipheral and central auditory processing, capable ofutilizing very small interaural time and interaural leveldi¡erences (ITD and ILD) (Je¡ress, 1972; Searle et al.,1976).Aging can a¡ect hearing in a variety of ways. Pres-

bycusis is sensory neural hearing loss associated withdegenerative changes due to aging. The degenerativee¡ects of aging can involve the inner ear as well asthe central nervous system. Presbycusis is manifestedby a steady decline in sensitivity to pure tones, espe-cially those of high frequency. Decrements in speechdiscrimination are frequently reported. The elderly ex-perience greater di⁄culty in understanding speech thanwould be predicted from the pure tone audiogram(Hull, 1995; Prosser et al., 1991). This di⁄culty in-creases under conditions of degraded speech. The re-sults of a recent study indicate that de¢cit in wordrecognition performance among the elderly may be as-sociated with poor spectral resolution of complex sig-nals (Phillips et al., 2000).Decline in other auditory functions is associated with

aging, e.g. changes in auditory fusion thresholds andbinaural masking level di¡erences (Willott, 1991). Eld-erly subjects exhibit poor duration discrimination (Fitz-gibbons and Gordon-Salant, 1995). Older individualsseem to su¡er de¢cit in tasks requiring auditory tempo-ral processing, such as gap detection and temporal or-der (Ludlow et al., 1982; McCrosky and Kasten, 1982;Muchnik et al., 1985; Schneider et al., 1998; Snell,1997; Strouse et al., 1998). These results have ledsome researchers to conclude that the age-relatedchanges in the peripheral and central auditory pathwaysespecially impact discrimination of time-related param-eters (Bergman, 1980; Von Wedel et al., 1990).Several earlier studies have provided evidence that

older individuals may have di⁄culty with free-¢eld spa-tial localization (Konig, 1969; Cranford et al.,1990,1993). Free-¢eld localization of vertically manipu-lated sound sources was found to be poorer in hearing-impaired older subjects (Colburn et al., 1982; Abel andHay, 1996). Several studies have compared younger andolder subjects on the interaural time and interaural leveldi¡erences (ITD, ILD jnds) needed to perceive changesin the location of a dichotic stimulus from the midline(Balogh and Leventhal, 1982; Herman et al., 1977).These studies reported de¢cit in ITD jnds due to agingbut not in the ILD jnds. Strouse et al. (1998) reportedhigher ITD thresholds for older subjects. The impact ofaging on lateralization could be caused by central audi-tory changes as well as peripheral sensorineural hearingloss, both of which are associated with aging. However,lateralization sensitivity, as measured from the midline,may not be the only relevant descriptor of the ability toperceive the location of sounds. Aharonson et al. (1998)

reported that normal ITD or ILD jnds can be accom-panied by abnormal lateralization functions. Thebroader, more general, question of the mapping of theperceived location of sounds (Algom et al., 1988), i.e.the lateralization functions, by the elderly has, to thebest of our knowledge, not been studied to date.In summary, the information on changes in laterali-

zation due to aging is limited. The data available onaging and lateralization presently focus on the ILDand ITD jnds from the midline (i.e. where the referenceILD and ITD=0). There has been no systematic studyof the e¡ect of aging on the lateralization functionaround the head. To date, there has been no systematicstudy of changes in lateralization over a broad agerange. The extant data relate to the comparison of lat-eralization in a group of young subjects with a group ofolder subjects. The transition from peak lateralizationsensitivity in the young to reduced sensitivity in theelderly over a broad range has not been described. Inthe present experiment, we systematically mapped thelateralization function of click trains based on ITD andon ILD in individuals, whose age ranged from 21 to 88years, whose hearing thresholds accorded with the pub-lished age-related norms of Lebo and Reddell (1972).

2. Materials and methods

2.1. Stimuli

Click trains were used as stimuli because it is possibleto perceive changes in location either by ITD or ILDwith the same stimuli (Babko¡, 1982). It is also easierfor untrained subjects to perceive the location of clicktrains than of either pure tones, noise bursts or singleclicks (Algom et al, 1988; Cranford et al., 1990, 1993;Fischrung-Abudy, 1993).All programming of stimulus arrays, presentations

and data collection were accomplished by an IBM-486PC, programmed in Visual Basic. Stimuli were gener-ated by a Tucker Davis Technologies Signal ProcessingSystem II with an Array Processor Card-AP2, whichprovided the basic timing and attenuation in each oftwo independent channels, and across the channels.The stimuli were trains of ¢ve dichotically presented

rarefaction clicks delivered at a rate of 10/s (inter-stim-ulus interval = 100 ms) with various ITDs or ILDs. Theclicks were generated by 100 Ws electrical pulses, con-verted to an electrical signal by a D/A converter, thendelivered to TDH-49 P headphones. The output of thetransducer was measured via a 6-cc £at plate coupler.The auditory stimulus had a low to mid frequency spec-trum with maximum energy in the 200^5000 Hz range.There was a sharp drop in energy (V16 dB) from 5 kHzto 6^8 kHz.

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2.2. Subjects

The original subject pool consisted of 130 individuals,aged 21^88, from whom subjects were chosen accordingto the following criteria. (1) All subjects ¢lled out a shortform, indicating their general medical history and an-swering speci¢c questions on hearing loss. Anyone witha history of complaints of hearing loss or ear surgerywas disquali¢ed. (2) All subjects were tested by puretone audiometry, ranging from 250 to 8000 Hz. Audio-metric pure tone and speech testing were performed witha Belton-112 Audiometer and TDH-39 phones. Only in-dividuals whose thresholds were within the norm fortheir age, determined according to the criteria of Leboand Reddell (1972) and whose thresholds did not di¡erby more than 5 dB between the two ears at any fre-quency were quali¢ed. (3) Speech reception thresholdswere within 5 dB of the averaged pure tone thresholds(0.5, 1 and 2 kHz) as an attempt to exclude individualswith central presbycusis from the testing sample. Onlysubjects with discrimination scores of 88% and above formonosyllabic words were quali¢ed (Jerger, 1973; Jergerand Jerger, 1981). (4) Monaural click thresholds weremeasured with 1-dB intervals. Any individual with adi¡erence between the two ears of 5 dB or more wasdisquali¢ed. Eighty-two out of the original 130 individ-uals in the subject pool quali¢ed according to these andthe training criteria (see below) and participated in thestudy. Data of four of the 82 subjects were incomplete ;the ¢nal analyses are based on 78 subjects.Two types of analyses were performed: (1) Regres-

sion analysis with age as a continuous variable;(2) ANOVA with age categories as a discrete variable.For the latter analyses, subjects were assigned to fourgroups according to age2. Distribution by age and sex isshown in Tables 1a and 1b. Audiometric data andthresholds for click trains are shown for the four agegroups in Table 2.

2.3. Procedure

Monaural click thresholds were determined prior tothe experiment by the method of limits. The click trainswere presented at 40 dB above the averaged monauralthresholds at the two ears (40 dB SL).

Interaural time di¡erences (ITDs) were varied from0 to 1.0 ms in steps of 250 Ws. Interaural level di¡er-ences (ILDs) were varied from 0 to 10 dB in steps of 2.5dB. In order to maintain the overall intensity level ofthe stimuli constant, all ILDs were generated so thatthe overall summed voltage level of the clicks was al-ways 46 dB SL. For example, when ILD=5 dB, thestimulus at one ear was 42.1 dB SL, and the stimulusat the other ear was 37.1 dB SL. When ILD=10 dB,the stimulus at one ear was 43.7 dB SL, and the stim-ulus at the other ear was 33.7 dB SL. Thus, there wereone diotic and 16 dichotic click stimuli. Each of thestimuli was presented 15 times in random order, result-ing in 255 trials per subject, per session. A 3-min restperiod was given after the ¢rst 155 trials.Subjects were seated facing the PC keyboard. All

keys were covered except for the upper row of keys,numbered 1^9 representing locations around the headfrom left ear (1) to right ear (9) with the number ‘5’corresponding to the midline. Subjects were instructedto listen to the stimuli on each trial, then to respond bypressing one of the keys on the keyboard numbered 1^9to indicate the ‘perceived location’ of the click train. Amodel ‘head’ was drawn on a board placed in front ofthe subject with positions marked in equidistant arcs

Table 1aDistribution of subject sex and age in years

Males Females

N Age range Mean (S.D.) N Age range Mean (S.D.)

9 22^29 25.11 (2.8) 8 21^32 25.75 (4.267)9 34^49 40.7 (6) 12 35^46 39.75 (3.81)9 52^64 57.2 (4.41) 9 48^65 56.22 (6)12 66^73 69.9 (2.39) 10 66^88 72.3 (6.91)

Table 1bGroups

Group N Mean age (S.D.)

1 17 25.4 (3.346)2 21 40.14 (4.75)3 18 56.7 (5.13)4 22 71 (5)

2 In the present experiment, age was generally treated as a continuousvariable. However, since in earlier studies of the e¡ect of aging ontemporal discrimination, age was sometimes treated as a discrete var-iable, we also assigned subjects to four age groups for additionalanalyses. In earlier studies, when younger and older subjects werecompared on lateralization and localization tasks, the di¡erence inage between the two groups was approximately 30^40 years (Cranfordet al., 1993, 1990; Herman et al., 1978). In the present experiment, thedi¡erence in age between the youngest and oldest groups was approx-imately 45 years. Consequently, subjects of ages between these ex-tremes were assigned to two groups so that the average age of eachgroup di¡ered from the next older by approximately 15 years (Table1b).

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and numbered 1^9 from the ‘left ear’ to the ‘right ear’.The ‘subjective distance’ between the perceived loca-tions as expressed by each of the numbers from 1^9was, therefore, 22.5‡. In the event that a subject wasuncertain as to a speci¢c ‘perceived location’, he/shewas encouraged to guess. The following trial only beganafter a response was made.

2.4. Training

The experiment began with a training session to ac-quaint the subjects with the proper use of the responsekeys. During this preliminary session, subjects received15 stimuli in a random order: ¢ve monaural click trainsto the right ear, ¢ve to the left ear and ¢ve binauralclick trains with ITD and ILD=0. To ascertain thatsubjects understood the use of the response keys prop-erly, they were required to respond by pressing the left-most key to the monaural click trains presented to theleft ear, the center key to the binaurally presented clicktrains and the right-most key to the monaural right earclick trains. Three of the subjects, who were unable torespond according to the instructions, were dismissed.The remaining subjects were then familiarized with thedi¡erent dichotic click trains and asked to match theresponse positions with the perceived click locationsaround the head. Subjects were told that during theexperiment they would receive stimuli located at variouspositions around the head and that they were to pressthe key that best represented the location of the per-ceived click trains. However, since none of the dichoticclick trains with ILD or ITD was perceived directly atthe ear receiving the leading or more intense stimulus(Furst and Algom, 1995), using monaural click trains toidentify the most lateralized positions in preliminarytraining may have introduced a bias. We, therefore,compared the results of the youngest group of subjectsin this experiment with those of another group of youngsubjects, trained only with dichotic click trains. Thedata, shown in Appendix, indicate that despite the dif-ferences in training procedures, the click lateralizationfunctions and the absolute scales used by the twogroups of subjects did not di¡er signi¢cantly.Testing took place in one of two places, either in a

sound-treated room at the Bar-Ilan University Auditorylaboratory or in a sound-proofed underground shelterat Kibbutz Hamadia in Emek Beit Shean, Israel. In the

Bar-Ilan sound-treated room overall ambient noise levelwas 38 dB SPL (A Scale) and 52 dB SPL (C Scale) ;while in Kibbutz Hamadia comparable levels were 42dB SPL (A Scale) and 48 dB SPL (C Scale).

3. Results

The data were examined by two di¡erent proceduresthat are reported separately. In one procedure, we an-alyzed the lateralization function around the head andreported judgments of perceived location as the depen-dent variable. Age was treated as a continuous variablefor these analyses. In the second procedure, we ana-lyzed the data in terms of the probability of a subjectassigning a speci¢c location to each of the ITDs andILDs. The results of the second analysis are reported interms of frequency histograms and response probabil-ity. Age was also treated as a discrete variable (groups)for the latter analyses.2

3.1. Age and click threshold

Binaural click threshold increases about 12 dB fromGroup 1 to Group 4 (Table 2) (F(3,77) = 27.23,P6 0.0001) and correlates signi¢cantly with subjectage (r(77) = 0.716; P6 0.001). Consequently, all of theregression analyses include binaural click threshold asan independent variable.

3.2. Mapping click train stimuli: full range lateralization

3.2.1. ILDThe results of a one-way (repeated) ANOVA indi-

cated a very signi¢cant e¡ect of ILD (F(8,560) = 404;P6 0.0001). The perceived stimulus location is plottedas a function of ILD (ILD lateralization function) inFig. 1. Negative ILD values (310 to 32.5 dB) representleft ear receiving the more intense click trains; whilepositive values (2.5 to 10 dB) represent right ear receiv-ing the more intense click trains. Analysis by the Gen-eral Linear Model showed that only the linear compo-nent was signi¢cant (F(1,77) = 533.4; P6 0.0001), sothat a straight line can ¢t the ILD lateralization func-tion very well.The linear slopes of the ILD functions were evaluated

for each subject separately. The linear slopes were then

Table 2Audiometric (pure tone, dB HL) and click train thresholds (dB re SPL)

Age group 250 Hz 500 Hz 1 kHz 2 kHz 4 kHz 8 kHz Click trains (average of two ears)

1 (25.4) 8.088 (5.19) 5.588 (4.377) 11.17 (4.6) 7.941 (4.35) 6.029 (5.52) 12.5 (7.705) 45.44 (4.82)2 (40.14) 6.905 (3.614) 5.476 (4.515) 10.833 (3.818) 8.33 (6.24) 10.476 (5.73) 16.785 (8.37) 47.761 (3.23)3 (56.7) 10 (4.372) 8.19 (5.803) 14.58 (4.13) 14.027 (5.432) 24.3 (10.1) 27.22 (12.33) 51.58 (4.229)4 (71) 15.11 (6.92) 12.27 (8.86) 19.54 (8.613) 22.95 (10.34) 43.295 (15.98) 65 (15.905) 57.86 (6.65)

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analyzed by stepwise linear regression with age, sex andclick threshold as independent variables. The linearslope of the ILD lateralization function does notchange either with age (L=30.201; t(74) = 1.228, NS),sex (L=30.115; t(74) = 1, NS) or with click threshold(L=0.072; t(74) = 0.435, NS).

3.2.2. ITDThe results of the one-way (repeated) ANOVA indi-

cated a very signi¢cant main e¡ect of ITD(F(8,560) = 543; P6 0.0001). The perceived stimulus lo-cation is plotted as a function of ITD (ITD lateraliza-

tion function) in Fig. 2. The negative ITD values (31ms to 30.250 ms) represent left ear leading in time;while the positive values (0.250 ms to 1 ms) representright ear leading in time. The ITD lateralization func-tion is S-shaped and best described by signi¢cant linearand cubic components (general linear model analysis,F(1,77) = 671.967, P6 0.0001, linear component;F(1,77) = 366.055, P6 0.0001, cubic component).The presence of an asymptote in the ITD lateraliza-

tion function divides the judgments of click locationsinto a ‘nearly linear’ range (within the asymptotes) andan ‘asymptotic’ range (around the asymptote). Within

Fig. 1. ILD-based lateralization function. Perceived location of click train is plotted as a function of interaural level di¡erences (ILD). Meandata S 1 S.E.M. are shown.

Fig. 2. ITD-based lateralization functions. Perceived location of click train is plotted as a function of interaural time di¡erence (ITD). Meandata S 1 S.E.M. are shown.

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the nearly linear range, manipulation of ITD results inchanges in perceived location; while in the asymptoticrange, click trains are apparently not perceived to‘move’ any more laterally despite increases in ITD.The asymptote was de¢ned as the inter-ITD intervalat which less than 0.1 unit perceived change in clicktrain location was reported between two adjacentITDs. For the total subject population (N=78), theinter-ITD intervals (both left and right leading) between750 Ws and 1 ms showed less than 0.1 unit change inclick train location. The interval 750 Ws^1.0 ms wasidenti¢ed as the asymptotic range. The nearly linearrange was identi¢ed as extending from right ear leadingby 750 Ws to left ear leading by 750 Ws.

3.2.3. The e¡ect of age on the lateralization functionwithin the linear range

The slope of the linear component of the lateraliza-tion function within the ITD range extending from leftear leading by 750 Ws to right ear leading by 750 Ws wasevaluated for each subject separately. The linear slopesof the ITD lateralization function were then analyzedby stepwise linear regression with age, sex and clickthreshold as independent variables. The slope of thelinear component of the perceived click location withinthe dynamic range decreased signi¢cantly with age(L=30.535; t(74) =33.503, P6 0.001) but did notchange signi¢cantly with sex (L=0.078; t(74) = 0.726,NS) or click threshold (L=0.246; t(74) = 1.603, NS).To illustrate the di¡erential e¡ects of aging on ITDand ILD, the ITD and ILD linear slopes were alsoanalyzed for the four groups2 by ANOVA. The resultsare shown in Fig. 3. The linear slope of the ITD later-alization function decreased signi¢cantly across the fourage groups (F(3,77) = 3.54, P6 0.019); while the linear

slopes of the ILD lateralization function did not changesigni¢cantly (F(3,77) = 2.48, NS).

3.2.4. The e¡ect of age on the use of the scale to describeperceived click location

The range of the scale used by subjects to describeperceived location within the nearly linear lateralizationrange (from left ear leading by 0.75 ms to right earleading by 0.75 ms) was also analyzed by stepwise linearregression with age and click threshold as independentvariables. The range of the scale to describe perceivedlocation for the ITD click trains reduced signi¢cantlywith age (L=30.366; t(76) =33.428, P6 0.001), butdid not change with click threshold (L=0.223;t(76) = 1.467, NS). A comparable analysis showed thatthe range of the scale to describe ILD click train loca-tion (from left ear 10 dB more intense to right ear 10 dBmore intense) did not change either with age(L=30.186; t(76) =31.141, NS) or with click threshold(L=0.019; t(76) = 0.117, NS). The di¡erential e¡ects ofaging on the range of the scale to describe perceivedlocation were also analyzed for the four groups2 byANOVA. The results are shown in Table 3. The rangeof the scale to describe click train location for ITD-based stimuli decreased signi¢cantly across the fourage groups (F(3,77) = 2.913, P6 0.04); while the com-

Fig. 3. A comparison of the linear slopes of the ITD-based lateralization (¢lled circles) and ILD-based lateralization (¢lled squares) functionsare plotted for the four age groups.

Table 3The range of the scale to describe perceived location of ITD-basedand ILD-based click trains for the four age groups

Age ITD-based click trains ILD-based click trains

25.4 4.79 (1.325) 3.737 (1.11)40.14 4.53 (1.61) 3.165 (1.28)56.7 3.94 (1.27) 2.97 (0.947)71 3.66 (1.126) 3.224 (1.015)

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parable range to describe ILD-based stimuli did notchange signi¢cantly (F(3,77) = 1.545, NS).

3.3. Response frequency distributions of click trainlocations

Lateralization response frequency histograms areshown in Fig. 4 for the four age groups. The left panelshows the frequency of responses to click trains lateral-ized by ITD; while the right panel shows the frequencyof responses to click trains lateralized by ILD. In bothpanels, the y-axis represents the nine response positionsde¢ned by the test procedure and the x-axis the inter-aural di¡erence. Each bin in the histograms in Fig. 4 isde¢ned as h(loc, ITD) or h(loc, ILD), which is equal tothe relative number of times the subject indicated acertain location (loc) for a given interaural di¡erence,ITD or ILD. The height of each bin represents thevalue of h(loc, ITD) or h(loc, ILD). ThusX

pos

X

ITD

hðpos; ITDÞ ¼ 1 andX

pos

X

ITD

hðpos; ILDÞ ¼ 1:

ð1Þ

The patterns of response distributions in the ITD-gen-erated histograms (left panels) are fairly clear and sep-arate out horizontally around the ITD=0 axis for all ofthe age groups. The ‘left’ response histograms (loca-tions 1, 2, 3, and 4) all coincide with ITDs favoringthe left ear (ranging from ITD=31 ms to 30.250ms), with very few responses falling into the rightITD bins. The ‘right’ response histograms (locations6, 7, 8 and 9) all coincide with ITDs favoring the rightear (ranging from ITD=0.250 ms to 1 ms), also with

very few responses falling into the left ITD bins. Incontrast, the pattern of response distributions in theILD-generated histograms shows much greater overlapand much less clear di¡erentiation of left and rightlocations with their respective ILD left and right bins.In general the horizontal spread of the ILD histogramsis greater than that of the ITD histograms. Anothervery clear di¡erence between the ITD- and ILD-gener-ated histograms can be seen at the response location 5(midline). The horizontal distribution of the ITD-gen-erated histograms at response position 5 is much moreconcentrated with the major weighting at the midline(ITD=0); while the horizontal distribution of the ILD-generated histograms around the midline (ILD=0) ismuch more di¡use with no clear emphasis on midlinedetection. The major change in this pattern across thefour age groups appears to be a more horizontally dif-fuse distribution of the ITD-generated histograms atthe midline (ITD=0, response position 5) in the oldergroup of subjects (Group 4) as compared to the young-er subjects. We suggest that this re£ects a di⁄culty forthe older subjects to discriminate between the ITD=0condition (response 5 = ‘center’) and the other condi-tions in which ITDs 0, with either left ear or rightear leading.To test this hypothesis, we analyzed the probability

of making an error response ‘center’ (‘5’) to click trainswith either ITDs 0 or ILDs 0, i.e. p(c=nc). The resul-tant value was used to estimate the spread of the re-sponse distributions (Fig. 4) around the position de-¢ned as midline (ITD=0, ILD=0). The ITD-basedp(c=nc) was tested by stepwise linear regression withage and click threshold as independent variables. TheITD-based p(c=nc) increased signi¢cantly with age(L=0.367; t(76) = 3.438, P6 0.001) but not with clickthreshold (L=30.045; t(76) =30.294, NS) indicatingthat the probability of wrongly identifying click trainswith ITDs 0, as being ‘centered’ increased with age.The comparable ILD-based p(c=nc) did not increase sig-ni¢cantly either with age (L=30.004; t(76) =30.025,NS) or with click train threshold (L=0.077;t(76) = 0.465, NS). The average data for the four agegroups are also shown in Table 4. p(c=nc)P for ITD-based click trains di¡ered signi¢cantly among the fourgroups (F(3,74) = 4.004, P6 0.01). There was no signi¢-cant reduction in p(c=nc) for ILD-based click trains withage (F(3,74) = 2.627, NS).

4. Discussion

4.1. ITD and ILD lateralization functions

In the present paper, we examined the ITD and ILDlateralization functions and also estimated the ITDs

Fig. 4. Response frequency plotted as histograms for ITD and ILDand for each of the four age groups separately. Left panel histo-grams based on ITD lateralizations; Right panel histograms basedon ILD lateralizations.

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and ILDs necessary to move from a midline location toa speci¢ed lateral left or right location. In addition, weanalyzed the probability distributions for estimating thediscriminability of midline-located click trains (ITD andILD=0) from lateralized click trains (ITDs 0 orILDs 0).Several observations of the general shape of the lat-

eralization functions describing the location of clicktrains as a function of ITD and ILD should be made.First, for both dichotic manipulations, the perceivedlocations are symmetric across the two sides of thehead as reported previously (Furst and Algom, 1995;Yost, 1981). Second, the ILD lateralization functiondi¡ers from the ITD function over the conditions andranges used in this study. For ILDs ranging from 10 dBfavoring the left ear to 10 dB favoring the right ear thefunction is linear (Fig. 1). In contrast, for ITD rangingfrom 1 ms favoring the left ear to 1 ms favoring theright ear, the ITD function is S-shaped (Fig. 2). Thisfunction has a clear, signi¢cant linear component in themid-ITD range and two asymptotes, describing a min-imal change or absence of change in perceived locationin the more extreme ITD range of 750^1000 Ws. WhenITDs 1 ms, depending on sensation level, the dichoticclicks may begin to break up and be heard at both ears(Babko¡ and Sutton, 1966). With ILDs greater than 10dB, lateralization of the click trains continues.

4.2. Aging and lateralization: earlier studies

Several earlier studies had reported that aging im-pacted negatively on lateralization and localizationjnds around the center of the head. Herman et al.(1977) compared older (60^72 years) to younger (22^32 years) males on the ILD and ITD lateralizationjnds from center for click pairs 60^66 dB above mon-aural threshold. They found that the older subjects re-quired twice the ITD than the younger subjects (ITDjnd= 28 Ws vs. 14 Ws) to lateralize dichotic clicks, butboth groups were approximately equally sensitive inlateralizing clicks by ILD (1.35^1.46 dB). Herman etal. (1978) compared the intelligibility of a speech signalin a background of speech noise presented under di-chotic and diotic listening conditions. Dichotic pres-entation resulted in less improvement in intelligibility

for older than for younger subjects. They interpretedthe relative de¢cit in the older subjects as due partlyto a reduced ability to perform the binaural signal anal-ysis necessary for selective ¢ltering of speech signalsbased on directional cues. Balogh and Leventhal(1982) also reported relative de¢cit in lateralizationITD jnds but not in ILD jnds for pairs of clicks at 75dB SL in an aged population. Strouse et al. (1998) mea-sured monaural gap detection thresholds and ITDthresholds in young and elderly subjects at three sensa-tion levels. They reported that elderly subjects hadhigher gap detection and ITD thresholds than theyounger subjects at all sound levels.Aging has also been reported to a¡ect the discrimi-

nation of sound location in the free ¢eld using a two-loudspeaker procedure (‘precedence e¡ect’), each placed45‡ laterally from midline. De¢cit due to aging wasreported in the inter-speaker temporal range of 0.3^0.7 ms (Cranford et al., 1990, 1993). However, since‘open air stimulation’ uses inter-speaker distance asthe test variable, any change in ITD is also accompa-nied by a change in ILD, especially when the stimuli arebroadband. It is thus not possible to determine whetheraging impacts on neural mechanisms that process ITDsor ILDs or both. In addition, with open air stimulation,even very small trial to trial variations in head positionhave the potential for producing signi¢cant errors inperformance, especially at the shorter inter-speaker dis-tances.

4.3. Aging and lateralization: the present study

The present study examined click train lateralizationfunctions, perceived movement from the center and theprobability of misidentifying a lateralized click as ‘cen-tered’. Our results indicate that the general shape of theITD and ILD lateralization functions is similar for allage groups. Nevertheless, aging signi¢cantly a¡ects theITD-based function. The slope of the linear componentof the ITD-based lateralization function becomes shal-lower with age. Furthermore, aging a¡ects the sensitiv-ity to changes in ITD and ILD around the midlinedi¡erently. The ITD necessary to move one subjectiveunit laterally to the left or right from midline increasessigni¢cantly as people age; while there is no signi¢cant

Table 4Probability of identifying click trains as ‘centered’ although ITDs 0 or ILDs 0

Group (age) p(c/nc) (S.D.) (ITD) 95% Conf. int. (ITD) p(c/nc) (S.D.) (ILD) 95% Conf. int. (ILD)

1 (25.4) 0.059 (0.0524) 0.0269 0.246 (0.13) 0.0672 (40.1) 0.0869 (0.095) 0.0431 0.3409 (0.165) 0.0753 (56) 0.1097 (0.0634) 0.0315 0.3625 (0.148) 0.07364 (71) 0.1504 (0.111) 0.049 0.272 (0.1251) 0.055

For all comparisons of p(c|nc) (ITD) vs. p(c|nc) (ILD) within each of the groups p(c|nc) (ITD) was signi¢cantly larger than p(c|nc) (ILD)(Group 1, t(16) = 5.601, P6 0.0001; Group 2, t(20) = 6.104, P6 0.0001; Group 3, t(17) = 6.656, P6 0.0001; Group 4, t(21) = 3.41, P6 0.0014).

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change in the smallest ILD necessary to move the per-ceived stimulus either left or right from the midline. Theability to discriminate midline-located click trains (ITDand ILD=0) from ITD-lateralized click trains deterio-rates with age; while the comparable ability to discrim-inate the midline-located click trains from ILD-lateral-ized click trains does not change with age. These data,thus, lend support to the idea that aging a¡ects ITDlocalization mechanisms more than it a¡ects ILD local-ization mechanisms.The peripheral and central changes that accompany

aging in most individuals (Cooper and Gates, 1991;Jerger et al., 1991; Rizzo and Gutnick, 1991) are likelyresponsible for the reported changes in a variety ofpsychophysical measures of supra-threshold hearing(Willott, 1991). Since the central nervous system isclearly involved in the comparison of the time cuesassociated with two dichotic stimuli, it is almost certainthat both peripheral and central parts of the auditorynervous system are involved in lateralization. Conse-quently, an argument may be made that the de¢cit inlateralization found in the elderly is, perhaps, secondaryto degraded auditory function associated with aging.However, a general deterioration of auditory functionwould not, by itself, predict that the elderly would havedi⁄culty with ITD-based lateralization, but not withILD-based lateralization. An alternative suggestion isthat the deterioration in the ability to localize stimuliin the horizontal plane found in the elderly representsanother speci¢c source of de¢cit, perhaps related to theability to discriminate time cues (Willott, 1991). Oldersubjects have been reported to have di⁄culty in local-izing stimuli with the precedence procedure when theITD is less than 1 ms although they do not demonstratesevere sensorineural hearing loss (Cranford et al., 1993).Mean auditory gap thresholds were reported to be high-er in older subjects than younger subjects, although allsubjects were within the normal hearing range of youngsubjects (Snell, 1997).The present report provides evidence that aging af-

fects the ability to perceive changes in stimulus locationwhen location is based on temporal cues. Our resultsindicated that the slope of the linear component ofITD-based lateralization functions is shallower in olderpeople. The regression analysis indicated that the slopeof the ITD-based lateralization function decreased asage increased. Although the click threshold increasedby 12 dB from the younger to the older subjects, onlyage was signi¢cantly related to the change in slope ofthe ITD-based lateralization functions, whereas clickthreshold was not. Thus, subject age, rather than in-creasing click threshold, was identi¢ed as the variablethat predicted a shallower slope in the ITD-based lat-eralization functions. Furthermore, there was a signi¢-cant reduction in the ability of older subjects to discrim-

inate between midline-centered click trains (ITD=0)and click trains lateralized by ITDs 0. This de¢citwas also unrelated to click threshold. None of thesechanges was found with the ILD-based lateralizationfunctions. Therefore, the present ¢ndings of shallowerslopes in the ITD-based lateralization functions of theelderly, and the reduced ability to discriminate midline-centered click trains (ITD=0) from click trains lateral-ized by ITD (ITDs 0), argue for a primary source ofde¢cit in ITD-based lateralization, not necessarily de-pendent on the overall sensorineural hearing loss asso-ciated with aging. The di¡erential e¡ect of aging onITD- versus ILD-based lateralization may be viewedas additional evidence supporting the argument thatthe two dichotic cues, intensity and time, are processedby di¡erent auditory nervous system mechanisms(Babko¡, 1982; Furst and Algom, 1995; Yamada etal., 1997). This may also imply that de¢cit in one mech-anism may not necessarily mean de¢cit in the othermechanism (Hausler et al., 1983; Levine et al., 1993).These results may also lend support to a more generalconclusion reached by some reviewers that age-relatedde¢cits in temporal performance measures are unaf-fected by the presence of presbycusic hearing loss (Fitz-gibbons and Gordon-Salant, 1996).Techniques almost identical to those used in the

present study have been used to study lateralizationfunctions in patients su¡ering from multiple sclerosis(MS) or stroke. Several di¡erent patterns of ITD andILD lateralization functions have been reported in pa-tients with MRI-identi¢ed brainstem lesions. Patientswith no lesions involving the pontine auditory systemyielded a normal pattern of ITD- and ILD-based later-alization functions. Patients with unilateral or bilateralcaudal lateral lemniscus lesions were unable to producea lateralization function either with ITD or ILD. Theirresponses were always lateralized to one of the sides, nomidline identi¢cations were made. Patients with lesionsinvolving the trapezoid body responded ‘midline’ to al-most all ITDs producing a very range-reduced midline-centered ‘function’ with hardly any lateralization. Someof these patients also responded ‘midline’ to click trainslateralized by ILD (Furst and Algom, 1995; Furst etal., 1995; Aharonson et al., 1998). We do not wish toimply that normal, healthy, elderly subjects su¡er fromlesions in the pontine auditory system. However, it isinteresting and perhaps instructive to note that the pat-tern of change in lateralization functions of the elderlyis in the direction of a generally reduced lateralizationrange accompanied by a di⁄culty in di¡erentiatingITD-lateralized click trains (ITDs 0) from midline-cen-tered click trains (ITD=0). This pattern is very remi-niscent of lateralization functions in patients with trap-ezoid body lesions (Furst and Algom, 1995; Furst et al.,1995; Aharonson et al., 1998).

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5. Summary

The ¢nding of shallower slopes relating the perceivedlocation of sounds to ITD as individuals age suggests ade¢cit in the ITD-based lateralization mechanism dueto aging. These data complement reports of reducedtime resolution in aging individuals which may applyto all of the senses and may a¡ect the capacity to dis-criminate basic features of stimulation (Stevens, 1992).The reduced sensitivity is more evident when lateraliza-tion is based on ITD. Aging signi¢cantly a¡ects theability to discriminate a midline-‘centered’ click train(ITD=0) from an ITD-based lateralized click train(ITDs 0), but not from an ILD-based lateralized clicktrain (ILDs 0). The latter ¢nding also lends support toa claim by Stevens and Choo (1996) that aging may nota¡ect all sensory thresholds uniformly, even within thesame modality.

Acknowledgements

This study was carried out in accordance with theprinciples of the Helsinki Convention.

Appendix

A supplementary experiment was performed to testthe e¡ect of the training procedure used in the mainexperiment on the range of values and the shape ofthe ITD lateralization function. The ITD-based lateral-ization functions of the youngest subjects in the mainexperiment (Group 1, mean age= 25.4 years, N=17)were compared to those of another group of age-

matched subjects (Group 1A, N=15) who received adi¡erent training procedure.In the training portion of the main experiment, sub-

jects received 15 stimuli in a random order: ¢ve mon-aural click trains to the right ear, ¢ve to the left ear and¢ve binaural click trains with ITD and ILD=0. Sub-jects were required to respond by pressing the left-mostkey to the monaural click trains presented to the leftear; by pressing the center key to the binaurally pre-sented click trains; and by pressing the right-most keyto the monaural right ear click trains. The subjects werethen familiarized with the di¡erent ITD and ILD di-chotic click trains and asked to match the responsepositions with the perceived click locations around thehead.Group 1A performed a similar lateralization task fol-

lowing a di¡erent one-session training procedure, dur-ing which they received all of the dichotic ILD-basedclick trains, randomly presented. Subjects in Group 1Awere asked: (1) to point to the location around thehead which most closely corresponded to where thesound was perceived; and (2) to press whichever ofthe nine response keys corresponding to that location.No monaural clicks were presented to this group.The mean ITD-based lateralization functions of

Group 1 and Group 1A are compared in Fig. A1.Note that despite the di¡erences in the training proce-dures, the data of the two groups overlap. There are nosigni¢cant di¡erences either in the absolute values usedto map click train lateralization, or in the shape of theITD-based lateralization functions (F(8,269) = 0.2854,NS).We conclude, therefore, that despite the exposure to

monaural clicks during training (as examples of totallylateralized stimuli), the mapping values used for ITD-

Fig. A1. ITD-based lateralization functions plotted for Group 1 (main experiment, N=17) and Group 1A (supplementary experiment, N=15).

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based click train lateralization functions are not af-fected. The training procedure used in the main experi-ment did not bias the shapes or absolute values of theITD-based lateralization functions.

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