Infants’ Perception of Timbre

8/12/2019 Infants Perception of Timbre

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INFANTS PERCEPTiON OF TiMBRE 301differences). These phenomena can be considered to be manifestationsof categorization (Quinn & Eimas, 1986; Reznick & Kagan, 1983) orequivalence classification (Bomstein, 1981), that is, the equivalent treat-ment of discriminably different stimuli based on their perceptual simi-larity (Bornstein, 1981, p. 40). It is of particular interest to ascertainthe extent to which such organizing processes are operative in infancy.In recent years, several investigators have attempted to gather evi-dence for categorization or perceptual equivalence classification in thefirst year of life. Because response limitations in this age group precludedirect measures of categorization, it has been necessary to use indirectdesigns that tap equivalent responding by infants to discriminably dif-ferent exemplars of an adult category. Generally, infants are familiarizedwith a set of variable exemplars from one category and then tested withmembers and nonmembers of this category. Similar responses to mem-bers of the familiarized category coupled with differential responses tononmembers provide inferential evidence of categorization on the basisof the perceived similarity of category exemplars.In the visual domain, such research has yielded evidence of infantcategorization of schematic faces and animals (e.g., Strauss, 1979;Younger & Cohen, 1983) dot patterns (e.g., Bomba & Siqueland, 1983;Quinn, 1987), color (Bomstein, Kessen, & Weiskopf, 1976), orientation(e.g., Bomba, 1984), and types of motion (Ruff, 1978). In the auditorydomain, the focus has been primarily on speech stimuli. This work hasrevealed, for example, that infants can categorize spoken syllables onthe basis of vowel identity despite irrelevant variations in pitch contour(monotone/rising/falling) and speaker (man/woman/child) (Kuhl, 1979,1983; Kuhl & Miller, 1982). Infants can also categorize syllables on thebasis of specific consonants or consonant features (Hillenbrand, 1983,1984). Moreover, they show evidence of categorizing adult voices on thebasis of sex of the speaker (Miller, 1983; Miller, Younger, & Morse,1982).Clarkson and Clifton (1985) ventured beyond the speech domain toconsider infants ability to categorize tonal complexes. They establishedthat infants could respond equivalently to sets of spectrally distinct tonalcomplexes that signal a common pitch for adults. This work implies thatinfants extract the pitch of complex auditory stimuli in the highly struc-tured manner that is characteristic of adults. Moreover, Trehub, Thorpe,and Morrongiello (1987) demonstrated that infants could extract the pitchcontour (i.e., pattern of directional changes in pitch) of a multitonesequence or melody and categorize different melodies on that basis.Beyond the psychological dimension of pitch, which relates to thefrequency of sound vibration, is the more amorphous psychological di-mension of timbre, which refers to the distinctive quality that differen-tiates one complex sound from another of identical pitch and loudness.


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302 TREHUB, ENDMAN, AND THORPEIn contrast to a pure tone with a single frequency component, a complexsound typically consists of several simultaneous frequencies, with a fun-damental or lowest tone (normally signalling the pitch of the sound) andovertones or harmonics at integer multiples of the fundamental. Thepattern of energy distribution or amplitude variation across these over-tones (i.e., the shape of the spectral envelope) gives rise to the distinc-tions between various spoken vowels and between various musical in-struments (Slawson, 1968). Many musical instruments are alsodistinguished by rapidly changing (transient) cues associated with soundonset (Saldanha & Corso, 1964); these aspects of timbre are consonant-like. It is clear, moreover, that adults conserve the disinctive qualitiesor timbres of vowels, consonants, or musical instruments over a relativelybroad range of pitch and loudness (see Dowling & Harwood, 1986).Timbre perception is critical to auditory pattern processing and hasbeen the subject of considerable research and discussion with adults(e.g., Bregman & Pinker, 1978; Grey, 1977; Plomp, 1976; Risset & Wes-sel, 1982; Slawson, 1968). Nevertheless, there has been relatively limitedconsideration of this dimension of auditory experience with infants. Evi-dence of timbre discrimination is implied by infants discrimination ofvowels (e.g., Kuhl, 1979; Swoboda, Morse, & Leavitt, 1976; Trehub,1973) but it is unclear whether infants note global differences in vowelquality or whether they rely simply on contrasting formant frequencies.Additional evidence comes from the recent finding of infants discrimi-nation of complex tones of identical pitch but contrasting spectral en-velope (Clarkson, Clifton, & Perris, 1988). However, the generalizabilityof this research remains unclear. Clarkson et al.s (1988) stimuli wererelatively unnatural in embodying few component frequencies, all ofequal amplitude. Because the range of frequencies differed for standardand comparison stimuli, infants may not have responded to differencesin overall sound quality (i.e., timbre) but to specific cues associated withfrequency range. There is evidence that infants encode information aboutthe frequency range of tone sequences and can use this information todifferentiate melodies (Trehub, Bull, & Thorpe, 1984; Trehub, Thorpe,& Morrongiello, 1985). Moreover, there are indications that they retaininformation about frequency range more readily than information aboutthe component frequencies of a sequence (Trehub et al., 1984). Anotherpotentially limiting factor in Clarkson et al.s research is that infantsreceived only one (repeating) exemplar of the standard and comparisonstimulus so that their performance could have been based on an irrelevantbut systematic cue such as loudness or a more direct cue such as thepresence or absence of a specific component frequency. Finally, althoughClarkson et als study does reveal infants discrimination of timbraldifferences. what remains unresolved is whether infants can use the


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INFANTS PERCEPTION OF TIMBRE 303timbre of complex tones as a basis for categorization or equivalenceclassification.The purpose of the present investigation was twofold. First, we soughtto replicate infants discrimination of the timbre of complex tones. In-stead of omitting many overtones and presenting the remaining few atequal intensity, as did Clarkson et al. (1988), we used stimuli with nu-merous frequency components and selected particular regions for energyemphasis, in line with examples of contrasting timbre in the naturalenvironment. Second, we sought to extend previous research by ex-ploring infants ability to use timbre as the basis for categorizing complextones. In constructing the two tone categories or sets of the presentexperiment, we used the vowels [a] and [i] to guide the selection ofregions for energy emphasis. The defining feature of one set of complextones (ah-like) was an overtone or spectral structure with energy peaks(regarded as formants in speech contexts) corresponding roughly to theovertone peaks in the vowel [a]. Members of this set embodied variationsin fundamental frequency, intensity, or duration. The contrasting set oftones (ee-like) had comparable variations, but had energy peaks sim-ilar to the overtone structure of the vowel [il.The general methodological approach (following Kuhl, 1979; Trehubet al., 1987) was to test for discrimination of two contrasting spectralstructures in the context of variations in extraneous cues. In order torule out the possibility that infants could accomplish the task by simplylearning the identity of specific exemplars, a further variation conditionwas included with arbitrary groupings of exemplars. If infants failed todifferentiate the two arbitrary categories and succeeded in differentiatingthe categories distinguished by spectral structure, this would provideclear evidence of equivalence classification of the latter categories.

METHODSubjects

The subjects were 71 normal, full-term infants, 7 to 8.5 months of age,all from white middle-class families who volunteered in response to mailsolicitation. Infants were excluded from the sample on the followingbases: failure to meet a predetermined training criterion (N = 19), fussingor crying (N = 1 ), and failure to turn on at least four of the six probetrials (N = 1). The final sample consisted of 40 infants (20 male, 20female), with a mean age of 7 months, 21 days (range: 7 to 8.5 months).Each infant was assigned to one of four conditions.

The equipment was controlled by a microcomputer (Commodore PET)and specially designed interface. Stimuli were presented via stereo tape


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304 TREHUB, ENDMAN, AND THORPEdeck (Tandberg 9200 XD), amplifier (Panasonic Technics SU 7300), andloudspeaker (Radio Shack Nova-6). The experimenter used a small con-trol box with two push buttons, one for initiating trials, the other forrecording head turns. The experimental site consisted of a sound atten-uating booth (Industrial Acoustics Co.) with an ambient noise level of42 dBC (27 dBA). The booth contained two chairs, one for the parentand infant and one for the experimenter. The loudspeaker was locatedapproximately 2 m to the left of the infant at a 45 angle. On top of theloudspeaker was a smoked Plexiglas box concealing the reinforcer, anelectronically activated toy animal.Stimuli

The stimuli were complex tones generated by a PDP/l l-40 computerand tone generator. The tones were equated for root mean square (rms)amplitude and had 25ms linearly ramped rise and decay times. Thefrequency components of each tone consisted of the fundamental fre-quency and its overtones up to 4000 Hz. To achieve the spectral structurecorresponding roughly to [a], we used two triangular bandpass filters,each having symmetrical 19 dB per octave log-linear roll off, with centerfrequencies (and therefore energy peaks) of 270 and 2300 Hz. The powerper cycle of the input to the filters was attenuated linearly with respectto frequency from 0 dB at 100 Hz to - 20 dB at 4000 Hz. The contrastingspectral structure, corresponding roughly to [i], was achieved by usingtwo triangular bandpass filters, each having log-linear roll of f of 113 dBper octave, with center frequencies at 570 and 840 Hz. Input to the filterwas of equal amplitude at all component frequencies, with a maximumfrequency of 4000 Hz. Roll-off continued to a maximum attenuation of30 dB for all filters. A schematic version of the contrasting overtonestructures is presented in Fig. 1. Audio tapes were prepared with ex-emplars of one stimulus on one channel and exemplars of the contrastingset in the identical position on the other channel of the same tape face.Training and test stimuli were presented at a rate of one per second.The training stimuli consisted of a single repeating token, with one spec-tral structure on one channel and the repeating contrasting stucture onthe other channel. All training stimuli had a fundamental frequency of200 Hz, duration of 500 ms, and interstimulus intervals of 500 ms. Thetest stimuli, consisting of the variable tokens of each stimulus category,were randomized over blocks of 200 with the constraint of no sequentialrepetition of the same irrelevant parameter value within or acrosschannels.There were four conditions: (1) frequency variation, with four valuesof fundamental frequency (100, 200, 300, and 400 Hz) for each spectralstructure, duration of 500 ms, and equal intensity; (2) duration variation,with four values of duration (100, 200, 400, and 500 ms) for each spectral


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INFANTS PERCEPTION OF TIMBRE 305

frequencyFIG. 1. Line spectra of the frequency variation stimuli. Frequency is shown on a linearscale, from 100 to 4000 Hz. Panels a, b, c, and d display one spectral shape for each offour different fundamental frequencies. Panels e, f , g, and h show the contrasting spectralshape with its four fundamentals. Stimuli from the duration and intensity variation con-ditions are shown in panels b and f. Stimuli from the arbitrarily arranged control conditionwere a, c, f, and h in one group and b, d, e, and g in the other.

structure, fundamental frequency of 200 Hz, and equal intensity (Fig.lb, f); (3) intensity variation, with four values of intensity (56, 62, 68,and 74 dBC) for each spectral structure, duration of 500 ms, and afundamental frequency of 200 Hz (Fig. lb, f); and (4) control, with theeight stimuli from the frequency variation condition (i.e., four exemplarsfor each spectral structure) rearranged such that the lOO- and 300-Hztones of one spectral structure and the 200- and 400-Hz tones of theother comprised one variable set (Fig. la, c, f, h), and the remainingfour stimuli (200- and 400-Hz tones of one spectral structure and lOO-and 300-Hz tones of the other) comprised the other (Fig. lb, d, e, g).As noted above, the variable exemplars were presented in modified ran-dom order. A schematic illustration of a sample test trial is given in Fig.2. With the exception of the intensity variation condition, all test stimuliwere presented at 65 dBC.Procedure

The infant was seated on the parents lap and presented repeatedlywith one of the training stimuli (i.e., one value of one spectral structure).This repeating stimulus constituted the background or standard stimulusand was presented at a rate of one repetition per second at an intensityof 65 dBC. When the infant had maintained a quiet alert state and gazeddirectly ahead for 4 s, a training trial was automatically initiated. Forthe 3-s duration of the trial, the contrasting spectral structure was pre-


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306 TREHUB. ENDMAN, AND THORPErem;~-- ~orienting to midline turn reinforcer

*-l ~AVBOVA00OvAHA@H11111111111111 l l lt (set)----,

FIG. 2. Schematic illustration of a sample test trial. Filled symbols represent exemplarsof one spectral shape (the background in this case): open symbols represent exemplars ofthe contrasting spectral shape. The trial begins after the infant has oriented to midline forat least 4 s. The infant turns (*) during the response window (3 s), leading to activationof the reinforcer for 4 s.sented at an intensity 5 dB higher than that of the previously heardbackground stimulus. If the infant turned 45 or more toward the loud-speaker during the presentation of the contrasting stimulus, the reinforcerwas illuminated and activated for 4 s. Head turns at other times andhead turns of less than 45 were not reinforced. If the infant respondedcorrectly on two consecutive training trials, the intensity of the con-trasting stimulus was reduced to the level of the background stimulus.Subsequently, the infant was required to meet a training criterion of fourconsecutive correct responses with background and test stimuli at equalintensity. An infant who did not turn to the contrasting stimulus on eitherof the first two trials was presented with the contrasting stimulus 10 dBhigher on subsequent trials. Testing continued at this level until the infantresponded correctly on two consecutive trials, at which time the intensityof the contrasting stimulus was reduced 5 dB, and so on, until the trainingcriterion was met with both stimuli at equal intensity. The session wasabandoned if the infant failed to meet the training criterion within 20trials. During the subsequent test phase, the variation was introducedfor both background and contrasting stimuli. Again, a trial was initiatedby the computer only when the experimenter had recorded continuousinfant attention at midline for 4 s. This ensured that the infant was notorienting frequently to the background variations and also that therewere at least four presentations of the background stimulus betweentrials. The infant was then presented with change trials (as in training)and no-change trials. The no-change trials provided an estimate of false-positive responses. The experimenter and parent wore headphones car-rying music to mask the nature of trials presented to the infant. The testphase consisted of 36 trials: 15 change trials, 15 no-change trials, and 6probe trials. The change and no-change trials were randomized for eachtest session with the constraint that no more than two no-change trialscould be presented consecutively. Every sixth trial was a probe trialduring which the contrasting stimulus was presented at an intensity 5dB above the background stimulus. Half of the infants in each condition


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INFANTS PERCEPTION OF TIMBRE 307were tested with one spectral structure as the background or standard,the remaining infants with the other spectral structure as background.

RESULTSHead turns on change and no-change trials are shown in Fig. 3. Notethe high rate of false-positive responses in the frequency variation andintensity variation conditions, reflecting the difficulty of the task. Thisrate of false-positive responding is comparable to that obtained by infantson difficult melodic (Cohen, Thorpe, & Trehub, 1987) and temporal(Thorpe & Trehub, 1989) discrimination tasks. Performance on probetrials was near perfect, indicating reasonable understanding of and at-tention to the task. There was no indication of poorer performance onthe first trial of the test phase compared to subsequent trials. Instead,the major effect of the introduction of variable exemplars was seen inthe dramatically increased time required for midline orientation (i.e.,readiness for a trial) compared to the immediately previous training trialsand the subsequent test trials.To eliminate possible effects of response bias, data from each infantwere transformed to d (d prime) values, which are assumed to be nor-mally distributed (see Green & Swets, 1966). Because the infants taskin the present experiment (go/no-go) is analogous to the adult yes/notask, we used tables for the yes/no task to determine d. The proportion

ChangeNo Change No Change No Change Nomange Change ChangeVariation: Frequency Duration intensity Mixed-Control

FIG. 3. Percentage of responses on change and no-change trials as a function ofcondition.


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308 TREHUB, ENDMAN, AND THORPEof turns on change trials provided an estimate of the probability of a hit;the proportion of turns on no-change trials provided an estimate of theprobability of a false alarm. (For further details regarding data treatment,see Thorpe, Trehub, Morrongiello, & Bull, 1988). Perfect scores wereconsidered to reflect a statistically infinite d rather than a truly infinited (Macmillan & Kaplan, 1985). Accordingly, perfect scores on changeand no-change trials were assigned proportions of .99 and .Ol, respec-tively. The d values were analyzed with r-tests to reveal whether thed scores were significantly greater than zero (see Table 1).Infants readily detected the change in spectral structure in the contextof variations in frequency, duration, and intensity. Note that the d valueis above 1.0 in each case. This is often chosen as the detection thresholdin signal-detection studies with adults (Green & Swets, 1966). In contrastto the experimental conditions, data from the control condition revealedthat infants were unable to differentiate the two groups of tokens arrangedin arbitrary groupings.

DISCUSSIONInfants 7 to 8.5 months of age clearly differentiated the timbre ofcomplex tones and they did so in the context of irrelevant variations infundamental frequency, duration, or intensity. Thus, they could attend

to cues associated with spectral structure and, at the same time, ignoreirrelevant acoustic cues. Infants failure to discriminate between setscomposed by arbitrarily grouping the exemplars (control condition) in-dicates that their performance was not based on memorization of ex-emplars in the background set. This does not preclude the possibilitythat memory facilitated infants differentiation of spectral structure inthe other conditions, for memorability is enhanced when the to-be-re-membered items can be organized in some way (Tulving & Donaldson,1972). Thus, if memorization contributed to infants performance in thevariation conditions, this was undoubtedly due to the perceived similarityof exemplars within each category. In short, infants displayed unequi-

TABLE 1PERFORMANCE AS A FUNC TION OF EXPERIMENTAL

CONDITIONCondition Mean d r(df = 9)Frequency variation 2.06 8.93*Duration variation 2.85 9.84*Intensity variation 1.91 7.32*Control-mixed variation 0.01 0.06

* p < .0005, one-tailed.


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INFAN TS PERCEEION OF TIMBRE 309vocal evidence of equivalence classification for some of the cues asso-ciated with timbre.It is clear that infants can differentiate complex tones on the basis ofcontrasting component frequencies, as in Clarkson et al. (1988), or onthe basis of differences in spectral structure or shape, as in the presentinvestigation. These findings also extend our knowledge of equivalenceclassification in infant audition. Not only can infants categorize spokensyllables on the basis of vowel (Kuhl, 1979, 1983) or consonant (Hillen-brand, 1984) identity, and auditory sequences on the basis of pitch con-tour (Trehub et al., 1987), component intervals (Cohen et al., 1987), orrhythmic patterning (Trehub & Thorpe, 1989), they can also categorizecomplex tones based on their inferred pitch (Clarkson & Clifton, 1985)or on their timbre (the present study).Infants ability to categorize static, single tones by their spectral struc-ture or shape has wide-ranging implications. It suggests provocative andadmittedly speculative parallels to their categorization of dynamic, mul-titone sequences (Trehub et al., 1987) on the basis of melodic contour(i.e., configuration of successive fundamental frequencies) and to theircategorization of spoken syllables on the basis of pitch contour (Kuhl& Hillenbrand, 1979). It is also consistent with evidence of infantsdiscrimination of voice onset time on the basis of spectral cues (Eilers,Morse, Gavin, & Oller, 1981; Miller & Eimas, 1983; Soli, 1983) and withthe development of prototypical spectral representations in infancy (Jus-czyk, 1985). A spectral basis for speech and timbre discriminations doesnot preclude the achievement of such discriminations on the basis oftemporal cues or the possibility of trading relations between spectral andtemporal cues (Best, Morrongiello, & Robson, 1981; Eimas, 1985b; Repp,1982).In the case of stimulus sequences such as melodies or running speech,there is evidence that infants attend principally to the overall frequencyconfiguration (Femald & Kuhl, 1987; Trehub, 1987; Trehub et al., 1987)as opposed to specific component frequencies or phonetic cues. Similarly,they may attend preferentially to the frequency configuration of steady-state stimuli, such as those of the present investigation.A frequency configuration strategy in the early months of life wouldprovide infants with a preliminary means for organizing successive aswell as simultaneous frequency components. Conceivably, prelinguisticlisteners could use spectral shape as a basis for processing speech, music,and other auditory input. This would provide a global alternative toanalytic accounts of speech perception that necessitate access to phoneticsegments or features (see Kuhl, 1985, 1987), and a parsimonious alter-native to special speech-processing mechanisms in the prelinguisticperiod (e.g., Eimas, 1985a). It would also provide infants with a basisfor distinguishing between male and female voices (Miller, 1983; Miller


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310 TREHUB, ENDMAN, AND THORPEet al., 1982). Although such voices typically differ in fundamental fre-quency, infants are nevertheless able to segregate male and female voicesthat cannot be distinguished by fundamental frequency (Miller et al.,1982). Formant frequencies and spectral shape are likely to contribute,as well, to male and female voice quality. If infants focused on spectralshape, this would also facilitate recognition of their mothers voice, afeat accomplished in the early weeks of life (Brown, 1979; DeCasper &Fifer, 1980; Mehler, Bertoncini, Barriere, & Jassik-Gerschenfeld, 1978).Finally, there are claims that adults, and possibly infants, representphonetic categories as prototypes (Miller, Connine, Schermer, & Kleun-der, 1983; Quinn & Eimas, 1986; Samuel, 1982). It is intriguing to spec-ulate whether these prototypic representations are based on goodspectral form, which may be analogous to good melodic form (Bhar-ucha, 1984; Cohen et al., 1987; Trehub, 1987; Trehub, Cohen, Thorpe,& Morrongiello, 1986), and which may entail comparable processingadvantages.

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Child Development, 54, 858-867.RECEIVED: June 5, 1989; REVISED: August 21, 1989.

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Infants’ Perception of Timbre