Perception & Psychophysics1974, Vol. 16 (3), 513-521

Auditory and linguistic processing of cuesfor place of articulation by infants*

PETER D. EIMAS

W. S. Hunter Laboratory ofPsychology, Brown University, Providence, Rhode Island 02912

Two- and.3-mo~th~0Id infants ~ere found to discriminate the acoustic cues for the phonetic feature ofplace of articulation III a categorical manner; that is, evidence for the discriminability of two syntheticsp~ech patter~s ~as. pre~~nt only when the stimuli signaled a change in the phonetic feature of place. Noevidence of discriminability was found when two stimuli, separated by the same acoustic difference,signaled acoustic variations of the same phonetic feature. Discrimination of the same acoustic cues in anonspeech context was found, in contrast, to be noncategorical or continuous. The results were discussedin terms of infants' ability to process acoustic events in either an auditory or a linguistic mode.

A major conclusion of the research on theperception of speech over the past two decades is thathuman listeners perceive speech quite differently fromthe way in which they perceive nonspeech sounds.Speech perception is said to occur in a linguistic orspeech mode as opposed to an auditory mode (Eimas,in press; Liberman, Cooper, Shankweiler, &Studdert-Kennedy, 1967; Liberman, 1970; Studdert­Kennedy, 1974). Evidence for this conclusion comesfrom studies in which listeners identified anddiscriminated series of synthetic speech sounds thatvaried continuously along a single acoustic dimension.Listeners were typically quite consistent in assigningphonetic labels to the various stimuli. Moreover, andmost importantly, the ability to discriminate pairs ofstimuli was strongly determined by the phoneticassignments. Thus, two stimuli that were acousticvariations of the same phonetic category or featurewere discriminated only slightly better than would beexpected by chance, whereas two stimuli that wereseparated by the same acoustic difference butmembers of different phonetic categories were highlydiscriminable. This form of perception has beentermed categorical and is particularly apparent forthose acoustic dimensions that distinguish the stopconsonants (Liberman, 1957; Liberman, Harris,Hoffman, & Griffith, 1957; Eimas, 1963; Mattingly,Liberman, Syrdal, & Halwes, 1971; Lisker &Abramson, 1970; Abramson & Lisker, 1970; Pisoni,1973). Categorical-like perception has also beenfound with other consonantal distinctions (e.g.,Miyawaki, Liberman, Fujimura, Strange, & Jenkins,

*The studies reported here were supported by Grant HD 05331from the National Institute of Child Health and HumanDevelopment. United States Public Health Service. I wish to thankJane Carlstrom. Jean Fechheimer, and Sheila Polofsky for theircapable assistance in the collection and analysis of the data. I alsowish to acknowledge my indebtedness to Franklin S. Cooper for hisgenerosity in making the facilities of the Haskins Laboratoriesavailable for the preparation of stimulus materials.

1972) and, to a considerably lesser extent, with vowelstimuli under some conditions (Pisoni, 1973; Stevens,Liberman, Studdert-Kennedy, & Ohman, 1969;Fujisaki & Kawashima, 1969).

Although these findings are of considerableimportance in and of themselves, they are particularlyrevealing when the discriminability functions for thesynthetic speech stimuli are compared with thediscriminability functions for the same acousticdimensions when presented in a nonspeech context.Mattingly et al (1971) showed that the discontinuitiesin the discriminability functions, so apparent whenthe synthetic sounds were perceived as speech, wereabsent when the stimuli were not perceived as speech.More specifically, they showed that variations in thestarting frequency and direction of the second­formant transition (which are sufficient for theperceived distinctions among the voiced stopconsonants [b, d , g) and their voiceless counterparts[po t, k)) were perceived categorically and continuouslyin speech and nonspeech contexts, respectively. Thelatter condition was arranged by presenting only thesecond-formant transitions which are heard asbird-like chirps. Similar effects have also beenobtained by Miyawaki et al (1972). In addition,Eimas, Cooper, and Corbit (1973) have shown theacoustic information signaling the onset of voicing isperceived differently when presented in a speech asopposed to a nonspeech context.

Although it is certainly the case that all sounds,speech or nonspeech, must undergo some commonauditory analysis, the sounds of speech would appearto undergo some additional, specialized processingthat permits the extraction of distinctive phoneticfeatures. It is the process of feature extraction that iscategorical in that it reduces the continuous variationin acoustic-auditory information to a set of discretefeature values. The actual phonetic experience, thatis, the perception of a particular phone involvesadditional higher processes, including most likely

513

514 EIMAS

matching a set of features with its appropriate label.Add itional evidence favoring a special speech

processor comes from studies of dichotic listening. Anumber of researchers have shown that speech signalsare better perceived by the left hemisphere (right ear)whereas nonspeech signals are better processed by theright hemisphere (left ear) (Kimura, 1961, 1964;Studdert-Kennedy &, Shankweiler, 1970). Electro­physiological studies of neural activity have also,supported this conclusion in adults (Wood, Goff, &Day, 1971). \J

Of particular interest have been the recentelectrophysiological (Molfese, 1972) and behavioralfindings (Eimas, Siqueland , Jusczyk , & Vigorito,1971) that the specialized speech processes may beoperative as early as the first few weeks of life. Eimaset al (1971) investigated the infant's ability todiscriminate small differences in voice onset time.Voice onset time, which is defined by Lisker andAbramson (1964) in articulatory terms as the timebetween the release burst and the onset of laryngealpulsing or voicing, is a sufficient cue fordistinguishing between the voiced and voiceless stopconsonants [b] vs [p], [d] vs [t], and [g] vs [k]. Theyfound that infants 1 and 4 months of age were betterable to discriminate a 20-msec difference in voiceonset time when the two stimuli to be discriminatedwere from different adult phonetic categories, [b] and[p], than when they were from the same adultphonetic category, [b] or [pl. As was true for adultlisteners (Abramson & Lisker, 1970), acousticvariations of the same phonetic feature were notdiscriminable. In later studies, Eimas (in press) wasable to replicate these results with the apical stops,[d, t], and to find some evidence for the categoricalperception by young infants of a voicing distinctionnot found in English.

Additional research has shown that infants betweenthe ages of 2 and 6 months are sensitive to differencesin place of articulation, acoustically represented byvariations in the second- and third-formanttransitions (Moffitt. 1971; Morse, 1972). AlthoughMorse has contended that infants process placedistinctions in a linguistically relevant manner, thesupporting evidence was not compelling. It is toward aresolution of this issue, that is, the manner in whichplace information is perceived by infants, that thepresent studies were directed. Another purpose was todetermine whether the mechanisms that are necessaryto decode information signaling place of articulationare, as is apparently the case with adult listeners, theunique property of the speech processor.

EXPERIMENT I

The purpose of Experiment I was to determinewhether the perception of place distinctions by younginfants was categorical. The stimuli were synthetic

speech sounds that varied in the starting frequencyand direction of the second- and third-formanttransitions. These variations correspond to thevariations in articulatory movements necessary for theproduction of bilabial stops [b, p] as opposed to apical[d, t] or velar [g, k] stops. Discrimination of aparticular acoustic difference was measured undertwo conditions: (1) when the acoustic variationsignifies a change in place of articulation as measuredby adult identification functions, and (2) when thevariation represents acoustic variants of the sameplace of articulation. Evidence of greater discrimin­ability in the first condition permits the inference thatinfants are capable of perceiving information relatedto place distinctions in a categorical fashion andhence in a linguistically relevant manner.

Method

Procedure. The procedure was a modification of the methodologydeveloped by Siqueland and DeLucia (1969). Each infant was testedindividually in a small sound-shielded room. moderatelyilluminated by the rear-projected image of a colorful object. Thevisual image, in addition to providing illumination, tended tomaintain the infant's orientation to the speaker, which was situatedjust above the screen, about 45 em from the infant's head. Theinfant was placed in a reclining seat and presented with a blindnipple. The nipple was held gently by one of the Es , who listened tomusic over a set of headphones. The second E monitored therecording apparatus and controlled the presentation of the speechpatterns'. During the first 2 or 3 min, the high-amplitude criterionand the baseline rate of high-amplitude sucking were established.The amplitude criterion was defined as the level which yielded abaseline sucking rate of approximately 20 or 30 responses/min. Bypermitting the amplitude criterion to vary from infant to infant, itwas possible to reduce the variability associated with baseline ratesof sucking as well as to establish a baseline rate for each infant suchthat changes in either direction could occur without seriouscontamination by either floor or ceiling effects. Immediately afterobtaining the baseline rate, the first speech sound was madecontingent upon high-amplitude sucking. If the time between eachhigh-amplitude sucking response was at least t sec, then each s'tichresponse produced one presentation of the stimulus pattern300 msec in duration plus 700 msec of silence. However, if theinfant produced a burst of sucking responses with interresponsetimes less than I sec, as was typical, then each response did notproduce one presentation of the stimulus. Rather, each responserecycled the timing apparatus and the I-sec on period began again.This limitation in the presentation of auditory feedback wasimposed to prevent the occurrence of the reinforcing sound longerthan t sec after the last response.

The presentation of an auditory stimulus in this manner typicallyresults in an increase in the infant's rate of high-amplitude sucking,compared with the baseline rate. After several minutes of thiscontingency (the time varies from infant to infant from about 4 or5 min to over 15 min), the infant usually shows a decrement inperformance, presumably as a result of a diminution in thereinforcing quality of the once novel speech stimulus. When the rateof sucking diminished by 20% or more for 2 consecutive minutescompared with the minute immediately preceding the first minuteof decrement, the feedback stimulus was changed withoutinterruption by switching the channel selector on the tape deck. Thesecond synthetic speech pattern was presented, likewise contingentupon high-amplitude sucking, for 4 min, after which the testingsession was terminated.

One group of infants, Group D, received two stimuli, one ofwhich signaled the adult phonetic category [d] and the other the

ACOUSTIC CUES FOR PLACE OF ARTICULATION IN INFANTS 515

phonetic category [g]. The second group of infants, Group S,likewise received two stimuli, but these sound patterns wereacoustic variations of the same adult phonetic category [dl. Theorder in which the two stimuli were presented was counterbalancedacross infants in both groups. The infants in the control condition,Group C, were randomly assigned one of the four stimuliadministered to Groups D and S. At the point at which a change instimulation would have occurred for the infants of Groups Sand D,the records of the control infants were marked, the channel selectorcontrol was switched, and the session continued for 4 min. Thus,the control records were completely comparable with those of theexperimental infants. Given that infants are highly responsive tonovel stimuli, the presentation of a new, discriminable stimuluswould be expected to result in a different rate of sucking whencompared with the sucking rates for infants who did not receive achange in stimulation. Thus, either an increase in response rateassociated with a change in stimulation greater than that shown bythe control infants or a decrease less than that of the controls istaken as inferential evidence that the infants perceived the twostimuli as different,

Stimuli. The stimuli were four synthetic speech sounds preparedby means of the parallel resonance synthesizere at the HaskinsLaboratories by Pisoni (1971). Each stimulus was 300 msec induration, with the initial 40 msec simulating a period of closurevoicing by means of a low-amplitude first formant centered at150 Hz. The next 40 msec was a period of transition during whichall three formants moved from their starting frequencies to theirterminal steady-state values of 743, 1,620, and 2,862 Hz for thefirst. second, and third formant, respectively. These steady-stateformant values are appropriate for the American English vowel[ae]. The steady-state portion of each pattern was 220 msec induration. The starting values for the second- and third-formantsare given in Table I for each ofthe two stimuli presented to GroupsD and S. along with their phonetic identification as determined byadult listeners (Pisoni, 1973). All four stimuli had the samefirst-formant starting values, ISO Hz. As may be seen in Table I.the only differences among the stimuli were in starting frequenciesand direction of the second- and third-formant transitions (and, ofcourse, in one instance. in the phonetic identification or place ofarticulation value). Of importance to note is the fact that theacoustic differences between the two stimuli used for Group D werevery nearly identical to the differences between the stimuli used forGroup S. Thus. any differences in discriminability between the twogroups cannot be attributed to an inequality in the acousticdifferences between the stimuli. For additional details concerningthe construction of these stimuli, the reader is referred to Pisoni(1971).

The stimuli were recorded on high-quality magnetic tape fromwhich continuous loops were made, with each 300-msec speechpattern separated by 700 msec of silence. There were several copiesof each tape loop. The loops used for Groups D and S had onestimulus on Channel I and the second stimulus on Channel 2. Thecontrol tapes contained the same stimulus on both channels. Thisarrangement of the stimuli permitted all infants to be treated in alike manner as well as the nearly instantaneous switching betweenstimuli.

Apparatus. Part of the apparatus was a blind nipple on which theinfant sucked. The positive pressure generated by the infant'ssucking was transduced to provide a record of all sucking responsesas well as a digital record of criterional high-amplitude suckingresponses by means of a HP 7202B polygraph and a Hunter digitaltimer. Additional equipment included a two-channel tape deck(Sony Model TC 850), a VM Model 33-1 speaker, power supply,HP 8805A preamplifier, and Lafayette 5710 event timer, arrangedto provide auditory feedback when the power supply was activatedby a criterion response. Each sucking response of criterionamplitude activated a power supply for 1 sec (or restarted a periodof l-sec activation), which resulted in a rapid increase in theintensity of auditory feedback stimulation from an inaudible level toone about 13 to 15 dB above the background noise level of 63 dB.

Table 1Starting Frequencies (in Hz) of the Second- and

Third-Formant Transitions: Experiment

Stimulus 1 Stimulus 2 Difference

Group DF-2 1845 1996 151F-3 2862 2525 337

Group SF-2 1541 1695 154F-3 2862 3195 333

Note-Adult listeners consistently identified the stimuli forGroup S and Stimulus 1 for Group D as [d], Stimulus 2 forGroup D was identified as fsl.

Intensity readings were made with a General Radio Type 155l-Csound-level meter, B scale. The background noise was producedalmost entirely by the room exhaust system and the slide projectorfan.

Subjects. The Ss were 24 2-month-old and 24 3-month-oldinfants from the Greater Providence, Rhode Island, area. Half ofthe infants at each age level were males and females. In order toobtain complete data on 48 infants, it was necessary to test 115infants. The success rate of approximately 40% is typical of thesatiation/release-from-satiation procedure that we have used toassess the speech processing capacities of young infants. There wereno reliable differences in failures due to experimental condition,age, or sex. The reasons for failure to complete testing were manyand included such factors as falling asleep (33%), crying (25%),initial failure to suck on the nipple (20%), ceasing to suck duringthe course of the experiment (7%), and a group of factors consistingoffailure to show satiation, an extremely erratic pattern of sucking,equipment failure, and E error (15%). The majority of the infants(78%) who failed to complete the session were eliminated prior tothe shift in stimulation. There were no reliable differences in theinfants who were eliminated after the shift as a function of sex orexperimental treatment, and 80% of these infants were eliminatedfor crying or falling asleep. The criterion for the former was loudcrying and a failure to take the nipple. whereas the criterion for thelatter was the characteristic sucking pattern associated with sleep.that is. brief bursts of very low-amplitude sucking. Most of thepostshift eliminations occurred during the first or second postshiftminute.

As often as possible, the infants were assigned randomly to thethree conditions. However, given the high attrition rate and therestrictions that the three groups (each with 16 infants) becounterbalanced with respect to age. sex, and order of stimulus

, presentation. random assignments could not always be made.

Results and DiscussionThe mean number of sucking responses is displayed

in Fig. 1 as a function of minutes and treatments. Ananalysis of the minute-by-minute sucking rates for the5 min immediately prior to the shift in stimulation (orat that point in time when a shift would have occurredin the case of the control infants) revealed nonreliabledifferences between groups. As would be expectedfrom the nature ofthe experiment, the mean responserate for the 2 min before shift (the satiation period)was significantly lower than the third minute beforestimulus change (p < .01).

Inasmuch as the measure of discriminability isbased on a change in response rate correlated with achange in stimulation, and given that there wereindividual differences in response rates during thesatiation period, the analyses of postshift performance

Fig. 1. Mean number of sucking responsesas a function of time and experimentalconditions. Time is measured with referenceto the moment of stimulus shift, which isindicated by the dashed line. The baseline rateof sucking is indicated by the letter"B."

I

IIIIIIIIII

:~IIIII

I I, I

1 1 3'.

S 5 4

•

GROUP CGROUP S

..L L...L....!.-l.....L-L-LJ...1.-..L~S 5 4 " 2 I 1 234

516 EIMAS

75GROUP 0

I'"w I'"z I0 60 I~

'" I......0';

o IZ

45 I:>2 I /"V:> ,...../,.'"u, ,0CIt 30 Iw IOIl

~ I:::> •Z IZ 15 I-<I:w I~

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8 5 " 3 2 11 1 2 3 4

TiME (min)

were conducted on difference scores. For each infant,the mean response rate for the 2 min immediatelypreceding the stimulus shift was subtracted from eachof the four l-rnin measures of postshift performance.An analysis of variance, Groups by Minutes,performed on these difference scores, revealed asignificant groups effect, F(2,45) = 4.5, p < .025,and a significant Groups by Minutes interaction,F(6,135) = 8.4, p < .001. Group D, which showed amean increment of 4.1 responses/min for the 4-minpostshift period, differed reliably from Groups SandC (p < .025 in each instance). Groups Sand C, whichshowed mean decrements of 6.1 and 4.8responses/min, respectively, did not differ signif­icantly. The significant interaction, as can be seen inFig. 1, was primarily a function of the increase in rateof responding shown by Group D over the 4-minpostshift period, whereas Groups Sand C tended toshow a decreasing rate of response with time.!

The cues for place of articulation, like the cues forvoice onset time, are discriminated in acategorical-like manner by young infants; that is tosay, the infant's ability to discriminate place cues isdetermined largely by whether or not the acousticstimuli signal the same or different values along thephonetic feature, place of articulation. Beforeconsidering the perceptual mechanisms that might beresponsible for processing of speech in a linguisticmode, we wish to consider whether these mechanismsare part of the more general auditory processingsystem or whether they are part of the specializedspeech processor.

EXPERIMENT II

Experiment II compared the infant's ability todiscriminate acoustic cues for place when these cues

are presented in a speech context, as in Experiment I,and when they are presented in a non speech context.The speech stimuli for this experiment weretwo-formant synthetic speech patterns, which variedin the starting frequency and direction of thesecond-formant transition only. AU other characteris­tics of the stimuli did not vary from one pattern toanother. Variations in the second-formant transitionare by themselves sufficient cues to signal variations inplace of articulation, and indeed, as Liberman et al(1967) have noted: " ... [the second-formanttransition] is probably the single most importantcarrier of linguistic information in the speech signal[p, 434]." Based on identification functions fromadult listeners obtained by Mattingly et al (1971),pairs of synthetic speech patterns, separated by aconstant acoustic difference, were selected such thatthe two stimuli were perceived either as differentphones, [b] and [d], or as the same phone, [b]. Thetwo different types of stimulus pairs corresponded tothe Group D and Group S stimuli of Experiment 1.In order to present the identical relevant acousticinformation in a non speech context, additionalpatterns were synthesized that eliminated all acousticfeatures, except the second-formant transitions of thespeech stimuli. These patterns, 40 msec in duration,are perceived as bird-like chirps by adult listeners(Mattinglyet aI, 1971). Thus, both sets of stimuli varyin precisely the same manner, but yet are perceived inradically different ways by adult listeners.

Should both sets ofstimuli be perceived in the samecategorical manner, then it is possible to concludethat, at least during infancy, the more generalauditory processing system is differentially sensitive tovariations in rapidly changing frequency values.Furthermore, it would seem that the points of greaterdiscriminability were selected to mark phonetic

ACOUSTIC CUES FOR PLACE OF ARTICULATION IN INFANTS 517

boundaries (ct. Stevens, 1972; Stevens et ai, 1969).However, should categorical-like perception becharacteristic of only the speech stimuli (as in adultlisteners), then categorical decisions regardingphonetic feature values cannot be ascribed to thefunctioning of the general auditory processing system,but rather must be considered to be a result of someform of additional linguistic processing to which onlythe sounds of speech are subjected.

Method

Table 2Starting Frequencies (in Hz) of the Second-Formant

Transitions: Experiment II

Stimulus Diffe-1 2 renee

Group 0Pair 1 F-2 1312 1541 229Pair 2 F-2 1386 1620 234

Group S F-2 1232 1465 233

Note-Adult listeners consistently identified the stimuli forGroup S and Stimulus 1 for both Group D pairs as [b],Stimulus 2 for both Group D pairs was identified as [d],

Speech +8.5 -4.6 -1.2Nonspeech +4.0* +7.9 +.6

"In a replication study with nine infants, the mean incrementin response rate was +7.3 responses/min.

Table 3Mean Recovery in Response Rate (Responses Per Minute) for

the Entire 4-Min Postshift Period: Experiment II

after the stimulus change did not differ reliably due to sex orexperimental treatment and most of the infants were eliminatedbefore the third postshift minute. When possible, infants wererandomly assigned to the six conditions. But, again, the highattrition rate and the restrictions that the six groups be balanced asclosely as possible for age, sex, order of stimulus presentation, and,in the case of the infants in Group D. for stimulus pair, completelyrandom assignments were simply not possible.

Experimental ConditionGroup 0 Group S Group CStimuli

Results and DiscussionAn analysis of the response rates for the 5 min

immediately preceding the stimulus change revealedno reliable differences due to experimental conditionsor type of stimulus, speech or nonspeech. All sixgroups showed a reliable increment in sucking rate atthe third minute before shift compared with thebaseline rate (p < .01) and a reliable decrementduring the final 2 min before shift, as would beexpected (p < .01).

Difference scores were again used in all analyses ofpostshift performance. The mean changes in responserates over the final 4 min are shown in Table 3.Preliminary analyses of the postshift data revealed nosignificant differences as a function of age, sex, orderof stimulus presentation, or, in the case of the infantsassigned to Group D, the stimulus pair. An analysisof variance, Type of Stimulus (speech vs nonspeech)by Groups (D vs S vs C) by Minutes yielded only onesignificant effect, namely, the Groups by Type ofStimulus interaction, F(2,90) 4.1, P < .025).Individual comparisons revealed the following:(l) When the stimuli were synthetic speech patterns,there was evidence for categorical perception in thatGroup D responded at a reliably higher rate than didGroups Sand C, which did not differ from each other.(2) When the stimuli where nonspeech sounds, there

Stimuli. The speech stimuli were six synthetic speech patternsconstructed by means of a parallel resonance synthesizer at theHaskins Laboratories by Mattingly et al (1971). Each stimulus was245 msec in duration. The initial 15 msec simulated closure voicingby means of a low-amplitude first formant at 150 Hz. The following40 msec was a transitional period, during which the first andsecond formants moved from their starting values to their terminalsteady-state values of 743 and 1,620 Hz, respectively. Thesteady-state portions were 190 msee in duration. These stimuli wereperceived as the voiced stops [b] or [d] plus the vowel [ae], Thenonspeech stimuli, 40 msec in duration, were likewise synthesizedby means of the parallel resonance synthesizer. Each non speechpattern corresponded exactly to one of the second-formanttransitions of the speech stimuli. Table 2 gives the startingfrequencies for the six second-formant transitions. The differencein the second-formant starting frequency between the two stimuli ofthe two pairs used for Group D and the single pair used forGroup S was approximately the same. This equality held, of course,for both the speech and nonspeech stimuli. The stimuli wererecorded and made into continuous loops, as in Experiment 1. Thesilent interval between successive stimuli was 755 msec for thespeech sounds and 960 msec for the nonspeech sounds.

Apparatus. The apparatus was the same as that used inExperiment 1.

Procedure. The general procedural details were the same as thoseused in Experiment 1. For each type of stimulus, speech ornonspeech, there were three groups of 16 infants, D, S, and C. Theinfants of Group D (both speech and non speech) received one oftwo pairs of stimuli. half of the infants receiving one pair and theremaining infants the other pair. When the stimuli were speechsounds. one stimulus of each pair signaled the phonetic category[bl. while the second stimulus signaled the phonetic category [d].Two different pairs were used to be sure that. in at least oneinstance, the two stimuli lay on opposite sides of the phoneticboundary. The infants in Group S received a single pair of stimuli,which, when speech sounds, were both representatives of thecategory [b]. The control infants (Group C) received one of the sixspeech or six non speech stimuli, randomly selected. All stimuli werepresented at approximately 15 dB above a background noise levelof63 dB (B scale, Type 1551-C General Radio sound-level meter).

Subjects. The Ss were 96 infants from the Greater Providence,Rhode Island, area. Approximately half of the infants were malesand half females. Likewise, approximately half of the sample were 2months of age and the remaining infants were 3 months old.Forty-four percent of the infants tested completed the experiment,and there were no reliable differences in the rate of failure due toage, sex, type of stimulus, or experimental treatment. Of the 122infants who failed to complete the experiment, 34% cried, 27% fellasleep. 12% failed to suck on the nipple at all. 80/0 stopped suckingat some time during the course of the experiment, and 18% wereeliminated for a variety of reasons. including failure to achieve thesatiation criterion, very erratic responding, and equipment failure.The majority (86%) of the infants who failed to complete theexperiment were eliminated before the stimulus change. and ofthose who were eliminated after the change, 83% either fell asleepor cried. As in the first experiment, the infants who were eliminated

518 EIMAS

was no evidence for categorical perception; Groups Dand S did not differ reliably. (3) Although Group S diddiffer from Group C, the infants of Group D did notdiffer significantly from the control infants when thestimuli were non speech sounds.

It is, of course, possible that the auditoryprocessing system finds some pairs of nonspeechsounds more discriminable than other pairs (cf,Mattingly et al, 1971). On the other hand, it islikewise possible that the slightly depressedperformance of the nonspeech Group D (comparedwith Group S) was due to sampling error. Indeed, thedifference between thq two groups could be attributedto the relatively poor performance of only four infantsin Group D. To test these alternatives, an additionalnine infants were tested on their ability todiscriminate the two pairs of nonspeech chirpspreviously used for Group D. The results indicate thatthese stimuli can be reliably discriminated and thatthe original estimate of discriminability may well havebeen underestimated due to sampling error: the meanincrease in response rate was 7.3 responses (p < .05),very near the performance level of Group S.

Regardless of the true level of discriminability forthe stimuli of the nonspeech Group D, it is the casethat infants discriminate variations in thesecond-formant transition quite differently when theyare presented in a speech context as opposed to anonspeech context. Further confirmation of thisconclusion comes from comparisons of Groups D andS, which received speech stimuli, with the samegroups which received nonspeech stimuli. A 2 by 2analysis of variance revealed a highly significantGroups by Type of Stimulus interaction, FO,60) =20.5, P < .001: Group S, which received speechstimuli, responded less frequently than did theremaining three groups, which, in turn, did not differsignificantly from one another.

In summary, the results of these two experimentsindicate that infants are capable of categorizingcontinuous variations in the acoustic dimension thatsignal adult phonetic distinctions corresponding toplace of articulation. Moreover, the manner in whichthis categorization takes place agrees perfectly withthe three major and possibly universal phoneticdistinctions based on place of articulation." Finally,the processing system that subserves perception of thephonetic feature of place differs from the system thatis capable of processing the same acoustic informationin a nonspeech context.

GENERAL DISCUSSION

It remains now to consider 0) the perceptualmechanisms that could be responsible for thecategorical perception of place cues in speechcontexts, (2) how acoustic differences that are

discriminable in one context (nonspeech) might not bediscriminable in another context (speech), and(3) what functions perception in a speech mode mightserve in the infant during the earliest stages oflanguage acquisition. .

The major theoretical positions pertaining to theperception of speech are complex models, in thatperception is assumed to be more directly linked tointernal mediating events than to external acousticevents or their immediate auditory transforms (see,for example, Liberman, 1957, 1970; Liberman et ai,1967; Stevens & House, 1972). Thus, in the motortheory of speech perception of Liberman and hisassociates, decoding of the incoming acoustic signal ismade by reference to invariant motoric commands tothe articulatory system, undoubtedly at the level ofneural activity. Exactly how the acoustic informationmakes contact with these central articulatorycommands has not as yet been fully explicated. In asimilar manner, the analysis-by-synthesis model ofStevens and his colleagues makes the assumption thatperception occurs when the temporarily stored speechevent, in auditory form, is matched by a second,internally generated or synthesized representation ofthe speech event in question. This process ofgeneration must necessarily involve considerableknowledge of production processes, more specifically,of the rules by which distinctive phonetic features areconverted into articulatory commands and of the rulesby which auditory patterns are associated with these

. commands.Although motor theories of speech perception are

possible models of adult speech perception, theapplication of this class of model to thespeech-processing capacities of the infant raisescertain difficulties. Most prominent of these is theneed to explain how the young, virtually inarticulateinfant has come to possess the complex knowledgerequired for the conversion of phonetic features toarticulatory commands, and how he has come toassociate particular auditory patterns with articula­tory commands. Without ascribing this knowledge tothe biological endowment of the infant, there wouldappear to be no way in which an infant could haveacquired this information by traditional means duringthe first few weeks or months of life. However, toattribute this knowledge to the infant's biologicalendowment would seem to extend considerably thecognitive competencies that we are willing to imputeto genetically determined factors.

A number of researchers have begun to consideralternate models of speech perception that are basedon feature detectors and that do not requireknowledge of production routines (Abbs & Sussman,1971; Cole & Scott, 1972; Cooper, in press; Eimas &Corbit. 1973; Eimas, Cooper. & Corbit, 1973;Lieberman, 1970; Stevens. 1972, 1973).3 Although

ACOUSTIC CUES FOR PLACE OF ARTICULATION IN INFANTS 519

feature detectors which permit phonetic decisionscould be either auditory or linguistic in nature, theweight of evidence favors linguistic feature detectors(but see Stevens, 1973, for an account of placediscrimination based on auditory property detectors).

Recently, Eimas and Corbit (1973) and Eimas,Cooper, and Corbit (1973) have presented evidenceconsistent with the hypothesis that there existphonetic feature detectors for the acousticconsequences of the two modes of voicing found inEnglish and numerous other languages (Lisker &Abramson, 1964). These detectors were assumed to besensitive to relatively restricted ranges of complexacoustic energy (voice onset time), part of the centralspeech processing system, and finally the directmediators of the categorization of voicinginformation. Similarly, Cooper (in press) and Cooperand Blumstein (1974) have found evidence for theexistence of phonetic feature detectors underlying theperception of the acoustic consequences of the threemajor place distinctions. If these phonetic featuredetectors exist and are operative early in the lives ofinfants, then it is possible to accommodate thecategorical perception of infants without recourse tomotoric theories of speech perception (Eimas, inpress; Cutting & Eimas, in press). Essentially, it hasbeen argued that, given the passive nature of a featuredetector system of analysis, the presentation of anacoustic signal with sufficient linguistic informationto activate the speech processor (cf. Molfese, 1972)will likewise activate those phonetic feature detectorsfor which there is an adequate stimulus. Continuedpresentation of the same speech sound eventuates inthe adaptation of the stimulated detectors. Thisadaptation may be related to the diminution of thereinforcing properties of novel speech stimuli as theybecome increasingly familiar and the subsequentreduction in effort to obtain the stimulus. Theintroduction of an acoustically different, butphonetically identical, speech pattern will not beexperienced as novel by the infant, whereas anacoustically and phonetically different speech soundwill be perceived as novel. From a phonetic featureanalysis and the infant's strong attraction to novelty,it is easily predicted that the postshift period ofGroup D should be marked by a substantialincrement in response rate to obtain the new stimulus.On the other hand, the postshift period of Group S(and, of course, of Group C) should reveal acontinued decrease in the rate of responding. As foraccommodating the continuous discrimination ofsecond-formant transition variations when presentedin a nonspeech context, we need only assume thatthere was not sufficient linguistic information in thesignal to activate the -speech processor. Hence, theanalysis of these sounds is restricted to the auditoryprocessing system, the discriminative capacity of

which is more closely tied to physical (acoustic)differences per se than is that of the speech analyzers.(For evidence pertaining to the insensitivity of thespeech processor to variations in the acousticdifference between two phonetically distinct sounds,the reader is referred to Eimas, in press.) In essence,then, .we have tried to argue that the discriminabilityof speech in infants is based primarily on the detectionof different values of one or more phonetic features,such as voice onset time or place of articulation. 4

The ability of infants (and adults) to discriminatecertain variations of second-formant transitions in anonspeech context and yet be unable to discriminatethe same acoustic information when it is presented ina speech context is a seemingly paradoxical finding.As mentioned above, it must be the case that acousticevents, regardless of their nature, initially undergo aconsiderable degree of common auditory analysis, butas the results indicate, some of the auditoryinformation from this common processing. isapparently lost or not available when the stimuli arespeech signals. Perhaps the information arising fromthe additional linguistic analysis that only the soundsof speech undergo takes precedence in some mannerover the output information of the auditory analyzers.Of equal likelihood, however, is the hypothesis thatlinguistic processing reduces the amount of auditoryinformation that can be held in a temporary, probablypreperceptual store (cf, Massaro, 1972), as a con­sequence of the additional time and/or processingcapacity required by the linguistic analysis. Thus,after linguistic processing, the relatively brief, butcomplex, auditory information associated withphonetic features becomes unavailable, for all intentsand purposes, to response decision units. Although atthis time the available evidence does not permit aresolution ofthis problem, it is of interest to note thata number of researchers have considered thecategorical nature of speech perception to be due, atleast in part, to memorial processes that affect thestored auditory analogs of the acoustic input (cf.Fujisak i & Kawashima, 1969; Pisoni, 1973).

The capacity of infants to perceive speech in alinguistic mode through the activation of a specialspeech processor with phonetic feature detectorsserves a number of functions. In the first place, thisform of analysis of speech provides the infant with anautomatic means for the recognition of speech. As aconsequence, speech signals will undergo specialprocessing, without conscious decisions on the part ofthe listener (infant or adult), provided only that thereis sufficient linguistic information in the signal toactivate the speech processor (cf. Eimas, Cooper, &Corbit, 1973; Mattingly et aI, 1971). Secondly, thespeech processing system, with its phonetic featureanalyzers, not only provides information concerningthe phonetic feature structure of the speech sounds

520 EIMAS

but also automatically acts to segment the continuousstream of speech into discrete elements-a necessarystep for the analysis of speech and language. Finally,this form of processing, in not requiring that theinfant actively learn to recognize speech or to segmentit into phonetic features must certainly hasten, andperhaps even make possible, the acquisition of humanlanguages.

REFERENCES

ABBS, 1. H., & SUSSMAN. H. M. Neurophysiological feature detec­tors and speech perception: A discussion of theoretical implica­tions. Journal ofSpeech & Hearing Research. 1971, 14.23-36.

ABRAMSON, A. S., & LISKER, L. Discriminability along with voic­ing continuum: Cross-language tests. In Proceedings of the SixthInternational Congress of Phonetic Sciences. Prague. 1967.Prague: Academia, 1970. Pp. 569-573.

COLE, R. A., & SCOTT, B. Phoneme feature detectors. Paperpresented at the meetings of the Eastern Psychological Associa­tion, Boston, Massachusetts, April 1972.

COOl~ER, W. E. Adaptation of phonetic feature analyzers for placeof articulation. Journal of the Acoustical Society of America,

in press.COOPER, W. E., & BLUMSTEIN, S. E. A "labial" feature analyzer in

speech perception. Perception & Psychophysics. 1974, 16,591-600.

CUTTING, 1. E., & EIMAS, P. D. Phonetic feature analyzers in theprocessing of speech by infants. In J. Kavanagh and J. E.Cutting (Eds.), The role of speech in language. Cambridge,Mass: M.LT. Press, in press.

EIMAS, P. D. The relation between identification and discrimina­tion along speech and nonspeech continua. Language & Speech,1963, 6, 206-217.

EIMAS, P. D. Speech perception in early infancy. In L. B. Cohenand P. Salapatek (Eds.), Infant perception. New York:Academic Press, in press.

EIMAS, P. D., COOPER, W. E., & CORBIT, J. D. Some properties oflinguistic feature detectors .•Perception & Psychophysics, 1973,13, 247-252.

EIMAS, P. 0 .. & CORBIT, J. D. Selective adaptation of linguisticfeature detectors. Cognitive Psychology, 1973, 4, 99-109.

EIMAS, P. D., SIQUELAND, E. R., JUSCZYK, P., & VIGORITO, J.Speech Perception in infants. Science, 1971, 171, 303-306.

FUJISAKI, H.. & KAWASHIMA, T. On the modes and mechanisms ofspeech perception. Annual Report of the Engineering ResearchInstitute, Vol. 28, Faculty of Engineering, University ofTokyo, Tokyo, 1969. Pp. 67-73.

KIMURA, D. Cerebral dominance and the perception of verbalstimuli. Canadian Journal of Psychology, 1961, 15, 166-171.

KIMURA, D. Left-right differences in the perception of melodies.Quarterly Journal of Experimental Psychulogy, 1964, 16,355-358.

LIBERMAN, A. M. Some results of research on speech perception.Journal of the Acoustical Society ofA merica, 1957, 29, 117-123.

LIBERMAN, A. M. The grammars of speech and language. Cogni­tive Psychology, 1970, 1, 301-323.

LIBERMAN, A. M., COOPER, F. S., SHANKWEILER, D. P., &STUDDERT-KENNEDY, M. Perception of the speech code.Psychological Review. 1967, 74,431-461. .

LIBERMAN, A. M., HARRIS, K. S., HOFFMAN, H. S., & GRIFFITH,B. C. The discrimination of speech sounds within and acrossphoneme boundaries. Journal of Experimental Psychology,1957, 54, 358-368.

LIEBERMAN, P. Towards a unitied phonetic theory. LinguisticInquiry, 1970, 1. 307-322.

LISKER, L., & ABRAMSON, A. S. A cross-language study of

voicing of initial stops: Acoustical measurements. Word, 1964,20, 384-422.

LISKER, L., & ABRAMSON, A. S. The voicing dimension: Someexperiments in comparative phonetics. in Proceedings of theSixth International Congress ofPhonetic Sciences. Prague, 1967.Prague: Academia, 1970. Pp. 563-567.

MASSARO, D. W. Preperceptual images, processing time, and per­ceptual units in auditory perception. Psychological Review, 1972,79, 124-145.

MATTINGLY, I. G .. LIBERMAN, A. M., SYRDAL, A. K., &HALWES, T. Discrimination in speech and nonspeech modes.Cognitive Psychology, 1971,2,131-157.

MIYAWAKI, K., LIBERMAN, A. M., FUJIMURA, 0., STRANGE, W.,& JENKINS, J. J. Cross-language study of the perception of theF3 cue for [r] versus [I] in speech and nonspeech patterns.Paper presented at the XXth International Congress ofPsychology, Tokyo, 1972. (Also in Status Report on SpeechResearch, Haskins Laboratories, New Haven, Connecticut,1973, SR-33. Pp. 67-76.)

MOFFITT, A. R. Consonant cue perception by twenty- to twenty­four-week-old infants. Child Development, 1971, 42, 717-731.

MOLFESE, D. L. Cerebral asymmetry in infants, children andadults: Auditory evoked responses to speech and noise stimuli.Unpublished PhD dissertation, The Pennsylvania State Univer­sity, 1972.

MORSE, P. A. The discrimination of speech and nonspeechstimuli in early infancy. Journal of Experimental ChildPsychology, 1972, 14,477-492.

PISONI, D. P. On the nature of categorical perception of speechsounds. Unpublished PhD dissertation, University of Michigan,1971. (Also in Supplement to Status Report on Speech Research,Haskins Laboratories, New Haven, Connecticut, November1971)

PISONI, D. B. AUditory and phonetic memory codes in thediscrimination of consonants and vowels. Perception & Psvcho..physics, 1973, 13, 253-260. -

SIQUELAND, E. R., & DELUCIA, C. A. Visual reinforcement of non­nutritive sucking in human infants. Science, 1969, 165,1144-1146.

STEVENS, K. N. The quantal nature of speech: Evidence fromarticulatory-acoustic data. In E. E. David, Jr., and P. B. Denes(Eds.). Human communication: A unified view. New York:McGraw-Hili, 1972, pp. 51-66.

STEVENS, K. N. The potential role of property detectors in theperception of consonants. Paper presented at Symposium onAuditory Analysis and Perception of Speech, Leningrad, USSR,August 1973.

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STUDDERT-KENNEDY, M. The perception of speech. In T. A.Sebeok (Ed.), Current trends in linguistics. Vol. XlI. The Hague:Mouton, 1974.

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NOTES

I. In one respect, the present findings differ from those obtainedwith the dimension of voice onset time: the recovery function forGroup 0 showed a rising function, whereas for voice onset time, theGroup 0 recovery function attained its maximum response rate

ACOUSTIC CUES FOR PLACE OF ARTICULATION IN INFANTS 521

during the initial 2 min. This effect appears to be real in that thesame rising function for Group D was found in a replication studyusing similar three-formant synthetic speech stimuli. The recoveryfunction for the comparable Group D in Experiment II, showed aless pronounced increment over time, with the peak response rateoccurring at the third postshift minute. The reason for the delay inthe postshift response peak with place cues is not clear (but seeCutting and Eimas, in press, for some possibly analogous findingsin the adult speech perception literature).

2. Comparison of the pattern of results of the first experimentwith that obtained by Pisani (1971, 1973) indicates that infants maybe bound by phonetic feature values in discriminating speechsignals even more than adult listeners. Pisani, using a variety ofpsychophysical procedures, found that the level of discriminabilityfor the stimuli of Group S was consistently above chance, albeitonly 7%-100/0. It will be recalled that the recovery function forGroup S did not differ from that of the control infants. Moreover,in Experiment II, an even larger acoustic difference than used byMattingly et al (1971) failed to produce any evidence ofwithin-phonetic category discrimination. Whether infants areactually more constrained than adult listeners by the phoneticfeature structure or whether the apparent difference is oneattributable to procedural differences remains to be determined. Inany event, there appears to be ample evidence that thediscrimination of speech signals by infants is controlled to a large

extent by the phonetic composition of the acoustic event.3. We recognize that there exist, as yet, a number of unresolved

problems in hypothesizing a model of speech perception based onthe extraction of phonetic information by feature detectors. Theseproblems include, for example, delineating the phonetic featuresand the invariant stimulus information necessary for the activationof the various detectors. The latter issue is particularly difficult,given the extent to which a number of cues sufficient for signalingphonetic feature distinctions are context-conditioned, and varygreatly from speaker to speaker (d. Liberman et aI, 1967).Whether these issues can be resolved is to a large extent anempirical problem. What we have tried to do in this paper andothers is to demonstrate that this form of analysis can accommodateat least a portion of the speech data, whether from infant or adultlisteners.

4. It should be borne in mind that the feature detector model isused in the present paper to accommodate the discriminability dataonly. It is obviously the case that additional, high-level processes ofintegration, as well as some form of matching sets of distinctivefeatures with appropriate labels, are necessary before anidentification response can be made.

(Received for publication March I, 1974;revision received July 6, 1974.)