Reflections on the Development of Speech Perception* SANDRA E.TREHUB

University of Toronto

skills have been generated largely on the basis of indirect comparisons of infant and adult data. The present paper will focus on these infant-adult comparisons, examining their role in current developmental formulations, and delineating some of their methodological problems. It will be argued that these problems cast doubt on current accounts of the development of speech perception. THE DATA ON DEVELOPMENT

ABSTRACT

The role of infant-adult comparisons in developmental accounts of speech perception is examined, and methodological problems associated with such comparisons are delineated. It is argued that the data that are appropriate for the evaluation of categorical perception in infancy are unavailable. Moreover, the view that language experience operates to eliminate discriminative abilities once present rather than to add abilities once absent is without clear-cut support. The serious confounding of age and method of testing casts doubt on current developmental accounts of speech perception based on comparisons of infant and adult. The finding that young infants manifest perceptual discontinuities in certain speech continua (Eimas, Siqueland, Jusczyk, & Vigorito, 1971; Eimas, 1974a, 1975a) which parallel adult discontinuities along these dimensions has been responsible for much interest and speculation regarding the biological preparation and experiential factors in the domain of speech perception. Despite the interest in such developmental considerations, there have been few attempts to investigate age directly as a variable in the design of empirical investigations. Instead, global accounts of the developmental course of speech perception

The phenomenon of categorical perception is central to the notion of perception in the 'speech mode,' that is, phonetic as opposed to auditory perception. The perception of certain consonants is said to be categorical in the sense that discontinuities in discrimination are related to differences in identification. Thus phonemes that are labelled differently are highly discriminable, whereas different instances of the same phoneme that reflect comparable physical differences are not discriminable (Liberman, Harris, Hoffman, & Griffith, 1957; Mattingly, Liberman, Syrdal, & Halwes, 1971; Studdert-Kennedy, 1973). This contrasts with continuous perception in which discrimination is not constrained by listeners' labelling or categorization of stimuli. Thus many more stimuli can be discriminated than can be categorized on an absolute basis (Fry, Abramson, Eimas, & Liberman, 1962; Pollack, 1952). It has been claimed that infants categorically perceive voicing (e.g., [ba] [pa]), place of articulation (e.g., [ba] [ga]), and liquid (e.g., [r] [1]) stimuli (Eimas, 1974a, 1975a; Eimas et al., 1971; Miller & Morse, 1976; Till, 1976). In contrast, infants show continuous perception of vowels (Swoboda, Morse, & Leavitt, 1976) as well as consonant cues presented in a non-speech context

*The preparation of this paper was assisted by financial support from the Natural Sciences and Engineering Research Council of Canada and the Medical Research Council of Canada. Thanks are due to Dale Bull, Carl Corter, Bruce Schneider, Richard Aslin, and David Pisoni for their comments on an earlier draft of this manuscript. Requests for reprints should be sent to the author, Centre for Research in Human Development, Erindale College, University of Toronto, Mississauga, Ontario, Canada, L5L iC6.

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(Eimas, 1975a), findings that are consistent with the adult data. The belief that infants engage in this 'linguistically relevant' mode of perception long before they manifest any productive or even receptive language competence prompted Eimas et al. (1971) to invoke innate mechanisms for recoding the acoustic variations of adult phonemes into appropriate phonetic categories. To account for this innate knowledge of linguistic features, Eimas (1974a, 1975b; Eimas &Tartter, 1979) proposed the existence of phonetic feature detectors which are operative in early infancy and are analogous to the neurophysiological detection mechanisms that have been identified in the human visual system (Blakemore & Campbell, 1969; McCollough, 1965). The activation of these detectors is presumed to be possible without any identification of linguistic segments or even judgments about the 'speechness' of a signal. Rather, they are activated automatically and unconsciously in the presence of acoustic information that corresponds directly with phonetic features. (For a critique of featuredetector models see Trehub, Bull, & Schneider, in press.) Despite the 'considerable overall similarity of the adult and infant discriminability data' (Eimas, 1975a, p. 341), some differences have become apparent as a result of several cross-language studies, most of which employed voicing distinctions. Three categories along the voicing dimension have been identified (Lisker & Abramson, 1964), although English uses only two of these phonemically, the voiced and voiceless consonants. Other languages have been found to use a prevoiced category, either in addition to the other two categories (e.g., Thai) or contrastively with the voiced category (e.g., Spanish).' This unequal distribution of categories across languages permits an evaluation of discrimination across category boundaries, presumably indepen-

dent of prior listening experience. Thus, Lasky, Syrdal-Lasky, and Klein (1975) evaluated the ability of 4- to 6-month-old Guatemalan (Spanish) infants to discriminate stimuli which crossed the English (voiced-voiceless) and Spanish (prevoicedvoiced) boundary and found evidence for both discriminations. Similarly, Streeter (1976) found that 2-month-old Kenyan infants from a Kikuyu-speaking environment discriminated voiced-voiceless stimuli which are not phonemic in Kikuyu, as well as prevoiced-voiced stimuli which are phonemic. A comparable attempt by Eimas (1975b) to evaluate these prevoiced-voiced distinctions with American infants yielded equivocal findings. Only one of his prevoiced comparison pairs yielded evidence of discrimination. Discrimination in this case may have been related to the sheer size of the voicing difference, 80 msec, which contrasts with the 20 msec difference in the voiced-voiceless exemplars of the Eimas et al. (1971) investigation. On the basis of such evidence, Morse (1978) has suggested that experience is unnecessary for infants' discrimination of the three voicing categories. Eimas (1978) concludes more generally that 'the specific phonetic environment of the infant appears to be relatively independent of the infant's ability to perceive speech in a linguistic mode' (p. 368). Eilers and her associates have, however, provided an alternative perspective on the developmental course of VOT discrimination, and, accordingly, on the role of experience. They (Eilers, Wilson, & Moore, 1979) tested infants from English-speaking backgrounds on a wide range of VOT stimuli and found discrimination of voicedvoiceless but not prevoiced-voiced stimuli. Their careful documentation of the VOT values and results of other infant studies of this dimension reveals that voiced-voiceless comparisons have typically been discriminated, whether infants were from English or

'Although prevoicing is not phonemic in English, it is, nevertheless, commonly used, especially in non-initial position (Lisker & Abramson, 1964, 1967).

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foreign-language communities. On the other hand, prevoiced-voiced stimuli have not provided comparable evidence of discrimination unless infants were from foreign-language backgrounds where these distinctions were phonemic. The single exception to this overall pattern is the previously noted Eimas (1975b) finding with large VOT differences. Eilers, Gavin, & Wilson (1979) provided further corroboration of the apparent relation between language environment and speech discrimination skill in a study of English- and SpanishAmerican infants. Both were presented with voiced-voiceless and prevoiced-voiced pairs, the former phonemic in English only, the latter in Spanish only. Although all infants discriminated the stimuli that crossed the English boundary, only the SpanishAmerican infants discriminated the stimuli that crossed the Spanish boundary.2 Thus, although listening experience appears to be unnecessary for distinguishing voiced from voiceless stimuli, it is apparently required for discriminating between prevoiced and voiced stimuli. Since the Kikuyu infants were only 2 months of age, the Guatemalan infants 5 months of age, and the SpanishAmerican infants 6 months of age, a minimal amount of such experience appears to be sufficient for the development of these discriminative abilities.3 On the basis of English adults' difficulty in discriminating prevoiced from voiced stimuli (Abramson & Lisker, 1970), a task apparently accomplished by Kikuyu and Guatemalan infants, Morse (1978) and Eimas (1978) have argued that certain category boundaries (e.g., prevoiced-voiced) may disappear developmentally. Eimas advances further support for this view in

his finding of greater categorical perception of the [r—1] contrast by English infants (Eimas, 1975a) than by adult Japanese speakers (Miyawaki, Strange, Verbrugge, Liberman, Jenkins, & Fujimura, 1975), who perform only slightly above chance levels for between- and within-category stimuli. Trehub (1976b, 1978) has also raised the issue of a developmental decrement in her finding of infant discrimination of the Czech [z-f] distinction and adult difficulty with this discrimination. In terms of the feature detector model, 'it is only necessary to postulate that if specific phonetic experience is not forthcoming within some broadly defined critical period (cf. Lenneberg, 1967), the phonetic feature detectors subserving the contrast in question will undergo some loss of sensitivity or receptivity' (Eimas, 1975a, p. 346). In more general terms, 'linguistic experience eventually operates on speech perception at the phonetic level to eliminate discriminative abilities that were originally present in the infant, rather than to build in discriminative abilities that were not originally present' (Eimas, 1978, p. 368). On the contrary, Eilers, Gavin, & Wilson (1979) have shown an apparently facilitative effect of experience on infants' discrimination of the prevoiced-voiced distinction. Moreover, Eilers, Wilson, and Moore (1979) have demonstrated that English adults can, under appropriate conditions, discriminate prevoiced from voiced stimuli. Finally, evidence for adult within-category discrimination (Eilers, Wilson, & Moore, 1979), notwithstanding its phonetic irrelevance, can also be construed as evidence in favour of age-related improvement as opposed to deterioration.

"Aslin, Hennessey, Pisoni, and Perey (1979) claim to have obtained evidence for the discrimination of prevoicedvoiced stimuli in English infants, but methodological considerations (see MacKain, 1979) preclude an unequivocal interpretation of their findings. 3 MacKain (1979) argues that the notion of experiential effects in the preverbal period requires numerous assumptions including the supposition that infants process speech phonetically. She contends that the only reasonable basis for experientially induced change is infants' recognition that sounds contrast to convey meaning. She maintains, therefore, that in early infancy when such linguistic considerations are irrelevant, listening experience would be unlikely to alter discriminative performance.

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METHODOLOGICAL PROBLEMS

This cursory review of the presumed developmental course of speech perception should suffice to illustrate the crucial role played by infant-adult comparisons in these formulations. Nevertheless, little attention has been accorded the problems inherent in these comparisons. Butterfield and Cairns (1974) have raised the issue, but only insofar as the interpretation of infant categorical perception is concerned. Their argument is not so much with infant-adult comparisons in general or in any particular case but rather with the paucity of data from which conclusions of infant categorical perception have been derived. The phenomenon of categorical perception requires not only within-category discrimination performance that is near chance and between-category performance that is near perfect, but also a sharp delineation of categories in identification (Studdert-Kennedy, Liberman, Harris, & Cooper, 1970). Moreover, identification and discrimination functions must be appropriately related so that discrimination peaks occur at the category boundaries and discrimination troughs, within categories. In infancy, judgments of categorical perception have been made on the basis of between-category discrimination coupled with no apparent evidence of within-category discrimination. Since there has been no effective means for obtaining categorical or labelling information from infants, investigators have generally sidestepped this issue.4 The requisite discrimination criteria have also been ignored, again for practical reasons. For near perfect between-category discrimination, investigators have substituted discrimination performance above chance levels. There are comparable difficulties in obtaining unequivocal evidence of non-

discrimination. The absence of verbal corroboration or of converging methodologies leaves open the possibility that the apparent non-discrimination reflects infant performance as opposed to competence factors. It should be clear that the evidence for categorical perception in infancy is, at the least, considerably different from that which has been provided for adults, thus calling into question the common underlying processes proposed by Eimas (1974b, 1975b). While Butterfield and Cairns (1974) were primarily concerned with Eimas's claims that infants' perception of speech was linguistically relevant or in the 'speech mode,' this issue seems less crucial in the current climate of doubt concerning adults' perception in such a mode. Such doubt has been generated by evidence of categorical-like perception by non-human species (Kuhl & Miller, 1975; Morse & Snowdon, 1975; Waters & Wilson, 1976), adult categorical perception of certain nonspeech continua5 (Burns & Ward, 1978; Cutting & Rosner, 1974; Miller, Pastore, Wier, Kelly & Dooling, 1976; Pisoni, 1977), evidence of adult within-category discrimination (Carney, Widin, & Viemeister, 1977; Eilers, Wilson, & Moore, 1979; Sinnott, Beecher, Moody, & Stebbins, 1976), and numerous methodological points of contention (Ades, 1977; Carney et al., 1977; Macmillan, Kaplan, & Creelman, 1977). The implications of these issues for auditory versus phonetic interpretations of infant speech perception are elaborated in Kuhl (1978) and Trehub et al. (in press). Regardless of interpretations of infant or adult data as linguistically relevant or irrelevant, the fact remains that infantadult comparisons have provided the data for contemporary accounts of development. More basic than, or perhaps inseparable from, the question of whether age

4

Aslin, Perey, Hennessey, and Pisoni (1977) have proposed a procedure for generating labelling data from infants, but the efficacy of their procedure remains to be determined. 5 Similar claims of infant categorical perception of non-speech continua have been made by Jusczyk, Rosner, Cutting, Foard, and Smith (1977) and Jusczyk, Pisoni, Walley, and Murray (1980).

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change is associated with improvement, deterioration, or no change, is the methodological question of whether such comparisons can yield meaningful statements concerning age-related change. A brief description of the discrimination techniques that have been employed with adults and infants will illustrate some of the problems. Adult Discrimination Techniques

The most commonly used discrimination procedures for adults are ABX and oddity techniques, ABX involves the presentation of sets of three stimuli, the first two differing from each other and the third being identical with the first or second stimulus. The listener must judge whether the third stimulus is the same as the first or the second stimulus. In the case of the oddity procedure, which also involves triads of two identical stimuli and one different stimulus, listeners must identify the position of the different stimulus. Carney et al. (1977) suggest that the randomization of stimuli within blocks, the absence of specific feedback, as well as the memory demands of these tasks contribute to the poor performance generally observed for within-category stimuli. Their introduction of procedural modifications which lessened uncertainty generated successful within-category discrimination. Similarly, Pisoni and Lazarus (1974) have demonstrated that simple manipulations of experimental procedures can yield dramatically different discrimination functions, the differences being reflected primarily in within-category discrimination. Other techniques which involve repeating standard stimuli (Eilers, Wilson, & Moore, 1979; Carney &Widin, ig76;Sinnott et al., 1976) have likewise shown good within-category discrimination. In addition, instructions designating the stimulus dimension as speech or non-speech have yielded drastically different performance levels (Cross & Lane, in Pisoni & Lazarus, 1974). While the usual discrimination peaks and troughs were evident in the speechinstructions case, chance performance

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characterized the non-speech case. This contrasts with Miyawaki et al.'s (1975) Japanese subjects, who showed better discrimination of the cues for the [r-1] contrast in a non-speech than in a speech context. The implications of these findings, while important for interpretations of adult speech perception data, are, nevertheless, beyond the scope of the present discussion. From the perspective of infant-adult comparisons, however, they raise serious questions. Which technique should provide the representative adult data? Should ABX and oddity data be used simply because the techniques have been used most frequently? Alternatively, should one use data from techniques which yield the most generous estimate of stimulus discriminability? If two experimentally valid techniques, X and Y, were used for infants, and only X yielded evidence of discrimination, the likely conclusion would be that infants could discriminate the stimuli in question. To verify this conclusion and exclude the possibility of artifact, one would attempt to provide converging evidence with still another technique. Although it would appear that these conditions have been met with respect to the evidence of adult within-category discrimination, the traditional paradigms continue to provide the adult reference data (Eimas, 1978; Eimas & Tartter, 1979; Morse, 1978). As an alternative to the technique which yields the highest performance level one might select a technique which matches, to some extent at least, the characteristics of infant techniques. Eilers, Wilson, and Moore (1979) have pursued this strategy and have identified a pattern of adultinfant findings which contrasts markedly with the formulations of Eimas and Morse. The principal feature of their technique, a repeating standard stimulus followed by a comparison stimulus, is to be found in several studies which have revealed withincategory discrimination (e.g., Carney & Widin, ig76;Sinnottetal., 1976). Compared to ABX and oddity designs, this procedure S.E. Trehub

involves a considerable reduction in memory load and in stimulus uncertainty. Since the adult participant must simply indicate when a change occurs without specifying the nature of the change, there is no necessity for stimulus recoding or identification. Infant Discrimination Techniques

An examination of infant discrimination techniques reveals further problems. The high-amplitude sucking (HAS) technique has been used in most demonstrations of infant categorical perception (Eimas, 1974a, 1975a; Eimas et al., 1971) as well as many other studies of infant speech discrimination (Eilers & Minifie, 1975; Morse, 1972; Swoboda, Kass, Morse, & Leavitt, 1978; Swoboda etal., 1976; Trehub, 1973b, 1976a, b; Trehub & Rabinovitch, 1972). The rationale for the use of this procedure is derived less from the logic of its design than from the simple fact that it works, which is to say that it has yielded evidence of discrimination (a reasonable feat, as infant techniques go). After a baseline period of non-nutritive sucking with no stimulus, infants are repeatedly presented with a speech stimulus that is contingent upon their high-pressure (high-amplitude) sucking. When infants meet a predetermined percentage criterion of sucking decrement, a contrasting sound is substituted for experimental infants, while comparison or control infants continue to receive the same stimulus. Sensitivity to sound change is inferred from significant post-decrement differences between experimental and control infants. The individualized decrement criterion results in experimental infants receiving vastly different amounts of familiarization with the standard stimulus prior to the sound change, the range of such stimulus presentations being roughly on the order of 120—1000. The contingent aspect of the design results in interstimulus intervals that vary considerably both between and within subjects. While this variation is likely to Development of speech perception

influence the course of familiarization, it has more serious implications for the evaluation of discrimination. At the point of sound change, the interval between the familiar and novel stimulus can be as small as 500 msec or as large as 30 seconds or more. In the former case the infant can compare sensory impressions of the novel and familiar stimulus; in the latter he must compare sense impressions of the novel stimulus with stored representations of the familiar stimulus. Thus the young infant's ability to recode and represent such information in an efficient manner will influence the outcome of this task. It is not surprising, then, that the rate of highamplitude sucking in the first minute following the sound change has been found to be related to the interval between the final familiar and first novel stimulus, the rate decreasing with longer intervals (Swoboda et al., 1978). In an effort to decrease the memorial demands of the HAS technique, Spring & Dale (1977) have modified the stimulus-change phase of the design. Instead of the usual repetitions of the novel stimulus, they alternate the novel and familiar stimuli, a procedure which permits several pairings of novel and familiar stimuli. From this description it should be clear that the HAS technique bears little resemblance to the ABX and oddity designs in use with adults, a fact that has not provoked particular concern on the part of its users. This is not to suggest that ABX be used with infants (an impossibility) or that adults be required to suck on nipples (an inconvenience). Trehub (1976b) did attempt to match some of the stimulus presentation parameters of the HAS paradigm in her evaluation of adults' discrimination of English and foreign speech contrasts. She presented adults with a series of stimulus repetitions followed by repetitions of the novel stimulus, all of these separated by variable interstimulus intervals (1-5 sec). Under these conditions, adults' discrimination of the English stimuli was significantly

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better than their discrimination of the foreign stimuli, with discrimination of the foreign stimuli slightly exceeding chance levels. When different foreign stimuli have been used in traditional adult designs (Abramson & Lisker, 1970; Miyawaki et al., 1975), no evidence of discrimination has been apparent. The second most commonly used technique in infant speech perception research is the heart rate (HR) habituationdishabituation procedure (Lasky et al., 1975; Moffitt, 1971; Miller & Morse, 1976; Leavitt, Brown, Morse, & Graham, 1976; Miller, Morse, & Dorman, 1977), which involves the presentation of a fixed number of familiarization trials with a standard stimulus followed by test trials with a novel stimulus. Each trial comprises several repetitions of the stimulus. Evidence for discrimination is provided by habituation of the cardiac (orienting) response to the standard stimulus followed by dishabituation or response recovery to the novel stimulus. Intertrial intervals are necessarily long (2030 sec) to allow the infant's heart rate to return to baseline levels between trials. The familiarization period generally comprises fewer than 100 stimulus presentations compared to the 120-1000 repetitions of the HAS procedure. For habituation or familiarization effects to become apparent, the infant must represent the stimulus in some manner and then recognize this stimulus on subsequent trials. Dishabituation on change trials is dependent on the infant noticing and responding to the discrepancy between the current stimulus and his representation of the standard stimulus. At present, it is still unclear whether detection of the change is a sufficient condition for an observable HR response to it. The HR habituation-dishabituation technique has failed to yield evidence of auditory discrimination with infants younger than 4 months of age (Berg, 1974; Miller et al., 1977; Leavitt et al., 1976), a result thought to be attributable to the stringent

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memorial demands of the procedure (Leavitt etal., 1976). This prompted Leavitt et al. (1976) to introduce a modification aimed specifically at lessening task demands by eliminating the long intertrial intervals. The modified technique comprises several repetitions of the standard stimulus followed by an equal number of repetitions of the novel stimulus, all of which are separated by very brief (e.g., 1 sec) interstimulus intervals. This modified procedure has yielded evidence of discrimination of place of articulation cues with infants younger than 4 months (Leavitt et al., 1976; Miller et al., 1977; Till, 1976), but has, nevertheless, failed to yield evidence of voicing discrimination with infants of comparable age (Roth & Morse, 1975; Miller & HankesRuzicka, 1978). It should be recalled that such evidence has been obtained with the HAS technique with infants as young as a few days of age (Butterfield & Cairns, 1974) as well as with infants 1 - 4 months ofage (Eimas et al., 1971; Streeter, 1976; Trehub & Rabinovitch, 1972). Although HR and HAS techniques yield findings which are similar in some instances and discrepant in others, the factors responsible for such similarities and differences are unknown. While the HR technique is potentially applicable to adults, it has, in practice, met with only limited success. In one study, for example, there was no evidence of discrimination of a clearly discriminate stimulus change except when adults were specifically instructed to attend to the change (Brown, Morse, Leavitt, & Graham, 1975). This finding draws attention to our limited understanding of the relation between behavioural and physiological responding. A recent addition to the repertoire of infant speech discrimination techniques is the Visually Reinforced Infant Speech Discrimination (VRISD) procedure (Eilers, Gavin, & Wilson, 1979; Eilers, Wilson, & Moore, 1977,1979) which involves the reinforcement of head-turn responses to a change in a repeating background stimu-

S.E. Trehub

lus. The background consists of repetitions of the standard stimulus separated by brief (1-2 sec) interstimulus intervals and is continuously available except during change intervals. The change consists of four or more repetitions of the novel stimulus. All stimuli are presented from a loudspeaker situated to the side of the infant and test trials are presented only when the infant is facing directly ahead. Appropriate turns towards the loudspeaker during change intervals are reinforced automatically with the presentation of animated toys. More frequent turns on change compared to nochange trials provide evidence of discrimination. In contrast to the other infant procedures, only VRISD provides feedback for infants' judgments of change, although this feedback is incomplete. Infants receive information about correct and false-positive responses, but not about false-negative responses. While the HAS and standard HR techniques provide data on only one stimulus change for each infant, the VRISD procedure can provide data on 20 or more changes from one infant in a single test session. Thus individual and within-subject comparisons are possible. The VRISD design minimizes memorial demands compared to the other infant procedures and, in that sense, would appear to be suitable for use with very young infants. However, the necessity of a reinforced localization or headturn response effectively precludes its use with infants younger than 5 months of age (Moore, Thompson, & Thompson, 1975). Thus the populations of the HAS and VRISD procedures cannot overlap. Beyond 5 months, however, the technique should be applicable throughout the life span, provided appropriate reinforcers are used. One of the most compelling features of the VRISD technique is its demonstrated applicability to adults (Eilers, Wilson, & Moore, 1979). The test environment and stimulus presentation parameters are identical for adult populations. Adults respond

Development of speech perception

to a perceived change by pressing a button rather than by turning their heads. It was with this procedure that Eilers et al. demonstrated English adults' discrimination of voiced-voiceless and prevoiced-voiced stimuli as well as within-category (voiceless) stimuli. Infants tested with this procedure, however, showed discrimination of voicedvoiceless stimuli but not prevoiced-voiced or within-category stimuli. A further technique which has had more limited use (Trehub, 1978) involves habituation and dishabituation of the localization response to laterally presented stimuli. This technique is of interest because it has also provided data on adults and infants. In the infant version, trials are initiated only when the infant is looking directly ahead. These trials consist of four repetitions of the standard stimulus presented on one of two loudspeakers (randomly determined) at the infant's left or right. Trials continue until a predetermined number (2 or 3) of successive trials with no localization response have occurred. Experimental subjects then receive a stimulus change, while control subjects continue to hear the same stimulus. Discrimination of the change is inferred from greater responding by experimental than control subjects on test trials. As with the HAS procedure there are variable intertrial intervals and a variable number of stimulus presentations in the familiarization period. While the decrement criterion of the HAS technique is not clearly related to underlying processes (Butterfield & Cairns, 1974; Trehub et al., in press), the habituation criterion in this case is thought to reflect the formation of an internal model or engram of the stimulus (Sokolov, i960). The duration of intertrial intervals is determined by the time an infant takes to re-establish midline orientation. The fact that this interval, in the case of the final familiarization stimulus and the first novel stimulus, is necessarily separated by a few seconds or more means that this technique requires representation and re-

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tendon of the standard stimulus and will, consequently, underestimate infant abilities compared to the VRISD design. In the adult variant of the procedure, there are 4—8 familiarization trials (randomly determined) followed by 2 test trials (change or no-change) with variable interstimulus intervals roughly parallel to those of infants. Adults verbally indicate the occurrence of a sound change. When adults and infants were tested with the Czech stimuli which had previously been used by Trehub (1976b), there was evidence of discrimination by the infants but not by the adults (Trehub, 1978). Eilers (in press) has reported that adults as well as infants can accomplish this discrimination when the VRISD procedure is used. This would suggest that the necessity for recoding such foreign stimuli in the localization procedure is more debilitating for adults, who may be constrained by the sound categories of their particular language, than for infants who are unconstrained in this respect. Thus, while infant processes of recoding and representation may be relatively inefficient compared to those of adults, these processes may, nevertheless, be more flexible. This interpretation is likewise applicable to the poor performance of Japanese adults on the [r—1] distinction (Goto, 1971; Miyawaki et al., 1975), contrasted with successful discrimination of these stimuli by young infants (Eimas, 1975a). Moreover, such an interpretation is more parsimonious than the proposed developmental process of deterioration (Eimas, 1975a, 1978; Morse, 1978; Trehub, 1976b). From this description of techniques, it can be seen that infant discrimination designs invariably evaluate the ability to detect a stimulus change as evidenced by a response decrement or increment, whereas traditional adult techniques typically assess discrimination by means of responses 6

which necessarily engage higher-order processes of considerable complexity. The task demands are clearly different, and while discrimination in ABX or oddity tests presupposes the ability to detect a stimulus change, it is likely that the simple detection of such a change is necessary but not sufficient for discriminative responding in ABX-type situations. Thus direct comparison of adult discrimination findings with infant difference-detection results are simply inappropriate. The fact that these data have been gathered at different times and in different laboratories further compounds the problems. It would appear that such criticism cannot be levelled against the VRISD6 and localization procedures. There are, however, grounds for caution in interpreting direct comparisons, even with these techniques. As noted, the memorial demands of the localization technique are likely to exert differential effects on infants and adults which may interact with stimulus familiarity. Moreover, although a detection procedure like VRISD has been applied in a roughly similar fashion to infants and adults (Eilers, Wilson, & Moore, 1979), it is impossible to assume that identical measures reflect the same processes at different ages (Botwinick, 1970; Comalli, 1970), especially in the present case where age and presumed organization of processes are dramatically different. In the VRISD procedure adults can, in principle, process the stimuli without phonetically recoding the auditory signal, but do they, in fact, do so? Would speech or non-speech instructions influence adult performance as with conventional adult discrimination techniques (e.g., Cross & Lane, in Pisoni 8c Lazarus, 1974; Miyawaki et al., 1975)? At this point, it is unclear whether the adult repeating-standard procedures engage different processes or whether their effects are merely facilitative with respect to the same processes involved

For a critique of the VRISD technique, see Aslin and Pisoni (1980) and the reply by Eilers, Gavin, and Wilson (1980).

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in alternative techniques. In any case, it should be apparent that adult-infant comparisons are necessarily problematic. REFLECTIONS ON THE PAST AND FUTURE

In the context of the methodological issues that have been raised, let us re-examine the two major developmental statements. First and foremost, infants are purported to perceive in the 'speech mode' (Eimas, 1974b, 1978; Eimas et al., 1971; Morse, 1978), largely on the basis of their categorical discrimination of several consonant continua (Eimas, 1974a, 1975a, 1975b; Eimas et al., 1971; Miller & Morse, 1976) viewed in relation to adults' perception of these same continua (Liberman et al., 1957; StuddertKennedy, 1973). It has been shown, however, that the data appropriate for the evaluation of categorical perception in infancy have not been gathered, nor can they be gathered, at present. Moreover, with respect to the adult-infant comparisons, age and method of testing have been confounded. The second developmental statement is that language experience operates to eliminate discriminative abilities once present rather than to add abilities once absent (Eimas, 1975a; Morse, 1978; Trehub, 1976b). Aside from the age/method confounding which is evident in these comparisons, the adult and infant findings on prevoiced-voiced comparisons have been disconfirmed (Eilers, Wilson, & Moore, 1979), thereby generating an alternative proposal that the appropriate description of infant to adult change in this domain is improvement. It has been shown, however, that while procedure is unconfounded with age in the latter case, there is no evidence that process is unconfounded. In short, there is no empirical basis for current accounts of the development of speech perception. If comprehensive accounts of development are to be pursued, and it seems worthDevelopment of speech perception

while to do so, then investigators cannot continue to ignore the intervening age groups (older infants, children) which have largely escaped attention in current formulations. Several important and potentially answerable questions have simply not been addressed. For example, what is the impact of the process of language acquisition, in its receptive and productive aspects, on speech discrimination? Shvachkin (1973) and Garnica (1973) have drawn attention to the inability of children in the second year of life to discriminate certain phonemic distinctions in natural speech (e.g., [ba] [pa], [da] [ga]). Infants have, nevertheless, shown discrimination of some of these differences. Again, comparisons are impossible because of the confounding of age and method. For obvious reasons, infants cannot be tested with the discrimination procedure used for children, but the reverse should be possible. If children would not show discrimination of these stimuli with the VRISD procedure (and if motivational factors could be ruled out), then one might suspect the operation of criterion differences occasioned by children's knowledge of language. On the other hand, if discrimination were evident, it would raise the possibility that certain speech differences, while detectable, could not yet be used discriminatively in a receptive language context. In the investigation of any of these age relations, the confounding of age and technique can be eliminated through the use of a uniform methodology such as VRISD, while the confounding of age and process can be minimized by the selection of relatively narrow age ranges. Does this mean that the past decade of research in infant speech perception has been fruitless? By no means! The various studies of infant speech discrimination, notwithstanding their failure to clarify the course of subsequent development, have, nevertheless, served to identify changes in synthesized and natural speech stimuli to which young infants are responsive. On the basis of such research we have learned that 377

infants can discriminate various consonants in initial (e.g. Eimas et al., 1971; Trehub & Rabinovitch, 1972), medial (Jusczyk & Thompson, 1978; Trehub, 1976a) and final (Eilers, 1977; Jusczyk, 1977) position. The range of stimuli studied has included stop consonants differing in voicing (Eimas et al., 1971; Miller & Hankes-Ruzicka, 1978; Trehub & Rabinovitch, 1972) and place of articulation (Eimas, 1974a; Moffltt, 1971; Miller etal., 1977; Morse, 1972), fricatives (Eilers, 1977; Eilers & Minifie, 1975; Eilers, Wilson, & Moore, 1977; Holmberg, Morgan, & Kuhl, 1977; Trehub, 1976b), vowels (Eilers, Oiler, & Gavin, 1978; Kuhl, 1976, 1977; Swoboda et al., 1976, 1978; Trehub, 1973b, 1976a), glides (Eilers et al., 1978; Jusczyk, Copan, & Thompson, 1977), and liquids (Eimas, 1975a). Quite aside from the segmental distinctions which have been the focus of the present discussion, infants have been shown to discriminate the suprasegmental dimensions of intonation and stress (Kaplan, 1969; Kuhl, 1976; Morse, 1972; Spring & Dale, 1977). Moreover, these segmental and suprasegmental abilities have been identified long before their use in production.

In short, research in infant speech perception has generated substantial leaps of knowledge over the past decade. Indeed, it is likely that the euphoria associated with these advances coupled with the Zeitgeist of biological hardware propelled the formulation of a premature but influential account of life-span development. RESUME

Etude du role des comparaisons enfant-adulte dans l'analyse genetique de la perception du Iangage et identification des problemes methodologiques lies a ces comparaisons. II est allegue que les donnees susceptibles de permettre revaluation de la perception categorielle du nourrisson manquent encore. De plus, l'idee que l'experience du langage a pour effet d'eliminer des apititudes discriminatives deja presentes plutot que d'en ajouter de nouvelles encore absentes n'est pas nettement confirmee. La confusion agemethode qui marque les etudes basees sur des comparaisons enfant-adulte ne peut que mettre en doute la valeur de ces recherches sur la perception du langage. REFERENCES ADES, A.E. Vowels, consonants, speech, and nonspeech. Psychol. Rev., 1977,84,524-530 ABRAMSON, A.S., & LISKER, L. Discriminability along the

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