COGNJTIVE PSYCHOLOGY 4, 99-109 ( 1973)

Selective Adaptation of Linguistic Feature Detectors1

PETER D. EIMAS~ AND JOHN D. CORBIT Brown University

Using a selective adaptation procedure, evidence was obtained for the existence of linguistic feature detectors, analogous to visual feature detec- tors. These detectors are each sensitive to a restricted range of voice onset times, the physical continuum underlying the perceived phonetic distinctions between voiced and voiceless stop consonants. The sensitivity of a partic- ular detector can be reduced selectively by repetitive presentation of its adequate stimulus. This results in a shift in the locus of the phonetic boundary separating the voiced and voiceless stops.

Coverging evidence from electrophysiological studies of single neurons in animals (e.g., Lettvin et al., 1959; Hubel & Wiesel, 1962) and from psychopyhsical studies of human perception (Blackmore & Campbell, 1969; Blackmore & Sutton, 1969) indicates that there are detector mechanisms in the brain that are uniquely sensitive to particular and relatively restricted patterns of stimulation. In this study of speech per- ception, we attempted to demonstrate the existence of feature detectors for linguistic information by use of a selective adaptation procedure. The acoustic dimension that was investigated was voice onset time (VOT), variations in which are sufficient for the perceived distinctions between the voiced and voiceless stop consonants of English in initial position3 ( Lisker & Abramson, 1970).

Voice onset time is defined as the time between the release burst and laryngeal pulsing (Lisker & Abramson, 1964). Very short lags in the onset of voicing are perceived in English as voiced stops, [b, d, g], whereas

1 We thank Dr. F. S. Cooper for generously making available the facilities at the Haskins Laboratories and Drs. R. M. Church, D. J. Getty, and A. M. Liberman for their critical comments. We also thank Mrs. Catherine G. Wolf for her assistance in conducting these experiments. Supported by PHS Grants HD 05331 and MH 16608.

* Department of Psychology, Brown University, Providence, Rhode Island 02912. ‘It should be noted that the cues underlying the voicing distinctions discussed in

the present paper apply to sound segments in absolute initial position. Although, as Lisker and Abramson (1964) noted, voice onset time does effectively separate stop categories in sentences, there is some effect of embedding the various stops in con- tinuous speech. As a consequence we have limited our research to voicing distinctions in initial positions, where voice onset time is relatively insensitive to contextual effects ( Lisker & Abramson, 1967).

99 Copyright @ 1973 by Academic Press, Inc. All rights of reproduction in any form reserved.

100 EIMAS AND CORBIT

relatively long lags in the onset of voicing are perceived in English as voiceless stops, [p, t, k]. It is possible to produce synthetic speech with variations in VOT. Figure 1 shows two spectrograms of synthetic speech which illustrate two values along the VOT continuum. The sound pattern shown in the upper spectrogram has a short (10 msec) lag in VOT and is perceived in its acoustic form as a voiced stop, in this instance as [b] plus the vowel [a]. The lower spectrogram depicts a sound with a longer (100 msec) lag in voicing, which is perceived as a voiceless stop, [p] in this case, plus the vowel [a].

The perception of series of synthetic speech varying continuously in

t 10 msec

t 100 msec FIG. 1. Spectrograms of synthetic speech showing two values of voice onset time:

a slight voicing lag represented by [ba] in the upper figure and a long voicing lag in the lower figure, represented by [pa]. The symbols F-l, F-2, and F-3 represent the first three formants, that is, the relatively intense bands of energy in the speech signal. (Courtesy of L. Lisker & A. S. Abramson. )

LINGUISTIC FEATURE DETECTORS 101

VOT alone has been found to be very nearly categorical for English listeners (Abramson & Lisker, 1970; Wolf, 1972). That is, for these series of stimuli the percept exists only in one of two stages: the voiced or voice- less stop. As a consequence, listeners are able to assign stimuli to phonetic categories with great consistency. Moreover, their ability to detect differences between stimuli is limited by their ability to assign differential labels to the stimuli. The latter is evidenced by marked peaks in the discriminability functions at the region of the phonetic boundary; that is, the discrimination of a given difference in VOT is considerably better when two stimuli lie in different phonetic categories than when the two stimuli are from the same phonetic category.

Of particular interest is the fact that the categorical nature of the perception of the VOT continuum appears to be universal. That is, it is characteristic not only of adult speakers of English but also of adult speakers of other languages, e.g., Thai, and more importantly it is present in preverbal human infants as young as 1 month of age (Eimas et ul., 1971). The apparent universality of this phenomenon suggests that it is a manifestation of the basic structure of the human brain.

Another line of evidence consistent with this idea comes from the cross- language research of Lisker and Abramson ( 1964). They found that the manner in which speakers of 11 diverse languages divided the VOT continuum was notably consistent. The phonetic tokens produced by these speakers, although not the same for all languages, nevertheless tended to fall at three modal values of VOT. Two of these values are used for the English voiced and voiceless stops. English does not use the third distinction, long voicing lead, found in Thai, for example.4 It would seem reasonable that this uniformity in producing voicing distinctions is matched by specialized perceptual structures. These structures might well take the form of detectors that are differentially tuned to the acoustic consequences of the modes of production (see Lieberman, 1970, for an extended discussion of the relation between the processes of speech production and speech perception).

EXPERIMENT 1

Our experimental plan to obtain evidence for these linguistic feature detectors was based on a selective adaptation procedure. We reasoned that if there are linguistic feature detectors mediating the perception of the voiced and voiceless stops, then repeated presentation of the feature (i.e., appropriate VOT value) to which a given detector is sensitive

‘Inasmuch as this distinction, long voicing lead, does not exist in English and in fact may not be detectable by adult Engish speakers, we have restricted our research to the two voicing distinctions found in English.

102 EIMAS AND CORJ3IT

should fatigue the detector and reduce its sensitivity. As a consequence, the manner in which stimuli are assigned to phonetic categories would be altered, especially for those stimuli near the phonetic boundary where both detectors may be somewhat sensitive to the same VOT values. In order to test this idea, we first obtained identification functions for two series of stop consonants, the bilabial stops [b, p] and the apical stops [d, t], when listeners were in the normal unadapted state. Next, identi- fication functions were obtained for the same series of stimuli after adaptation by repeated presentations of good exemplars of both modes of voicing.

Method Stimuli. The stimuli were two series of 14 synthetic speech stimuli pre-

pared by means of a computer-controlled parallel resonance synthesizer by Lisker and Abramson ( 1970). For greater detail concerning the construction of synthetic speech by a computer-controlled synthesizer the reader is referred to .Mattingly ( lQ6S). To produce variations in VOT in the context of the English stop consonants, the onset of the first fonnant relative to the onset of the second and third formants is varied and the second and third formants are excited by a noise source rather than a periodic source when the first formant is absent (see Fig. 1. for two ex- amples of VOT). In the [b, p] series the VOT values ranged from - 10 msec (short voicing lead) to + 60 msec (relatively long voicing lag) in 5msec steps except for the final two stimuli which were separated by 10 msec. The [d, t] series had VOT values ranging from 0 msec (voicing and first formant onset coincident with the onset of the second and third formants) to +80 msec. The difference between stimuli was 5 msec except for the final four stimuli in which the difference was 10 msec. The acoustic differences between the two series were in the starting frequency and direction of the second- and third-formant transitions, these dif- ferences being sufficient cues for the perceived phonetic differences, that is, for the perceived differences between [b] and [d] and between [p] and [t] (Liberman et al., 1967).

Procedure. To obtain the initial identification functions when the listeners were in an unadapted state, the 14 stimuli from each series were presented binaurally by means of a tape recorder at a comfortable listening level. For both series, which were presented separately, the order of presentation was randomized, and 50 identification responses were obtained for each stimulus. The interval between stimuli was always 3 sec. Next, identification functions were obtained after selective adaptation. For example, if the detector assumed to underlie perception of the voiced stops were to be adapted, listeners were exposed to

LINGUISTIC FEATURE DETECTORS 103

repetitive presentations of a [b] with a VOT value of - 10 msec or to a [d] with a VOT value of 0 msec for 2 min at the beginning of each session, and then for 1 min before each stimulus was to be identified. When adapting the detector for the voiceless stops, either a [p] with a VOT value of + 60 msec or a [t] with a VOT value of +80 msec was repeatedly presented before each identification response. There were eight adaptation conditions in all: each of the two series was identified after adaptation with [b], [d], [p], and [t]. In any single adaptation session, listeners heard 2 min ( 150 presentations) of the adapting sound pattern, with each presentation being 500 msec in duration and separated by 300 msec of silence. Next 70 adaptation trials were administered in which each individual adaptation trial consisted of 1 min of the adapting stimulus (75 presentations), followed by 500 msec of silence and then a single stimulus to be identified. Five seconds elapsed before the next trial occurred, and short breaks were given every 14 trials. For any session, the adapting stimulus and series to be identified were randomly deter- mined as was the order in which the individual stimuli were presented for identification. There was a total of 16 adaptation sessions, with at least 24 hr between sessions, yielding 10 identification responses to each stimulus of both series under each of the four adaptation conditions,

Subjects. The subjects were two undergraduate students and one graduate student at Brown University who were paid for their participa- tion. TWO of the subjects had previous experience in listening to synthetic speech.

Results and Discussion

In Fig. 2, the identification functions for a single subject are shown. In each instance, adaptation caused a notable shift in the phonetic boundary and moreover, the direction of the shifts in the locus of the phonetic boundary were uniformly consistent; the boundary moved closer to the adapting stimulus indicating a greater number of identifica- tion responses representing the unadapted mode of voicing had occurred. After adaptation with a voiced stop, the listener gave more identification responses belonging to the voiceless category, especially when attempting to identify stimuli near the original phonetic boundary. Conversely, after adaptation with a voiceless stop, a greater number of identification re- sponses belonged to the voiced category. Again the effect was most pronounced for stimuli near the phonetic boundary.

Of particular interest was the finding that the shifts in the locus of the phonetic boundary occurred when the adapting stimulus and identifica- tion stimuli were from different series. For example,, adaptation with a

EIMAS AND CORBIT

20 -

-10 0 IO 20 30 40 SO 60

Voice onset time (msec)

FIG. 2. Percentages of voiced identification responses ( [b or d]) obtained with and without adaptation for a single subject. The functions for the [b, p] series are on the left and those for the [d, t] series are on the right. The solid lines indicate the unadapted identification functions and the dotted and dashed lines, the identi- fication functions after adaptation. The phonetic symbols indicate the adapting stimulus.

bilabial stop produced an approximately equivalent effect on the identi- fication of both bilabial and apical stops. These cross-series effects rule out explanations based on adaptation of the sound pattern as a phonetic unit. If this were the case, it is difficult to understand, for example, how alterations in the system underlying perception of [b] would likewise affect perception of the apical stops. In addition, given the acoustic

LINGUISTIC FEATURE DETECTORS 105

differences between the two series with respect to the second- and third-formant transitions, and the cross-series adaptation effects, it is unlikely that what was, in fact, selectively adapted were detectors for simple acoustic information, Rather the evidence indicates that detectors for those complex aspects of the sound pattern that both series had in common, namely, voice onset time, were selectively adapted.

The identification functions for the remaining two subjects were very similar to those shown in Fig. 2. In all, there were 24 instances of at- tempted adaptation, eight adapting conditions for each of three subjects. In each instance there was a shift in the phonetic boundary, and further- more the direction of the shift was always toward the voicing distinction that had been adapted, i.e., more identification responses belonged to the unadapted mode of voicing. The individual data are shown in Table 1. The mean shift in the locus of the phonetic boundary was 8.0 msec. It should be noted that the effects of adaptation were not sym- metrical: the mean shift was 6.1 msec after adaptation with voiced stops and 10.0 msec after adaptation with voiceless stops. In addition for all listeners the mean magnitude of the boundary shift was only slightly (less than 2 msec) contingent upon the adapting stimulus and the identification series belonging to the same class of stop consonants (i.e., bilabial or apical).

Although we have not systematically investigated the time-course of recovery from adaptation, some preliminary investigations indicated that recovery is no more than 50% complete at the end of 90 set and that complete recovery will require 30 min or more.

TABLE 1 Shift in the Locus of the Phonetic Boundary in Milliseconds of VOT for the

Identification Experiment

Adapting stimulus Identification

series Subjects Lb1 iPI I4 [tl

1 (29.0)” f6.3 -118.8 + .5 -15.0 [b, PI 2 (28.8) +7.5 - 6.8 + 2.8 -16.3

3 (30.0) +7.5 - 7.5 +10.0 - 7.5

R (29.3) +7.1 - 9.4 + 4.4 -12.9

1 (42.5) +3.x - x.0 + 3.5 -10.0 b-4 tl 2 (37.5) +5.3 - 4.8 + 8.0 -14.5

3 (37.5) +5.5 - 6.0 +12.0 -10.0

2 (39.2) +4.9 - 6.3 + 7.8 -11.5

~1 The number in parentheses shows the locus of the unadapted phonetic boundary.

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EXPERIMENT 2

Given the categorical nature of the perception of the stop consonants as most markedly evidenced by a peak in the discriminability function of adjacent stimuli at the region of the phonetic boundary, we reasoned that the peak would shift after selective adaptation of one of the voicing de- tectors. That is, inasmuch as the ability to discriminate these stimuli has been found to be closely related to the ability to apply differential phonetic labels, then any shift in the locus of the phonetic boundary should be par- alleled by a corresponding shift in the peak of the discriminability func- tions. To verify this we obtained discriminability functions for the [b, p] series before and after adaptation with the voiceless stop [p].

Method

Stimuli. The stimuli to be discriminated were 11 synthetic speech patterns taken from the [b, p] series of Experiment 1. The VOT values ranged from 0 to +50 msec in 5msec steps. The adapting stimulus had a VOT value of +60 msec and was perceived uniformly as [p] plus the vowel [a].

Procedure. The psychophysical method of ABX was used to measure discriminability. For any set of three stimuli, the first stimulus A differed from the second stimulus B, and the third stimulus X was identical to the first stimulus or to the second stimulus. The listeners’ task was to indicate whether the third stimulus was the same as the first or the second stimulus. Sets of stimuli to be discriminated were arranged by pairing each stimulus with the stimulus two steps (10 msec) removed. That is, the discriminability of VOT values 0 and + 10, + 5, and + 15, and so forth was measured. There are nine such pairs in all and four permutations for each pair (ABA, ABB, BAB, and BAA) for a total 36 possible triads.

To obtain the discriminability function without adaptation the 36 triads were presented to the listeners in random order. The stimuli within each triad were separated by 1.5 set and each triad was separated by 5 sec. A total of 24 measures were obtained for each stimulus pair.

The procedure used to obtain the discriminablity function when the listeners were adapted was the same as that used during the identification study, except that in place of a single stimulus to be identified a randomly selected ABX triad was presented for discrimination. Twenty-four mea- sures were obtained from each listener for each pair of stimuli.

Subjects. Two of the subjects had served as listeners in the first ex- periment and together with the third subject had had extensive experi- ence in listening to synthetic speech.

LINGUISTIC FEATURE DETECTORS 107

Results Figure 3 depicts the mean discriminability function for the three sub-

jects. We have used an average function in this instance since the individual functions were more variable than were the individual identi- fication functions. This variability was most likely a function of the greater difficulty of the discrimination task. However, the effects evident in the group function also appear in each of the individual functions. Exposure to the voiceless stop [p] radically altered the discrimination function. There was a shift in the peak of the discrimination function that corresponded to the shift in the locus of the phonetic boundary, demonstrating in a novel manner that discrimination of the stop con- sonants is closely related to the ability to differentially identify the stimuli. The magnitude of the shift was 5 msec for two listeners and 10 msec for the third listener. The discriminability function clearly shows that after adaptation, when the likelihood is increased that the stimuli

Voice onset time (msec)

FIG. 3. The group discriminability function. The points are plotted midway be- tween the two values of voice onset time being discriminated. The dashed line represents the discriminabihty function after adaptation with [p]. The arrows indicate the locus of the phonetic boundaries found from identification functions with (dashed arrow) and without (solid arrow) adaptation with [p].

108 EIMAS AND CORBIT

being discriminated belong to the same phonetic class, discriminability is at or very near chance. Conversely, for those stimuli which after adaptation have a greater probability of being assigned to different phonetic categories, there is a marked increase in the level of dis- crimination.

GENERAL DISCUSSION

The fact that repeated presentation of a member of one of the voicing categories dramatically reduces the sensitivity of the system to members of that category may be explained by assuming two linguistic feature detectors, each of which is tuned to a restricted range of VOT values and mediates the perception of one of the two voicing distinctions found in the stop consonants.

To explain how these detectors might operate to produce the identi- fication and discrimination functions that are obtained with and without experimentally induced adaptation, the following assumptions are needed: (a) There exist detectors that are differentially sensitive to a range of VOT values with greatest sensitivity (as might be measured, in principle, by the output signal of the detector) occurring at the modal production value for a particular voicing distinction (Lisker & Abramson, 1964). (b) Some VOT values excite both detectors, but, all other things being equal, only the output signal with the greater strength reaches higher centers of processing and integration. (c) The phonetic boundary will lie at the VOT value that excites both detectors equally, all other factors being equal. (d) After adaptation, the sensitivity of a detector is lessened; that is, the output signal is weakened or decreased.’ Furthermore, for purposes of simplicity, the signal strength is assumed to decrease equally for the entire range of VOT values to which the detector is sensitive. From this it follows that selective adaptation shifts the phonetic boundary by shifting the point of equilibrium along the VOT continuum. If we further assume (e) that no distinction is made by higher-order processing elements between two output signals from the same detector, that is, no distinction is made when the same detector is excited by two different values of VOT, then the peaked discriminability functions are readily accounted for.

The existence of linguistic feature detectors for the voicing distinctions among the stop consonants has a number of important implications. First, it provides a mechanism whereby infants can perceive the VOT continuum in a nearly categorical manner. Second, it adds credence to theoretical descriptions of the basic sound units of language based on distinctive features (Halle, 1962; Jakobson, Fant, & Halle, 1963). And finally it provides an example of a complex analysis of linguistic informa-

LINGUISTIC FEATURE DETEfXORS 109

tion in a manner at least analogus to that previously demonstrated in the visual system.

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(Accepted May 23, 1972)