Continuous and categorical perception of a fricative-affricate continuum

Journal of Phonetics (1982) 10, 231-244

Continuous and categorical perception of a fricative- affricate continuum F. E. Ferrero, G. M. Pelamatti and K. Vagges Centro di Studio perle Ricerche di Fonetica (C.N.R.), Padova, Italy

C.Umilta' Istituto di Fisiologia Umana, Facolta di Medicina, Universita di Parma, Parma, Italy Received 1st August 1981

Abstract:

The experiments reported here aimed to examine the mode of perception of a series of synthetic speech stimuli /sa-tsa/ and /fa-t fa/, differing systematically in the duration of the frication. It was thought that a continuous perception could be shown for the fricatives if their acoustic characteristics were rendered more prominent for the discrimination task, by including stimuli of shortened duration in the continuum. The results indicated a rather categorical perception suggesting that the availability of acoustic information in auditory memory is immaterial for producing continuous perception in a discrimination task. However, definitive conclusions with regard to the perception of this particular continuum could not be drawn. The results obtained in both experiments for the stimuli having the shortest frication showed a significant difference between obtained and predicted discrimination functions. This effect may be attributed to the ratio between the duration of the frication noise and the duration of the entire stimulus.

Introduction Several experiments on the perception of speech sounds have revealed that stop consonants are perceived in a special mode , that is in a way different from that of other auditory stimuli. liberman et al. (1957) found , by using synthetic speech stimuli, that the changes of shape in second formant transition brought about perceptual changes from /b/ to /d/ to /g/. In other words, such stimuli were identified as belonging to three different phonetic categories rather than to a series gradually changing from one phonetic category to another. It was also found that listeners ignored acoustic differences within categories not only in the identification task but also in the discrimination task. When the discrimination function may be predicted from the identification function, that is, when the discrimination is based exclusively on the phonetic category labels, as is the case for stop consonants, the perception is said to be "categorical" (liberman et al., 19 57; Studdert-Kennedy et al., 1970; Pisoni, 1971). Obtained discrimination functions for isolated vowels, on the other hand, are much better 0095-44 70/82/030231 + 14 $03 .00/0

© 1982 Academic Press Inc. (London) Ltd.

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than those predicted from identification functions, indicating that listeners discriminate many more differences than they identify. This type of perception is said to be "continuous" (Stevens et at., 1969; Pisoni, 1971; Fry et at., 1972). These differences in perceptual processes between stop consonants and vowels may be accounted for by the motor theory of speech perception. In fact, it was argued that while vowels are perceived much as any other acoustic stimulus, stops are perceived by reference to the discrete articulatory gestures from which they arise (Liberman et at., 1967). Another explanation of the difference between continuous and categorical perception may be related to the role of auditory memory in speech discrimination (Fujisaki & Kawashima, 1969, 1970; Pisoni, 1971, 1973, 1975; Darwin & Baddeley, 1974). Fujisaki & Kawashima (1969) and Pisoni (1971) affirm that discrimination for vowels is better than that for stops because auditory memory of the acoustic properties is more readily available for use in the discrimination in the case of vowels than in the case of stops. Empirical evidence in favour of this notion can be found in the observation that the acoustic cues of stops are of brief duration and their representation in auditory memory is not stable, whereas vowels have steady-state acoustic characteristics whose representation in auditory memory is much more robust and can be utilized more easily in a discrimination task(Crowder, 1971, 1973a, b). Despite the fact that fricative consonants have a rather stable representation in auditory memory, similar to that of vowels (Crowder, 1971, 1973b), Fujisaki & Kawashima (1968) found that these consonants are perceived in a categorical mode, that is, like stops. Thus the results obtained for the fricatives seem to contradict the notion that auditory memory plays a decisive role in speech discrimination tasks. The aim of the first experiment of the present study was to investigate whether it could be possible to bring about a continuous mode of perception of the fricatives as the hypothesis of auditory memory seems to imply. Previous studies (Fujisaki & Kawashima, 1969, 1970; Pisoni, 1971 , 1973, 1975; Cutting et at., 1976; Repp et at., 1979) showed a more nearly categorical perception of vowels by shortening their duration, that is, by making their aco1:1stic properties less stable in auditory memory. Fallowing the same line of reasoning it could be possible to bring about a continuous perception of fricatives by lengthening their duration, that is, by making their acoustic properties more stable and thus more available to auditory memory. However, in the present study this possibility was discarded since a preliminary experiment had shown that, beyond a certain limit, the noise of the frication would render the stimulus unidentifiable as speech. An alternative way of bringing about a continuous perception of fricatives might be the following. Since it is well known that the acoustic properties of fricatives are present in auditory memory (Crowder, 1971 , 1973b ), the subjects could be induced to utilize them in a discrimination task by rendering acoustic information more prominent. The acoustic properties of some stimuli are likely to become more prominent when the stimuli are presented intermixed with other stimuli in which the frication noise has been shortened. We thought of inducing the subjects to utilize the acoustic information held in auditory memory in a context of stimuli where the duration of the frication was systematically shortened. In this connection, it must be considered that the finding of a categorical perception of fricatives by Fujisaki & Kawashima (1968) might be attributed to their use of stimuli changing along a frequency continuum instead of a duration continuum. In other words it was hypothesized that for long duration of the frication, subjects should

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be induced to use the auditory memory in the discrimination task and they should show a more continuous mode of perception. If this is the case, then the role of auditory memory in speech discrimination would be confirmed in accordance with the explanation of continuous perception given by Fujisaki & Kawashima (1969 , 1970), Pisoni (1971, 1973, 1975), Darwin & Baddeley (1974).

Experiment 1 Method Stimuli All stimuli were produced by the terminal analog speech synthesizer OVE Illb under the control of a HP 21 OOA computer.

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The speech stimuli where /sa/, / Ja/ and /fa/. As shown in Fig. 1 the syllables consisted of a frication period followed by a transition to the steady-state vowel. The duration of the frication for all three syllables was 140 ms, a value which on the basis of pretest judgements yielded 100% correct identification (Ferrero et al, 1979). The duration of the transition plus the steady-state was 160 ms. The frication noise of each syllable was systematically shortened by steps of 10 ms, from 140 ms (total length of the syllable 300 ms) to 50 ms (total length of the syllable 210 ms). Previous experiments have shown that shortening in equal increments the duration of the fricative noise yields a continuum of stimuli having a fricative (long frication duration) and corresponding affricate (short frication duration) at its extremes (Gerstman, 1957; Huntington & Miller, 1972; Cole & Cooper, 1975; Ferrero etal., 1979 ; Dorman eta!. , 1979). Thus each of the 10 syllables of the continuum belonged either to the fricative class (long

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frication noise) or to the corresponding affricate (short frication noise). The three continua are respectively /sa-tsa/ , / f a-tf a/ and /fa-pfa/. Experimental tapes Identification tapes. Three different 100-item identification test sequences were prepared, one for the /sa-tsa/, one for the If a-tf a/ and one for the /fa-pfa/ series. Each sequence contained 10 randomized repetitions of the 10 stimuli of the series. The stimuli were recorded with a 4 s interstimulus interval and 10 s after every 10 presentations: ABX discrimination tapes . Six different 68-item test sequences were constructed , two for the /sa-tsa/ , two for the / f a-tf a/ and two for the /fa-pfa/ continuum. Members of each AB comparison were made by pairing each stimulus with the next stimulus one or two steps apart along the continuum, i.e. 140-130 ms is a one-step pair while 140-120 ms is a twostep pair . There were nine possible one-step comparisons (with stimula having a difference of 10 ms) and eight two-step comparisons (with stimuli having a difference of 20 ms) which produced 17 AB pairs. For each AB comparison there were four possible ABX arrangements: ABA, ABB, BAA, BAB. The stimuli within each triad were separated by 1 s while successive triads were separated by 4 s. Subjects The subjects were six university students, three females and three males, who reported no past history of a hearing disorder. They were paid for their services. None of the subjects had ever heard any synthetic speech before the experiment. Procedure All experimental tapes were reproduced on a Revox A77 tape recorder and were presented binaurally through ELA DT 48 headphones. Subjects were tested separately on five consecutive days. Each day subjects were tested on all identification tapes followed by the corresponding ABX discrimination tapes. Three of the six discrimination tapes were used each day. The order of the presentations was randomized. At the end of the sessions each subject provided 50 identification responses to each of the 10 stimuli of every continuum and 20 judgements for each of the AB discrimination comparisons. For the identification tests, subjects were required to identify the syllable either as a fricative or an affricate (i .e. /sa/ or /tsa/ , /fa/ or /tfa/ and /fa/ or /pfa/). In the ABX discrimination task the subjects were asked to decide whether the third stimulus of each triad was identical to either the first or the second . Subjects were told that they should respond on every trial. Results Identification The average identification functions for the three series /sa-tsa/, / f a-tf a/ and /fa-pfa/ are shown in Fig. 2. Subjects partitioned the continua into two distinct phonetic categories, the fricatives /sa/, /fa/ and /fa/ , and the affricates /tsa/ , /t fa/ and /pfa/, respectively . An analysis of variance , with type of syllable and duration of noise as within-subjects factors, and number of correct responses as dependent variable, confirmed this result, F 9, 15 = 35.58 p < 0.001 for the main factor duration of noise. Also the interaction type of syllable x duration of noise reached significance, F 18, 90 = 76.38 p < 0.001. The effect depended on the continuum /fa-pfa/. In fact, as Fig. 2 clearly shows, the identification function of /fa-pfa/ is quite different from that of /sa-tsa/ and /Ja-tfa/.

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The identification functions for /sa-tsa/ and / Ja-tJ a/ (see Fig. 2.) showed that the category shift, which occurs at a frication noise of approximately 80 ms, was more abrupt and clear cut than that for /fa-pfa/. The /fa-pfa/ stimuli were not consistently identified as belonging to one phonetic category or the other. In particular, three subjects consistently classified as affricate tlie stimuli with the longest frications , one those with the shortest frications and two gave chance responses . This result for the /fa-pfa/ identification function is probably due to the fact that the /pfa/ sound is unknown to the Italian Phonematic System (Ferrero et al., 1979) where it does not appear either within or across morpheme boundaries . A previous experiment (Ferrero eta!., 1979) with the same syllables had shown that when subjects are unaware of the possibility of the affricate response, the shift to the corresponding affricate does not occur at all. ABX discrimination The /fa-pfa/ series was not examined because, as previously explained, subjects did not use consistent criteria for the phonetic category label in the identification task. The closeness of fit between the obtained discrimination function and that predicted from the identification data, is a meaningful measure of the degree of categorical perception only when the test stimuli are consistently attributed to one phonetic category or the other. The one ai:J.d two-step ABX discrimination functions for the series /sa-tsa/ and IJa-tJ a/ are shown in Fig. 3 where the results are averaged across the subjects. Figure 4 shows the one and two-step ABX discrimination functions, averaged across the subjects and the continua /sa-tsa/ and IJa-t J a/. The solid line shows the obtained discrimination functions, while the broken line shows the discrimination functions predicted from the identification data, obtained according to the method provided by Liberman et al. (1957) and averaging the individual predictive scores for all the subjects.

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Figure 5 shows the one and two-step predicted and obtained discrimination functions of the lsa-tsal and I fa-tfal continua for subject S.C. One-step condition . An analysis of variance was carried out with the three following withinsubjects factors : type of continuum (/sa-tsal vs If a-tf a/) , stimulus comparison along the continuum (nine one-step pairs), and discrimination functions (predicted vs obtained). The dependent variable was the number of correct responses. For both continua the obtained discrimination functions were significantly better than the predicted, F 1, 5 = 12.91 p < 0.02. This result did not hold true for all pairs along the continuum . In fact , the obtained func tions were better than the predicted only for the stimuli with short frication noise, as shown by the interaction stimulus continuum X discrimination functions , F 8, 40 = 4.90 p < 0.001 (see also Fig. 4). Two-step condition. An analysis of variance was carried out with the same three factors. In this case the factor stimulus comparison had eight levels instead of nine. As in the one-step condition, the obtained discrimination function was better than the predicted, F 1, 5 = 32.48 p < 0 .005, and this was particularly true for stimuli with short frication noise , as shown by the interaction stimulus continuum x discrimination functions, F 7, 35 = 14.59 p < 0.001 (see again Fig. 4.).

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Discussion Categorical perception assumes that listeners can discriminate two sounds only to the extent that they can identify them as different. Since discrimination is thus based exclusively on the phonetic category labels the obtained discrimination function must be non-significantly different from that predicted from the identification data. When the obtained discrimination function is better than the predicted it means that the subjects discriminate differences which they cannot identify and perception is said to be continuous (Liberman eta!., 1957; Fujisaki & Kawashima, 1969; Studdert-Kennedy eta!., 1970; Pisoni, 1971, 1973, 1975). The results of this experiment failed to indicate clearly whether the perception of fricatives was categorical or continuous. The obtained discrimination function always revealed a close correspondence with the predicted only for the stimuli in the middle range and at the left end of the continuum (long frication noise). This result seems to indicate a categorical mode of perception . On the contrary, the obtained discrimination function was better than the predicted at the right end of the series (short frication noise) indicating a continuous mode of perception. This result is surprising since the acoustic characteristics of the stimuli with short frication noise are more similar to those of stop consonants, and yet they seem to be perceived in a continuous mode. The following explanation can be proposed for the findings of the present study. (1) Two different modes of perception can be involved, that is, fricatives (long frication noise) are perceived continuously. In order to test this hypothesis more stimuli belonging to the affricate category should be employed. If affricates are really perceived in a continuous mode then all the affricate stimuli should show the significant difference between obtained and predicted discrimination functions. In Experiment 1 only three stimuli were consistently attributed to the affricate category, therefore any conclusion in their mode of perception is uncertain.

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Perception of a fricative-affricate continuum

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(2) The stimuli at the right end of the continuum could be more discriminable owing to the fact that the effect of the shortening of the frication depends on the duration of the entire stimulus. In other words , subtracting 10 ms from an already short frication could have a different effect than subtracting 10 ms from a long frication. If this is true, the difference between obtained and predicted discrimination functions should be observed also when new stimuli with an even shorter frication are added at the right end of the continuum. (3) The difference between the obtained and the predicted discrimination functions observed at the right end of the continuum is due to a phonetic boundary as predicted by the categorical mode of perception. In fact , it is known that the performance is better when the discrimination takes place between stimuli drawn from different phonetic categories than from the same category (Liberman eta!. , 1957 ; Studdert-Kennedy eta!., 1970; Pisoni, 1971). By adding new stimuli at the right end of the continuum it should be possible to test this hypothesis . In fact, the obtained discrimination function at the right end should match the predicted showing that the perception is categorical. Experiment 2 replicated Experiment 1 with the only exception that three new stimuli with short frication were added at the right end of the continuum. Experiment 2

Method Stimuli The stimuli were the same as Experiment 1, with the addition of three stimuli at the right end of the continua, resulting thus in a series of 13 stimuli with frication duration noise from 140 to 20 ms. The /fa-pfa/ continuum was not examined in this experiment. Experimental tapes Identification. Two different 65-item identification sequences were prepared one for the /sa-tsa/ and one for the If a-tf a/ continuum. Each sequence contained five different randomizations of the entire series of 13 stimuli. The recording procedures were identical to those outlined in Experiment 1. ABX discrimination. Four different 91-item test sequences were prepared, two for the /sa-tsa/ and two for the If a-t fa/ continuum. Members of each AB comparison were made by pairing each stimulus with the next stimulus, one or two steps apart along the continuum . There were 12 possible one-step comparisons, with stimuli having a difference of 10 ms, and 11 two-step comparisons with pairs having a difference of 20 ms, which gives 23 AB pairs. For each AB comparison there were four possible ABX arrangements, where X was identical to either A or B. The recording procedures were identical to those outlined in Experiment 1. Subjects The subjects were eight students, four males and four females, who reported no past history of hearing disorder. None of the subjects had ever heard any synthetic speech before the experiment. They were paid of their services. Procedure The recording procedures and the instructions for the identification and the ABX discrimination tests were identical to those used in Experiment 1. Subjects were run in groups of four for three consecutive days. Each day subjects were tested twice on both continua. Discrimination tests were presented after the corresponding identification tests. Each group received a different random arrangement of the tapes. Overall each subject gave 30

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identification responses to each of the 13 stimuli of every continuum , and 20 judgements for each of the AB discrimination comparisons.

Results Identification The average identification function for the /sa-tsa/ and /Ja-tfa/ series is shown in Fig. 6. The continua were partitioned in two distinctive phonetic categories, the fricatives /sa/, /fa/ and affricates /tsa/, /tJa/. An analysis of variance with the same factors of Experiment 1 was carried out on correct responses. It showed that the category shift was abrupt and clear cut and occurred approximately at 80 ms, F 12, 84 = 14.96 p < 0.001 .

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ABX discrimination The one and two-step ABX discrimination functions for the series /sa-tsa/ and / Ja-tf a/ are shown in Fig. 7 where the results are averaged across the subjects. Figure 8. shows the one and two-step ABX discrimination functions averaged across subjects and continua. One-step condition. A three-factor within-subjects analysis of variance was carried out. The variables under consideration were : type of continuum (/sa-tsa/ vs / Ja-tf a/), stimulus comparison along the continuum (12 one-step pairs) and discrimination functions (predicted vs obtained). The dependent variable was the number of correct responses. The predicted discrimination functions were calculated , as in Experiment 1, according to the method provided by Liberman eta!. (1957).

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No source of variability reached statistical significance. For both continua the correspondence between predicted and obtained functions was closer than in Experiment 1, even at the right end of the series. The overall performance in the new extended continuum was not significantly different from that predicted by the categorical perception . Two-step condition. An analysis of variance with the same three factors described above was conducted on correct responses . The stimulus comparison factor had 11 levels. The results of Experiment 1 were replicated. The obtained discrimination function is significantly different from that predicted, F 1, 7 = 7.99 p < 0 .02 and this difference being apparent at the right end of the series, F 10, 70 = 10.10 p < 0 .001. Discussion Two of the three hypotheses discussed at the end of Experiment 1 predicted a difference between the two discrimination functions in correspondence to the new stimuli introduced in the present experiment. The third hypothesis predicts no difference between obtained and predicted functions along the whole continuum. The results of the one-step analysis were in agreement with the prediction of the last

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hypothesis, however this same hypothesis was disconfirrned by the results of the two-step analysis. In fact , while no discrepancy was found between observed and predicted functions in the first analysis, a significant discrepancy was shown by the second analysis. It must be kept in mind that all previous studies have considered the two-step analysis to be more conclusive than the one-step analysis for discriminating between categorical and continuous perception (see e.g. Liberman et al. , 1967; Pisoni, 1971). Furthermore, a clearcut even though non-significant discrepancy between the two discrimination functions was apparent also in the one-step analysis. On the other hand the significant difference between the obtained and predicted functions found in the two-step analysis cannot be considered as confirmatory of the other two hypotheses since they predict that those stimuli at the right end of the continua employed in both experiments should demonstrate a difference between the two functions also in Experiment 2. To sum up , none of the three hypotheses was confirmed. However a modified version of the second hypothesis could be accomodated to the results at least of the more important two-step analysis. The results of the present experiment showed that the difference between obtained and predicted functions was displaced to the newly introduced stimuli. Probably, the stimuli with the sho rtest frications were more distinctive and were perceived as being more different . In other words, we propose that the stimuli with the shortest frications play a distinctive role , owing to the differential effect of shortening the frication of an already very short stimulus . This distinctive role is assumed always by the shortest stimuli irrespective of their absolute duration. Therefore the effect can shift towards the end of the continuum when new and shortest stimuli are added. It should be noted that the acoustic characteristics of the shortest stimuli were very

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similar to those of stop consonants, even though they were identified as affricates. The discrimination task was thus facilitated, because the subjects were probably comparing stimuli belonging to different categories.

Conclusions The aim of this study was to examine the mode of perception of a series of synthetic speech stimuli differing systematically in the duration of the frication noise, that is, the /sa-tsa/ and the / Ja-t Ja/ series. It was thought that fricatives could show a continuous mode of perception when their acoustic characteristics were rendered more prominent for the discrimination task . To this effect stimuli of shortened duration were included in the continuum. Overall, the results indicated a categorical perception of fricatives confirming previous findings obtained with different procedures (Fujisaki & Kawashima, 1969). It seems thus, that the availability of acoustic information in auditory memory is immaterial for producing continuous perception in a discrimination task. In fact, Crowder (1973b) has shown that the acoustic characteristics of fricatives are held in auditory memory and despite this, fricatives seem to be perceived categorically. It was not possible to draw any definitive conclusions with regard to the perception of this particular continuum, that is, fricatives plus corresponding affricates, because of the surprising results at the right end of the series. In both experiments only the shortest stimuli showed a significant difference between the obtained and the predicted discrimination functions. Moreover, the difference was displaced to the right when the continuum was extended by adding three stimuli of shorter duration. The most plausible explanation is that this effect is strongly connected with the particularly distinctive role the shortest stimuli assume in comparison with the other stimuli of the continuum. This effect may be attributed to the ratio between the duration of the frication noise and the duration of the entire stimulus. References Cole, R. A. & Cooper, W. E. "(1975). Perception of voicing of English affricates and fricatives. Journal of Acoustical Society of America, 58 , 1280-1287. Crowder, R. G. (1971). The sound of vowels and consonants in immediate memory. Journal of Verbal Learning and Verbal Behavior, 10, 587-596. Crowder, R. G. (1973a). Precategorical acoustic storage for vowels of short and long duration. Perception and Psychophysics, 13, 502-506. Crowder, R. G. (1973b). Representation of speech sounds in precategorical acoustic storage. Journal of Experimental Psychology, 98 , 14-24. Cutting, J. E., Rosner, B.S. & Foard, C. F . (1976). Perceptual categories for musiclike sounds: implication for the theories of speech perception. Quarterly Journal of Experimental Psychology, 28, 361-378 . Darwin , C. J. & Baddeley, A. D. (1974). Acoustic memory and the perception of speech. Cognitive Psychology, 6, 41-60. Dorman, F . M., Raphael, L. S. & Isenberg, D. (1979). Acoustic cues for a fricative-affricate contrast in word-final position. Haskins Laboratories, Status Report on Speech Research, SR-57, 217-229. Ferrero, F . E., Pelamatti, G. M. & Vagges, K. (1979). Perceptual category shift of voiceless Italian fricatives as a function of duration shortening. In: Frontiers of Speech Communication Research (Lindblom, B. & Ohman, S. Eds), New York: Academic Press, pp. 159-165. Fry , D. D. , Abramson, A. S., Eimas, P. D. & Liberman, A. M. (1962) . The identification of synthetic vowels. Language and Speech, 5, 171-18 9. Fujisaki, H. & Kawashima, T. (1968) . The influence of various factors on the identification and discrimination of synthetic speech sounds. Reports of 6th International Congress on Acoustics, Tokyo, B-3-6. Fujisaki, H. & Kawashima, T. (1969). On the modes and mechanisms of speech perception. Annual Report of the Engineering Research Institute, University of Tokyo, 28, pp. 67-73. Fujisaki, H. & Kawashima, T. (1970). Some experiments on speech perception and a model for the

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