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Pulsation threshold patterns of synthetic vowels: Study of the second formant emergence and the “center of gravity” effects
Speech Communication 4 (1985) 189-198 North-Holland
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PULSATION THRESHOLD PATTERNS OF SYNTHETIC VOWELS: STUDY OF THE SECOND FORMANT EMERGENCE A N D THE "CENTER OF GRAVITY" EFFECTS* P. E S C U D I E R
and J.L. SCHWARTZ
lnstitut de la Communication Parl~e - INPG, E N S E R G , 46 avenue Felix Viallet, 38031 Grenoble Cedex, France
Received 31 July 1984 Revised 15 October 1984
Abstract. Very small spectral irregularities may be detected by the auditory system, which suggests the existence of a kind of frequency derivation-like mechanism. On the other hand, the "center of gravity" phenomenon suggests the existence of an integration-like mechanism. Our goal here is to study these two facts through the determination of psychophysical estimates of the internal representations of static harmonic spectra. It is commonly agreed that the pulsation threshold technique provides estimates of the results of the peripheral spectral analysis of such signals, accounting for the lateral suppression phenomena. The first part of this study consisted of the determination of a good experimental procedure for such a task. We report some observations we made about the pulsation threshold test and the solutions adopted. Then we present a preliminary set of results showing how the internal representation of a one-formant static sound is modified by the emergence of a second formant or by the value of the lateral slopes of the formant. The main conclusion is that smoothing of the spectrum due to non-infinite frequency selectivity dominates the lateral suppression effects in both cases.
Zusammenfassung. Das Geh6r kann sehr kleine Unregelm~i6gkeiten des Spektrums erkennen, wodurch das Vorhandensein einer Art Frequenzgradientenmechanismus nahegelegt wird. Andererseits deutet der Effekt der Schwerpunktbiidung (center of gravity) auf einen integrativen Mechanismus bin. Unser Ziel ist es, die beiden Tatsachen zu untersuchen, indem psychiphysikalische Sch/itzungen der internen Repr~isentation statischer harmonischer Spektren durchgeffihrt werden. Es besteht allgemeine Obereinstimmung darin, dab die Pulsationsschwellenmethode Sch~itzwerte der Ergebnisse peropherer Spektralanalyse derartiger Signale liefert, wobei laterale Suppression berficksichtigt wird. Der erste Teil der Arbeit bestand darin, eine gute Experimentiermethode ffir diese Aufgabe zu ermitteln. Wir berichten fiber einige Beobachtungen, die wir fiber die Pulsationsschwellenmessung machten, sowie fiber eingeschlagene L6sungswege. Danach beschreiben wit einige vorl~iufige Ergebnisse, aus denen hervorgeht, wie die interne Repr/isentation eines einformantigen statischen Schalles durch das Auftauchen eines zweiten Formanten oder die seitlichen Flanken des Formanten beeinflul3t wird. Die haupts~ichliche Schlul3folgerung besteht darin, da6 eine spektrale Gl/ittung, verursacht durch endliche Frequenzselektivit~it, die lateralen Suppressionseffekte in beiden F~illen beherrscht. R6sam6. De tr6s petites irr6gularit6s spectrales peuvent 6tre d6tect6es par le syst6me auditif, ce qui iaisse supposer l'existence d'un m6canisme de type d6rivation. A l'oppos6, le ph6nom6ne de "centre de gavit6" sugg6re l'existence d'un m6canisme d'int6gration. Notre objeetif est d'analyser les r6sultats pr6c6dents/~ l'aide du test du seuil de pulsation, outil d'6valuation des repr6sentations internes de spectres de signaux statiques. II est g6n6ralement admis que cette m6thode permet d'estimer le r6sultat de i'analyse spectrale du signal realis6e par le syst6me auditif p6riph6rique, en tenant compte des effets de suppression lat6rale. La premi6re partie de ce travail a consist6/~ d6terminer ies conditions exp6rimentales favorables pour effectuer nos mesures. Nous d6crivons les r6sultats de cette 6rude pr61iminaire ainsi que les solutions adopt6es. Nous montrons ensuite, sur un premier ensemble de donn6es exp6rimentales, comment la repr6sentation interne d'un signal hun formant est modifi6e avec l'6mergence d'un second formant ou la variation de la valeur num6rique des pentes lat6rales du formant. La principale conclusion est que les m(~canismes de lissage spectral li(~s b. l'existence d'une s61ectivit6 fr6quentielle non infinie dominent les effets de suppression lat6rale darts ces deux cas exp6rimentaux. Keywords. Pulsation threshold, synthetic vowels, center of gravity, psychoacoustics. This work has been presented at the 107th Meeting of the Acoustical Society of America (see J. Acoust. Soc. A m . , Voi. 75, Suppl. 1, 1984, p. 83). 0 1 6 7 - 6 3 9 3 / 8 5 / $ 3 . 3 0 © 1985, E l s e v i e r S c i e n c e P u b l i s h e r s B . V . ( N o r t h - H o l l a n d )
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1. Introduction The work we shall describe in this paper has been carried out in the Speech Communication Institute of Grenoble, where collaboration with the Pavlov Institute of Leningrad has been developed for several years. This collaboration has been mainly pursued in one research area: the study of the perception of stationary vowel-like signals, the objective being the accumulation of experimental data and the testing of perception models. This work has progressed in two directions. ~ First, the study of the detection of spectral irregularities on a reference pattern (Chistovich et al. [4], Lublinskaya et al. [14]). We have been able to show that very small spectral irregularities can be detected, and may, in certain contexts, be used in a phonetic task. The mechanisms responsible for this very efficient detection of small local objects on the spectrum could be found in the phenomenon of lateral suppression , which, as commonly agreed, is capable of reinforcing spectral contrasts. This first fact is described by the Leningrad group as a spectral derivation-like process (Chistovich [6]). Secondly, the quantitative study of the "center of gravity effect" (Chistovich and Lublinskaya [5], Lublinskaya et al. [15]). This can be described as a large scale spectral integration-like process and appears to play an important part in the description of perceptual space of stationary vowels (Bladon [2]). In Grenoble, we are more specially concerned with peripheral processes of the auditory treatment of sound. Thus, we decided to study what were the internal representations of the stationary harmonic signals that we had used in the previously described experiments, with special concern for two main problems: i) For the detection of small spectral irregularities, what really is the role of lateral suppression'? Are contrasts reinforced by such spectral configurations? Can we imagine what kind of 1 Of course, a lot of papers deal with the general topic of perception of stationary vowel-like signals, and more specifically with the different issues addressed here. A special accent has been given in this paper on the works of Ludmilla Chistovich's group in Leningrad. Speech Communication
decision mechanisms could be applied to those representations so as to reproduce our results about detection of small irregularities? ii) What can be the influence of the formant pattern on the weight of the formant in the center of gravity effect? More particularly, how do the lateral sppression mechanisms act on the perceived formant amplitude when the lateral slopes, and thus, the formant bandwidth, are modified? To sum up these preliminary considerations, our purpose was to estimate internal representations of static harmonic vowel-like signals with special emphasis on the role of lateral suppression. Hence, we decided to obtain these estimates by determining nonsimultaneous masking patterns of the signals. This project raised two series of questions. First, what kind of nonsimultaneous masking could we use? This leads to a longstanding and still open debate between forward masking and pulsation threshold detection. We have chosen the second one mainly for its greater sensitivity, very important for our project, but we shall see that the use of this psychoacoustic tool is not simple, and requires considerable care by anyone who needs good precision of measurement, which was our case. More generally, what information is contained in a pulsation threshold pattern? What is an internal representation? What is the relationship between these two notions? We feel that these questions are most important, and are not clearly answered. But we also believe that the best way to progress in this field is to systematically use the tools in order better to know how they function, and to test the validity of the models of representation of our knowledge at the present time. In this paper, we shall stay in the area of the current assumptions: that the pulsation threshold patterns are a correct representation of the pattern of activity of nerve fibers in the auditory nerve, which can be considered as the internal projection of the signal, on which more central processes act to detect and treat the main features of the spectrum. We shall begin with a description of the procedure we have elaborated to obtain good experimental conditions, together with some observations we have made on our experimental corpus and the problems linked with the use of the pulsation threshold test. Then we shall
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describe a first set of results, which were obtained by virtue of the adopted procedure, in two different experiments linked to the two "main problems", cited above.
2. Experimental procedure We must begin with a preliminary statement. We planned an experiment where we wanted to measure the influence of small spectral modifications on pulsation threshold patterns, and we therefore had special need of very good precision in our measurements. Yet the observation of experimental results described in the literature show a very striking interindividual and sometimes intraindividual dispersion. Some authors report differences between subjects of more than 20 dB for the same determination. These results suggest the hypothesis that there exist different criteria for achieving the estimates of pulsation threshold. Making this point even more probable, Duifhuis [9] has shown that intrasession standard deviations for such experiments were much lower than the intersession ones, and he argues that one subject can change his/her criterion between two experiments. Therefore, we decided to use the following procedure (Fig. 1). In a single session, the subjectestimates the pulsation threshold of a test tone of fixed frequency FT pulsating with one of four different maskers M h je{1, 2, 3, 4}, presented three times each in a random order - - the subjects operated in two phases for each measure: a first "gross" adjustment was followed by a 2 dB up-down Bekesy tracking (see [12]). We were then able to compute the mean differences between the thresholds for two maskers in one single session and for the fixed test-tone frequency FT. This provided us a way of accounting for any change of criterion, since this bias is eliminated by the computation of those differences if we assume that the criterion is almost invariable within a session, as it appears in Duifhuis's results. After three sessions with the same test-tone and masker set, we were able to propose a final estimate of the differences. All the signals were prepared on a PDP-11 computer and synthesized with a 14 bit D/A converter. The tests were carried out by two subjects
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(the two authors) with normal audiograms and previous experience in these experiments, listening monaurally through TDH-39 earphones.in a soundproof booth. The temporal characteristics of the signals were the standard parameters of the pulsation threshold technique (see Verschuure et al [17]): equal duration of 125 ms and equal rise/ fall times of 10 ms for both signals (masker and probe). The masker bursts were repeated without interruption while the test-tone bursts were interrupted once every four cycles to assist the subjects in the task.
3. Observations about the pulsation threshold data With this procedure the subjects reported the subjective impression that the threshold was increasing, in a fatigue-like effect, within each session. We decided to systematically study the two biasing effects reported, namely the criterion change, which is an intersession bias, and this fatigue-like phenomenon, which is an intrasession bias. This led us to prepare a complete and detailed study of the pulsation threshold procedure, where we described our observations about some general characteristics of the experimental data obtained with this test and we proposed a special processing of experimental data in order to account for these observations. This work is detailed elsewhere (see Escudier and Schwartz [19]) and we shall only give here the main facts and assumptions that allowed us to use the pulsation threshold method in "good conditions". Our study began with the following set of assumptions. Let Si specify the session, n represent the number of the measure within one session (a "measure" has been defined in Section 2), n going from 1 to 12, and Mj and Fx respectively characterize the masker and the test-tone. We assume in Eq. 1 that there exists an hypothetical "exact value" L(M h FT) of the pulsation threshold level for masker M i and test-tone FT and that our experimental determinations, namely L*, depend not only on Mj and FT, but also on n, by the bias of the fatigue-like effect, and Si, by the bias of the criterion change, X being a measure alea of mean value 0:
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L*(Mj,FT,Si,n ) = L(Mj,FT) + ALintra(n)+ + zXL~n,or(S,) + X.
L(MhFT) was 0.6 dB for subject JLS and 1.2 dB for subject PE. This gives for the estimates of the differences L(Mj,FT) - L(M~,FT) (after 9 measures for each condition, namely 3 sessions of 3 measures each, see section 2) a precision better than 1 dB. We shall consider this value of 1 dB as the minimal "pertinent" value for the discussion. 2 °) A change of criterion (characterized by the component ALinter(Si) in Eq. (1))'was clearly attested. Indeed, some measurements carried out with the same experimental conditions and the
(1)
Then, with additional statistical assumptions and with a recursive procedure we attempted to decorrelate these different terms and we finally obtained the following conclusions: 1°) Our model described by equation (1) seemed acceptable, in the sense that our procedure allowed us good estimates of the values L(MhFT) - L(Ma,FT), with small standard deviations: the mean standard deviation for each estimate of
MASKER SET Ml, M2, M3, M.
i EXTRACTION OF ONE ELEMENT (IN A RANDOMORDER)
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THE SESSION IS COMPLETED WHENEACH MASKERHAS BEEN EXTRACTEDTHREE TIMES
FINAL PREClSE ESTIMATE DUE TO 2 dB UP-DOWN BEKESY TRACKING
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FINAL RESULTS << Mj > -< M1 > >
Fig. 1. Description of the test procedure. Speech Communication
[ SESSION 3...
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same subject showed the same scale of relative estimates, with coherent values of the differences between thresholds for the same pair of maskers in two different sessions, but the absolute values of threshold for each masker clearly displayed the existence of a systematic bias from one session to the other. Thus, Duifhuis's result is confirmed. Our procedure seems to be able to provide a good way of accounting for this bias, as only mean differences within each session are considered. 3°) Within one single session, there appears a systematic fatigue-like effect, with a mean increase in threshold of 1 to 2 dBs between the 1st and the 12th measurement (the duration of the whole session being about 15-20 mn). This result has been obtained by a statistical computation based on 50 to 100 sessions for each subject (around 1000 measurements), with the same result for both subjects. These observations suggest a few questions about the different models proposed to explain the origin of the pulsation threshold (this is more completely discussed in the article cited above, see [10]). In the present paper, the important point is the fact that we now have at our disposal a procedure allowing good precision in the determination of differences between pulsation threshold spectra obtained for different maskers. We shall now apply this procedure in the two following sections.
steepness of the spectrum slope (see [4], [15]), which suggests a treatment containing some kind of spectral differentiation of the excitation pattern. In this process would be involved the lateral suppression mechanisms (Karnickaya et al. [13]). We shall describe here the changes of pulsation threshold patterns of a one formant harmonic synthetic signal (the "reference") when an irregularity emerges on the high frequency side of the formant, and hence may appear as a phonetically important "second formant".
4.1. Description of the signals We have used exactly the same signals as those which had been already studied in our previous experiments about irregularity detection [14] (see Fig. 2). The reference consists of the first 30 harmonics of a fundamental frequency 100 Hz, with a triangular pattern of the spectrum in the dBBark plane, the maximum (the "formant") being fixed at 400 Hz. The equal lateral slopes were fixed in this experiment at 3 dB/Bark and two levels have been used: 50 and 70 dB SPL. The 30 harmonics are all added in sine-phase relation. The spectral irregularity is a triangular cluster with lateral slopes of 6 dB/Bark emerging on the
I
4. Emergence of a spectral irregularity The Leningrad group adopts the classical description of the auditory processing of steady-state vowels which begins with a peripheral spectral analysis followed by the extraction of local maxima, corresponding in part to the formants in the sound and in part to the lower harmonics of the fundamental tone [1]. The basic problem remains the detection of these formants. Some experiments have been carried out showing that the appearance of a barely detectable second maximum on the high frequency side of the single formant of a synthetic vowel-like harmonic signal changes the phonemic quality of the vowel [13]. The detection threshold of such spectrum irregularities depends on both the formant spacing and the
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[acoustic level in dB)
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~ =0.5 16.9 11000 Hz~ 13000 Hz) or 10.1 11300 Hz) Fig. 2. Structure o f the maskers relating to Section 4. AIMa~ characterizes the emergence o f the irregularity: (A/Ma x = O) corresponds to the reference with a single formant, Mz.
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reference, the frequency Fe and amplitude A/Max of the irregularity maximum being varied in the different experiments.
ences of the acoustic spectra of these signals. All the measures are made for test frequencies at harmonic multiples of 100 Hz. The results for one subject and one specific condition, namely a level of the masker reference of 50 dB SPL and an irregularitY frequency equal to 1300 Hz are shown in Fig. 3. In this figure, we present the comparison between acoustic spectra differences and pulsation threshold spectra differences for various irregularity levels. The first conclusions can be drawn, and they would apply to all the results we have
4.2. Results
For each value of the frequency position of the irregularity, we computed the differences between pulsation threshold patterns of the reference plus irregularities of different levels, and pulsation threshold patterns of the reference alone, and we compared them with the differ-
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Fig. 3. a) Differences between the acoustic spectra of maskers M2, M~, M4 and reference masker M1 in the vicinity of the spectra] irregularity. A/M= is defined in Fig. 2. b) corresponding differences for the pulsation threshold spectra (subject JLS). Speech Communication
P. Escudier, J.L. Schwartz / Pulsation threshold of vowels
obtained for both subjects: 1°) A suppression area does not appear where the pulsation threshold would be lower for the (reference + irregularity) condition than for the reference alone condition. This fact is systematic for one subject; Sometimes, for the other subject, a suppression area seemed to appear, but the depth of this area was very small (less than 2 dBs) and so was its width (generally one or two harmonics). 2°) The maximum value of the pulsation threshold spectra differences is smaller than the maximum value of the acoustic spectra differences. By contrast, the area of positive differences appears a little larger for pulsation threshold spectra than for acoustic spectra. 3°) Finally, the contrasts between spectra do not appear better displayed in the internal representation than in the acoustic spectra. In this
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figure, it appears just the opposite and it is a general feature of all .our results. We then tried to draw out the systemacity in our results by plotting the value of the emergence of the irregularity in pulsation thresholds patterns that we defined by the data of the maximum value ALMax of the differences between pulsation threshold spectra obtained for the reference plus irregularity and for the reference alone, see Fig. 3 - - as a function of its acoustic emergence A/Max defined in Fig. 2. The results are presented for one subject, the data for the other one are less complete but display the same trends. On these curves, which provide a kind of psychophysical input-output function in response to the emergence of the second formant (Fig. 4), we may notice three main facts: 1°) The curves can be approximated by straight lines, fitted by eye in this figure. -
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Fig. 4. Psychophysical input-output functions in response to the emergence of a second formant (see text) for subject JLS. The level indicated in parameter in each drawing is the acoustic level of the reference masker.
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P. Escudier, J.L. Schwartz / Pulsation threshold of vowels
2°) The slopes of these lines are less than one. This accords well with our previous assertion that contrasts do not appear to be reinforced on pulsation threshold spectra in these experiments. 3°) The intersection of these lines with the Xaxis occurs for values A/Max which are systematically positive, around 0.5-1 dB. This may be a kind of threshold effect on pulsation threshold spectra: indeed, these threshold values are of the same order of magnitude as the direct estimates of the irregularity detection thresholds. Therefore, it could be that these detection thresholds are mainly due to peripheral effects linked with processing of the spectrum in the peripheral auditory system. Nevertheless, the precision of pulsation threshold data, and the small size of such values (about 1 dB) must make us cautious about this hypothesis. The main result of this section is the fact that the emergence of the spectral irregularity does not appear to be reinforced in the pulsation threshold spectra compared with the acoustic spectra. We could explain this by saying that in the "struggle" between smoothing of the spectrum due to non-infinite peripheral frequency selectivity and reinforcement of contrasts due to lateral suppression phenomena, smoothing finally wins. Suppression due to the first formant also certainly plays an important part, as was shown by Stelmachowicz et al. [16]. 5. Perceived amplitude of a formant as a function of its lateral slopes
In the study of the center of gravity effects we had to estimate what was the "weight" of each formant in the integration-like process, and one of the questions was: what happens if the lateral slopes of the formant pattern become steeper? Will it reduce the weight of the formant because the amount of energy in its neighbourhood diminishes? Or, on the contrary, will this weight increase because the effects of lateral suppression lessen, which reinforces the spectral peak, making it more accentuated? The results we obtained rather pointed towards the first description (Lublinskaya et al. [15]), but we have tried here to obtain a direct confirmation of this fact. Speech Communication
For this purpose, we used maskers of the same type as in the first experiment: harmonic signals of fundamental F0, with a triangular pattern of equal slopes in the dB-Bark plane, the peak being F 1 of 400 Hz, 700 Hz, 1000 Hz or 1300 Hz, and the equal lateral slopes of 1 to 6 dB/Bark. The reference in the estimates of the pulsation threshold differences was the signal with slopes of 3 dB/Bark, and a level of 50 or 70 dB SPL. The test-tone frequency was always fixed to F1 in this experiment: therefore, we estimated the formant amplitude in the pulsation threshold pattern, and we studied the variations of this amplitude with the lateral slopes of the formant. The results are displayed in Fig. 5. The diffe-
IdB re 3dB/Barkl
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SO dB SPL F~=t~O0Hz
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1
&L 70 dB SPL f~=~O0 Hz ,
, , Slope
=1000 Hz
Fig. 5. Variations in the formant amplitude of the pulsation threshold pattern with the lateral slopes of the masker formant. On the left, the level of the reference (masker obtained for lateral slopes of 3 dB/Bark) is 50 dB SPL. On the right, this level is 70 dB SPL. All data in this experiment are only for subject JLS.
P. Escudier, J.L. Schwartz/Pulsation threshold of vowels
ent curves correspond to the different experimental conditions for F1 and for the overall acoustic level of the reference masker. For a given slope, a negative value of AL corresponds to a perceived formant amplitude lower than the value obtained for a slope of 3 dB/Bark. The result is obvious: the larger the slope, the smaller the amplitude. Once more, we could say that suppression effects are weaker than integration effects: the more energy in the vicinity of the formant, the larger its amplitude in the internal representation. The influence of the lateral slopes appears greater for high F1 values than for low ones. This is in good agreement with the role probably played by integration mechanisms in this experiment. As a matter of fact, for a given value of the formant, the main contribution to the perceived formant amplitude due to spectral integration comes from the low frequency side of the spectrum, because of the well-known low-pass characteristics of the inner ear. For the lowest formant frequencies, this side is reduced to a small number of components (only three for FI = 400 Hz). Hence, its weight in the formant amplitude of the pulsation pattern is not considerable, so this amplitude is not very dependent on the influence of the lateral slopes. On the contrary, for the highest F1 values, the contribution of the lateral low frequency components is important, and changes very much with the lateral slopes.
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due to spectral integration in the peripheral auditory system. This makes less easy the description of the mechanism responsible for the extraction of maxima and irregularities of the spectrum as a derivation-like process. In fact, the quantitative characteristics of lateral suppression phenomena are complex and still not very well known at present, and it appears rather difficult to predict how they may act in a given "new" experimental situation. This is the reason why the systematic accumulation of experimental data such as those exposed in this paper remains, in our opinion, very important. For the same reason, quantitative models accounting for peripheral tuning characteristics and two-tone suppression data, such as those proposed by the groups of Leningrad and Grenoble (see [3], [7], [8[, [13]) may also be useful for the prediction of the influence of a given parameter on an internal representation. These two axes will be the basis of the continuation of this preliminary work.
Acknowledgements The authors would like to thank Dr. Michel Boulogne for assistance in software development. This research was supported by the Centre National de la Recherche Scientiflque (L.A. no. 368).
References 6. Conclusions This study allowed us to progress in two main directions. First, since we had special need of very good precision in pulsation threshold measurements, we had to specify a special procedure. This led us to elaborate a complete method of processing of pulsation threshold data [10] and it will provide the opportunity for a further reflexion on the pulsation threshold technique and its theoretical basis. Second, we applied this technique in the field of perception of static harmonic signals. The main result of this preliminary study is the following: in the experimental situations where we have operated, the effects of lateral suppression are generally "dominated" by smoothing of the spectrum
[1] Ya.A. Bedrov, L.A. Chistovich and R.L. Sheikin, "Frequency position of the 'center of gravity' of formants as a useful feature in vowel perception", Sov. Acoust., Vol. 24, 1978, pp. 275-278. [2] A. Bladon, "Two-formant models of vowel perception: Shortcomings and enhancements", Speech Comm., Vol. 2, 1983, pp. 304-313. [3] I.A. Chistovich, M.P. Granstrem, V.A. Kozhevnikov, L.W. Lesogor, V.S. Shupljakov, P.A. Taljasin and W.A. Tjulkov, "A functional model of signal processing in the peripheral auditory system, Acustica, Vol. 31, 1974, pp. 34%353. [4] L.A. Chistovich, R.L. Sheikin and V.V. Lublinskaya, "Centers of gravity and spectral peaks as the determinants of vowel quality", in: Frontiers of Speech Communication Research, edited by B. Lindblom and S. Ohman, Academic, London, 1979, pp. 143-158. [5] L.A. Chistovich and V.V. Lublinskaya, "The 'center of gravity' effect in vowel spectra and critical distance beVol, 4, Nos, 1-3, August 1985
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[5] L.A. Chistovich and V.V. Lublinskaya, "The 'center of gravity' effect in vowel spectra and critical distance between the formants: Psychoacoustical study of the perception of vowel-like stimuli", Hearing Res., Vol. 1, 1979, pp. 185-195. [6] L.A. Chistovich, "Auditory processing of speech", Language and Speech, Vol. 23, 1980 pp. 67-73. [7] J.M. Dolmazon, L. Bastet and V.S. Shuplakov, "A functional model of peripheral auditory system in speech processing", Proc. I.E.E.E. Inter. Conf. on Acoustics, Speech and Signal Processing, Hartford, 1977, pp. 261264. [8] J.M. Dolmazon and M. Boulogne, "Interaction phenomena in a model of mechanical to neural transduction in the ear", Speech Comm., Vol. 1, 1982, pp. 55-73. [9] H. Duifhuis, "Level effects in psychophysical two-tone suppression", J. Acoust. Soc. Am., Vol. 67, 1980, pp. 914-927. [10] P. Escudier and J.L. Schwartz, "Pulsation threshold test: Evidence for the existence of two biases and discussion of their implications", to be published in the J. Acoust. Soc. Am., 1985. [11] T. Houtgast, "Psychophysical evidence for lateral inhibition in hearing", J. Acoust. Soc. Am., Vol. 51, 1972, pp. 1885-1894.
SpeechCommunication
[12] T. Houtgast, "Lateral inhibition in hearing", PhD Thesis, Academische Pers. B.V. Amsterdam, 1974. [13] E.G. Karnickaya, V.N. Mushnikov and N,V. Slepokurova, "Auditory processing of steady-state vowels", in: G. Fant and M.A.A. Tatham, eds., Auditory Analysis and Perception of Speech, Academic Press, London, 1975, pp. 55-82. [14] V.V. Lublinskaya, P. Escudier and R. Carr6, "Study of the formant detection thresholds", J. Acoust. Soc. Am., Vol. 67, Suppl. 1, 1980, pp. 102. [15] V.V. Lublinskaya, R. Carr6 and P. Escudier, "The auditory perception of the phonetically meaning spectral cues of steady-state synthetic vowels", in: Actes du Symposium Franco-Sovi#tique sur la parole, edited by the GALF, 1981, pp. 19-31. [16] P.G. Stelmachowicz, A.M. Small and P.J. Abbas, "Suppression effects for complex stimuli", J. Acoust. Soc. Am., Vol. 71, 1982, pp. 410--420. [17] J. Verschuure, M. Rodenburg and A.J.J. Maas, "Presentation conditions of the pulsation threshold method", Acustica, Vol. 35, 1976, pp. 47-54.
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PULSATION THRESHOLD PATTERNS OF SYNTHETIC VOWELS: STUDY OF THE SECOND FORMANT EMERGENCE A N D THE "CENTER OF GRAVITY" EFFECTS* P. E S C U D I E R
and J.L. SCHWARTZ
lnstitut de la Communication Parl~e - INPG, E N S E R G , 46 avenue Felix Viallet, 38031 Grenoble Cedex, France
Received 31 July 1984 Revised 15 October 1984
Abstract. Very small spectral irregularities may be detected by the auditory system, which suggests the existence of a kind of frequency derivation-like mechanism. On the other hand, the "center of gravity" phenomenon suggests the existence of an integration-like mechanism. Our goal here is to study these two facts through the determination of psychophysical estimates of the internal representations of static harmonic spectra. It is commonly agreed that the pulsation threshold technique provides estimates of the results of the peripheral spectral analysis of such signals, accounting for the lateral suppression phenomena. The first part of this study consisted of the determination of a good experimental procedure for such a task. We report some observations we made about the pulsation threshold test and the solutions adopted. Then we present a preliminary set of results showing how the internal representation of a one-formant static sound is modified by the emergence of a second formant or by the value of the lateral slopes of the formant. The main conclusion is that smoothing of the spectrum due to non-infinite frequency selectivity dominates the lateral suppression effects in both cases.
Zusammenfassung. Das Geh6r kann sehr kleine Unregelm~i6gkeiten des Spektrums erkennen, wodurch das Vorhandensein einer Art Frequenzgradientenmechanismus nahegelegt wird. Andererseits deutet der Effekt der Schwerpunktbiidung (center of gravity) auf einen integrativen Mechanismus bin. Unser Ziel ist es, die beiden Tatsachen zu untersuchen, indem psychiphysikalische Sch/itzungen der internen Repr~isentation statischer harmonischer Spektren durchgeffihrt werden. Es besteht allgemeine Obereinstimmung darin, dab die Pulsationsschwellenmethode Sch~itzwerte der Ergebnisse peropherer Spektralanalyse derartiger Signale liefert, wobei laterale Suppression berficksichtigt wird. Der erste Teil der Arbeit bestand darin, eine gute Experimentiermethode ffir diese Aufgabe zu ermitteln. Wir berichten fiber einige Beobachtungen, die wir fiber die Pulsationsschwellenmessung machten, sowie fiber eingeschlagene L6sungswege. Danach beschreiben wit einige vorl~iufige Ergebnisse, aus denen hervorgeht, wie die interne Repr/isentation eines einformantigen statischen Schalles durch das Auftauchen eines zweiten Formanten oder die seitlichen Flanken des Formanten beeinflul3t wird. Die haupts~ichliche Schlul3folgerung besteht darin, da6 eine spektrale Gl/ittung, verursacht durch endliche Frequenzselektivit~it, die lateralen Suppressionseffekte in beiden F~illen beherrscht. R6sam6. De tr6s petites irr6gularit6s spectrales peuvent 6tre d6tect6es par le syst6me auditif, ce qui iaisse supposer l'existence d'un m6canisme de type d6rivation. A l'oppos6, le ph6nom6ne de "centre de gavit6" sugg6re l'existence d'un m6canisme d'int6gration. Notre objeetif est d'analyser les r6sultats pr6c6dents/~ l'aide du test du seuil de pulsation, outil d'6valuation des repr6sentations internes de spectres de signaux statiques. II est g6n6ralement admis que cette m6thode permet d'estimer le r6sultat de i'analyse spectrale du signal realis6e par le syst6me auditif p6riph6rique, en tenant compte des effets de suppression lat6rale. La premi6re partie de ce travail a consist6/~ d6terminer ies conditions exp6rimentales favorables pour effectuer nos mesures. Nous d6crivons les r6sultats de cette 6rude pr61iminaire ainsi que les solutions adopt6es. Nous montrons ensuite, sur un premier ensemble de donn6es exp6rimentales, comment la repr6sentation interne d'un signal hun formant est modifi6e avec l'6mergence d'un second formant ou la variation de la valeur num6rique des pentes lat6rales du formant. La principale conclusion est que les m(~canismes de lissage spectral li(~s b. l'existence d'une s61ectivit6 fr6quentielle non infinie dominent les effets de suppression lat6rale darts ces deux cas exp6rimentaux. Keywords. Pulsation threshold, synthetic vowels, center of gravity, psychoacoustics. This work has been presented at the 107th Meeting of the Acoustical Society of America (see J. Acoust. Soc. A m . , Voi. 75, Suppl. 1, 1984, p. 83). 0 1 6 7 - 6 3 9 3 / 8 5 / $ 3 . 3 0 © 1985, E l s e v i e r S c i e n c e P u b l i s h e r s B . V . ( N o r t h - H o l l a n d )
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P. Escudier, J.L. Schwartz / Pulsation threshold of vowels
1. Introduction The work we shall describe in this paper has been carried out in the Speech Communication Institute of Grenoble, where collaboration with the Pavlov Institute of Leningrad has been developed for several years. This collaboration has been mainly pursued in one research area: the study of the perception of stationary vowel-like signals, the objective being the accumulation of experimental data and the testing of perception models. This work has progressed in two directions. ~ First, the study of the detection of spectral irregularities on a reference pattern (Chistovich et al. [4], Lublinskaya et al. [14]). We have been able to show that very small spectral irregularities can be detected, and may, in certain contexts, be used in a phonetic task. The mechanisms responsible for this very efficient detection of small local objects on the spectrum could be found in the phenomenon of lateral suppression , which, as commonly agreed, is capable of reinforcing spectral contrasts. This first fact is described by the Leningrad group as a spectral derivation-like process (Chistovich [6]). Secondly, the quantitative study of the "center of gravity effect" (Chistovich and Lublinskaya [5], Lublinskaya et al. [15]). This can be described as a large scale spectral integration-like process and appears to play an important part in the description of perceptual space of stationary vowels (Bladon [2]). In Grenoble, we are more specially concerned with peripheral processes of the auditory treatment of sound. Thus, we decided to study what were the internal representations of the stationary harmonic signals that we had used in the previously described experiments, with special concern for two main problems: i) For the detection of small spectral irregularities, what really is the role of lateral suppression'? Are contrasts reinforced by such spectral configurations? Can we imagine what kind of 1 Of course, a lot of papers deal with the general topic of perception of stationary vowel-like signals, and more specifically with the different issues addressed here. A special accent has been given in this paper on the works of Ludmilla Chistovich's group in Leningrad. Speech Communication
decision mechanisms could be applied to those representations so as to reproduce our results about detection of small irregularities? ii) What can be the influence of the formant pattern on the weight of the formant in the center of gravity effect? More particularly, how do the lateral sppression mechanisms act on the perceived formant amplitude when the lateral slopes, and thus, the formant bandwidth, are modified? To sum up these preliminary considerations, our purpose was to estimate internal representations of static harmonic vowel-like signals with special emphasis on the role of lateral suppression. Hence, we decided to obtain these estimates by determining nonsimultaneous masking patterns of the signals. This project raised two series of questions. First, what kind of nonsimultaneous masking could we use? This leads to a longstanding and still open debate between forward masking and pulsation threshold detection. We have chosen the second one mainly for its greater sensitivity, very important for our project, but we shall see that the use of this psychoacoustic tool is not simple, and requires considerable care by anyone who needs good precision of measurement, which was our case. More generally, what information is contained in a pulsation threshold pattern? What is an internal representation? What is the relationship between these two notions? We feel that these questions are most important, and are not clearly answered. But we also believe that the best way to progress in this field is to systematically use the tools in order better to know how they function, and to test the validity of the models of representation of our knowledge at the present time. In this paper, we shall stay in the area of the current assumptions: that the pulsation threshold patterns are a correct representation of the pattern of activity of nerve fibers in the auditory nerve, which can be considered as the internal projection of the signal, on which more central processes act to detect and treat the main features of the spectrum. We shall begin with a description of the procedure we have elaborated to obtain good experimental conditions, together with some observations we have made on our experimental corpus and the problems linked with the use of the pulsation threshold test. Then we shall
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describe a first set of results, which were obtained by virtue of the adopted procedure, in two different experiments linked to the two "main problems", cited above.
2. Experimental procedure We must begin with a preliminary statement. We planned an experiment where we wanted to measure the influence of small spectral modifications on pulsation threshold patterns, and we therefore had special need of very good precision in our measurements. Yet the observation of experimental results described in the literature show a very striking interindividual and sometimes intraindividual dispersion. Some authors report differences between subjects of more than 20 dB for the same determination. These results suggest the hypothesis that there exist different criteria for achieving the estimates of pulsation threshold. Making this point even more probable, Duifhuis [9] has shown that intrasession standard deviations for such experiments were much lower than the intersession ones, and he argues that one subject can change his/her criterion between two experiments. Therefore, we decided to use the following procedure (Fig. 1). In a single session, the subjectestimates the pulsation threshold of a test tone of fixed frequency FT pulsating with one of four different maskers M h je{1, 2, 3, 4}, presented three times each in a random order - - the subjects operated in two phases for each measure: a first "gross" adjustment was followed by a 2 dB up-down Bekesy tracking (see [12]). We were then able to compute the mean differences between the thresholds for two maskers in one single session and for the fixed test-tone frequency FT. This provided us a way of accounting for any change of criterion, since this bias is eliminated by the computation of those differences if we assume that the criterion is almost invariable within a session, as it appears in Duifhuis's results. After three sessions with the same test-tone and masker set, we were able to propose a final estimate of the differences. All the signals were prepared on a PDP-11 computer and synthesized with a 14 bit D/A converter. The tests were carried out by two subjects
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(the two authors) with normal audiograms and previous experience in these experiments, listening monaurally through TDH-39 earphones.in a soundproof booth. The temporal characteristics of the signals were the standard parameters of the pulsation threshold technique (see Verschuure et al [17]): equal duration of 125 ms and equal rise/ fall times of 10 ms for both signals (masker and probe). The masker bursts were repeated without interruption while the test-tone bursts were interrupted once every four cycles to assist the subjects in the task.
3. Observations about the pulsation threshold data With this procedure the subjects reported the subjective impression that the threshold was increasing, in a fatigue-like effect, within each session. We decided to systematically study the two biasing effects reported, namely the criterion change, which is an intersession bias, and this fatigue-like phenomenon, which is an intrasession bias. This led us to prepare a complete and detailed study of the pulsation threshold procedure, where we described our observations about some general characteristics of the experimental data obtained with this test and we proposed a special processing of experimental data in order to account for these observations. This work is detailed elsewhere (see Escudier and Schwartz [19]) and we shall only give here the main facts and assumptions that allowed us to use the pulsation threshold method in "good conditions". Our study began with the following set of assumptions. Let Si specify the session, n represent the number of the measure within one session (a "measure" has been defined in Section 2), n going from 1 to 12, and Mj and Fx respectively characterize the masker and the test-tone. We assume in Eq. 1 that there exists an hypothetical "exact value" L(M h FT) of the pulsation threshold level for masker M i and test-tone FT and that our experimental determinations, namely L*, depend not only on Mj and FT, but also on n, by the bias of the fatigue-like effect, and Si, by the bias of the criterion change, X being a measure alea of mean value 0:
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L*(Mj,FT,Si,n ) = L(Mj,FT) + ALintra(n)+ + zXL~n,or(S,) + X.
L(MhFT) was 0.6 dB for subject JLS and 1.2 dB for subject PE. This gives for the estimates of the differences L(Mj,FT) - L(M~,FT) (after 9 measures for each condition, namely 3 sessions of 3 measures each, see section 2) a precision better than 1 dB. We shall consider this value of 1 dB as the minimal "pertinent" value for the discussion. 2 °) A change of criterion (characterized by the component ALinter(Si) in Eq. (1))'was clearly attested. Indeed, some measurements carried out with the same experimental conditions and the
(1)
Then, with additional statistical assumptions and with a recursive procedure we attempted to decorrelate these different terms and we finally obtained the following conclusions: 1°) Our model described by equation (1) seemed acceptable, in the sense that our procedure allowed us good estimates of the values L(MhFT) - L(Ma,FT), with small standard deviations: the mean standard deviation for each estimate of
MASKER SET Ml, M2, M3, M.
i EXTRACTION OF ONE ELEMENT (IN A RANDOMORDER)
I
THE SESSION IS COMPLETED WHENEACH MASKERHAS BEEN EXTRACTEDTHREE TIMES
FINAL PREClSE ESTIMATE DUE TO 2 dB UP-DOWN BEKESY TRACKING
I FIRST GROSSDETERMINATION } OF THE PULSATIONTHRESHOLDI
I
I
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RESULTS MI
M2
M3
M4
COMPUTATION OF THE MEANVALUES < M1> < M2 >.
< Ma> < M4>
COMPUTATION OF THE DIFFERENCES
...SESSION 2
SESSION i
FINAL RESULTS << Mj > -< M1 > >
Fig. 1. Description of the test procedure. Speech Communication
[ SESSION 3...
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same subject showed the same scale of relative estimates, with coherent values of the differences between thresholds for the same pair of maskers in two different sessions, but the absolute values of threshold for each masker clearly displayed the existence of a systematic bias from one session to the other. Thus, Duifhuis's result is confirmed. Our procedure seems to be able to provide a good way of accounting for this bias, as only mean differences within each session are considered. 3°) Within one single session, there appears a systematic fatigue-like effect, with a mean increase in threshold of 1 to 2 dBs between the 1st and the 12th measurement (the duration of the whole session being about 15-20 mn). This result has been obtained by a statistical computation based on 50 to 100 sessions for each subject (around 1000 measurements), with the same result for both subjects. These observations suggest a few questions about the different models proposed to explain the origin of the pulsation threshold (this is more completely discussed in the article cited above, see [10]). In the present paper, the important point is the fact that we now have at our disposal a procedure allowing good precision in the determination of differences between pulsation threshold spectra obtained for different maskers. We shall now apply this procedure in the two following sections.
steepness of the spectrum slope (see [4], [15]), which suggests a treatment containing some kind of spectral differentiation of the excitation pattern. In this process would be involved the lateral suppression mechanisms (Karnickaya et al. [13]). We shall describe here the changes of pulsation threshold patterns of a one formant harmonic synthetic signal (the "reference") when an irregularity emerges on the high frequency side of the formant, and hence may appear as a phonetically important "second formant".
4.1. Description of the signals We have used exactly the same signals as those which had been already studied in our previous experiments about irregularity detection [14] (see Fig. 2). The reference consists of the first 30 harmonics of a fundamental frequency 100 Hz, with a triangular pattern of the spectrum in the dBBark plane, the maximum (the "formant") being fixed at 400 Hz. The equal lateral slopes were fixed in this experiment at 3 dB/Bark and two levels have been used: 50 and 70 dB SPL. The 30 harmonics are all added in sine-phase relation. The spectral irregularity is a triangular cluster with lateral slopes of 6 dB/Bark emerging on the
I
4. Emergence of a spectral irregularity The Leningrad group adopts the classical description of the auditory processing of steady-state vowels which begins with a peripheral spectral analysis followed by the extraction of local maxima, corresponding in part to the formants in the sound and in part to the lower harmonics of the fundamental tone [1]. The basic problem remains the detection of these formants. Some experiments have been carried out showing that the appearance of a barely detectable second maximum on the high frequency side of the single formant of a synthetic vowel-like harmonic signal changes the phonemic quality of the vowel [13]. The detection threshold of such spectrum irregularities depends on both the formant spacing and the
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[acoustic level in dB)
11 . . . . .
.
.
.
.......6 dB/Bark
12
1.1 1100 Hz]
~ =~,.1 I~00Hzl
~ =0.5 16.9 11000 Hz~ 13000 Hz) or 10.1 11300 Hz) Fig. 2. Structure o f the maskers relating to Section 4. AIMa~ characterizes the emergence o f the irregularity: (A/Ma x = O) corresponds to the reference with a single formant, Mz.
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reference, the frequency Fe and amplitude A/Max of the irregularity maximum being varied in the different experiments.
ences of the acoustic spectra of these signals. All the measures are made for test frequencies at harmonic multiples of 100 Hz. The results for one subject and one specific condition, namely a level of the masker reference of 50 dB SPL and an irregularitY frequency equal to 1300 Hz are shown in Fig. 3. In this figure, we present the comparison between acoustic spectra differences and pulsation threshold spectra differences for various irregularity levels. The first conclusions can be drawn, and they would apply to all the results we have
4.2. Results
For each value of the frequency position of the irregularity, we computed the differences between pulsation threshold patterns of the reference plus irregularities of different levels, and pulsation threshold patterns of the reference alone, and we compared them with the differ-
AI IdB) 7 ...................
6
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DIFFERENCES BETWEEN THE ACOUSTICSPECTRA
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Fig. 3. a) Differences between the acoustic spectra of maskers M2, M~, M4 and reference masker M1 in the vicinity of the spectra] irregularity. A/M= is defined in Fig. 2. b) corresponding differences for the pulsation threshold spectra (subject JLS). Speech Communication
P. Escudier, J.L. Schwartz / Pulsation threshold of vowels
obtained for both subjects: 1°) A suppression area does not appear where the pulsation threshold would be lower for the (reference + irregularity) condition than for the reference alone condition. This fact is systematic for one subject; Sometimes, for the other subject, a suppression area seemed to appear, but the depth of this area was very small (less than 2 dBs) and so was its width (generally one or two harmonics). 2°) The maximum value of the pulsation threshold spectra differences is smaller than the maximum value of the acoustic spectra differences. By contrast, the area of positive differences appears a little larger for pulsation threshold spectra than for acoustic spectra. 3°) Finally, the contrasts between spectra do not appear better displayed in the internal representation than in the acoustic spectra. In this
/
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figure, it appears just the opposite and it is a general feature of all .our results. We then tried to draw out the systemacity in our results by plotting the value of the emergence of the irregularity in pulsation thresholds patterns that we defined by the data of the maximum value ALMax of the differences between pulsation threshold spectra obtained for the reference plus irregularity and for the reference alone, see Fig. 3 - - as a function of its acoustic emergence A/Max defined in Fig. 2. The results are presented for one subject, the data for the other one are less complete but display the same trends. On these curves, which provide a kind of psychophysical input-output function in response to the emergence of the second formant (Fig. 4), we may notice three main facts: 1°) The curves can be approximated by straight lines, fitted by eye in this figure. -
/
IF2 =1000 Hz) 50 dB SPL
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195
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Fig. 4. Psychophysical input-output functions in response to the emergence of a second formant (see text) for subject JLS. The level indicated in parameter in each drawing is the acoustic level of the reference masker.
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P. Escudier, J.L. Schwartz / Pulsation threshold of vowels
2°) The slopes of these lines are less than one. This accords well with our previous assertion that contrasts do not appear to be reinforced on pulsation threshold spectra in these experiments. 3°) The intersection of these lines with the Xaxis occurs for values A/Max which are systematically positive, around 0.5-1 dB. This may be a kind of threshold effect on pulsation threshold spectra: indeed, these threshold values are of the same order of magnitude as the direct estimates of the irregularity detection thresholds. Therefore, it could be that these detection thresholds are mainly due to peripheral effects linked with processing of the spectrum in the peripheral auditory system. Nevertheless, the precision of pulsation threshold data, and the small size of such values (about 1 dB) must make us cautious about this hypothesis. The main result of this section is the fact that the emergence of the spectral irregularity does not appear to be reinforced in the pulsation threshold spectra compared with the acoustic spectra. We could explain this by saying that in the "struggle" between smoothing of the spectrum due to non-infinite peripheral frequency selectivity and reinforcement of contrasts due to lateral suppression phenomena, smoothing finally wins. Suppression due to the first formant also certainly plays an important part, as was shown by Stelmachowicz et al. [16]. 5. Perceived amplitude of a formant as a function of its lateral slopes
In the study of the center of gravity effects we had to estimate what was the "weight" of each formant in the integration-like process, and one of the questions was: what happens if the lateral slopes of the formant pattern become steeper? Will it reduce the weight of the formant because the amount of energy in its neighbourhood diminishes? Or, on the contrary, will this weight increase because the effects of lateral suppression lessen, which reinforces the spectral peak, making it more accentuated? The results we obtained rather pointed towards the first description (Lublinskaya et al. [15]), but we have tried here to obtain a direct confirmation of this fact. Speech Communication
For this purpose, we used maskers of the same type as in the first experiment: harmonic signals of fundamental F0, with a triangular pattern of equal slopes in the dB-Bark plane, the peak being F 1 of 400 Hz, 700 Hz, 1000 Hz or 1300 Hz, and the equal lateral slopes of 1 to 6 dB/Bark. The reference in the estimates of the pulsation threshold differences was the signal with slopes of 3 dB/Bark, and a level of 50 or 70 dB SPL. The test-tone frequency was always fixed to F1 in this experiment: therefore, we estimated the formant amplitude in the pulsation threshold pattern, and we studied the variations of this amplitude with the lateral slopes of the formant. The results are displayed in Fig. 5. The diffe-
IdB re 3dB/Barkl
[
SO dB SPL F~=t~O0Hz
1
Ct,•L 1
1
&L 70 dB SPL f~=~O0 Hz ,
, , Slope
=1000 Hz
Fig. 5. Variations in the formant amplitude of the pulsation threshold pattern with the lateral slopes of the masker formant. On the left, the level of the reference (masker obtained for lateral slopes of 3 dB/Bark) is 50 dB SPL. On the right, this level is 70 dB SPL. All data in this experiment are only for subject JLS.
P. Escudier, J.L. Schwartz/Pulsation threshold of vowels
ent curves correspond to the different experimental conditions for F1 and for the overall acoustic level of the reference masker. For a given slope, a negative value of AL corresponds to a perceived formant amplitude lower than the value obtained for a slope of 3 dB/Bark. The result is obvious: the larger the slope, the smaller the amplitude. Once more, we could say that suppression effects are weaker than integration effects: the more energy in the vicinity of the formant, the larger its amplitude in the internal representation. The influence of the lateral slopes appears greater for high F1 values than for low ones. This is in good agreement with the role probably played by integration mechanisms in this experiment. As a matter of fact, for a given value of the formant, the main contribution to the perceived formant amplitude due to spectral integration comes from the low frequency side of the spectrum, because of the well-known low-pass characteristics of the inner ear. For the lowest formant frequencies, this side is reduced to a small number of components (only three for FI = 400 Hz). Hence, its weight in the formant amplitude of the pulsation pattern is not considerable, so this amplitude is not very dependent on the influence of the lateral slopes. On the contrary, for the highest F1 values, the contribution of the lateral low frequency components is important, and changes very much with the lateral slopes.
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due to spectral integration in the peripheral auditory system. This makes less easy the description of the mechanism responsible for the extraction of maxima and irregularities of the spectrum as a derivation-like process. In fact, the quantitative characteristics of lateral suppression phenomena are complex and still not very well known at present, and it appears rather difficult to predict how they may act in a given "new" experimental situation. This is the reason why the systematic accumulation of experimental data such as those exposed in this paper remains, in our opinion, very important. For the same reason, quantitative models accounting for peripheral tuning characteristics and two-tone suppression data, such as those proposed by the groups of Leningrad and Grenoble (see [3], [7], [8[, [13]) may also be useful for the prediction of the influence of a given parameter on an internal representation. These two axes will be the basis of the continuation of this preliminary work.
Acknowledgements The authors would like to thank Dr. Michel Boulogne for assistance in software development. This research was supported by the Centre National de la Recherche Scientiflque (L.A. no. 368).
References 6. Conclusions This study allowed us to progress in two main directions. First, since we had special need of very good precision in pulsation threshold measurements, we had to specify a special procedure. This led us to elaborate a complete method of processing of pulsation threshold data [10] and it will provide the opportunity for a further reflexion on the pulsation threshold technique and its theoretical basis. Second, we applied this technique in the field of perception of static harmonic signals. The main result of this preliminary study is the following: in the experimental situations where we have operated, the effects of lateral suppression are generally "dominated" by smoothing of the spectrum
[1] Ya.A. Bedrov, L.A. Chistovich and R.L. Sheikin, "Frequency position of the 'center of gravity' of formants as a useful feature in vowel perception", Sov. Acoust., Vol. 24, 1978, pp. 275-278. [2] A. Bladon, "Two-formant models of vowel perception: Shortcomings and enhancements", Speech Comm., Vol. 2, 1983, pp. 304-313. [3] I.A. Chistovich, M.P. Granstrem, V.A. Kozhevnikov, L.W. Lesogor, V.S. Shupljakov, P.A. Taljasin and W.A. Tjulkov, "A functional model of signal processing in the peripheral auditory system, Acustica, Vol. 31, 1974, pp. 34%353. [4] L.A. Chistovich, R.L. Sheikin and V.V. Lublinskaya, "Centers of gravity and spectral peaks as the determinants of vowel quality", in: Frontiers of Speech Communication Research, edited by B. Lindblom and S. Ohman, Academic, London, 1979, pp. 143-158. [5] L.A. Chistovich and V.V. Lublinskaya, "The 'center of gravity' effect in vowel spectra and critical distance beVol, 4, Nos, 1-3, August 1985
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