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An electropalatographic and acoustic study of consonant-to-vowel coarticulation
Journal of Phonetics (1991) 19, 177-192
An electropalatographic and acoustic study of consonant-to-vowel coarticulation Daniel Recasens CEDI, Institut d 'Estudis Catalans, c/ Carme 47, 08001 Barcelona, and Departament de Filologia Catalana, Universitat Autonoma de Barcelona, Bellaterra, Barcelona, Spain Received 8th December 1989, and in revised form 2nd May 1990
Coarticulatory effects from consonants on vowels were investigated using electropalatography and acoustical analysis . The electropalatographic data conform with an articulatory classification of vowels into front and back categories. Moreover, vowels are shown to differ in their degree of sensitivity to coarticulatory effects from the adjacent consonants. In general, consonant-dependent effects are larger upon those articulatory regions which do not intervene in the formation of a vowel constriction, although compensatory variation in other gestures (e .g., lip rounding) must be invoked to interpret the highly variable pattern of lingual coarticulation for [u). The paper also concludes that the degree of C-to-V coarticulation is influenced by the production requirements on the consonant; that explains why the degree of linguopalatal contact for [i] is more affected by the velarized alveolar lateral [t) than by other consonants. The electropalatographic data are generally correlated with acoustic (F2 ) data, with greater variation in F 2 frequency in those vowels showing greater variation in linguopalatal contact area .
1. Introduction While there are a good number of studies on vowel-to-consonant coarticulation, rather less attention has been paid to the effects of consonants on vowels. Much of the literature on the articulation of vowels refers to the configuration and activation of the speech organs involved in their production (Alfonso & Baer, 1982; Ladefoged, 1977) or to the location of their places of constriction (Wood, 1979) without regard to the consonantal context. The studies that do examine C-to-V effects almost without exception provide data only from acoustic analyses (Pols, 1977; Recasens, 1985; Stevens & House, 1963) so that the articulatory correlates of the observed C-to-V effects must be inferred from the acoustic data. This paper reports some articulatory data on C-to-V coarticulation in linguopalatal contact collected by means of electropalatography. Such data are needed to gain a more complete understanding of the production mechanisms used by speakers when uttering CV or VC syllables. They should also help differentiate among general models of speech production that have been proposed. The prevalence of studies on V-to-C coarticulation over studies on C-to-V effects may originate in certain broadly held theoretical assumptions about the articulatory 0095-4470/91/020177 + 16 $03.00/0
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constraints used by speakers during the production of the two phonetic categories. There are theories that claim that vowels are resistant to coarticulation because they are produced by means of global vocal tract shapes which require articulatory control upon the entire tongue body configuration whereas consonants involve only local constrictions which leave other articulatory regions free to coarticulate (e.g., Fowler, Rubin, Remez & Turvey, 1980) . This paper will show that vowels are subject to coarticulatory effects from consonants in a way which is very much like that attested in V-to-C coarticulatory effects . If we attribute primary constriction locations to vowels (Wood, 1979), these data can be explained with a model of C-to-V coarticulation in which some tongue regions are more sensitive to coarticulatory effects than others. This approach is essentially the same as that advocated by Ohman (1966) to explain V-to-C effects. Thus, not only consonants, but also vowels may turn out to be more or less resistant to coarticulatory effects depending on similar properties such as the tongue region involved in their primary constriction. A first goal of this paper therefore is to characterize the tongue region which is primarily involved in the formation of the constriction for each vowel in order to predict its relative resistance to place modifications induced by different consonants. More specifically, an experimental basis will be sought for a classification of vowels into a front and a back category using palatography. Recent production studies using other analysis techniques confirm the validity of this articulatory division for a language other than Catalan- namely, American English (Alfonso & Baer, 1982; Baer, Alfonso & Honda, 1988). A more basic question underlying this division is what gestures do speakers use in the vowel production process. Palatography allows us to infer the positioning of the tongue dorsum for different articulations through a measure of linguopalatal contact. While a correlation between tongue dorsum raising and vowel height is well documented in the case of front vowels (see X-ray data for several languages in Wood, 1982) , more data are needed to understand differences in the positioning of the entire tongue dorsum for different back vowels. To account for C-to-V effects, then, this study tests the hypothesis that the amount of C-to-V coarticulation for a particular articulator is inversely related to the involvement of that articulator in the formation of the vowel constriction. If this is true, variability in the degree of contact between the tongue and the hard palate ought to be less for front vowels than for back vowels. Data from the literature provide some support for this hypothesis. For example, X-ray microbeam data reported by Perkell & Nelson (1985) for the English vowel (i] show similar contextual variability for pellets positioned at the blade and the mediodorsum. The major component of variability was along an axis parallel to the hard palate surface and there was only a small variability in the dimension perpendicular to the palate. For [a], on the other hand, the major variation in these pellets was in the perpendicular dimension . This suggests that those two lingual regions are positioned as a unit to form the palatal constriction of the vowel (i]. Kiritani , Itoh, Hirose & Sawashima (1977) similarly found for Japanese that there is less context-dependent variability at the blade and mediodorsum than further back in the tongue for the two front vowels (i] and [e], as opposed to larger context-dependent effects at the blade and mediodorsum than further back in the tongue for the three back vowels [a], [o] and (u]. In order to provide a more extensive test of the hypothesis, the present study carries out a precise analysis of the variation in linguopalatal contact patterns during
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vowels as a function of consonants differing in place and manner of articulation in nonsense words produced by a Catalan speaker. The analysis will provide information concerning the demands on the production not only of the vowels but of the neighboring consonants as well. Several different patterns of interaction are expected. First, the vowel and the consonant both may be specified for high demands on tongue dorsum activity. Moreover, these demands may be complementary (as for high front vowels and palatal consonants, where the two articulations both involve tongue dorsum raising and fronting), or they may be antagonistic (as for high front vowels and velarized [t), where the consonant involves lowering of the tongue predorsum and the formation of a postdorso-velopharyngeal constriction). While little C-to-V coarticulation is expected to take place in the first case, the outcome of the second interaction is not clear and thus is of particular interest. Second, only one of the segments might be specified for high demands on tongue dorsum activity. That is, the vowel may be specified for high demands and the consonant for low demands (e.g. [i] and [p]) or the reverse (e.g. [::l] and [j]). In these circumstances the vowel gesture should override the consonantal gesture in the first case and the consonantal gesture should override the vowel gesture in the second case. Thus , very slight C-to-V effects are expected in the first combination whereas substantial C-to-V effects are expected to show up in the latter combination. A particular question that the analysis will address is the extent to which those tongue dorsum regions which are anterior to the place of constriction for back vowels adapt to adjacent consonants. If speakers exert articulatory control upon the entire tongue body, little C-to-V coarticulation is expected to take place on these regions. On the other hand, if those particular tongue regions are left free to coarticulate because they do not intervene actively in the formation of the vowel constriction, substantial C-to-V effects ought to occur. Data on C-to-V coarticulation for low vowels in the literature show some support for the latter alternative. For example, Carney & Moll (1971) report X-ray data showing more tongue-dorsum raising for [o] when it is adjacent to [s] in the sequence [hiso] than when adjacent to [v] in the sequence [hivo]. Gay (1974) reports X-ray data showing similar effects in [VCo] sequences; the dorsum was higher after [t] than after [k] than after [p] . A similar trend is also documented for rounded vowels of different degrees of height. For example, Carney & Moll (1971) give X-ray data for English which show more fronting for [u] in the sequence [husi] than in the sequence [hufi]; Butcher & Weiher (1976) give EPG data for German which show lateral contact up to the front alveolar zone (one speaker) and up to the prepalatal zone (another speaker) in the case of [u] in the sequence [utu]; and Kiritani eta/. (1977) give X-ray microbeam data for Japanese which show the tongue stretching out and becoming flat during the production of [o] and [u] in symmetrical eve sequences with dental and alveolar consonants. The analysis of coarticulation presented here is both electropalatographic (EPG) and acoustic (F2). An inconvenience with the palatographic technique is that it provides accurate linguopalatal impressions for front vowels but little or no information for back vowels with primary constrictions behind the posterior edge of the electropalate. This problem becomes less acute when, as in the present investigation, back vowels are embedded in a consonantal context. In these circumstances consonant-dependent fronting and raising effects upon lingual activity cause linguopalatal contact during the production of the vowel; it thus becomes
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possible to analyze some C-to-V effects. The relationship between EPG and acoustic data is also studied in this paper. Only F 2 data will be analyzed in view of the fact that there is a main correlation between the values of this formant and the degree and location of linguopalatal contact (Fant, 1960). 2. Method The speech material consisted of nonsense disyllables which conform to the phonology of Catalan and have either first-syllable stress (['CVC;}]) or secondsyllable stress ([C;}'Ca]). The V slot in the ['CVC;}) sequences was filled with [i], [e], [c], [a], [::>], [o] and [u]. Only [;}] was used for the comparable slot in the [C;}'Ca) sequences, since the other two vowels which may appear in unstressed position in Catalan (i.e. [i) and [u]) are expected to show similar articulatory characteristics to their stressed correlates. In all sequences, C 1 was identical to Cz. The following Catalan consonants were chosen in order to analyze a large variety of C-to-V effects: the bilabial stop [p]; the apicodental stop [t]; the alveolar fricative [s] (which is usually !aminal for the speaker used in this study); the lamino-postalveolar fricative [f); the predorso-postalveolo-prepalatal nasal stop [p); the dorsopalatal approximant [j] (which is articulated with a prepalato-mediopalatal constriction with the predorsum and the mediodorsum); the postdorso-postpalatal (in the context of front vowels) or dorsovelar (in the context of back vowels) stop [k]; the velarized apicoalveolar lateral [t). The consonant [t) in Catalan is produced with a double articulation, namely, a primary apicoalveolar closure and a secondary dorsal constriction at the velar region or at the upper pharynx. A large cavity is found between these two places of articulation since the tongue dorsum is actively lowered at that vocal tract location (see Giles & Moll, 1975, and Fant, 1960, for similar realizations of velarized [t] in American English and in Russian, respectively). Those aforementioned articulatory descriptors for the consonants of Catalan used in this study have been documented elsewhere (Recasens, 1986). Simultaneous electropalatographic and acoustic recordings were made. The overall number of sequences was 640 (8 vowels X 8 consonants X 10 repetitions). One Catalan speaker (the author) uttered the sequences ten times in the carrier sentence Diu_ ("He says_") with a custom-made artificial palate in place. The electropalatographic system (Rion Electropalatograph model DP-01) is equipped with 63 equidistant electrodes arranged in five semicircular rows (see Fig. 1, left) and enables the recording of one pattern of contact every 15.6 ms. Since the electrodes are not arranged horizontally into rows (as is the case for other EPG systems) but into arcs, I will use the latter term instead of the former one throughout the paper. The distance between adjacent electrodes on the artificial palate used in the experiment reported here varies between 5 and 6 mm. It can be seen that the number of electrodes decreases as the arcs become more central. The figure also shows that some electrodes fall along a median line and, more specifically, that the two backmost electrodes on that line are not associated with any of the five arcs. The articulatory descriptions reported in the Results section will take into account the anterior-posterior dimension of contact as well as the peripheral-central dimension of contact. To allow for the former, a division into four articulatory zones (alveolar, prepalatal, mediopalatal , postpalatal) has been superimposed upon the
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Median line Median line
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Figure 1. (Left) Original representation of the artificial palate with electrodes
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surface of the electropalate. (Occasionally I will refer to the front or to the back of one of these zones for the sake of precision.) To allow for the latter, a differentiation is made between peripheral arcs (the three outermost arcs) and central arcs (the two innermost arcs). Although EPG directly shows only the location of linguopalatal contact on the palate, X-ray data on similar articulations in many languages allow us to assume that a given lingual articulator will be in contact with any given region on the palate surface . The terminology used to characterize the assumed lingual articulators is consistent with that proposed in Catford (1977): tongue tip for the apical articulations in the dental or alveolar zones; tongue blade for !aminal articulations typical of the alveolar zone; tongue predorsum for articulations in the prepalatal zone; tongue mediodorsum for contact in the mediopalatal zone; and tongue postdorsum for contact in the postpalatal zone. The linguopalatal configurations reported in this paper correspond to averages across ten repetitions . They will be displayed on polygons like the one represented in Fig. 1 (right). This graph is a straightened out version of the original graph (Fig. 1, left) in which the arcs of electrodes have been transformed into vertical columns. The reason for having this alternative representation lies in the need to better differentiate variation in linguopalatal contact along the anterior-posterior dimension from that along the peripheral- central dimension. In the original representation the confusion between both measures of variation is particularly noticeable as linguopalatal contact approaches the median line. The EPG frame corresponding to the vowel midpoint (as detected on waveform displays) was selected for measurement . The measure used for the contact place was the average number of "on" electrodes on each arc across repetitions; since all electrodes behind the frontmost electrode on each arc were always "on" for all the vowels in all the consonantal contexts, each average number corresponds to the point of maximal fronting in that arc. F 2 measurements were taken at the vowel midpoint by means of an LPC analysis program available in the ILS (Interactive Laboratory System) package; F 2 data presented in this paper represent averages across three repetitions for each sequence .
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3. Electropalatographic data 3.1. Vowel-dependent linguopalatal configurations 3. 1.1. Articulatory characteristics
Since the bilabial stop involves no active tongue movement during its production it should have a minimal effect upon the tongue movement of the adjacent vowel. Figure 2 displays linguopalatal contact for each vowel in the context [pVp] . It shows two graphs, which are analogous to those in Fig. 1. The right-hand graph is the polygon representation that will be used throughout the paper and the left-hand graph shows the same data arranged on stylized arcs to help orient the reader in interpreting such graphs for the first time. The contour lines within each graph connect the points of maximum linguopalatal fronting on each arc. Linguopalatal contact takes place over the area between the contour lines and the sides of the figure. The median line of electrodes separates patterns of linguopalatal contact at the right and left sides of the artificial palate; the division in articulatory zones coincides with that in Fig. 1. As the figure shows, the place of maximum constriction for the front vowels [i], [e] arid [c] is towards the mediopalate and postpalate. The figure also shows a decrease of contact area all over the palatal surface in the progression [i] > [e] > [c]. These three front vowels differ in the degree of contact along both the anterior/posterior and the peripheral/central dimensions: the tongue sides contact the palate well into the alveolar zone for [i], hardly so for [e], and not at all for [c]; central contact decreases in the same progression (i.e. [i] > [e] > [c]). Interpreting the first pattern in terms of the assumed articulator, we can conclude that tongue fronting decreases with decreasing tongue height. These EPG data are consistent with the view that the fronting and raising of the tongue body for the production of front vowels are executed by means of a single gesture (see also Harshman, Ladefoged & Goldstein, 1977). The vowels [a],[:)], [o] and [u] are articulated farther back on the palatal surface. Indeed, frontmost contact in this case does not even reach the prepalate; these are back vowels. It can also be seen that the contour lines for [:)] and [o] lie behind those for [a] and [u]. If constrictions posited by Wood (1979) are assumed, it
Figure 2. Lingu.opalatal contact for each of the Catalan vowels in the context (pVp]. Representations are analogous to those in Fig. 1, namely, semicircular (left) and polygonal (right).
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appears that vowels which are articulated with an upper pharyngeal constriction (i.e. [;,] and [o]) may show less palatal contact than those that are articulated either at the lower pharynx (i.e. [a]) or at the velar region (i.e. [u]). This contrast makes sense if we look at the overall tongue body configurations found, for example, in Fant's (1960) X-ray diagrams for Russian: the tongue dorsum is lowered and retracted during the production of [u], [;,] and [o], and lowered but not retracted during the production of [a]. As a result of these mechanisms the tongue dorsum for [a] is flat and may be more fronted than for back rounded vowels. The presence of a larger amount of dorsal contact for [u] than for [;,] and [o], on the other hand, is consistent with a higher position of the jaw and tongue body. It should be added that the Catalan low back vowel appears to be rather fronted and thus closer to IP A [a] than to IPA [o]; languages or dialects having a lower and more retracted variety would be expected to show little or no linguopalatal contact at all. The linguopalatal configuration for the schwa is more similar to that for back vowels (particularly [a]) than to that for front vowels. This is so because no raising of the tongue dorsum is required for the production of [;}], particularly when the schwa is coarticulated with a low vowel, as it would be in the [p;}'pa] sequences in this study. These data suggest an articulatory basis for dividing vowels into a front and a back class. Moreover they show consistent differences in the configuration of dorsal contact across vowels within each class. 3.1.2. Variability of linguopalatal contact and linguopalatal asymmetry Graphs in Fig. 3 give the means and standard deviations of contact patterns on the two sides of the palate for the eight vowels across all consonantal contexts. Standard deviation values give an estimate of the variability in degree of linguopalatal fronting on each row. The graphs show an asymmetry in degree of palatal contact for all vowels on all rows; the mean number of "on" electrodes on the right side of the palate is always larger than that on the left side. These data indicate that lingual asymmetry in linguopalatal contact does not occur only for consonants (Hamlet, Bunnell & Struntz, 1986; Marchal & Espesser, 1989) but also for vowels. The standard deviation values allow a grouping of the vowels into two categories, front and back. The group of front vowels (i.e. [i], [e], [c]) shows little variability in the configuration of the contour lines as a function of consonantal context for these vowels (standard deviations are always less than one electrode). The group of back vowels (i .e. [a], [;,], [o], [u]), on the other hand, shows a large degree of linguopalatal variability across consonantal environments (with standard deviations often above one electrode). These data confirm our intuitions about the relationship between the degree of constraint in the articulatory control for a given tongue region and the amount of variability in the corresponding area of linguopalatal contact. In the case of front vowels, the fact that the standard deviation values are equally low on all rows suggests that placement of the entire tongue dorsum is subject to a high degree of constraint. For back vowels, on the other hand, high degree of contact variability in area of palatal contact is predictable from their more posterior constriction locations. The ventral surface of the tongue and much of the tongue dorsum is left quite free to coarticulate with the contextual consonants, since the vowel constric-
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Figure 3. Mean and standard deviations of linguopalatal contact for each vowel across consonantal contexts on each side of the palate .
tion is formed with the back of the tongue. In agreement with studies on acoustic variability (American English: Stevens & House, 1963; Catalan: Recasens, 1985}, standard deviation values are higher for [u] than for [a], [;,] and [o]. The large degree of variability exhibited by [u] is tolerated presumably because its acoustic consequences can be offset by compensatory variability in the lip rounding gesture (see Section 3.2.2 for further explication of this point). Consonant-dependent variability in linguopalatal contact is also large for the schwa. This is an expected trend since [~] is unconstricted and should be highly sensitive to changes in the vocal tract configuration induced by the surrounding consonants. 3.2. Consonant-dependent effects in linguopalatal contact To show more clearly the precise effect of the different consonants on the vowels, Fig. 4 gives mean patterns of linguopalatal contact for each vowel in each consonantal context. In the panels of this figure, the solid lines show the average number of "on" electrodes calculated for each row across the right and left sides of the palate. For reference, each graph also displays linguopalatal contact for each
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Figure 4. Configurations of linguopalatal contact for each vowel as a function of each consonant. Contact values have been averaged across the two sides , right and left, of the artificial palate.
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vowel in the [pVp] context (dashed line) . The difference between the solid and dashed lines, then, represents the amount of deviation in linguopalatal contact area caused by each consonant with respect to a neutral context which is highly unspecified for lingual activity in the bilabial consonant. The magnitude of the deviation can be interpreted as a measure of the degree of C-to-V coarticulation. Also , the deviation can be positive or negative , indicating that the consonant causes an increase or a decrease, respectively , in amount of linguopalatal contact in the vowel compared with the contact configuration for the same vowel in the neutral context. 3. 2.1. Effects on front vowels
In general, coarticulatory effects on front vowels are quite small, except in the context of the velarized lateral [t]. Also, dental [t] causes some increase of contact at the prepalatal zone with the more open vowels [e] and [e] but not with [i]. This contact increase is probably due to the apicodental articulation (with simultaneous laminoalveolar contact) of the consonant as opposed to the lowered position of the blade and predorsum for the two vowels in the [pVp] context. Alveolar [s] and postalveolar [f] cause some positive effects at the prepalatal zone since the tongue front is higher for these consonants than for all three vowels . These C-to-V effects in linguopalatal contact suggest that the two fricatives are subject to special requirements on the formation of a front constriction . They are consistent with X-ray data showing that the central groove for English [i] in the sequence [hizo] is narrower than that in the sequence [hivo] (Carney & Moll, 1971) . In addition, [f] causes some increase of dorsal contact at the postpalate in the context of the more open front vowels [e] and [e]; this effect follows from the fact that this consonant is produced with some tongue dorsum raising , and is consistent with a lower tongue dorsum position for [e] and [e] than for [i] . When adjacent to front vowels, the velarized alveolar lateral [t] has a negative effect on all rows. Moreover, the figure shows that the degree of coarticulation due to [t] is greater than that for any other consonant. Ohala & Kawasaki (1979) also found a decrease of the medial groove for [i] when adjacent to [t] in American English. Overall, these data reflect the existence of antagonistic mechanisms during the articulation of front vowels and velarized [t]: while front vowels r~quire fronting and raising of the tongue predorsum and mediodorsum, velarized [t] requires active lowering of those tongue regions to bring about a "dark" acoustic quality and presumably to facilitate lateral airflow as well. A decrease of contact at the prepalate is observed for all three front vowels in the context of [j] and [k], but not in the context of [Jl]. This pattern is consistent with the fact that the two former consonants (prepalato-mediopalatal and postpalatal, respectively) are produced with less dorsal fronting than the latter (alveoloprepalatal) . Moreover, [Jl] causes some increase of contact at the mediopalatal and postpalatal zone for [e], presumably as a result of some tongue dorsum raising. 3.2.2. Effects on back vowels and schwa
Inspection of the graphs in Fig. 4 reveals a different pattern of C-to-V coarticulation in the case of the back vowels [a], [::>], [o] and [u], and of the neutral vowel [;:)]. First, almost no negative effects are found. Thus, except for [::>] in the
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context of the velar [k) and [u] in the context of [k) and of the velarized alveolar [t], all the vowels undergo only positive effects in all contexts. As discussed later on in this section, these coarticulatory effects accord well with the low positioning of the tongue predorsum and mediodorsum for back vowels. The schwa shows a similar pattern to that exhibited by [u] except for the absence of negative effects and the larger size of positive effects at the central arcs of the palatal surface. Large positive effects are found from dental [t], alveolar [s] and postalveolar [f). Thus the dorsum is fronted and/or raised during the production of back vowels and [;}] when adjacent to consonants involving an apical closure or a !aminal constriction . As indicated in the Introduction section, studies for other languages in the literature suggest that positive effects on [a] reflect coarticulation in tongue dorsum raising, while positive effects on back rounded vowels reflect coarticulation in tongue fronting and raising. It can be seen in the figure that for the rounded vowels, as vowel height decreases, the fronting effect of the [t] context also decreases, whereas that of the [s] context remains the same. Thus a difference in the amount of a positive effect between the two contexts emerges and becomes larger going from [u] to [o] to [::>). The same generalization holds true for the [t] context relative to the [f) context, although the positive effect of [f) itself is also somewhat diminished in the two lower vowels. This different interaction between fronting effects and vowel height can be interpreted in terms of greater constraints on the shaping of the tongue body for the coronal fricatives as opposed to the stop. The entire ventral surface of the tongue must be high to channel the airflow into the narrow constriction of an alveolar or postalveolar fricative . The consistently large fronting effects in [u] are more similar to those for [;}] and [a] than they are to those for [o) and [::>]. Large tongue fronting effects rna y be tolerated in the case of [u] because of the associated lip rounding, which is more considerable for this vowel than for the two other back rounded vowels. It can be hypothesized that speakers do not care much about the precise location of the lingual constriction because the spectral properties of [u] are already accounted for by the lip rounding gesture; if so, it seems a natural strategy to allow for C-to-V coarticulation in tongue positioning along the palatal surface. Large coarticulatory effects on the schwa can be explained because of the inherently low requirements on the tongue configuration during the production of this vowel. The velarized alveolar lateral [t], which has the only large systematic effect on the front vowels, causes few coarticulatory effects on these back vowels. The absence of coarticulatory effects on pharyngeal vowels [a], [o], and [::>] is presumably due to the similarity in tongue dorsum configuration; indeed, all these articulations are produced with a back lingual constriction and a lowered position of the predorsomediodorsal region of the tongue. Negative effects at the postpalatal region for [u] are associated with the raised tongue dorsum in the case of [u] as opposed to the other vowels. Alveolopalatal [Jl] and palatal [j] have generally positive effects but the size of the effect varies widely among the vowels. For the rounded vowels in these contexts the positive coarticulatory effects increase as the vowel becomes higher, both towards the alveolar region and the median line. Positive effects are also particularly salient in the case of the schwa. This pattern indicates that, similarly to adjacent dentals and alveolars, adjacent alveolopalatal and palatal consonants cause some stretching
D. Recasens
1
of the tongue dorsum during the production of highly rounded back vowels and [;:)], and pull it forward within the oral cavity . By contrast to the dentals and alveolars, alveolopalatal [J1] and palatal [j] cause little C-to-V coarticulatory influence upon [:::>] and [a]. In the case of [;:)], [o] and [u], however, those two consonants do cause a substantial increase in contact towards the front as well as the median line. Since the production of both consonants involves a large degree of tongue dorsum contact (Recasens, 1984), C-to-V effects may affect the front or the back regions of the tongue dorsum depending on the height of the vowel. C-to-V effects from [k] on back vowels are either absent (in the case of [a], [o] and [:::>]) , or are rather small (there is a small positive effect at the postpalatal zone for [;)] and a small negative one for [u]) . It should be pointed out that in the context of this consonant there is more contact in [a] and [;)] than in the back rounded vowels presumably because [k] is produced with a more fronted place of articulation in those two vowel contexts. 4. Articulatory-acoustic relations
Figure 5 shows F2 data for all vowels in all consonantal contexts. Differences between these F2 values and those in the [pVp] context are not given since F2 for a vowel preceded and followed by [p] reflects other articulatory factors besides tongue activity. It should also be noted that the correlation between the articulatory and acoustic measurements will be necessarily low for those vowels showing little palatal contact since the lingual configuration cannot be inferred with precision from the EPG data in these circumstances. ~ --------------------------------------------r---------------.
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Figure 5. F2 data for all vowels in all symmetrica l eve contexts.
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4.1. Front vowels A systematic relationship between the F 2 data and the EPG data obtains in the case of front vowels with adjacent velarized [t] . In this case, the large negative C-to-V effects in degree of dorsopalatal contact (described above in Section 3.2.1) are associated with a significant decrease in F2 . This appears to be the most salient C-to-V coarticulatory effect for front vowels both in the EPG and in the F2 data reported in this paper. Since Wood (1982) has shown that an increase in the distance between the tongue and the palate at the place of primary constriction for front vowels causes appreciable F2 changes, this systematic relationship can be interpreted as a causal one . There may also be a similar causal relationship between the positive C-to-V effects in dorsopalatal contact of [e] and [c] when adjacent to alveolopalatal [p], palatal [j] and postpalatal [k] (see Section 3.2.1) and the observed F 2 frequency. In these three contexts F 2 is significantly higher than in the context of [s], [t] and [f]. On the other hand, it could be argued instead that it is the dentoalveolars and labials (rather than the alveolopalatals and palatals) which cause some deviation (i .e. lowering) from the ideal F 2 values for those target vowels. This would accord with the interpretation of Stevens & House (1963) for their American English data. Notice that the fricative [f] causes a lower F 2 frequency than the stop of a similar place of articulation (i.e. [t]) at the midpoint of [i] and [e]; a similar trend is observed for [s] vs. [t] upon [e] (see also Stevens & House, 1963). A possible explanation for this pattern is that fricatives cause some tongue dorsum lowering along the median line during the production of the vowel and thus contribute to an F 2 decrease. Overall, these C-to-V effects suggest that F 2 for front vowels is particularly sensitive to changes in the conductivity index of the dorsopalatal passage (Fant , 1960). 4.2. Back vowels and schwa Again, the F2 patterns are similar to those in Stevens & House (1963) and are clearly related to the C-to-V effects in the EPG data. Thus, positive coarticulatory effects in linguopalatal contact from palatal consonants on [~], [o] and [u], and from dentals and alveolars on all back vowels and schwa, cause a F 2 frequency increase with respect to the neutral [pVp] context. This increase in contact and in F 2 frequency for the vowel reflects some tongue dorsum raising and fronting induced by the consonantal context. Consistent with the data reported here, Wood (1982) has shown that advancing the dorsal constriction for [u] from the back or mid soft palate to the front soft palate or to even more anterior region causes a large F 2 increase; Maeda (1990) reports a similar finding for the vowel [o] . Notice , however , that there is also a high F 2 frequency for [a] and [::>] in the context of palatal consonants, but this is not correlated with an increase of linguopalatal contact with respect to the neutral [pVp] context. This discrepancy suggests that there may not be enough fronting and raising of the tongue dorsum to leave a trace on the palatal surface in this contextual condition. Therefore data on linguopalatal contact are not always informative about the speech gestures themselves and should be complemented with data from other methods of observing articulatory patterns.
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The figure also shows that F 2 is higher in palatal contexts not just by comparison with the neutral bilabial context, but also by comparison with dentals and alveolars. This is true for the· back vowels and even more so for [~]. Inspection of the EPG data in Section 3.2.2 suggests that this pattern may be due to an increase in linguopalatal contact at the back palate towards the median line, particularly for[~], [u] and [o] . Again tongue dorsum raising for lower vowels such as [a] and [;)] in this context may not be enough to cause appreciable contact changes on the palatal surface. Figure 5 shows that F 2 for a back vowel and schwa is lowest when the adjacent consonant is the velarized [t]; indeed EPG data reported in Section 3.2.2 indicate that contact for the vowel remains further back and is not affected positively by this adjacent consonant. The F2 values are somewhat higher for the vowels in the sequence [kVk] by comparison to the neutral [pVp] context. This difference is larger in the case of [a] and[~] than of the rounded vowels; this is consistent with the EPG data which reveal some positive C-to-V effects at the back of the palate for the schwa. Overall these data confirm the view that the place of articulation for [k] next to [a] and[~] is intermediate between its place when adjacent to front vowels and its place when adjacent to back rounded vowels (see also Recasens, 1985). In general, F2 appears to be well correlated with front cavity size (Fant, 1960), and therefore with the degree of tongue dorsum raising towards the palate. An increase of F2 frequency results from a front cavity reduction caused by tongue dorsum fronting and raising with respect to its positioning in the neutral [pVp] context. Analogously to the EPG data, the schwa shows the largest spread of F 2 frequencies of all vowels across consonantal contexts. 5. Summary and conclusions Patterns of linguopalatal contact across consonantal contexts show differences in contact size between vowels of different height that are highly consistent within the front series ([i], [e], [c]) but not within the back series ([ u], [a], [o], [;)]). This apparent anomaly can be attributed to the position of the tongue dorsum, which may be higher for low back vowels than for higher back vowels. A systematic contrast in constriction location on the palatal surface was obtained for front vs. back vowels which is consistent with the tongue regions presumably under active control in the two groups. This grouping of vowels into a front and a back category has been shown to be not only related to patterns of linguopalatal contact but also to patterns of articulatory variability. Variability for [~] is the highest of all vowels. The finding that [i] is subject to a low degree of variability in palatal contact is consistent with the idea that [i] has a particularly stable articulation due to "saturation" (Fujimura & Kakita, 1979; Perkell & Nelson, 1985) . According to this view, vertical displacement at the front regions of the tongue dorsum is small because the sides of the tongue blade are pushed against and restrained by the hard palate; on the other hand , horizontal variability at the front and, more so, at the back of the tongue dorsum is caused presumably by contraction of the posterior genioglossus. High standard deviation values in EPG activity across the consonantal contexts for [u] and[~] suggest a clear trend for the degree of coarticulation to vary inversely with the degree of constraint on tongue dorsum activity. Less variability for [a], [o]
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and [:)] than for [u] suggests that lip rounding may play a role in the presence of C-to-V effects by compensating for lingual precision. Coarticulatory data reported in this paper may be useful for gaining some understanding of the nature of the articulatory strategies used by speakers for the production of adjacent vowels and consonants. It should be pointed out in this respect that C-to-V effects are subject to production constraints which are similar to those observed in V-to-C effects. Analogously to consonants, those lingual regions which are not involved in the formation of a vocalic constriction are left freer to coarticulate than those which are actively involved. Effects on front vowels are quite small due to the requirements on the tongue dorsum to achieve a palatal constriction. The most significant consonantal effects are caused by velarized [t] and result in an increase in tongue dorsum lowering. In general, those articulatory contrasts associated with palatal constriction width have an effect on F 2 frequency. Coarticulatory effects for back vowels and [;J] are larger than those observed for front vowels . Positive coarticulatory effects in this case reflect mainly tongue dorsum raising (in the case of [a]) and tongue dorsum fronting and raising (in the case of the other back vowels) as a function of adjacent dental, alveolar and palatal consonants. The consonants [k] and velarized [t] cause little C-to-V coarticulation. These articulatory differences have an effect on F2 frequency differences. Patterns of C-to-V coarticulation may yield information about the constraints on the production of a consonantal gesture. Thus, when a consonantal gesture is highly constrained, C-to-V effects ought to be already present during the production of an adjacent vowel (if the articulation of the vowel allows them). An interesting case is the increase in fronting of the contact for [a] and [:)] in the context of [s] compared with [t) in spite of the fact that the dental stop is produced with a fronter place of articulation than the alveolar fricative. It may be that this difference occurs because the necessity of forming the central groove for the fricative requires a high degree of articulatory control. Overall, these effects reflect contrasts in degree of articulatory constraint associated with different phonetic segments. Large effects on back vowels from dentoalveolar and palatal consonants show that the constriction requirements on the front dorsum for the consonant dominate in the absence of articulatory requirements for the vowel; on the other hand, coarticulatory effects from velarized [t] on back vowels are small since the front dorsum configurations for that consonant and those vowels are highly similar. In the case of front vowels, strict requirements on tongue dorsum lowering-backing for velarized [t) override to some extent tongue dorsum raising-fronting for front vowels; also, coarticulatory effects from fricatives on front vowels are rather restricted because fricative production demands high precision of the tongue configuration.
This research has been supported by NINCDS Grant NS-13617 to Haskins Laboratories, the ESPRIT-ACCOR project (BRA Action 3279) from the European Community, a research grant PB87-0774-C02-01 from the Spanish Government (DGICYT) and a fellowship from the Catalan Government (CIRIT) . I would like to thank Mary E. Beckman, Carol A. Fowler , Shinji Maeda, Ignatius G. Mattingly and three anonymous reviewers for comments on previous versions of this paper.
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Alfonso, P. J. & Baer, T. (1982) Dynamics of vowel articulation, Language and Speech, 25, 151-173. Baer , T., Alfonso, P. J." & Honda, K. (1988) Electromyography of tongue muscles during vowels in /~pVp/ environment, Annual Bulletin of the Research Institute of Logopedics and Phoniatrics, University of Tokyo, 22, 7-19. Butcher, A. & Weiher, E. (1976) An electropalatographic investigation of coarticulation in VCV sequences, Journal of Phonetics, 4, 59-74. Carney, P. J. & Moll, K. L. (1971) A cinefluorographic investigation of fricative consonant-vowel coarticulation, Phonetica, 23, 193-202. Catford, J. C. (1977) Fundamental problems in phonetics. Edinburgh: Edinburgh University Press . Fant, G. (1960) Acoustic theory of speech production. The Hague: Mouton. Fowler, C. A., Rubin, P., Remez, R . E. & Turvey, M. T. (1980) Implications for speech production of a general theory of action. In Language production (B. Butterworth, editor), pp. 373-420. New York: Academic Press. Fujimura, 0. & Kakita, Y. (1979) Remarks on the quantitative description of lingual articulation. In Frontiers of speech communication research (B. Lindblom & S. Ohman, editors), pp. 17-24. London: Academic Press. Gay, T. (1974) A cinefluorographic. study of vowel production, Journal of Phonetics, 2, 255-266. Giles, S. B. & Moll, K. L. (1975) Cinefluorographic study of selected allophones of English /1/, Phonetica, 31, 206-227. Hamlet, S. L., Bunnell , H. T. & Struntz, B. (1986) Articulatory asymmetries, Journal of the Acoustical Society of America, 79, 1164-1169. Harshman, R., Ladefoged, P. & Goldstein, L. (1977) Factor analysis of tongue shapes, Journal of the Acoustical Society of America, 62, 693-707. Kiritani, S., ltoh, K., Hirose , H. & Sawashima, M. (1977) Coordination of the consonant and vowel articulations-X-ray micro bean study on Japanese and English, Annual Bulletin of the Research Institute of Logopedics and Phoniatrics, University of Tokyo, 11, 11-21. Ladefoged, P. (1977) The description of tongue shapes. In Dynamic aspects of speech production (M. Sawashima & F. S. Cooper, editors), pp. 209-222. Tokyo: University of Tokyo Press. Maeda, S. (1990) Compensatory articulation during speech: Evidence from the analysis and synthesis of vocal-tract shapes using an articulatory model. In Speech production and speech modelling (W . J. Hardcastle & A. Marchal, editors), pp. 131-150. Dordrecht: Kluwer Academic Publishers. Marchal, A. & Espesser, R. (1989) L'asymetrie des appuis linguo-palatins, Journal d'Acoustique, 2, 53-57. Ohala, J. & Kawasaki, H. (1979) The articulation of [i)'s. Paper presented at the 98th Meeting of the Acoustical Society of America, Salt Lake City. Ohman, S. E. (1966) Coarticulation in VCV utterances: Spectrographic measurements, Journal of the Acoustical Society of America, 39, 151-168. Perkell, J. S. & Nelson, W. L. (1985) Variability in production of the vowels /i/ and /a/, Journal of the Acoustical Society of America, 77, 1889-1895. Pols, L. C. W. (1977) Spectral analysis and identification of Dutch vowels in monosyllabic words. Soesteberg: Institute for Perception TNO . Recasens, D . (1984) V-to-C coarticulation in Catalan VCV sequences: An articulatory and acoustical study, Journal of Phonetics, 12, 61-73. Recasens, D . (1985) Coarticulatory patterns and degrees of coarticulatory resistance in Catalan CV sequences, Language and Speech, 28, 97-114. Recasens, D. (1986) Estudis de Fonetica Experimental del Catalii Oriental Central. Barcelona: Publicacions de l'Abadia de Montserrat . Stevens, K. N. & House, A. S. (1963) Perturbation of vowel articulation by consonantal context: An acoustical study, Journal of Speech and Hearing Research, 6, 111-128. Wood, S. (1979) A radiographic analysis of constriction locations for vowels, Journal of Phonetics, 7, 25-43 . Wood, S. (1982) Radiographic and model studies of the palatal vowels. In X-ray model studies of vowel articulation, Vol. 23, pp. 119-155. Working Papers, University of Lund.
An electropalatographic and acoustic study of consonant-to-vowel coarticulation Daniel Recasens CEDI, Institut d 'Estudis Catalans, c/ Carme 47, 08001 Barcelona, and Departament de Filologia Catalana, Universitat Autonoma de Barcelona, Bellaterra, Barcelona, Spain Received 8th December 1989, and in revised form 2nd May 1990
Coarticulatory effects from consonants on vowels were investigated using electropalatography and acoustical analysis . The electropalatographic data conform with an articulatory classification of vowels into front and back categories. Moreover, vowels are shown to differ in their degree of sensitivity to coarticulatory effects from the adjacent consonants. In general, consonant-dependent effects are larger upon those articulatory regions which do not intervene in the formation of a vowel constriction, although compensatory variation in other gestures (e .g., lip rounding) must be invoked to interpret the highly variable pattern of lingual coarticulation for [u). The paper also concludes that the degree of C-to-V coarticulation is influenced by the production requirements on the consonant; that explains why the degree of linguopalatal contact for [i] is more affected by the velarized alveolar lateral [t) than by other consonants. The electropalatographic data are generally correlated with acoustic (F2 ) data, with greater variation in F 2 frequency in those vowels showing greater variation in linguopalatal contact area .
1. Introduction While there are a good number of studies on vowel-to-consonant coarticulation, rather less attention has been paid to the effects of consonants on vowels. Much of the literature on the articulation of vowels refers to the configuration and activation of the speech organs involved in their production (Alfonso & Baer, 1982; Ladefoged, 1977) or to the location of their places of constriction (Wood, 1979) without regard to the consonantal context. The studies that do examine C-to-V effects almost without exception provide data only from acoustic analyses (Pols, 1977; Recasens, 1985; Stevens & House, 1963) so that the articulatory correlates of the observed C-to-V effects must be inferred from the acoustic data. This paper reports some articulatory data on C-to-V coarticulation in linguopalatal contact collected by means of electropalatography. Such data are needed to gain a more complete understanding of the production mechanisms used by speakers when uttering CV or VC syllables. They should also help differentiate among general models of speech production that have been proposed. The prevalence of studies on V-to-C coarticulation over studies on C-to-V effects may originate in certain broadly held theoretical assumptions about the articulatory 0095-4470/91/020177 + 16 $03.00/0
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constraints used by speakers during the production of the two phonetic categories. There are theories that claim that vowels are resistant to coarticulation because they are produced by means of global vocal tract shapes which require articulatory control upon the entire tongue body configuration whereas consonants involve only local constrictions which leave other articulatory regions free to coarticulate (e.g., Fowler, Rubin, Remez & Turvey, 1980) . This paper will show that vowels are subject to coarticulatory effects from consonants in a way which is very much like that attested in V-to-C coarticulatory effects . If we attribute primary constriction locations to vowels (Wood, 1979), these data can be explained with a model of C-to-V coarticulation in which some tongue regions are more sensitive to coarticulatory effects than others. This approach is essentially the same as that advocated by Ohman (1966) to explain V-to-C effects. Thus, not only consonants, but also vowels may turn out to be more or less resistant to coarticulatory effects depending on similar properties such as the tongue region involved in their primary constriction. A first goal of this paper therefore is to characterize the tongue region which is primarily involved in the formation of the constriction for each vowel in order to predict its relative resistance to place modifications induced by different consonants. More specifically, an experimental basis will be sought for a classification of vowels into a front and a back category using palatography. Recent production studies using other analysis techniques confirm the validity of this articulatory division for a language other than Catalan- namely, American English (Alfonso & Baer, 1982; Baer, Alfonso & Honda, 1988). A more basic question underlying this division is what gestures do speakers use in the vowel production process. Palatography allows us to infer the positioning of the tongue dorsum for different articulations through a measure of linguopalatal contact. While a correlation between tongue dorsum raising and vowel height is well documented in the case of front vowels (see X-ray data for several languages in Wood, 1982) , more data are needed to understand differences in the positioning of the entire tongue dorsum for different back vowels. To account for C-to-V effects, then, this study tests the hypothesis that the amount of C-to-V coarticulation for a particular articulator is inversely related to the involvement of that articulator in the formation of the vowel constriction. If this is true, variability in the degree of contact between the tongue and the hard palate ought to be less for front vowels than for back vowels. Data from the literature provide some support for this hypothesis. For example, X-ray microbeam data reported by Perkell & Nelson (1985) for the English vowel (i] show similar contextual variability for pellets positioned at the blade and the mediodorsum. The major component of variability was along an axis parallel to the hard palate surface and there was only a small variability in the dimension perpendicular to the palate. For [a], on the other hand, the major variation in these pellets was in the perpendicular dimension . This suggests that those two lingual regions are positioned as a unit to form the palatal constriction of the vowel (i]. Kiritani , Itoh, Hirose & Sawashima (1977) similarly found for Japanese that there is less context-dependent variability at the blade and mediodorsum than further back in the tongue for the two front vowels (i] and [e], as opposed to larger context-dependent effects at the blade and mediodorsum than further back in the tongue for the three back vowels [a], [o] and (u]. In order to provide a more extensive test of the hypothesis, the present study carries out a precise analysis of the variation in linguopalatal contact patterns during
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vowels as a function of consonants differing in place and manner of articulation in nonsense words produced by a Catalan speaker. The analysis will provide information concerning the demands on the production not only of the vowels but of the neighboring consonants as well. Several different patterns of interaction are expected. First, the vowel and the consonant both may be specified for high demands on tongue dorsum activity. Moreover, these demands may be complementary (as for high front vowels and palatal consonants, where the two articulations both involve tongue dorsum raising and fronting), or they may be antagonistic (as for high front vowels and velarized [t), where the consonant involves lowering of the tongue predorsum and the formation of a postdorso-velopharyngeal constriction). While little C-to-V coarticulation is expected to take place in the first case, the outcome of the second interaction is not clear and thus is of particular interest. Second, only one of the segments might be specified for high demands on tongue dorsum activity. That is, the vowel may be specified for high demands and the consonant for low demands (e.g. [i] and [p]) or the reverse (e.g. [::l] and [j]). In these circumstances the vowel gesture should override the consonantal gesture in the first case and the consonantal gesture should override the vowel gesture in the second case. Thus , very slight C-to-V effects are expected in the first combination whereas substantial C-to-V effects are expected to show up in the latter combination. A particular question that the analysis will address is the extent to which those tongue dorsum regions which are anterior to the place of constriction for back vowels adapt to adjacent consonants. If speakers exert articulatory control upon the entire tongue body, little C-to-V coarticulation is expected to take place on these regions. On the other hand, if those particular tongue regions are left free to coarticulate because they do not intervene actively in the formation of the vowel constriction, substantial C-to-V effects ought to occur. Data on C-to-V coarticulation for low vowels in the literature show some support for the latter alternative. For example, Carney & Moll (1971) report X-ray data showing more tongue-dorsum raising for [o] when it is adjacent to [s] in the sequence [hiso] than when adjacent to [v] in the sequence [hivo]. Gay (1974) reports X-ray data showing similar effects in [VCo] sequences; the dorsum was higher after [t] than after [k] than after [p] . A similar trend is also documented for rounded vowels of different degrees of height. For example, Carney & Moll (1971) give X-ray data for English which show more fronting for [u] in the sequence [husi] than in the sequence [hufi]; Butcher & Weiher (1976) give EPG data for German which show lateral contact up to the front alveolar zone (one speaker) and up to the prepalatal zone (another speaker) in the case of [u] in the sequence [utu]; and Kiritani eta/. (1977) give X-ray microbeam data for Japanese which show the tongue stretching out and becoming flat during the production of [o] and [u] in symmetrical eve sequences with dental and alveolar consonants. The analysis of coarticulation presented here is both electropalatographic (EPG) and acoustic (F2). An inconvenience with the palatographic technique is that it provides accurate linguopalatal impressions for front vowels but little or no information for back vowels with primary constrictions behind the posterior edge of the electropalate. This problem becomes less acute when, as in the present investigation, back vowels are embedded in a consonantal context. In these circumstances consonant-dependent fronting and raising effects upon lingual activity cause linguopalatal contact during the production of the vowel; it thus becomes
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possible to analyze some C-to-V effects. The relationship between EPG and acoustic data is also studied in this paper. Only F 2 data will be analyzed in view of the fact that there is a main correlation between the values of this formant and the degree and location of linguopalatal contact (Fant, 1960). 2. Method The speech material consisted of nonsense disyllables which conform to the phonology of Catalan and have either first-syllable stress (['CVC;}]) or secondsyllable stress ([C;}'Ca]). The V slot in the ['CVC;}) sequences was filled with [i], [e], [c], [a], [::>], [o] and [u]. Only [;}] was used for the comparable slot in the [C;}'Ca) sequences, since the other two vowels which may appear in unstressed position in Catalan (i.e. [i) and [u]) are expected to show similar articulatory characteristics to their stressed correlates. In all sequences, C 1 was identical to Cz. The following Catalan consonants were chosen in order to analyze a large variety of C-to-V effects: the bilabial stop [p]; the apicodental stop [t]; the alveolar fricative [s] (which is usually !aminal for the speaker used in this study); the lamino-postalveolar fricative [f); the predorso-postalveolo-prepalatal nasal stop [p); the dorsopalatal approximant [j] (which is articulated with a prepalato-mediopalatal constriction with the predorsum and the mediodorsum); the postdorso-postpalatal (in the context of front vowels) or dorsovelar (in the context of back vowels) stop [k]; the velarized apicoalveolar lateral [t). The consonant [t) in Catalan is produced with a double articulation, namely, a primary apicoalveolar closure and a secondary dorsal constriction at the velar region or at the upper pharynx. A large cavity is found between these two places of articulation since the tongue dorsum is actively lowered at that vocal tract location (see Giles & Moll, 1975, and Fant, 1960, for similar realizations of velarized [t] in American English and in Russian, respectively). Those aforementioned articulatory descriptors for the consonants of Catalan used in this study have been documented elsewhere (Recasens, 1986). Simultaneous electropalatographic and acoustic recordings were made. The overall number of sequences was 640 (8 vowels X 8 consonants X 10 repetitions). One Catalan speaker (the author) uttered the sequences ten times in the carrier sentence Diu_ ("He says_") with a custom-made artificial palate in place. The electropalatographic system (Rion Electropalatograph model DP-01) is equipped with 63 equidistant electrodes arranged in five semicircular rows (see Fig. 1, left) and enables the recording of one pattern of contact every 15.6 ms. Since the electrodes are not arranged horizontally into rows (as is the case for other EPG systems) but into arcs, I will use the latter term instead of the former one throughout the paper. The distance between adjacent electrodes on the artificial palate used in the experiment reported here varies between 5 and 6 mm. It can be seen that the number of electrodes decreases as the arcs become more central. The figure also shows that some electrodes fall along a median line and, more specifically, that the two backmost electrodes on that line are not associated with any of the five arcs. The articulatory descriptions reported in the Results section will take into account the anterior-posterior dimension of contact as well as the peripheral-central dimension of contact. To allow for the former, a division into four articulatory zones (alveolar, prepalatal, mediopalatal , postpalatal) has been superimposed upon the
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Median line Median line
Alveolar reg ion
Prepa lata l region -..........
•
Periphera l
.._ Postpa latal region
.. .. .. .. . . .. .. .. .. . .. . ... . .. •
•
•
•
•
•
•
....._~---~=;::==:j~=-=.' Central
Periphera l
Figure 1. (Left) Original representation of the artificial palate with electrodes
arranged in semicircular rows; (right) polygonal representation of the artificial palate with electrodes arranged in vertical columns.
surface of the electropalate. (Occasionally I will refer to the front or to the back of one of these zones for the sake of precision.) To allow for the latter, a differentiation is made between peripheral arcs (the three outermost arcs) and central arcs (the two innermost arcs). Although EPG directly shows only the location of linguopalatal contact on the palate, X-ray data on similar articulations in many languages allow us to assume that a given lingual articulator will be in contact with any given region on the palate surface . The terminology used to characterize the assumed lingual articulators is consistent with that proposed in Catford (1977): tongue tip for the apical articulations in the dental or alveolar zones; tongue blade for !aminal articulations typical of the alveolar zone; tongue predorsum for articulations in the prepalatal zone; tongue mediodorsum for contact in the mediopalatal zone; and tongue postdorsum for contact in the postpalatal zone. The linguopalatal configurations reported in this paper correspond to averages across ten repetitions . They will be displayed on polygons like the one represented in Fig. 1 (right). This graph is a straightened out version of the original graph (Fig. 1, left) in which the arcs of electrodes have been transformed into vertical columns. The reason for having this alternative representation lies in the need to better differentiate variation in linguopalatal contact along the anterior-posterior dimension from that along the peripheral- central dimension. In the original representation the confusion between both measures of variation is particularly noticeable as linguopalatal contact approaches the median line. The EPG frame corresponding to the vowel midpoint (as detected on waveform displays) was selected for measurement . The measure used for the contact place was the average number of "on" electrodes on each arc across repetitions; since all electrodes behind the frontmost electrode on each arc were always "on" for all the vowels in all the consonantal contexts, each average number corresponds to the point of maximal fronting in that arc. F 2 measurements were taken at the vowel midpoint by means of an LPC analysis program available in the ILS (Interactive Laboratory System) package; F 2 data presented in this paper represent averages across three repetitions for each sequence .
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3. Electropalatographic data 3.1. Vowel-dependent linguopalatal configurations 3. 1.1. Articulatory characteristics
Since the bilabial stop involves no active tongue movement during its production it should have a minimal effect upon the tongue movement of the adjacent vowel. Figure 2 displays linguopalatal contact for each vowel in the context [pVp] . It shows two graphs, which are analogous to those in Fig. 1. The right-hand graph is the polygon representation that will be used throughout the paper and the left-hand graph shows the same data arranged on stylized arcs to help orient the reader in interpreting such graphs for the first time. The contour lines within each graph connect the points of maximum linguopalatal fronting on each arc. Linguopalatal contact takes place over the area between the contour lines and the sides of the figure. The median line of electrodes separates patterns of linguopalatal contact at the right and left sides of the artificial palate; the division in articulatory zones coincides with that in Fig. 1. As the figure shows, the place of maximum constriction for the front vowels [i], [e] arid [c] is towards the mediopalate and postpalate. The figure also shows a decrease of contact area all over the palatal surface in the progression [i] > [e] > [c]. These three front vowels differ in the degree of contact along both the anterior/posterior and the peripheral/central dimensions: the tongue sides contact the palate well into the alveolar zone for [i], hardly so for [e], and not at all for [c]; central contact decreases in the same progression (i.e. [i] > [e] > [c]). Interpreting the first pattern in terms of the assumed articulator, we can conclude that tongue fronting decreases with decreasing tongue height. These EPG data are consistent with the view that the fronting and raising of the tongue body for the production of front vowels are executed by means of a single gesture (see also Harshman, Ladefoged & Goldstein, 1977). The vowels [a],[:)], [o] and [u] are articulated farther back on the palatal surface. Indeed, frontmost contact in this case does not even reach the prepalate; these are back vowels. It can also be seen that the contour lines for [:)] and [o] lie behind those for [a] and [u]. If constrictions posited by Wood (1979) are assumed, it
Figure 2. Lingu.opalatal contact for each of the Catalan vowels in the context (pVp]. Representations are analogous to those in Fig. 1, namely, semicircular (left) and polygonal (right).
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appears that vowels which are articulated with an upper pharyngeal constriction (i.e. [;,] and [o]) may show less palatal contact than those that are articulated either at the lower pharynx (i.e. [a]) or at the velar region (i.e. [u]). This contrast makes sense if we look at the overall tongue body configurations found, for example, in Fant's (1960) X-ray diagrams for Russian: the tongue dorsum is lowered and retracted during the production of [u], [;,] and [o], and lowered but not retracted during the production of [a]. As a result of these mechanisms the tongue dorsum for [a] is flat and may be more fronted than for back rounded vowels. The presence of a larger amount of dorsal contact for [u] than for [;,] and [o], on the other hand, is consistent with a higher position of the jaw and tongue body. It should be added that the Catalan low back vowel appears to be rather fronted and thus closer to IP A [a] than to IPA [o]; languages or dialects having a lower and more retracted variety would be expected to show little or no linguopalatal contact at all. The linguopalatal configuration for the schwa is more similar to that for back vowels (particularly [a]) than to that for front vowels. This is so because no raising of the tongue dorsum is required for the production of [;}], particularly when the schwa is coarticulated with a low vowel, as it would be in the [p;}'pa] sequences in this study. These data suggest an articulatory basis for dividing vowels into a front and a back class. Moreover they show consistent differences in the configuration of dorsal contact across vowels within each class. 3.1.2. Variability of linguopalatal contact and linguopalatal asymmetry Graphs in Fig. 3 give the means and standard deviations of contact patterns on the two sides of the palate for the eight vowels across all consonantal contexts. Standard deviation values give an estimate of the variability in degree of linguopalatal fronting on each row. The graphs show an asymmetry in degree of palatal contact for all vowels on all rows; the mean number of "on" electrodes on the right side of the palate is always larger than that on the left side. These data indicate that lingual asymmetry in linguopalatal contact does not occur only for consonants (Hamlet, Bunnell & Struntz, 1986; Marchal & Espesser, 1989) but also for vowels. The standard deviation values allow a grouping of the vowels into two categories, front and back. The group of front vowels (i.e. [i], [e], [c]) shows little variability in the configuration of the contour lines as a function of consonantal context for these vowels (standard deviations are always less than one electrode). The group of back vowels (i .e. [a], [;,], [o], [u]), on the other hand, shows a large degree of linguopalatal variability across consonantal environments (with standard deviations often above one electrode). These data confirm our intuitions about the relationship between the degree of constraint in the articulatory control for a given tongue region and the amount of variability in the corresponding area of linguopalatal contact. In the case of front vowels, the fact that the standard deviation values are equally low on all rows suggests that placement of the entire tongue dorsum is subject to a high degree of constraint. For back vowels, on the other hand, high degree of contact variability in area of palatal contact is predictable from their more posterior constriction locations. The ventral surface of the tongue and much of the tongue dorsum is left quite free to coarticulate with the contextual consonants, since the vowel constric-
184
D. Recasens
Figure 3. Mean and standard deviations of linguopalatal contact for each vowel across consonantal contexts on each side of the palate .
tion is formed with the back of the tongue. In agreement with studies on acoustic variability (American English: Stevens & House, 1963; Catalan: Recasens, 1985}, standard deviation values are higher for [u] than for [a], [;,] and [o]. The large degree of variability exhibited by [u] is tolerated presumably because its acoustic consequences can be offset by compensatory variability in the lip rounding gesture (see Section 3.2.2 for further explication of this point). Consonant-dependent variability in linguopalatal contact is also large for the schwa. This is an expected trend since [~] is unconstricted and should be highly sensitive to changes in the vocal tract configuration induced by the surrounding consonants. 3.2. Consonant-dependent effects in linguopalatal contact To show more clearly the precise effect of the different consonants on the vowels, Fig. 4 gives mean patterns of linguopalatal contact for each vowel in each consonantal context. In the panels of this figure, the solid lines show the average number of "on" electrodes calculated for each row across the right and left sides of the palate. For reference, each graph also displays linguopalatal contact for each
~
~
~
G
(Q
[!]
I 'I
\
G
El
Figure 4. Configurations of linguopalatal contact for each vowel as a function of each consonant. Contact values have been averaged across the two sides , right and left, of the artificial palate.
186
D. Recasens
vowel in the [pVp] context (dashed line) . The difference between the solid and dashed lines, then, represents the amount of deviation in linguopalatal contact area caused by each consonant with respect to a neutral context which is highly unspecified for lingual activity in the bilabial consonant. The magnitude of the deviation can be interpreted as a measure of the degree of C-to-V coarticulation. Also , the deviation can be positive or negative , indicating that the consonant causes an increase or a decrease, respectively , in amount of linguopalatal contact in the vowel compared with the contact configuration for the same vowel in the neutral context. 3. 2.1. Effects on front vowels
In general, coarticulatory effects on front vowels are quite small, except in the context of the velarized lateral [t]. Also, dental [t] causes some increase of contact at the prepalatal zone with the more open vowels [e] and [e] but not with [i]. This contact increase is probably due to the apicodental articulation (with simultaneous laminoalveolar contact) of the consonant as opposed to the lowered position of the blade and predorsum for the two vowels in the [pVp] context. Alveolar [s] and postalveolar [f] cause some positive effects at the prepalatal zone since the tongue front is higher for these consonants than for all three vowels . These C-to-V effects in linguopalatal contact suggest that the two fricatives are subject to special requirements on the formation of a front constriction . They are consistent with X-ray data showing that the central groove for English [i] in the sequence [hizo] is narrower than that in the sequence [hivo] (Carney & Moll, 1971) . In addition, [f] causes some increase of dorsal contact at the postpalate in the context of the more open front vowels [e] and [e]; this effect follows from the fact that this consonant is produced with some tongue dorsum raising , and is consistent with a lower tongue dorsum position for [e] and [e] than for [i] . When adjacent to front vowels, the velarized alveolar lateral [t] has a negative effect on all rows. Moreover, the figure shows that the degree of coarticulation due to [t] is greater than that for any other consonant. Ohala & Kawasaki (1979) also found a decrease of the medial groove for [i] when adjacent to [t] in American English. Overall, these data reflect the existence of antagonistic mechanisms during the articulation of front vowels and velarized [t]: while front vowels r~quire fronting and raising of the tongue predorsum and mediodorsum, velarized [t] requires active lowering of those tongue regions to bring about a "dark" acoustic quality and presumably to facilitate lateral airflow as well. A decrease of contact at the prepalate is observed for all three front vowels in the context of [j] and [k], but not in the context of [Jl]. This pattern is consistent with the fact that the two former consonants (prepalato-mediopalatal and postpalatal, respectively) are produced with less dorsal fronting than the latter (alveoloprepalatal) . Moreover, [Jl] causes some increase of contact at the mediopalatal and postpalatal zone for [e], presumably as a result of some tongue dorsum raising. 3.2.2. Effects on back vowels and schwa
Inspection of the graphs in Fig. 4 reveals a different pattern of C-to-V coarticulation in the case of the back vowels [a], [::>], [o] and [u], and of the neutral vowel [;:)]. First, almost no negative effects are found. Thus, except for [::>] in the
Palatal contact in C-to-V coarticulation
187
context of the velar [k) and [u] in the context of [k) and of the velarized alveolar [t], all the vowels undergo only positive effects in all contexts. As discussed later on in this section, these coarticulatory effects accord well with the low positioning of the tongue predorsum and mediodorsum for back vowels. The schwa shows a similar pattern to that exhibited by [u] except for the absence of negative effects and the larger size of positive effects at the central arcs of the palatal surface. Large positive effects are found from dental [t], alveolar [s] and postalveolar [f). Thus the dorsum is fronted and/or raised during the production of back vowels and [;}] when adjacent to consonants involving an apical closure or a !aminal constriction . As indicated in the Introduction section, studies for other languages in the literature suggest that positive effects on [a] reflect coarticulation in tongue dorsum raising, while positive effects on back rounded vowels reflect coarticulation in tongue fronting and raising. It can be seen in the figure that for the rounded vowels, as vowel height decreases, the fronting effect of the [t] context also decreases, whereas that of the [s] context remains the same. Thus a difference in the amount of a positive effect between the two contexts emerges and becomes larger going from [u] to [o] to [::>). The same generalization holds true for the [t] context relative to the [f) context, although the positive effect of [f) itself is also somewhat diminished in the two lower vowels. This different interaction between fronting effects and vowel height can be interpreted in terms of greater constraints on the shaping of the tongue body for the coronal fricatives as opposed to the stop. The entire ventral surface of the tongue must be high to channel the airflow into the narrow constriction of an alveolar or postalveolar fricative . The consistently large fronting effects in [u] are more similar to those for [;}] and [a] than they are to those for [o) and [::>]. Large tongue fronting effects rna y be tolerated in the case of [u] because of the associated lip rounding, which is more considerable for this vowel than for the two other back rounded vowels. It can be hypothesized that speakers do not care much about the precise location of the lingual constriction because the spectral properties of [u] are already accounted for by the lip rounding gesture; if so, it seems a natural strategy to allow for C-to-V coarticulation in tongue positioning along the palatal surface. Large coarticulatory effects on the schwa can be explained because of the inherently low requirements on the tongue configuration during the production of this vowel. The velarized alveolar lateral [t], which has the only large systematic effect on the front vowels, causes few coarticulatory effects on these back vowels. The absence of coarticulatory effects on pharyngeal vowels [a], [o], and [::>] is presumably due to the similarity in tongue dorsum configuration; indeed, all these articulations are produced with a back lingual constriction and a lowered position of the predorsomediodorsal region of the tongue. Negative effects at the postpalatal region for [u] are associated with the raised tongue dorsum in the case of [u] as opposed to the other vowels. Alveolopalatal [Jl] and palatal [j] have generally positive effects but the size of the effect varies widely among the vowels. For the rounded vowels in these contexts the positive coarticulatory effects increase as the vowel becomes higher, both towards the alveolar region and the median line. Positive effects are also particularly salient in the case of the schwa. This pattern indicates that, similarly to adjacent dentals and alveolars, adjacent alveolopalatal and palatal consonants cause some stretching
D. Recasens
1
of the tongue dorsum during the production of highly rounded back vowels and [;:)], and pull it forward within the oral cavity . By contrast to the dentals and alveolars, alveolopalatal [J1] and palatal [j] cause little C-to-V coarticulatory influence upon [:::>] and [a]. In the case of [;:)], [o] and [u], however, those two consonants do cause a substantial increase in contact towards the front as well as the median line. Since the production of both consonants involves a large degree of tongue dorsum contact (Recasens, 1984), C-to-V effects may affect the front or the back regions of the tongue dorsum depending on the height of the vowel. C-to-V effects from [k] on back vowels are either absent (in the case of [a], [o] and [:::>]) , or are rather small (there is a small positive effect at the postpalatal zone for [;)] and a small negative one for [u]) . It should be pointed out that in the context of this consonant there is more contact in [a] and [;)] than in the back rounded vowels presumably because [k] is produced with a more fronted place of articulation in those two vowel contexts. 4. Articulatory-acoustic relations
Figure 5 shows F2 data for all vowels in all consonantal contexts. Differences between these F2 values and those in the [pVp] context are not given since F2 for a vowel preceded and followed by [p] reflects other articulatory factors besides tongue activity. It should also be noted that the correlation between the articulatory and acoustic measurements will be necessarily low for those vowels showing little palatal contact since the lingual configuration cannot be inferred with precision from the EPG data in these circumstances. ~ --------------------------------------------r---------------.
---o--
::r:"
2000
.5
---
e
---Q-
E
--- a 1000
---+---
u
0 +---~---.--~---,,---~--,---~--~--~----r---------------~
.P
J
s
k
p
+
Figure 5. F2 data for all vowels in all symmetrica l eve contexts.
Palatal contact in C-to- V coarticulation
189
4.1. Front vowels A systematic relationship between the F 2 data and the EPG data obtains in the case of front vowels with adjacent velarized [t] . In this case, the large negative C-to-V effects in degree of dorsopalatal contact (described above in Section 3.2.1) are associated with a significant decrease in F2 . This appears to be the most salient C-to-V coarticulatory effect for front vowels both in the EPG and in the F2 data reported in this paper. Since Wood (1982) has shown that an increase in the distance between the tongue and the palate at the place of primary constriction for front vowels causes appreciable F2 changes, this systematic relationship can be interpreted as a causal one . There may also be a similar causal relationship between the positive C-to-V effects in dorsopalatal contact of [e] and [c] when adjacent to alveolopalatal [p], palatal [j] and postpalatal [k] (see Section 3.2.1) and the observed F 2 frequency. In these three contexts F 2 is significantly higher than in the context of [s], [t] and [f]. On the other hand, it could be argued instead that it is the dentoalveolars and labials (rather than the alveolopalatals and palatals) which cause some deviation (i .e. lowering) from the ideal F 2 values for those target vowels. This would accord with the interpretation of Stevens & House (1963) for their American English data. Notice that the fricative [f] causes a lower F 2 frequency than the stop of a similar place of articulation (i.e. [t]) at the midpoint of [i] and [e]; a similar trend is observed for [s] vs. [t] upon [e] (see also Stevens & House, 1963). A possible explanation for this pattern is that fricatives cause some tongue dorsum lowering along the median line during the production of the vowel and thus contribute to an F 2 decrease. Overall, these C-to-V effects suggest that F 2 for front vowels is particularly sensitive to changes in the conductivity index of the dorsopalatal passage (Fant , 1960). 4.2. Back vowels and schwa Again, the F2 patterns are similar to those in Stevens & House (1963) and are clearly related to the C-to-V effects in the EPG data. Thus, positive coarticulatory effects in linguopalatal contact from palatal consonants on [~], [o] and [u], and from dentals and alveolars on all back vowels and schwa, cause a F 2 frequency increase with respect to the neutral [pVp] context. This increase in contact and in F 2 frequency for the vowel reflects some tongue dorsum raising and fronting induced by the consonantal context. Consistent with the data reported here, Wood (1982) has shown that advancing the dorsal constriction for [u] from the back or mid soft palate to the front soft palate or to even more anterior region causes a large F 2 increase; Maeda (1990) reports a similar finding for the vowel [o] . Notice , however , that there is also a high F 2 frequency for [a] and [::>] in the context of palatal consonants, but this is not correlated with an increase of linguopalatal contact with respect to the neutral [pVp] context. This discrepancy suggests that there may not be enough fronting and raising of the tongue dorsum to leave a trace on the palatal surface in this contextual condition. Therefore data on linguopalatal contact are not always informative about the speech gestures themselves and should be complemented with data from other methods of observing articulatory patterns.
190
D. Recasens
The figure also shows that F 2 is higher in palatal contexts not just by comparison with the neutral bilabial context, but also by comparison with dentals and alveolars. This is true for the· back vowels and even more so for [~]. Inspection of the EPG data in Section 3.2.2 suggests that this pattern may be due to an increase in linguopalatal contact at the back palate towards the median line, particularly for[~], [u] and [o] . Again tongue dorsum raising for lower vowels such as [a] and [;)] in this context may not be enough to cause appreciable contact changes on the palatal surface. Figure 5 shows that F 2 for a back vowel and schwa is lowest when the adjacent consonant is the velarized [t]; indeed EPG data reported in Section 3.2.2 indicate that contact for the vowel remains further back and is not affected positively by this adjacent consonant. The F2 values are somewhat higher for the vowels in the sequence [kVk] by comparison to the neutral [pVp] context. This difference is larger in the case of [a] and[~] than of the rounded vowels; this is consistent with the EPG data which reveal some positive C-to-V effects at the back of the palate for the schwa. Overall these data confirm the view that the place of articulation for [k] next to [a] and[~] is intermediate between its place when adjacent to front vowels and its place when adjacent to back rounded vowels (see also Recasens, 1985). In general, F2 appears to be well correlated with front cavity size (Fant, 1960), and therefore with the degree of tongue dorsum raising towards the palate. An increase of F2 frequency results from a front cavity reduction caused by tongue dorsum fronting and raising with respect to its positioning in the neutral [pVp] context. Analogously to the EPG data, the schwa shows the largest spread of F 2 frequencies of all vowels across consonantal contexts. 5. Summary and conclusions Patterns of linguopalatal contact across consonantal contexts show differences in contact size between vowels of different height that are highly consistent within the front series ([i], [e], [c]) but not within the back series ([ u], [a], [o], [;)]). This apparent anomaly can be attributed to the position of the tongue dorsum, which may be higher for low back vowels than for higher back vowels. A systematic contrast in constriction location on the palatal surface was obtained for front vs. back vowels which is consistent with the tongue regions presumably under active control in the two groups. This grouping of vowels into a front and a back category has been shown to be not only related to patterns of linguopalatal contact but also to patterns of articulatory variability. Variability for [~] is the highest of all vowels. The finding that [i] is subject to a low degree of variability in palatal contact is consistent with the idea that [i] has a particularly stable articulation due to "saturation" (Fujimura & Kakita, 1979; Perkell & Nelson, 1985) . According to this view, vertical displacement at the front regions of the tongue dorsum is small because the sides of the tongue blade are pushed against and restrained by the hard palate; on the other hand , horizontal variability at the front and, more so, at the back of the tongue dorsum is caused presumably by contraction of the posterior genioglossus. High standard deviation values in EPG activity across the consonantal contexts for [u] and[~] suggest a clear trend for the degree of coarticulation to vary inversely with the degree of constraint on tongue dorsum activity. Less variability for [a], [o]
Palatal contact in C-to- V coarticulation
191
and [:)] than for [u] suggests that lip rounding may play a role in the presence of C-to-V effects by compensating for lingual precision. Coarticulatory data reported in this paper may be useful for gaining some understanding of the nature of the articulatory strategies used by speakers for the production of adjacent vowels and consonants. It should be pointed out in this respect that C-to-V effects are subject to production constraints which are similar to those observed in V-to-C effects. Analogously to consonants, those lingual regions which are not involved in the formation of a vocalic constriction are left freer to coarticulate than those which are actively involved. Effects on front vowels are quite small due to the requirements on the tongue dorsum to achieve a palatal constriction. The most significant consonantal effects are caused by velarized [t] and result in an increase in tongue dorsum lowering. In general, those articulatory contrasts associated with palatal constriction width have an effect on F 2 frequency. Coarticulatory effects for back vowels and [;J] are larger than those observed for front vowels . Positive coarticulatory effects in this case reflect mainly tongue dorsum raising (in the case of [a]) and tongue dorsum fronting and raising (in the case of the other back vowels) as a function of adjacent dental, alveolar and palatal consonants. The consonants [k] and velarized [t] cause little C-to-V coarticulation. These articulatory differences have an effect on F2 frequency differences. Patterns of C-to-V coarticulation may yield information about the constraints on the production of a consonantal gesture. Thus, when a consonantal gesture is highly constrained, C-to-V effects ought to be already present during the production of an adjacent vowel (if the articulation of the vowel allows them). An interesting case is the increase in fronting of the contact for [a] and [:)] in the context of [s] compared with [t) in spite of the fact that the dental stop is produced with a fronter place of articulation than the alveolar fricative. It may be that this difference occurs because the necessity of forming the central groove for the fricative requires a high degree of articulatory control. Overall, these effects reflect contrasts in degree of articulatory constraint associated with different phonetic segments. Large effects on back vowels from dentoalveolar and palatal consonants show that the constriction requirements on the front dorsum for the consonant dominate in the absence of articulatory requirements for the vowel; on the other hand, coarticulatory effects from velarized [t] on back vowels are small since the front dorsum configurations for that consonant and those vowels are highly similar. In the case of front vowels, strict requirements on tongue dorsum lowering-backing for velarized [t) override to some extent tongue dorsum raising-fronting for front vowels; also, coarticulatory effects from fricatives on front vowels are rather restricted because fricative production demands high precision of the tongue configuration.
This research has been supported by NINCDS Grant NS-13617 to Haskins Laboratories, the ESPRIT-ACCOR project (BRA Action 3279) from the European Community, a research grant PB87-0774-C02-01 from the Spanish Government (DGICYT) and a fellowship from the Catalan Government (CIRIT) . I would like to thank Mary E. Beckman, Carol A. Fowler , Shinji Maeda, Ignatius G. Mattingly and three anonymous reviewers for comments on previous versions of this paper.
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D. Recasens References
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