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The contact index method of electropalatographic data reduction
Journal of Phonetics (1994) 22, 141 - 154
The contact index method of electropalatographic data reduction J. Fontdevila, M. D. Pallares and D. Recasens CEDI, lnstitut d 'Estudis Catalans, c/ Carme 47, 08001 Barcelona, Spain, and Departament de Filologia Catalana , Universitat Autonoma de Barcelona, Bellaterra, Barcelona, Spain Received Jrd April 1992, and in revised form &h July 1993
This paper presents a new method of electropalatographic data reduction , namely, the contact index method . It provides accurate information about the distribution and size of linguopalatal contact based on values for an anteriority index , a posteriority index and a centrality index . These values are calculated using coefficients specific to each row of electrodes along the sagittal dimension and to each column of electrodes along the coronal dimension. The contact index method is applied to the analysis of two research topics: V-to-C coarticulation effects at the palatal zone for consonants differing in place and manner of articulation characteristics; and intraconsonantal variability across repetitions of a given VCV sequence. Results are evaluated in the light of the theory of speech production.
1. Introduction Electropalatography (EPG) provides a detailed representation of linguopalatal contact over time and is thus a useful analysis technique for the study of lingual coarticulation. In the Reading EPG system (on which the content of this article is based) each linguopalatal configuration contains 62 data points corresponding to the 62 electrodes embedded in the artificial palate; one temporal frame lasts 5 ms (see Fig. 1; also Hardcastle, Jones, Knight, Trudgeon & Calder, 1989). It should be noticed that the EPG data are arrangements of binary values (recording only the presence vs. absence of activation for each electrode) which renders their use for quantitative analysis quite difficult. In order to solve this problem, several EPG data reduction methods have been proposed in the literature. Some EPG data reduction methods concentrate on the temporal evolution of linguopalatal contact with reference to a selected number of frames which represent crucial articulatory events (Butcher, 1989; Engstrand, 1989; Hardcastle, Gibbon & Nicolaidis, 1991). Simple counts of contact frequency for each electrode over some time interval can be included here (Mitzutani, Hashimoto, Wakumoto, Hamada & Miura, 1988; Matsuno, 1989; Gibbon, 1990) . Nevertheless, there appears to be a clear bias towards spatial methods of characterization of EPG patterns. Based upon a single linguopalatal contact configuration (usually the EPG frame showing maximum contact), these methods account for contact patterns and coarticulation by means of some numerical reduction criteria. As an example, simple articulatory 0095-4470/94/020141 + 14 $08.00/0
© 1994 Academic Press Limited
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142
---cc--CA
Zones
Subzones
Rl
.---1---t--+-+-+-+-t--, H-t--t-+-+-+-+---JR2 Alveolar R3 1--1---t--+-+-+-+-t---1 R4 1---1---t--+-+-+-+-t---I RS R6
l--ll--l---t---t--+--+--+~ R7
CP
I _ _[___J'------J___l_____l__.L__.L___J
Front alveolar Postal veolar Prepalatal
Palatal
Mediopala tal
RS
Postpalatal
C!C2 C3C4C4C3C2 Cl (a)
(b)
(c)
Figure 1. (a) Distribution of rows Rl through RS along the anteriority (CA) and posteriority (CP) dimensions , and of columns Cl through C4 along the centrali ty (CC) dimension on both sides of the electropalate . (b) Articulatory zones and subzones on the electropalate . (c) Vocal tract representation with articulatory zones and subzones , and tongue regions: 1, alveola r; 2, prepalatal ; 3, mediopalatal ; 4, postpala tal; 5, tongue tip and to ngue blade ; 6, predorsum ; 7, mediodorsum ; 8, postdorsum .
parameters have been proposed for fricatives (i.e ., positiOn of maximum constriction , constriction width) and also measures of contact variation (i.e., front and back gradients; Engstrand , 1989; Hoole, Ziegler, Hartmann & Hardcastle , 1989). Other spatial methods rely on more complex numerical indices which allow for statistical analysis of the EPG data. Three indices are particularly relevant in this respect: (1) The center of gravity index (COG) (Mitzutani et al. , 1988) was designed to locate the highest concentration of activated electrodes on a given EPG frame. The coefficient numbers associated with the different rows are arbitrarily selected in order to give preferential weight to the anterior contact along the sagittal axis . Thus, coefficient values increase as rows become more anterior. COG = [0.5(R 8 ) + 1.5(R 7 ) + 2.5(R 6 ) + 3.5(R 5 ) + 4.5(R 4 ) + 6.5(R 2 ) + 7.5(R 1)/total number of contacts,
+ 5.5(R 3 )
where R; = number of " on" electrodes on row i. (2) Faber's anteriority index (Faber , 1989) gives distributional preference to anterior or posterior electrodes along the sagittal axis by means of a series of coefficient numbers calculated on a specific area of the Rion artificial palate (Shibata, Ino, Yamashita, Hiki, Kiritani & Sawashima, 1978). Positive values provide a measure of anterior contact whereas negative values indicate posterior contact. Anteriority = [36(R 1)
+ 6(R 2 ) + (R 3 )] - [(R 4 ) + 6(R 5 ) + 36(R 6 + R 7 )/2],
where R; = number of " on" electrodes on row i.
Contact indices for EPG data reduction
143
(3) The coarticulation index (CI) (Farnetani , Hardcastle & Marchal, 1989) measures the difference in electrode activation for a phonetic segment (i.e., a consonant) when uttered in two different contextual environments (i.e. , two VCV sequences) . It is expressed in percentages of electrode contact and is usually calculated on a row-by-row basis . However, CI may also be presented as a mean value across the four front rows (anterior zone), across the four back rows (posterior zone) or across all rows (anterior and posterior zone) of the artificial palate.
Cltotal
=
(Clantcrior + Clposterior)/2,
Clanterior = (IR lA - R IBI + IR 2A - R2sl + IR3A- R3s l + IR4A- R4s l)/4 , Clpostcrior
= (IRsA -
Rssl + IR6A - R6sl + IR7A- R?s l + IRsA- Rss l) / 4,
where RiA= percentage(%) of "on" electrodes on row i for context A, and RiB = percentage( %) of "on" electrodes on row i for context B . In short, the center of gravity index and Faber's anteriority index have proved useful in giving a rough estimation of the distribution of linguopalatal contact. On the other hand, the coarticulation index is a measure of variation in degree of contact for the same consonant across different vowel contexts. This paper presents the contact index method, a new quantitative method of EPG data reduction which differs significantly from the three indices described above. According to this method, linguopalatal contact patterns are represented with reference to three index values, namely, contact anteriority (CA), contact posteriority (CP) and contact centrality (CC). The CA and CP index values vary along the sagittal axis of the palate, thus reflecting the degree of linguopalatal contact fronting and backing ; the CC index values vary along the coronal axis and represent the degree of linguopalatal contact from both sides of the palatal surface towards its median line regardless of whether contact distribution is at the front or at the back of the palate . This set of indices is capable of measuring both degree and distribution of linguopalatal contact. Consequently , two linguopalatal configurations showing the same number but a different distribution of "on" electrodes will be precisely distinguished by the CA, CP and/or CC index values. Since these indices give an accurate measure of contact distribution, they can be used for the characterization of variations in linguopalatal contact associated with different speech sounds (e.g., consonants of different place and manner of articulation) and contextual conditions (e.g., vowel-dependent effects on consonants).
2. The contact index defined In order to calculate the three contact indices , the electropalate is divided into eight rows (R) along the sagittal dimension and into eight columns (C) along the coronal dimension (see Fig. 1). Rows are numbered from 1 through 8 from front to back; columns are numbered from 1 through 4 from the periphery to the median line on both halves of the artificial palate. The value of the CA and CP indices is computed from a count of all " on" electrodes on a row-by-row basis; the CC index value is calculated from a count of all " on" electrodes on a column-by-column basis (see Fig. 1 for the row and column numbers).
J. Fontdevila et al.
144
The three contact indices are in essence weighted sums of activated electrodes that take into account the spatial distributions of the rows or columns along the electropalate. In addition, they are constructed mathematically so that the contribution of a single activated electrode on a given row or column is always higher than the joint contribution of all the activated electrodes on previous back rows (CA index), front rows (CP index) or lateral columns (CC index). Each row or column is assigned a coefficient number which gives a characteristic weight to all normalized electrodes on that row or column. For example, the contact anteriority index (CA) over the entire palate might be calculated by means of the following mathematical expression: (initial formulation of) CA = [A(R 8 / Rs,) + B(R 7 / R7 ,) + C(R 6 / R 6,) + D(R 5 / R5 ,) + E(R4/ R4,) + F(R3/ R3,) + G(Rz/ Rz,) + H(R 1/ R 1,))/(A + B + C + D + E + F + G +H) , where RJ R;, is the normalized value for each activated electrode on row i:
_ (number of activated electrodes on row
~ I Ru-
(total number of electrodes on row i)
i)
.
This procedure normalizes each row's intrinsic contribution to the total number of electrodes available on that row. Thus the maximum value for any row should be 1 if all electrodes on that row are activated. The intent of this normalization is that two rows should not contribute differently to an index value due to their differing in absolute number of electrodes but only because of their relative location along the artificial palate. The eight coefficient numbers A-H are chosen so that the activation of all electrodes at and behind a given row i always yields a lower index value than the activation of any single electrode at more anterior rows, as follows: A
= 1 Row 8 should be given the lowest weight in the construction of the CA index
since it is the most posterior row on the artificial palate. A coefficient number of 1 is chosen arbitrarily for this row [i.e., 1(R 8 /8) ranges from 0 when no electrodes are active, to 1 when all eight are). B = 9 A single "on" electrode on row 7 should yield a higher index value than all possible activated electrodes on row 8. If B(1 "on" electrode/8 electrodes on row 7) > A(8 "on" electrodes/8 electrodes on row 8), that is, B /8 >A, then B =(SA)+ 1 = 9. A quantity of 1 is added to the equation in order to obtain coefficient B (i.e. , when R7 = 1, and R 8 = 8, B(R 7 /8) = 1.125 > A(R 8 /8) = 1). The general formula for the calculation of the remaining coefficients is as follows: C = 81 (total number of electrodes on row 6)(A +B)+ 1 D = 729 (total number of electrodes on row 5)(A + B +C)+ 1 E = 6561 (total number of electrodes on row 4)(A + B + C +D)+ 1 F = 59049 (total number of electrodes on row 3)(A + B + C + D +E)+ 1 G = 531441 (total number of electrodes on row 2)(A + B + C + D + E +F)+ 1 H = 3587227 (total number of electrodes on row 1)(A + B + C + D + E + F +G+)+l. .
Contact indices for EPG data reduction
145
In order to balance its highly exponential tendency, the initial mathematical ex pression of the CA index is submitted to a logarithmic transformation which allows for a linear increment of its values. Finally, the CA index is divided by its maximum possible value (the log of the sum of all coefficients) so that a range from 0 to 1, where 0 stands for no contact and 1 for complete contact, is always obtained: CA = [log[[1(R 8 /8) + 9(R 7 /8) + 81(R 6 /8) + 729(R 5 /8) + 6561(R 4 /8)
+ 59049(R 3 /8) + 531441(R 2 /8) + 3587227(R 1 /6)] + 1]]/[log(4185098 + 1)]. Similar reasoning is used to calculate the contact posteriority (CP) and contact centrality (CC) index coefficients: CP = [log[[1(R 1/6) + 9(R 2 /8) + 81(R 3 /8) + 729(R 4 /8) + 6561(R 5 /8)
+ 59049(R 6 /8) + 531441(R 7 /8) + 4782969(R 8 /8)] + 1]] /[log(5380840 + 1)], CC = [log[[1(C,/14) + 17(C2 /16) + 289(C 3 /16) + 4913(C 4 /16)] + 1]] /[log( 5220 + 1)]. The CP index is obtained in the same way as the CA index except that the rows are taken in reverse order in the calculation process. Also, since in the Reading artificial palate the first row has 6 electrodes while all the others have 8 electrodes, the CA coefficient H is smaller than the corresponding CP coefficient H. The normalization procedure of the CC index takes into account all activated electrodes on a given column as well as those on the corresponding symmetrical column on the other half of the artificial palate. This measure of contact centrality reflects the distribution of contact along the lateral-central dimension. In addition, more local contact indices can be formulated by taking a subset of rows or columns; e.g., the palatal contact anteriority index (CAp) involves the application of the contact index method to the four back rows or palatal zone: CAp= [log[[1(R 8 /8) + 9(R 7 /8) + 81R 6 /8) + 729(R 5 /8)] + 1]]/[log(820 + 1)]. The contact index method allows us to recover linguopalatal patterns quite accurately. It should be emphasized in this respect that a fixed range of index values will always be obtained provided that linguopalatal contact does not go beyond a certain row or column. Table I( a) and (b) shows the range of CA, CP and CC values for all rows and columns. For each interval in one cell of the table, the minimum value reflects the activation of one electrode on a single row or column across the entire palate; the maximum value results from all electrodes being activated on the referred row or column and on all previous ones (where "previous" means less anterior for CA , more anterior for CP, and more lateral for CC) . Therefore, CA, CP and CC index values within each interval indicate the same degree of anterior, posterior and central contact on the surface of the electropalate , respectively . Contact index values increase primarily as a function of the activation of successively more electrodes on a given row or column and, to a lesser extent , of variations in overall linguopalatal contact pattern . Thus, for example, a CA index value of 0.563 [see Fig. 2(a) , middle] indicates the following three facts : (1) Anteriormost contact occurs on row 4. We know this because 0.563 is within the range of CA index values for row 4 according to Table I.
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1. Fontdevila et al.
I. (a) and (b) Intervals of CA, CP and CC index values indicating the same degree of anterior, posterior and central contact, respectively (see text for explanation). (c) CA index values as a function of electrode activation on row 4 when no contact occurs on the other seven rows of the artificial palate (a)
TABLE
Row
CA
CP
1 2 3 4 5
[0.87237467-1.00000000] [0.72826728-0.87237465] [0.58416691-0. 72826715] [0.44012969-0.58416580] [0.29665732-0.44011970] [0.15801065-0.29656827] [0 .04943725-0.15726955] [0 .00772498-0.04546109]
[0.00994626-0.04472392] [0.04863560-0.15471934] [0.15544842-0.29175925] [0 .29184686-0.43298292] [0 .43299274-0.57469323] [0.57469433-0.71645790] [0.71645802-0.85822862] [0.85822863-1.00000000]
6 7
8
(b) Column
cc
1 2 3 4
[0 .66939-1.00000] [0 .34434-0.66937] [0.08457-0.34396] [0. 00806-0. 08097] (c)
Number of "on" electrodes
1
2
3
4
5
6
7
8
CA (row 4)
0.440
0.486
0.512
0.531
0.546
0.558
0.568
0.576
(2) There are six activated electrodes on row 4. We know the number is fewer than 7 because the minimum value for CA in that case is 0.568 as shown in the top right configuration. (3) At least part of the palatal surface behind row 4 is activated. We know this because 0.563 is higher than 0.558, the CA index value for a contact pattern showing six "on" electrodes on row 4 and no "on" electrodes behind that row [see Fig. 2(a) , left] . In short, a given CA index value provides information about three aspects of linguopalatal contact : row number of anteriormost contact, degree of electrode activation within the frontmost row, and general level of linguopalatal contact behind the frontmost row along the sagittal dimension. Notice, however, that the CA index itself does not capture the distribution of contact along the coronal dimension; thus, two linguopalatal configurations may have identical CA index values and, nevertheless, show different contact patterns from the sides towards the median line. In order to determine the amount and distribution of central contact, a CA or a CP index value should be associated with a CC index value . Figure 2(b) shows ideal linguopalatal configurations of a fricative consonant (left) and a lateral consonant (right) . Though both linguopalatal patterns yield the same CA index value (0.563), they differ with respect to the CC index
147
Contact indices for EPG data reduction I I I I I I
..,.
I I
,.,.I. • ,. ,.,.1
•
• ,. 1 • ,. 1 I
CA= 0·558
<
1.1. 1.1. 1. 1•
I I I
1.1 •
I
CA= 0·563
I
I
I
I
'
CA = 0·568
<
(a)
I
•1• •1 •I• •I • • I• •I• i I
.... ......
1• •1• •••I,
••• •• ••I ••I ••I
••I, . 1. 1
,
CA= 0·563 cc = 0-453
•I• , ,
CA= 0·563
cc =0·782 (b)
Figure 2. (a) Ideallinguopalatal configurations and corresponding CA index values. (b) Ideal linguopalatal configurations for a fricative consonant (left) and a lateral consonant (right) , and corresponding CA and CC index values.
value . Thus, the CC index value is higher for the lateral (0.782) than for the fricative (0.453) since there is midsagittal contact during the production of the former consonant but not of the latter. TheCA, CP, and CC indices are similar to the COG index in that they are more sensitive to changes in linguopalatal contact distribution than to variations in size of contact area. However, these contact indices characterize contact directionality more accurately than the COG index since they are capable of weighing the contribution of a single activated electrode regardless of its distance from the bul k of linguopalatal contact. In other words, although the numerical value of each of the three contact indices increases progressively and steadily with linguopalatal contact size, it is mostly an indicator of maximum contact fronting (CA), backing (CP) or centrality (CC). On the other hand, though contact indices and Faber's anteriority index operate on a similar mathematical principle, linguopalatal configurations can be more easily defined according to our method since it involves no subtraction between the anterior and posterior half of the palate. It should be noticed that if applied to the characterization of a variety of articulations this subtraction could ascribe the same numerical value to different EPG patterns. In order to illustrate the validity of the contact indices explained above we will report some V-to-C coarticulatory effects in Catalan. Since most Catalan consonants are produced either at the dentoalveolar or at the alveolopalatal zone we will analyze coarticulation at the palatal zone only . During the production of many of those consonants the tongue dorsum is quite free to coarticulate because it does not intervene directly in the formation of the primary place of articulation . For that purpose the artificial palate is subdivided into an alveolar zone (the four front rows) and a palatal zone (the four back rows) (see Fig. 1). There is a one-to-one correspondence between each of these two articulatory zones and the tongue articulators: alveolar consonants are articulated at the alveolar zone with the
J. Fontdevila et al.
148
tongue tip and/or the tongue blade , and palatal sounds are produced at the palatal zone with the tongue dorsum . 3. Illustrative EPG data analysis 3.1. Intrasegmental variability Figure 3(a) plots EPG linguopalatal contact configurations for [n] as a function of adjacent [i], [u] and [a] in symmetrical VCV sequences with stress on the first syllable (one of these sequences is transcribed as [an~] since unstressed I a/ is realized as [~] in Catalan). These linguopalatal contact patterns were measured at PMC or point of maximum contact, i.e., the temporal frame showing the maximum number of "on" electrodes along each VCV sequence. They represent averages across five repetitions produced by one Catalan speaker (DR, the third co-author of this paper). Different shades in the EPG patterns indicate that electrode activation occurs in 80-100% (black), 40-80% (grey) or fewer than 40% (white) of the repetitions. As in Fig. 1, the horizontal line crossing the EPG graphs separates the alveolar zone from the palatal zone. The numerical values of the three contact indices were calculated at the palatal zone (four posterior rows) for each VCV sequence . This articulatory zone has been chosen for contact analysis since it is particularly sensitive to vowel-dependent
.....
[n]
•••••• •••••••• ,,. •• •• • •• ••
.. " ··· ... ••
!;; .
~I'i
••
••
.,;
"'•••••••• ••••••I'" ••• • ••• • • • ·••·=·' "'•••
•••••••• • ••••••• ••• •• I"' • •• • • •
[u]
[a]
[i]
(a)
CPp
6
08
i/u
c~ " CAp
.
CPp
.
CCp
.
i/a
u/a
0·6 CCp
0·4 0·2 -
. .
OL---~--~--~------~~~
[i]
[u]
[a]
(b)
Figure 3. (a) Average linguopalatal configurations at PMC for [n] with adjacent [i], [u] and [a] (speaker DR ; five repetitions) . Different shades in the EPG patterns indicate that electrode activation is 80-HJO% (black), 40-80% (grey) or less than 40% (white) . (b) Graphical representation of the mean contact index values across five repetitions (left) , with significant V-to-C effects listed to the right (Scheffe, p < 0.05).
Contact indices for EPG data reduction
149
coarticulatory effects during the production of [n]. The labels CAp, CPp, and CCp stand for palatal contact anteriority , palatal contact posteriority and palatal contact centrality, respectively. Contact index values are represented graphically in the lower graph of the figure . Significant vowel-dependent effects (Scheffe, p < 0.05) for the three contact indices are shown in the table at the right of the graph. The linguopalatal patterns (palatal zone only) are characterized by the contact index values as follows: (1) The palatal contact anteriority index value (CAp) for [ini] is significantly higher than that for [unu] and [an;:J]. Indeed, as the EPG patterns at the top of the figure show, the front vowel [i] causes more electrode activation at the front palatal zone than do the back vowels [a] and [u]. No significant difference was found for [unu] vs. [an;:J], which also accords with what the configurations above show about contact patterns at the front palatal zone in the two sequences. (2) The palatal contact posteriority index value (CPp) shows significant differences between all pairs of the three VCV sequences (i .e., [ini], [unu], [an;:J]). As shown in the lower graph of the figure, index values are highest for [ini] and lowest for [an;:J], those for [unu] falling in between. This finding is consistent with the presence of different degrees of contact at the back palatal zone in the progression [ini] > [unu] > [an;:J]. (3) Palatal contact centrality (CCp) also shows significant differences among [ini] (highest value) , [an;:J] (lowest value) and (unu) (intermediate value). Consistently with these index values, the linguopalatal contact area decreases towards the central zone in the progression [ini] > [unu] > [an;:J]. As shown in the lower graph, there is a steeper increase in CCp index values than in CAp and CPp index values across vowel contexts. This is due to the fact that, while an increase in CCp index value is conveyed by the filling of new columns , an increase in CAp and CPp index values results from the activation of new electrodes within the same row. In conclusion , contact index values at the palatal zone indicate the presence of more linguopalatal contact for [n] in the sequence [ini] than in the sequences [unu] and [an;:J] along the three dimensions (i.e., anteriority, posteriority and centrality). The consonant shows more contact posteriority and contact centrality in the sequence [unu] than in the sequence [an;:J] . This general description accords with the observation that alveolars are highly sensitive to coarticulatory effects at the palatal zone since the tongue dorsum does not intervene in the formation of the alveolar place of articulation. Moreover, coarticulatory trends are consistent with the articulatory specification for the adjacent vowels; indeed, they occur in tongue dorsum raising and fronting for front vs. back vowels and in tongue dorsum raising but not in tongue dorsum fronting as a function of back vowels. 3.2. Inter-segmental variability Mean values for CAp, CPp and CCp were also calculated across five repetitions of VCV sequences with the alveolar fricative [s] and the alveolopalatal nasal stop [Jl] in productions by the same Catalan speaker. In order to compare [n], [s] and [p], we excluded the fifth row of electrodes from the index calculations for [Jl], since this row is involved in the place of articulation of this consonant but not of [n] and [s].
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08 [n]
~ CPp CAp ~
0·2 0
i/a
u/a
CAp
0·6 0-4
i/u
"
~ [i] Ci
[u]
.
CPp CCp
.
.
[a] i/u
==:::::::::::::!====-il
0·8 CAp
i/a
u/a
i/a
u/a
.
[s] 0·6 0-4 0·2 0
~
CPp CCp
[i]
[u]
.
[a] i/u
0·8 CAp
0·6 [Jl] CPp
0-4 0·2 0
CCp
[i]
[u]
[a]
Figure 4. Mean contact index values at the palatal zone measured at PMC for [n], [s] and [Jl] as a function of vowel context (speaker DR; five repetitions). Significant V-to-C effects are given in the table at the right of each graph (Scheffe , p < 0.05) .
Indeed , alveolopalatal [Jl] is usually articulated at the back alveolar zone and at the front prepalatal zone with the back of the blade and the front predorsum. The three graphs in Fig . 4 report the numerical index values for the three consonants, i.e ., [n] (top) , [s] (middle) and [Jl] (bottom), in the symmetrical sequences [iCi], [uCu] and [aC~]. As in Fig. 3, the amount of V-to-C coarticulation in the graphs is directly related to the steepness of the slope of the lines connecting the context vowels. A joint consideration of all three contact index values reveals differences in degree of V-to-C coarticulatory sensitivity among the consonants in the order [n] > [s] > [Jl]. Less tongue dorsum coarticulation for [s] than for [n] is presumably due to a high degree of articulatory control on lingual activity for the former consonant vs. the latter (Wolf, Fletcher, McCutcheon & Hasegawa , 1976; McCutcheon , Hasegawa & Fletcher, 1980). Alveolopalatal [Jl] allows no significant vowel-dependent effects in tongue dorsum contact, thus suggesting that this articulator is highly constrained; higher articulatory requirements for alveolopalatals than for alveolars are presumably associated with more tongue dorsum raising for the former consonants than for the latter (Recasens, 1983). Table II shows significant V-to-C effects in mean contact index values across five repetitions for a larger subset of Catalan consonants (again in symmetrical
Contact indices for EPG data reduction
151
TA BLE II. Statistical results for some Catalan consonants (speaker DR). Asterisks indicate significant vowel-dependent effects (Scheffe , p < 0.05) in contact index values at the palatal zone
CAp
CPp
CCp '\
i/a
i/u
u/a
Apicodental oral stop , [t] Apicoalveolar nasal stop , [n]
*
*
i/a
i/u
u/a
i/a
i/u
*
*
*
*
*
*
*
*
*
*
u/a
*
Velarized apicoalveolar lateral,
[I] Apico- or laminoalveolar fricative , [s]
*
* *
*
*
*
*
*
*
*
*
Lamino-postalveolar fricative,
[f)
*
*
Alveolopalatal affricate (stop component only), [tf]
*
*
Alveolopalatallateral, [,.<;]
*
*
*
Alveolopalatal nasal stop, [.n]
sequences [iCi], [uCu] and [aC~]; speaker DR). For each pair of context vowels , asterisks indicate those differences which were found to be statistically significant (Scheffe, p < 0.05). In the table, the consonants are ordered from frontmost to backmost place of articulation from the apicodental oral stop [t] to the alveolopalatal nasal stop [Jl]. The fifth row of electrodes was excluded from the calculation of the contact indices in the sequence [u..\u] for the same reason as for [Jl] above . Data on significant effects in the table reveal three interesting trends: First, CPp shows more effects than CAp for all consonants except for [1]. The tongue dorsum front (i.e., predorsum) is less free to coarticulate than the tongue dorsum back (i.e., postdorsum) because the primary place of articulation for all these consonants is closer to the prepalate than to the postpalate. Second, an overall account of significant effects for all contact indices reveals that dentoalveolars coarticulate more freely than alveolopalatals. This is because the tongue dorsum front is involved in the formation of the place of articulation for the latter but not for the former. Larger coupling effects between front dorsum and back dorsum for alveolopalatals vs. dentoalveolars cause more tongue dorsum raising and less coarticulation over the entire palatal zone. Third, generally speaking, vowel effects occur for [i] > [u] > [a] in the case of dentoalveolars and for [i] = [u] >[a] in the case of alveolopalatals . Therefore, coarticulation in linguopalatal fronting among high vowels is found during the production of the former consonants but not of the latter. Both consonantal categories show effects related to differences in vowel height. Significant vowel-dependent effects reported in Table II suggest that the contact
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III. Coefficient of variation values across five repetitions of each VCV sequence as a function of consonant, vowel context and contact index at the palatal zone (speaker DR). No tongue contact was detected at the palatal zone for the sequence [a!;:,]
TABLE
iCi
[t l [n]
[I] [s] [f] [tf]
[.-\] [.n l
aC;:,
uCu
CAp
CPp
CCp
CAp
CPp
CCp
CAp
CPp
CCp
2.90 2.29 25.50 0.00 0.00 2.76 1.03 1.50
2.42 1.54 55.53 0.00 1.86 1.07 1.54 2.18
11.80 7.00 73.49 0.00 3.29 11.93 8.07 4.49
4.67 3.67 7.56 3.14 2.15 1.03 1.82 1.82
0.00 0.00 135.70 0.00 0.00 0.00 3.63 1.05
9.58 12.77 89.47 7.65 3.59 8.41 16.15 8.27
5.42 0.00
0.00 0.00
25.68 0.00
2.58 1.53 1.46 1.03 1.82
2.33 0.00 2.52 4.35 2.17
17.84 10.86 17.20 9.03 4.45
index method described in this paper is quite successful in gauging coarticulation . The following trends are consistent with phonetic theory: less coarticulatory sensitivity for dentoalveolars and alveolopalatals at the front palatal zone than at the back palatal zone; less tongue dorsum coarticulation for alveolopalatals than for dentoalveolars; tongue dorsum coarticulatory effects in vowel fronting and height for dentoalveolars but only in vowel height for alveolopalatals. In addition, coefficients of variation (CV) for the. three indices at the palatal zone were calculated across the five repetitions of each VCV sequence [CV = (sIX) x 100]. Table III shows these variation coefficient values displayed as a function of consonant, vowel context and contact index. According to the table, variation coefficients are quite high for [!] in the sequences [iii] and [ulu] since considerable contact variability occurs at the sides of the palatal zone in this case. Variation coefficient values for all consonants are usually higher in the case of the CCp index than of the CAp and CPp indices. Indeed , electrode activation across repetitions of a particular VCV sequence entails contacting new columns rather than more anterior and more posterior rows , thus causing important CCp index value fluctuations; moreover, while lateral contact at the palatal zone remains highly constant during the production of lingual consonants , central contact may reflect changes in tongue dorsum height. The table also shows a trend towards greater variability in CAp index values for dentoalveolars than for alveolopalatals, mostly when adjacent to back vowels. This finding is consistent with the presence of larger V-to-C effects at the prepalate for dentoalveolars than for alveolopalatals (see Table II) which is in accordance with differences in prepalatal contact size between the two consonantal categories. Manner requirements also affect the amount of variability, as shown by lower variation coefficient values for the alveolar fricative [s] than for dentoalveolar stops .
4. Conclusions This paper shows that a new method of EPG data reduction, i.e., the contact index method, can be used quite successfully for capturing linguopalatal contact configurations . This method involves the calculation of three indices, i.e., contact anteriority (CA), posteriority (CP) and centrality (CC), which reflect the distribution of tongue
Contact indices for EPG data reduction
153
contact over the palatal surface. Index values are sensitive to the number of frontmost (CA), backmost (CP) and centralmost (CC) activated electrodes as determined by the weight of row- and column-specific coefficients. The contact index method of EPG data reduction can be applied to the measurement of contact characteristics at the alveolar and at the palatal zone independently. The validity of this method has been illustrated in this paper for V-to-C effects in tongue dorsum contact at the palatal zone during the production of a consonant (i .e., [n]) which is highly sensitive to dorsopalatal coarticulation. Contact index values allow us to capture differences in contact location within the palatal zone; changes in contact centrality as a function of variations in contact fronting and backing can also be evaluated. In addition to its use in studying context-dependent contact variations for the same consonant, the contact index method has been used to measure differences in coarticulatory sensitivity between dentoalveolar and alveolopalatal consonants at the palatal zone. Results conform to previous findings in the literature in showing larger vowel-dependent coarticulatory effects for dentoalveolars than for alveolopalatals. Contact index values reveal that effects at the back palatal zone are larger than effects at the front palatal zone , mostly for alveolopalatals which allow no prepalatal effects at all. It can be claimed that during the production of these consonants the predorsum is less free to coarticulate than the medio-postdorsum because it is coupled with the primary articulator, i.e., the tongue front. Also, while dentoalveolars are sensitive to effects in vowel fronting and vowel height , alveolopalatals are sensitive to effects in vowel height only . While alveolopalatals involve less dorsal contact as a function of low vs high vowels, differences in vowel fronting among high vowels cause no adaptation problems since the production of alveolopalatals also requires a good deal of tongue dorsum raising. This work was supported by project ESPRIT-ACCOR BRA 3729 , project ESPRIT-SPEECH MAPS BRA 6975, and ESPRIT-Working Group 7098.
References Butcher , A. (1989) Measuring coarticulation a nd variability in tongue contact patterns , Clinical Linguistics and Phonetics , 3 , 39-47. Engstrand, 0. (1989) Towards an e lectropalatographic specification of consonant articulation in Swedish, Phonetic Experimental Research at the Institute of Linguistics, University of Stockholm, 10, 115-156. Faber, A. (19S9) Studies in sibilant articulation. Unpublished manuscript. Farnetani, E. , Hardcastle , W. J. & Marchal , A. (1989) Cross-language investigation of lingual coarticulatory processes using EPG. In Eurospeech '89 (J. P. Tubach & J. J . Mariani , editors), pp. 429-432. Paris : European Conference on Speech Communication and Technology. Gibbo n, F. (1990) Lingual activity in two speech disordered children's attempts to produce ve lar/a lveolar stop contrasts: evidence from Electropalatographic (EPG) data, Reading University Speech Research Laboratory Work in Progress, 6, 1-14. Hardcastle , W. J., Jon es, W. , Knigh t, C., Trudgeon, A. & Calder, G. ( 1989) New developments in electropalatography : a state-of-the-art report, Clinical Linguistics and Phonetics, 3 , 1-38. Hardcastle , W. J. , Gibbon , F. & Nicolaidis , K. (1991) EPG data reduction methods and their implications for studi es of lingual coarticulation , Journal of Phonetics, 19, 251-266. Hoole, P. , Ziegler, W., Hartmann , E. & Hardcastle , W. J. (1989) Parallel electropalatographic and acoustic measures of fricatives, Clinical Linguistics and Phonetics , 3 , 59-69. Matsuno , K. (1989) An e lectropalatographic study of Japanese palatal sounds , The Bulletin , The Phonetic Society of Japan, 190, 8-17 . McCutcheon, M. , Hasegawa, A . & Fletcher, S. (1980) Effects of palatal morphology on /s, z/ articu lation, Biocommunication Research Report, University of A labama , Birmingham, 3 , 38-47.
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Mitzutani, T., Hashimoto, K., Wakumoto , M ., Hamada , H. & Miura, T. (1988) Analysis of tongue motion for the dental consonants based on the high-speed palatographic data, Proceedings of the Joint Meeting of the Acoustical Society of Japan, Hawaii , 1-5. Recasens , D. (1983) Coarticulation in Catalan VCV sequences: an articulatory and acoustical study, PhD Dissertation , The University of Connecticut. Shibata, S., Ino, A., Yamashita , S. , Hiki, S. , Kiritani , S. & Sawashima, M. (1978) A new portable type unit for electropalatography , Annual Bulletin of the Research Institute of Logopedics and Phoniatrics, University of Tokyo, 12, 5-10. Wolf, M., Fletcher, S., McCutcheon , M. & Hasegawa , A. (1976) Medial groove width during /s/ sound production , Biocommunication Research Reports, University of Alabama, Birmingham, 1, 57-66.
The contact index method of electropalatographic data reduction J. Fontdevila, M. D. Pallares and D. Recasens CEDI, lnstitut d 'Estudis Catalans, c/ Carme 47, 08001 Barcelona, Spain, and Departament de Filologia Catalana , Universitat Autonoma de Barcelona, Bellaterra, Barcelona, Spain Received Jrd April 1992, and in revised form &h July 1993
This paper presents a new method of electropalatographic data reduction , namely, the contact index method . It provides accurate information about the distribution and size of linguopalatal contact based on values for an anteriority index , a posteriority index and a centrality index . These values are calculated using coefficients specific to each row of electrodes along the sagittal dimension and to each column of electrodes along the coronal dimension. The contact index method is applied to the analysis of two research topics: V-to-C coarticulation effects at the palatal zone for consonants differing in place and manner of articulation characteristics; and intraconsonantal variability across repetitions of a given VCV sequence. Results are evaluated in the light of the theory of speech production.
1. Introduction Electropalatography (EPG) provides a detailed representation of linguopalatal contact over time and is thus a useful analysis technique for the study of lingual coarticulation. In the Reading EPG system (on which the content of this article is based) each linguopalatal configuration contains 62 data points corresponding to the 62 electrodes embedded in the artificial palate; one temporal frame lasts 5 ms (see Fig. 1; also Hardcastle, Jones, Knight, Trudgeon & Calder, 1989). It should be noticed that the EPG data are arrangements of binary values (recording only the presence vs. absence of activation for each electrode) which renders their use for quantitative analysis quite difficult. In order to solve this problem, several EPG data reduction methods have been proposed in the literature. Some EPG data reduction methods concentrate on the temporal evolution of linguopalatal contact with reference to a selected number of frames which represent crucial articulatory events (Butcher, 1989; Engstrand, 1989; Hardcastle, Gibbon & Nicolaidis, 1991). Simple counts of contact frequency for each electrode over some time interval can be included here (Mitzutani, Hashimoto, Wakumoto, Hamada & Miura, 1988; Matsuno, 1989; Gibbon, 1990) . Nevertheless, there appears to be a clear bias towards spatial methods of characterization of EPG patterns. Based upon a single linguopalatal contact configuration (usually the EPG frame showing maximum contact), these methods account for contact patterns and coarticulation by means of some numerical reduction criteria. As an example, simple articulatory 0095-4470/94/020141 + 14 $08.00/0
© 1994 Academic Press Limited
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---cc--CA
Zones
Subzones
Rl
.---1---t--+-+-+-+-t--, H-t--t-+-+-+-+---JR2 Alveolar R3 1--1---t--+-+-+-+-t---1 R4 1---1---t--+-+-+-+-t---I RS R6
l--ll--l---t---t--+--+--+~ R7
CP
I _ _[___J'------J___l_____l__.L__.L___J
Front alveolar Postal veolar Prepalatal
Palatal
Mediopala tal
RS
Postpalatal
C!C2 C3C4C4C3C2 Cl (a)
(b)
(c)
Figure 1. (a) Distribution of rows Rl through RS along the anteriority (CA) and posteriority (CP) dimensions , and of columns Cl through C4 along the centrali ty (CC) dimension on both sides of the electropalate . (b) Articulatory zones and subzones on the electropalate . (c) Vocal tract representation with articulatory zones and subzones , and tongue regions: 1, alveola r; 2, prepalatal ; 3, mediopalatal ; 4, postpala tal; 5, tongue tip and to ngue blade ; 6, predorsum ; 7, mediodorsum ; 8, postdorsum .
parameters have been proposed for fricatives (i.e ., positiOn of maximum constriction , constriction width) and also measures of contact variation (i.e., front and back gradients; Engstrand , 1989; Hoole, Ziegler, Hartmann & Hardcastle , 1989). Other spatial methods rely on more complex numerical indices which allow for statistical analysis of the EPG data. Three indices are particularly relevant in this respect: (1) The center of gravity index (COG) (Mitzutani et al. , 1988) was designed to locate the highest concentration of activated electrodes on a given EPG frame. The coefficient numbers associated with the different rows are arbitrarily selected in order to give preferential weight to the anterior contact along the sagittal axis . Thus, coefficient values increase as rows become more anterior. COG = [0.5(R 8 ) + 1.5(R 7 ) + 2.5(R 6 ) + 3.5(R 5 ) + 4.5(R 4 ) + 6.5(R 2 ) + 7.5(R 1)/total number of contacts,
+ 5.5(R 3 )
where R; = number of " on" electrodes on row i. (2) Faber's anteriority index (Faber , 1989) gives distributional preference to anterior or posterior electrodes along the sagittal axis by means of a series of coefficient numbers calculated on a specific area of the Rion artificial palate (Shibata, Ino, Yamashita, Hiki, Kiritani & Sawashima, 1978). Positive values provide a measure of anterior contact whereas negative values indicate posterior contact. Anteriority = [36(R 1)
+ 6(R 2 ) + (R 3 )] - [(R 4 ) + 6(R 5 ) + 36(R 6 + R 7 )/2],
where R; = number of " on" electrodes on row i.
Contact indices for EPG data reduction
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(3) The coarticulation index (CI) (Farnetani , Hardcastle & Marchal, 1989) measures the difference in electrode activation for a phonetic segment (i.e., a consonant) when uttered in two different contextual environments (i.e. , two VCV sequences) . It is expressed in percentages of electrode contact and is usually calculated on a row-by-row basis . However, CI may also be presented as a mean value across the four front rows (anterior zone), across the four back rows (posterior zone) or across all rows (anterior and posterior zone) of the artificial palate.
Cltotal
=
(Clantcrior + Clposterior)/2,
Clanterior = (IR lA - R IBI + IR 2A - R2sl + IR3A- R3s l + IR4A- R4s l)/4 , Clpostcrior
= (IRsA -
Rssl + IR6A - R6sl + IR7A- R?s l + IRsA- Rss l) / 4,
where RiA= percentage(%) of "on" electrodes on row i for context A, and RiB = percentage( %) of "on" electrodes on row i for context B . In short, the center of gravity index and Faber's anteriority index have proved useful in giving a rough estimation of the distribution of linguopalatal contact. On the other hand, the coarticulation index is a measure of variation in degree of contact for the same consonant across different vowel contexts. This paper presents the contact index method, a new quantitative method of EPG data reduction which differs significantly from the three indices described above. According to this method, linguopalatal contact patterns are represented with reference to three index values, namely, contact anteriority (CA), contact posteriority (CP) and contact centrality (CC). The CA and CP index values vary along the sagittal axis of the palate, thus reflecting the degree of linguopalatal contact fronting and backing ; the CC index values vary along the coronal axis and represent the degree of linguopalatal contact from both sides of the palatal surface towards its median line regardless of whether contact distribution is at the front or at the back of the palate . This set of indices is capable of measuring both degree and distribution of linguopalatal contact. Consequently , two linguopalatal configurations showing the same number but a different distribution of "on" electrodes will be precisely distinguished by the CA, CP and/or CC index values. Since these indices give an accurate measure of contact distribution, they can be used for the characterization of variations in linguopalatal contact associated with different speech sounds (e.g., consonants of different place and manner of articulation) and contextual conditions (e.g., vowel-dependent effects on consonants).
2. The contact index defined In order to calculate the three contact indices , the electropalate is divided into eight rows (R) along the sagittal dimension and into eight columns (C) along the coronal dimension (see Fig. 1). Rows are numbered from 1 through 8 from front to back; columns are numbered from 1 through 4 from the periphery to the median line on both halves of the artificial palate. The value of the CA and CP indices is computed from a count of all " on" electrodes on a row-by-row basis; the CC index value is calculated from a count of all " on" electrodes on a column-by-column basis (see Fig. 1 for the row and column numbers).
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The three contact indices are in essence weighted sums of activated electrodes that take into account the spatial distributions of the rows or columns along the electropalate. In addition, they are constructed mathematically so that the contribution of a single activated electrode on a given row or column is always higher than the joint contribution of all the activated electrodes on previous back rows (CA index), front rows (CP index) or lateral columns (CC index). Each row or column is assigned a coefficient number which gives a characteristic weight to all normalized electrodes on that row or column. For example, the contact anteriority index (CA) over the entire palate might be calculated by means of the following mathematical expression: (initial formulation of) CA = [A(R 8 / Rs,) + B(R 7 / R7 ,) + C(R 6 / R 6,) + D(R 5 / R5 ,) + E(R4/ R4,) + F(R3/ R3,) + G(Rz/ Rz,) + H(R 1/ R 1,))/(A + B + C + D + E + F + G +H) , where RJ R;, is the normalized value for each activated electrode on row i:
_ (number of activated electrodes on row
~ I Ru-
(total number of electrodes on row i)
i)
.
This procedure normalizes each row's intrinsic contribution to the total number of electrodes available on that row. Thus the maximum value for any row should be 1 if all electrodes on that row are activated. The intent of this normalization is that two rows should not contribute differently to an index value due to their differing in absolute number of electrodes but only because of their relative location along the artificial palate. The eight coefficient numbers A-H are chosen so that the activation of all electrodes at and behind a given row i always yields a lower index value than the activation of any single electrode at more anterior rows, as follows: A
= 1 Row 8 should be given the lowest weight in the construction of the CA index
since it is the most posterior row on the artificial palate. A coefficient number of 1 is chosen arbitrarily for this row [i.e., 1(R 8 /8) ranges from 0 when no electrodes are active, to 1 when all eight are). B = 9 A single "on" electrode on row 7 should yield a higher index value than all possible activated electrodes on row 8. If B(1 "on" electrode/8 electrodes on row 7) > A(8 "on" electrodes/8 electrodes on row 8), that is, B /8 >A, then B =(SA)+ 1 = 9. A quantity of 1 is added to the equation in order to obtain coefficient B (i.e. , when R7 = 1, and R 8 = 8, B(R 7 /8) = 1.125 > A(R 8 /8) = 1). The general formula for the calculation of the remaining coefficients is as follows: C = 81 (total number of electrodes on row 6)(A +B)+ 1 D = 729 (total number of electrodes on row 5)(A + B +C)+ 1 E = 6561 (total number of electrodes on row 4)(A + B + C +D)+ 1 F = 59049 (total number of electrodes on row 3)(A + B + C + D +E)+ 1 G = 531441 (total number of electrodes on row 2)(A + B + C + D + E +F)+ 1 H = 3587227 (total number of electrodes on row 1)(A + B + C + D + E + F +G+)+l. .
Contact indices for EPG data reduction
145
In order to balance its highly exponential tendency, the initial mathematical ex pression of the CA index is submitted to a logarithmic transformation which allows for a linear increment of its values. Finally, the CA index is divided by its maximum possible value (the log of the sum of all coefficients) so that a range from 0 to 1, where 0 stands for no contact and 1 for complete contact, is always obtained: CA = [log[[1(R 8 /8) + 9(R 7 /8) + 81(R 6 /8) + 729(R 5 /8) + 6561(R 4 /8)
+ 59049(R 3 /8) + 531441(R 2 /8) + 3587227(R 1 /6)] + 1]]/[log(4185098 + 1)]. Similar reasoning is used to calculate the contact posteriority (CP) and contact centrality (CC) index coefficients: CP = [log[[1(R 1/6) + 9(R 2 /8) + 81(R 3 /8) + 729(R 4 /8) + 6561(R 5 /8)
+ 59049(R 6 /8) + 531441(R 7 /8) + 4782969(R 8 /8)] + 1]] /[log(5380840 + 1)], CC = [log[[1(C,/14) + 17(C2 /16) + 289(C 3 /16) + 4913(C 4 /16)] + 1]] /[log( 5220 + 1)]. The CP index is obtained in the same way as the CA index except that the rows are taken in reverse order in the calculation process. Also, since in the Reading artificial palate the first row has 6 electrodes while all the others have 8 electrodes, the CA coefficient H is smaller than the corresponding CP coefficient H. The normalization procedure of the CC index takes into account all activated electrodes on a given column as well as those on the corresponding symmetrical column on the other half of the artificial palate. This measure of contact centrality reflects the distribution of contact along the lateral-central dimension. In addition, more local contact indices can be formulated by taking a subset of rows or columns; e.g., the palatal contact anteriority index (CAp) involves the application of the contact index method to the four back rows or palatal zone: CAp= [log[[1(R 8 /8) + 9(R 7 /8) + 81R 6 /8) + 729(R 5 /8)] + 1]]/[log(820 + 1)]. The contact index method allows us to recover linguopalatal patterns quite accurately. It should be emphasized in this respect that a fixed range of index values will always be obtained provided that linguopalatal contact does not go beyond a certain row or column. Table I( a) and (b) shows the range of CA, CP and CC values for all rows and columns. For each interval in one cell of the table, the minimum value reflects the activation of one electrode on a single row or column across the entire palate; the maximum value results from all electrodes being activated on the referred row or column and on all previous ones (where "previous" means less anterior for CA , more anterior for CP, and more lateral for CC) . Therefore, CA, CP and CC index values within each interval indicate the same degree of anterior, posterior and central contact on the surface of the electropalate , respectively . Contact index values increase primarily as a function of the activation of successively more electrodes on a given row or column and, to a lesser extent , of variations in overall linguopalatal contact pattern . Thus, for example, a CA index value of 0.563 [see Fig. 2(a) , middle] indicates the following three facts : (1) Anteriormost contact occurs on row 4. We know this because 0.563 is within the range of CA index values for row 4 according to Table I.
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1. Fontdevila et al.
I. (a) and (b) Intervals of CA, CP and CC index values indicating the same degree of anterior, posterior and central contact, respectively (see text for explanation). (c) CA index values as a function of electrode activation on row 4 when no contact occurs on the other seven rows of the artificial palate (a)
TABLE
Row
CA
CP
1 2 3 4 5
[0.87237467-1.00000000] [0.72826728-0.87237465] [0.58416691-0. 72826715] [0.44012969-0.58416580] [0.29665732-0.44011970] [0.15801065-0.29656827] [0 .04943725-0.15726955] [0 .00772498-0.04546109]
[0.00994626-0.04472392] [0.04863560-0.15471934] [0.15544842-0.29175925] [0 .29184686-0.43298292] [0 .43299274-0.57469323] [0.57469433-0.71645790] [0.71645802-0.85822862] [0.85822863-1.00000000]
6 7
8
(b) Column
cc
1 2 3 4
[0 .66939-1.00000] [0 .34434-0.66937] [0.08457-0.34396] [0. 00806-0. 08097] (c)
Number of "on" electrodes
1
2
3
4
5
6
7
8
CA (row 4)
0.440
0.486
0.512
0.531
0.546
0.558
0.568
0.576
(2) There are six activated electrodes on row 4. We know the number is fewer than 7 because the minimum value for CA in that case is 0.568 as shown in the top right configuration. (3) At least part of the palatal surface behind row 4 is activated. We know this because 0.563 is higher than 0.558, the CA index value for a contact pattern showing six "on" electrodes on row 4 and no "on" electrodes behind that row [see Fig. 2(a) , left] . In short, a given CA index value provides information about three aspects of linguopalatal contact : row number of anteriormost contact, degree of electrode activation within the frontmost row, and general level of linguopalatal contact behind the frontmost row along the sagittal dimension. Notice, however, that the CA index itself does not capture the distribution of contact along the coronal dimension; thus, two linguopalatal configurations may have identical CA index values and, nevertheless, show different contact patterns from the sides towards the median line. In order to determine the amount and distribution of central contact, a CA or a CP index value should be associated with a CC index value . Figure 2(b) shows ideal linguopalatal configurations of a fricative consonant (left) and a lateral consonant (right) . Though both linguopalatal patterns yield the same CA index value (0.563), they differ with respect to the CC index
147
Contact indices for EPG data reduction I I I I I I
..,.
I I
,.,.I. • ,. ,.,.1
•
• ,. 1 • ,. 1 I
CA= 0·558
<
1.1. 1.1. 1. 1•
I I I
1.1 •
I
CA= 0·563
I
I
I
I
'
CA = 0·568
<
(a)
I
•1• •1 •I• •I • • I• •I• i I
.... ......
1• •1• •••I,
••• •• ••I ••I ••I
••I, . 1. 1
,
CA= 0·563 cc = 0-453
•I• , ,
CA= 0·563
cc =0·782 (b)
Figure 2. (a) Ideallinguopalatal configurations and corresponding CA index values. (b) Ideal linguopalatal configurations for a fricative consonant (left) and a lateral consonant (right) , and corresponding CA and CC index values.
value . Thus, the CC index value is higher for the lateral (0.782) than for the fricative (0.453) since there is midsagittal contact during the production of the former consonant but not of the latter. TheCA, CP, and CC indices are similar to the COG index in that they are more sensitive to changes in linguopalatal contact distribution than to variations in size of contact area. However, these contact indices characterize contact directionality more accurately than the COG index since they are capable of weighing the contribution of a single activated electrode regardless of its distance from the bul k of linguopalatal contact. In other words, although the numerical value of each of the three contact indices increases progressively and steadily with linguopalatal contact size, it is mostly an indicator of maximum contact fronting (CA), backing (CP) or centrality (CC). On the other hand, though contact indices and Faber's anteriority index operate on a similar mathematical principle, linguopalatal configurations can be more easily defined according to our method since it involves no subtraction between the anterior and posterior half of the palate. It should be noticed that if applied to the characterization of a variety of articulations this subtraction could ascribe the same numerical value to different EPG patterns. In order to illustrate the validity of the contact indices explained above we will report some V-to-C coarticulatory effects in Catalan. Since most Catalan consonants are produced either at the dentoalveolar or at the alveolopalatal zone we will analyze coarticulation at the palatal zone only . During the production of many of those consonants the tongue dorsum is quite free to coarticulate because it does not intervene directly in the formation of the primary place of articulation . For that purpose the artificial palate is subdivided into an alveolar zone (the four front rows) and a palatal zone (the four back rows) (see Fig. 1). There is a one-to-one correspondence between each of these two articulatory zones and the tongue articulators: alveolar consonants are articulated at the alveolar zone with the
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tongue tip and/or the tongue blade , and palatal sounds are produced at the palatal zone with the tongue dorsum . 3. Illustrative EPG data analysis 3.1. Intrasegmental variability Figure 3(a) plots EPG linguopalatal contact configurations for [n] as a function of adjacent [i], [u] and [a] in symmetrical VCV sequences with stress on the first syllable (one of these sequences is transcribed as [an~] since unstressed I a/ is realized as [~] in Catalan). These linguopalatal contact patterns were measured at PMC or point of maximum contact, i.e., the temporal frame showing the maximum number of "on" electrodes along each VCV sequence. They represent averages across five repetitions produced by one Catalan speaker (DR, the third co-author of this paper). Different shades in the EPG patterns indicate that electrode activation occurs in 80-100% (black), 40-80% (grey) or fewer than 40% (white) of the repetitions. As in Fig. 1, the horizontal line crossing the EPG graphs separates the alveolar zone from the palatal zone. The numerical values of the three contact indices were calculated at the palatal zone (four posterior rows) for each VCV sequence . This articulatory zone has been chosen for contact analysis since it is particularly sensitive to vowel-dependent
.....
[n]
•••••• •••••••• ,,. •• •• • •• ••
.. " ··· ... ••
!;; .
~I'i
••
••
.,;
"'•••••••• ••••••I'" ••• • ••• • • • ·••·=·' "'•••
•••••••• • ••••••• ••• •• I"' • •• • • •
[u]
[a]
[i]
(a)
CPp
6
08
i/u
c~ " CAp
.
CPp
.
CCp
.
i/a
u/a
0·6 CCp
0·4 0·2 -
. .
OL---~--~--~------~~~
[i]
[u]
[a]
(b)
Figure 3. (a) Average linguopalatal configurations at PMC for [n] with adjacent [i], [u] and [a] (speaker DR ; five repetitions) . Different shades in the EPG patterns indicate that electrode activation is 80-HJO% (black), 40-80% (grey) or less than 40% (white) . (b) Graphical representation of the mean contact index values across five repetitions (left) , with significant V-to-C effects listed to the right (Scheffe, p < 0.05).
Contact indices for EPG data reduction
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coarticulatory effects during the production of [n]. The labels CAp, CPp, and CCp stand for palatal contact anteriority , palatal contact posteriority and palatal contact centrality, respectively. Contact index values are represented graphically in the lower graph of the figure . Significant vowel-dependent effects (Scheffe, p < 0.05) for the three contact indices are shown in the table at the right of the graph. The linguopalatal patterns (palatal zone only) are characterized by the contact index values as follows: (1) The palatal contact anteriority index value (CAp) for [ini] is significantly higher than that for [unu] and [an;:J]. Indeed, as the EPG patterns at the top of the figure show, the front vowel [i] causes more electrode activation at the front palatal zone than do the back vowels [a] and [u]. No significant difference was found for [unu] vs. [an;:J], which also accords with what the configurations above show about contact patterns at the front palatal zone in the two sequences. (2) The palatal contact posteriority index value (CPp) shows significant differences between all pairs of the three VCV sequences (i .e., [ini], [unu], [an;:J]). As shown in the lower graph of the figure, index values are highest for [ini] and lowest for [an;:J], those for [unu] falling in between. This finding is consistent with the presence of different degrees of contact at the back palatal zone in the progression [ini] > [unu] > [an;:J]. (3) Palatal contact centrality (CCp) also shows significant differences among [ini] (highest value) , [an;:J] (lowest value) and (unu) (intermediate value). Consistently with these index values, the linguopalatal contact area decreases towards the central zone in the progression [ini] > [unu] > [an;:J]. As shown in the lower graph, there is a steeper increase in CCp index values than in CAp and CPp index values across vowel contexts. This is due to the fact that, while an increase in CCp index value is conveyed by the filling of new columns , an increase in CAp and CPp index values results from the activation of new electrodes within the same row. In conclusion , contact index values at the palatal zone indicate the presence of more linguopalatal contact for [n] in the sequence [ini] than in the sequences [unu] and [an;:J] along the three dimensions (i.e., anteriority, posteriority and centrality). The consonant shows more contact posteriority and contact centrality in the sequence [unu] than in the sequence [an;:J] . This general description accords with the observation that alveolars are highly sensitive to coarticulatory effects at the palatal zone since the tongue dorsum does not intervene in the formation of the alveolar place of articulation. Moreover, coarticulatory trends are consistent with the articulatory specification for the adjacent vowels; indeed, they occur in tongue dorsum raising and fronting for front vs. back vowels and in tongue dorsum raising but not in tongue dorsum fronting as a function of back vowels. 3.2. Inter-segmental variability Mean values for CAp, CPp and CCp were also calculated across five repetitions of VCV sequences with the alveolar fricative [s] and the alveolopalatal nasal stop [Jl] in productions by the same Catalan speaker. In order to compare [n], [s] and [p], we excluded the fifth row of electrodes from the index calculations for [Jl], since this row is involved in the place of articulation of this consonant but not of [n] and [s].
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08 [n]
~ CPp CAp ~
0·2 0
i/a
u/a
CAp
0·6 0-4
i/u
"
~ [i] Ci
[u]
.
CPp CCp
.
.
[a] i/u
==:::::::::::::!====-il
0·8 CAp
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i/a
u/a
.
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~
CPp CCp
[i]
[u]
.
[a] i/u
0·8 CAp
0·6 [Jl] CPp
0-4 0·2 0
CCp
[i]
[u]
[a]
Figure 4. Mean contact index values at the palatal zone measured at PMC for [n], [s] and [Jl] as a function of vowel context (speaker DR; five repetitions). Significant V-to-C effects are given in the table at the right of each graph (Scheffe , p < 0.05) .
Indeed , alveolopalatal [Jl] is usually articulated at the back alveolar zone and at the front prepalatal zone with the back of the blade and the front predorsum. The three graphs in Fig . 4 report the numerical index values for the three consonants, i.e ., [n] (top) , [s] (middle) and [Jl] (bottom), in the symmetrical sequences [iCi], [uCu] and [aC~]. As in Fig. 3, the amount of V-to-C coarticulation in the graphs is directly related to the steepness of the slope of the lines connecting the context vowels. A joint consideration of all three contact index values reveals differences in degree of V-to-C coarticulatory sensitivity among the consonants in the order [n] > [s] > [Jl]. Less tongue dorsum coarticulation for [s] than for [n] is presumably due to a high degree of articulatory control on lingual activity for the former consonant vs. the latter (Wolf, Fletcher, McCutcheon & Hasegawa , 1976; McCutcheon , Hasegawa & Fletcher, 1980). Alveolopalatal [Jl] allows no significant vowel-dependent effects in tongue dorsum contact, thus suggesting that this articulator is highly constrained; higher articulatory requirements for alveolopalatals than for alveolars are presumably associated with more tongue dorsum raising for the former consonants than for the latter (Recasens, 1983). Table II shows significant V-to-C effects in mean contact index values across five repetitions for a larger subset of Catalan consonants (again in symmetrical
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151
TA BLE II. Statistical results for some Catalan consonants (speaker DR). Asterisks indicate significant vowel-dependent effects (Scheffe , p < 0.05) in contact index values at the palatal zone
CAp
CPp
CCp '\
i/a
i/u
u/a
Apicodental oral stop , [t] Apicoalveolar nasal stop , [n]
*
*
i/a
i/u
u/a
i/a
i/u
*
*
*
*
*
*
*
*
*
*
u/a
*
Velarized apicoalveolar lateral,
[I] Apico- or laminoalveolar fricative , [s]
*
* *
*
*
*
*
*
*
*
*
Lamino-postalveolar fricative,
[f)
*
*
Alveolopalatal affricate (stop component only), [tf]
*
*
Alveolopalatallateral, [,.<;]
*
*
*
Alveolopalatal nasal stop, [.n]
sequences [iCi], [uCu] and [aC~]; speaker DR). For each pair of context vowels , asterisks indicate those differences which were found to be statistically significant (Scheffe, p < 0.05). In the table, the consonants are ordered from frontmost to backmost place of articulation from the apicodental oral stop [t] to the alveolopalatal nasal stop [Jl]. The fifth row of electrodes was excluded from the calculation of the contact indices in the sequence [u..\u] for the same reason as for [Jl] above . Data on significant effects in the table reveal three interesting trends: First, CPp shows more effects than CAp for all consonants except for [1]. The tongue dorsum front (i.e., predorsum) is less free to coarticulate than the tongue dorsum back (i.e., postdorsum) because the primary place of articulation for all these consonants is closer to the prepalate than to the postpalate. Second, an overall account of significant effects for all contact indices reveals that dentoalveolars coarticulate more freely than alveolopalatals. This is because the tongue dorsum front is involved in the formation of the place of articulation for the latter but not for the former. Larger coupling effects between front dorsum and back dorsum for alveolopalatals vs. dentoalveolars cause more tongue dorsum raising and less coarticulation over the entire palatal zone. Third, generally speaking, vowel effects occur for [i] > [u] > [a] in the case of dentoalveolars and for [i] = [u] >[a] in the case of alveolopalatals . Therefore, coarticulation in linguopalatal fronting among high vowels is found during the production of the former consonants but not of the latter. Both consonantal categories show effects related to differences in vowel height. Significant vowel-dependent effects reported in Table II suggest that the contact
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III. Coefficient of variation values across five repetitions of each VCV sequence as a function of consonant, vowel context and contact index at the palatal zone (speaker DR). No tongue contact was detected at the palatal zone for the sequence [a!;:,]
TABLE
iCi
[t l [n]
[I] [s] [f] [tf]
[.-\] [.n l
aC;:,
uCu
CAp
CPp
CCp
CAp
CPp
CCp
CAp
CPp
CCp
2.90 2.29 25.50 0.00 0.00 2.76 1.03 1.50
2.42 1.54 55.53 0.00 1.86 1.07 1.54 2.18
11.80 7.00 73.49 0.00 3.29 11.93 8.07 4.49
4.67 3.67 7.56 3.14 2.15 1.03 1.82 1.82
0.00 0.00 135.70 0.00 0.00 0.00 3.63 1.05
9.58 12.77 89.47 7.65 3.59 8.41 16.15 8.27
5.42 0.00
0.00 0.00
25.68 0.00
2.58 1.53 1.46 1.03 1.82
2.33 0.00 2.52 4.35 2.17
17.84 10.86 17.20 9.03 4.45
index method described in this paper is quite successful in gauging coarticulation . The following trends are consistent with phonetic theory: less coarticulatory sensitivity for dentoalveolars and alveolopalatals at the front palatal zone than at the back palatal zone; less tongue dorsum coarticulation for alveolopalatals than for dentoalveolars; tongue dorsum coarticulatory effects in vowel fronting and height for dentoalveolars but only in vowel height for alveolopalatals. In addition, coefficients of variation (CV) for the. three indices at the palatal zone were calculated across the five repetitions of each VCV sequence [CV = (sIX) x 100]. Table III shows these variation coefficient values displayed as a function of consonant, vowel context and contact index. According to the table, variation coefficients are quite high for [!] in the sequences [iii] and [ulu] since considerable contact variability occurs at the sides of the palatal zone in this case. Variation coefficient values for all consonants are usually higher in the case of the CCp index than of the CAp and CPp indices. Indeed , electrode activation across repetitions of a particular VCV sequence entails contacting new columns rather than more anterior and more posterior rows , thus causing important CCp index value fluctuations; moreover, while lateral contact at the palatal zone remains highly constant during the production of lingual consonants , central contact may reflect changes in tongue dorsum height. The table also shows a trend towards greater variability in CAp index values for dentoalveolars than for alveolopalatals, mostly when adjacent to back vowels. This finding is consistent with the presence of larger V-to-C effects at the prepalate for dentoalveolars than for alveolopalatals (see Table II) which is in accordance with differences in prepalatal contact size between the two consonantal categories. Manner requirements also affect the amount of variability, as shown by lower variation coefficient values for the alveolar fricative [s] than for dentoalveolar stops .
4. Conclusions This paper shows that a new method of EPG data reduction, i.e., the contact index method, can be used quite successfully for capturing linguopalatal contact configurations . This method involves the calculation of three indices, i.e., contact anteriority (CA), posteriority (CP) and centrality (CC), which reflect the distribution of tongue
Contact indices for EPG data reduction
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contact over the palatal surface. Index values are sensitive to the number of frontmost (CA), backmost (CP) and centralmost (CC) activated electrodes as determined by the weight of row- and column-specific coefficients. The contact index method of EPG data reduction can be applied to the measurement of contact characteristics at the alveolar and at the palatal zone independently. The validity of this method has been illustrated in this paper for V-to-C effects in tongue dorsum contact at the palatal zone during the production of a consonant (i .e., [n]) which is highly sensitive to dorsopalatal coarticulation. Contact index values allow us to capture differences in contact location within the palatal zone; changes in contact centrality as a function of variations in contact fronting and backing can also be evaluated. In addition to its use in studying context-dependent contact variations for the same consonant, the contact index method has been used to measure differences in coarticulatory sensitivity between dentoalveolar and alveolopalatal consonants at the palatal zone. Results conform to previous findings in the literature in showing larger vowel-dependent coarticulatory effects for dentoalveolars than for alveolopalatals. Contact index values reveal that effects at the back palatal zone are larger than effects at the front palatal zone , mostly for alveolopalatals which allow no prepalatal effects at all. It can be claimed that during the production of these consonants the predorsum is less free to coarticulate than the medio-postdorsum because it is coupled with the primary articulator, i.e., the tongue front. Also, while dentoalveolars are sensitive to effects in vowel fronting and vowel height , alveolopalatals are sensitive to effects in vowel height only . While alveolopalatals involve less dorsal contact as a function of low vs high vowels, differences in vowel fronting among high vowels cause no adaptation problems since the production of alveolopalatals also requires a good deal of tongue dorsum raising. This work was supported by project ESPRIT-ACCOR BRA 3729 , project ESPRIT-SPEECH MAPS BRA 6975, and ESPRIT-Working Group 7098.
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Mitzutani, T., Hashimoto, K., Wakumoto , M ., Hamada , H. & Miura, T. (1988) Analysis of tongue motion for the dental consonants based on the high-speed palatographic data, Proceedings of the Joint Meeting of the Acoustical Society of Japan, Hawaii , 1-5. Recasens , D. (1983) Coarticulation in Catalan VCV sequences: an articulatory and acoustical study, PhD Dissertation , The University of Connecticut. Shibata, S., Ino, A., Yamashita , S. , Hiki, S. , Kiritani , S. & Sawashima, M. (1978) A new portable type unit for electropalatography , Annual Bulletin of the Research Institute of Logopedics and Phoniatrics, University of Tokyo, 12, 5-10. Wolf, M., Fletcher, S., McCutcheon , M. & Hasegawa , A. (1976) Medial groove width during /s/ sound production , Biocommunication Research Reports, University of Alabama, Birmingham, 1, 57-66.