Peripheral motor commands in labial articulation - KTH Speech

55 Pages·2007·3.73 MB·English

by  Öhman, S.

Dept. for Speech, Music and Hearing
Quarterly Progress and
Status Report
Peripheral motor commands
in labial articulation
¨
Ohman, S.
journal: STL-QPSR
volume: 8
number: 4
year: 1967
pages: 030-063
http://www.speech.kth.se/qpsr

STL-QPSR 4/1967
B. PERIPHERAL MOTOR COMMANDS IN LABIAL ARTICULATION
S. t)hman++
Almost all modern electromyographic (EMG) (I) studies of speech
production (2) have attempted to establish or to refute some simple in-
variance concept for phonemes or rather, major allophones, at the
1
peripheral neural level.
I
However, linguistic units such as allophones are those aspects of
the human speech communication process that remain invariant under
an introspective analysis performed by the linguist (3). These allo-
phones represent constancies of the linguist' s subjective impressions
when he makes systematic observations of what he does in speaking or
of what he hears in understanding spoken utterances. I
On the other hand, the picture of motor control that is emerging
from recent work in neurophysiology (4) proves the subjective unity
and simplicity of even the most elementary movements to be quite illu-
sory when discussed in physiological terms. Motor control involves
complex arrangements of neural networks operating on diffuse as well
as highly specific forms of sensory feed-back at many levels along
several parallel channels. It is therefore unlikely that direct one-to-
one correspondences between linguistic units and peripheral signals
will be found.
The assumption is sometimes made, implicity or explicity, that
closer physiological correlates of phonological units are ultimately to
be found at some higher neuro-motor level; in particular, that these
units are nothing but abstract labels of neural input signals to the
speech production mechanisms.( 5)
This is a very strong assumption that has nevertheless gained some
support from model experiments carried out during the last few years
of research (6). Yet, the models developed in these experiments have
only a gross functional relationship to the microscopic neurophysio-
logical processes of speech production and the refore do not constitute
conclusive evidence. I
To arrive at a deeper understanding of speech production it seems
necessary first of all to establish physiological factors of a universal
5 ' This work was carried out in collaboration with Doc. A. Persson,
Dept. of Clinical Neurophysiology, Karolinska Sjukhuset, Stockholm,
and Dr. R. Leandersson, Phoniatric Clinic, Karolinska Sjukhuset,
Stockholm.
,
STL-QPSR 4/1967 31.
I
nature that limit the articulatory possibilities of human speech, and,
in so far as possible, to show how these factors constrain the phonetic
repertoires of various languages. With this sort of knowledge to start
from, it may perhaps be possible in the future to approach the higher
neural organization of speech with more powerful methods. I
The purpose of the present paper is therefore to describe and to
analyze electromyographic data that appear to have some relevance in
I
the direction indicated in the last paragraph above.
1
The data to be reported here relate to the motor unit activity of
the facial muscles during speech. The movements of the lips play an
important r ale in speech production and exhibit all features of coarticu-
lation, undershoot, and reorganization that are known to characterize
speech in general. Since the facial muscles are moreover easily ac-
cessible to electromyographic observation, they were selected as a
basis for the studies to which we now turn.
Methods and procedures
The data to be presented below have been collected by examining
the speech activity of the facial muscles of a male Swedish speaker.
More than half a dozen preliminary experiments have been done in or-
der to gain experience with electrode placement, choice of suitable
.
1
(7
speech materials, and data processing
Electrode positions at which signals from individual facial mus-
cles can be isolated have been localized by means of systematic rc-
I
cording of EMG activity at closely adjacent points over most of the
,
facial area. The electrical activity was recorded with thin concentric
needle electrodes (DISA 13K59, diameter 0.30 and 0.45 mm) and after
amplification (Grass preamplifier P9) fed into the vertical beam of an
oscilloscope for monitoring purposes and into a tape recorder for later
data analysis. The sound pressure wave was picked up by a micro-
phone (Sennheiser MD 421/2) placed in front of the subject and record-
ed on another channel of the tape. In the earliest phases of our work
we used a Revox G36 two-channel tape recorder. Later on, a 14-
channel Ampex FR1300 FM tape recorder was acquired by means of
which signals from many muscles could be registered simultaneously.
STL-QPSR 4/1967 32.
A recording session would typically proceed as follows. The sub-
ject is seated comfortably in a resting chair with a list of nonsense ut-
terances in front of him. An electrode is then inserted at one of the
preselected electrode positions whereupon the subject reads through
the list. The electrode is then adjusted in the in/out direction and the
list is read again. This process is repeated until an electrode depth
is found that gives a maximal activity response for those speech sounds
that are assumed to employ muscles in the neighborhood of the elec-
trode position being tested. When all electrodes (in general no more
than eight) have been inserted in the manner just described, the EMG
and the acoustic signals are recorded on the magnetic tape while the
1
subject reads the list five times.
The recorded data are then processed in various ways. In general,
we first obtain oscillographic traces of the raw EMG signals by means
of a Mingograph ink writer. When more-quantitative estimates of the
amount of activity are desired we also rectify and smooth (50 msec 1
time constant) the signals and then take averages (8) over the five
readings of the same utterance. By analyzing the many records pro-
duced in this way it has been possible to determine whether an electrode
has picked up activity from a single muscle only or from more than
one. This information has then been used (and checked) in later re-
cording sessions when the electrodes were to be inserted into the mus-
cles. By connecting the EMG preamplifiers and the microphone am-
plifier directly to the Mingograph and having the subject speak certain
test utterances, we can check whether the expected EMG pattern ob-
tains, and, if not, adjust the electrode until it does. i
-
In Fig. I-B-1 the best electrode positions according to our ex-
-
perience for the observation of the facial muscles has been indicated
I
by filled circles.
1
The speech material that we have studied consists of vowel-con-
sonant-vowel (VCV) or consonant-vowel-consonant (cvC) combinations
embedded in a fixed linguistic frame. This choice was motivated by
our interest in the mutual interaction of vowels and consonants. Lin-
guistic frames involving a uniform labial activity have turned out to be
very useful in that they provide a roughly constant EMG background a-
gainst which the patterns of the test words may be contrasted.
The
I
I
ELECTRODE POSITIONS
Fig. I-B-1.
STL-QPSR 4/1967 i 33.
I
(9)
frame most commonly used was
which has approximately uniform labial spreading (except, of course,
for the embedded VCV component).
Results
A considerable amount of electromyographic material has come
out of the several recording sessions referred to above. Two subsets
of the data that seem to be of particular interest will be discussed in
-
some detail here. These two sets are illustrated in Figs. I-B-2
I-B-6 (lo) and Fig. I-B-16, respectively. Important details of these
figures are shown enlarged in other figures of the paper.
i
-
Figs. I-B-2 I-B-6 will be examined first. Each of these dis-
plays has been divided up into little squares, that represent individual
utterances. Seven synchronous records are shown for each utterance,
namely, the acoustic sound trace (top) and six EMG traces represent-
ing muscles indicated in the figures (and, by the filled circles in Fig.
I-B-1). I
-
The VCV parts of the utterances in any column of Figs. I-B-2 I
I-B-6 have the same initial vowel and those of any row have the same
final vowel. The vowels are Swedish [i], [y], and [u] which have ap-
cy]
proximately the same tongue articulation but differ labially in that
and [u] are rounded versions of [i]. The vowel [y] is more protruded
than [H] and the latter has a much smaller mouth orifice area than the
former. Moreover, [w] is characterized by a slight labial diphthongi-
zation, being more rounded at the end than at the beginning (11)
-
Only one medial consonant occurs in each of Figs. I-B-2 I-B-6.
The consonants are [h], [p], [b], [f], and [v].
General observations
I
-
In many of the records shown in Figs. I-B-2 I-B-6 electromyo-
graphic activity is visible prior to and after the interval during which
1
the actual utterances occur. This is also illustrated in Fig. I-B-7
where a tonic (I2) EMG activity can be seen to precede by several hund-
red milliseconds the activity due to the articulatory movements of the

Fig. I-B-3.
Fig. I-B-4.