SPATIAL PROPERTIES OF SPEECH MOVEMENTS

If there is one characteristic of speech that has plagued speech production theorists for years, it is variability. Acoustic correlates of a given phoneme and by inference vocal tract configurations exhibit variability arising from a number of sources. The present experiment was designed to examine the degree of spatial variation among a limited set of phonetic segments. Results suggest that variability in vocal tract positioning may be sound dependent reflecting different degrees of perception/production stability. INTRODUCTION An issue of theoretical importance in speech production is to determine the precision at which articulatory actions are being controlled. One characteristic of speech, however, that has plagued the development of a realistic understanding of control precision is variability. Variation may arise from many sources and it’s understanding is crucial to the development of a realistic perspective on speech motor control. A general perspective can be obtained from consideration of the structure and function of the human nervous system as an'information processing device. As pornted out by von Nuemann [l] the nervous system is an analog device that IS ideally suited for reliable operation not precrston. In this context it can be suggested that articulatory performance ts good enough without incurring excessive “costs” [2] with the degree of prectsion inherently dependent on the listener's ability to extract meaning from the speech code. Alternatively, variation may be related to certain articulatory/acoustic relations such as those reflected in Stevens quanta] theory [3,4] which implicitly assumes that certain consonants should exhibit more or less articulatory variability as a function of the proximity of so-called primary articulators to sensitive regions of the vocal tract in which small changes in articulation have large acoustic consequences. Evaluation of spatial precision assumes that speech movements may in fact have spatial targets associated with them. The concept of spatial targets for speech was suggested by Lashley [5] in discussing space coordinate systems for controlling serial movements such as those for speech. In spite of the intuitiveness of speech motion planning in a spatial reference frame, the notion of spatial targets for speech have received little attention. One reason for the limited attention, again, appears to be related to the presence of variability in the observable signal in which variable vocal tract shapes yield acceptable acoustic signals [6] However, as noted above, this limitation may be related more to an unrealistic expectation regarding the precision of the articulatory target for speech. A recent perspective has been offered by Guenther [7] in which spatial targets for speech are viewed as regions rather than points (convex hulls) in orosensory space and conceptually is more attractive than target points. While the available data is limited it is difficult to imagine that speech is not planned to some extent in a spatial coordinate frame since inappropriately placed articulators will produce seriously compromised sounds. The purpose of the present experiment was to examine the spatial variation of a few of the phonetic segments of the language to determine how speech movement control varies as a function of phonetic identity. PROCEDURE The experimental group consisted of four subjects (two males, two females). Movements of the lips, jaw, and four points on the surface of the tongue were obtained using an electromagnetic transduction device [8]. The tongue receivers were placed approximately 1 cm behind the tongue tip and spaced approximately 1 cm apart. Data were hardware low pass filtered (200 Hz) and sampled at 625 Hz (12 bit resolution). Following the digitization, the voltages were digitally smoothed (25 points with a 3 dB pom! at 18 Hz) and the voltages were conICPhS 95 Stockholm verted to positions in the midsagittal plane of the device. All data were rotated to the subjects' occlusal plane. Subiects repeated a number of words embedded in the carrier phrase “Say again. " ten times. In order to examine the spatial variation associated with different phonetic segments. the two dimensional positions of the four tongue receivers were obtained at the time of zero (or minimum) speed associated with the target acoustic segment [9]. RESULTS Shown in Figure l is the average tongue shape estimated from a cubic spline interpolation of the four average receiver locations for one subject for the three phones N, /r/. and /ae/ along With one standard deviation bars. For these comparisons the variability reflects the variation associated with repeating each word ten times. The words represented are Isl-“sack". /r/—“rack". and Mel—“had". In considering the spatial variation there are two points of note: the degree of variation is quite different for the three segmea and the different tongue regions display different degrees of vanabihty.