Characterizing American Sign Language Fluency Using The Spatiotemporal Index
Presentation Languages: American Sign Language and English
Relatively little is known about the incidence of communication disorders in sign language users
(Quinto-Pozos et al., 2011). Although there are anecdotal reports of production fluency disorders
(Cosyns et al., 2009; Montgomery and Finch, 1988), as yet no one has provided a kinematic
characterization of what it means to be fluent in a sign language (although, see Mirus et al.
(2001)). In contrast, fluency has been characterized with more precision for speech and
particularly for the disorder of stuttering (Starkweather, 1987). The central and defining
symptom of stuttering is disfluency that perturbs the forward flow of speech (Bloodstein and
Bernstein-Ratner, 2008). Stuttering has a biological basis (Chang et al., 2009; Choo et al., 2011)
stemming from a genetic predisposition (Fedyna et al., 2011). As part of a longer-term approach
to characterizing fluency disorders in sign language, the reported study used techniques from the
study of speech movement variability in spoken language. The aim was to assess the kinematics
of American Sign Language (ASL) production in 4 fluent Deaf native signers and 12 hearing
college students studying ASL as a second language at three different proficiency levels of an
American university curriculum. The dependent measure, called the spatiotemporal index (STI),
characterizes the kinematic variability of fluent utterances (Smith et al., 1995). Higher STI
values indicate higher variability and suggest that whole-utterance kinematics are less stable
(Kleinow and Smith, 2000). The fluent Deaf native signers provided a benchmark for fluency,
and an estimate of the degree of variability to be explained within fluent production. The hearing
sign language learners were chosen because they knew enough ASL to be able to produce the
required sentences, and they provided a measure of fluency in the absence of communication
disorder stemming from a lack of familiarity and practice with the language. Hearing learners
ranged from those with a few weeks' experience of ASL, to those who had received 30-40 weeks
of instruction. Participants were required to produce target signs in carrier phrases (using the
stimuli reported by Emmorey et al., 2009) to elicit utterances with greater ecological validity.
Three-dimensional limb kinematics were obtained by placing markers on a participant's
shoulder, elbow, wrist and hand, and capturing the movements using infrared cameras (Motion
Analysis Corporation, Santa Rosa, CA). The start and end points of individual signs were
operationally identified using kinematic data and inter-utterance variability was then determined
following the established method of Smith et al. (1995). Figure 1 shows kinematic traces from
repeated utterances of a single target sign for a Deaf native signer and a hearing learner, along
with corresponding STI values. This novel approach to characterizing fluency in the context of
ASL utterances has relevance for the identification of potential ASL production disorders as well
as for assessing L2 acquisition in the context of teaching sign languages.
References
Bloodstein, O. and Bernstein-Ratner, N. (2008) A handbook on stuttering. Independence, KY: DelmarCengage.
Chang, S.E., Kenney, M.K., Loucks, T.M., and Ludlow, C.L. (2009). Brain activation abnormalities
during speech and non-speech in stuttering speakers. Neuroimage, 15, 201-212.
Choo, A.L., Kraft, S.J., Olivero, W., Ambrose, N.G., Sharma, H., Chang, S.E., and Loucks, T.M. (2011).
Corpus callosum differences associated with persistent stuttering in adults. Journal of
Communication Disorders, 44, 470-477.
Cosyns, M., Herreweghe, A.V., Christiaens, G., and Borsel, J.V. (2009). Stutter-like dysfluencies in
Flemish Sign Language Users. Clinical Linguistics & Phonetics, 23, 742-750.
Emmorey, K., Gertsberg, N., Korpics, F., and Wright, C.E. (2009). The influence of visual feedback and
register changes on sign language production: A kinematic study with deaf signers. Applied
Psycholinguistics, 30, 187-203.
Fedyna, A., Drayna, D., and Kang, C. (2011). Characterization of a mutation commonly associated with
persistent stuttering: evidence for a founder mutation. Journal of Human Genetics, 56, 80-82.
Kleinow, J., and Smith, A. (2000). Influences of length and syntactic complexity on the speech motor
stability of the fluent speech of adults who stutter. Journal of Speech, Language, and Hearing
Research, 43, 548-559.
Mirus, G., Rathmann, C., and Meier, R.P. (2001). Proximalization and distalization of sign movement in
adult learners. In V. Dively, M. Metzger, S. Traub, and A.M. Baer (Eds.), Signed languages:
discoveries from international research. Washington, DC: Gallaudet University Press.
Montgomery, B., and Finch, J. (1988). The prevalence of stuttering in the hearing-impaired school age
population. Journal of Speech and Hearing Disorders, 53, 131-135.
Quinto-Pozos, D., Forber-Pratt, A.J., and Singleton, J.L. (2011). Do developmental communication
disorders exist in the signed modality? Perspectives from professionals. Language, Speech, and
Hearing Services in Schools, 42, 423-443.
Smith, A., Goffman, L., Zelaznik, H.N., Ying, G., and McGillem, C. (1995). Spatiotemporal stability and
patterning of speech movement sequences. Experimental Brain Research, 104, 493-501.
Starkweather, W.C. (1987). Fluency and stuttering. Englewood Cliffs, NJ: Prentice-Hall.
Figure 1: Wrist traces (in x-dimension) for multiple productions of the target sign STRAIGHT produced
by a Deaf native signer (A) and a hearing learner of ASL (B). The corresponding STI values (after outlier
removal) indicate the variability of the kinematics, with high values indicating less stability in the
underlying kinematics.