Second language acquisition across modalities: Production variability in adult L2 learners of American Sign Language

A study was conducted to examine production variability in American Sign Language (ASL) in order to gain insight into the development of motor control in a language produced in another modality. Production variability was characterized through the spatiotemporal index (STI), which represents production stability in whole utterances and is a function of variability in effector displacement waveforms (Smith et al., 1995). Motion capture apparatus was used to acquire wrist displacement data across a set of eight target signs embedded in carrier phrases. The STI values of Deaf signers and hearing learners at three different ASL experience levels were compared to determine whether production stability varied as a function of time spent acquiring ASL. We hypothesized that lower production stability as indexed by the STI would be evident for beginning ASL learners, indicating greater production variability, with variability decreasing as ASL language experience increased. As predicted, Deaf signers showed significantly lower STI values than the hearing learners, suggesting that stability of production is indeed characteristic of increased ASL use. The linear trend across experience levels of hearing learners was not statistically significant in all spatial dimensions, indicating that improvement in production stability across relatively short time scales was weak. This novel approach to characterizing production stability in ASL utterances has relevance for the identification of sign production disorders and for assessing L2 acquisition of sign languages.