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Purpose We investigated short-term practice and retention of nonwords in 10 adults who stutter (Mean age = 30.7 years, SD = 15.1) and age and sex-matched 10 control participants (Mean age = 30.8 years, SD = 14.9). Methods Participants were required to repeat nonwords varying in length (3, 4, and 6 syllables), phonotactic constraint (PC vs. NPC, on 3-syllable nonwords only), and complexity (simple, complex). They were tested twice with 1 h gap between sessions. Results Logistic mixed model of speech accuracy revealed that the AWS showed a significantly lower probability of correct responses with increasing length and complexity. Analysis of speech kinematics revealed practice effects within Session 1 in AWS seen as a reduction in movement variability for the 3-syllable nonwords; the control group was performing at ceiling at this length. For the 4-syllable nonwords, the control group showed a significant reduction in movement variability with practice, and retained this reduction in Session 2, while the AWS group did not show practice or retention. Group differences were not evident at the 6-syllable level. Conclusions Group differences in speech accuracy suggest differences in phonemic encoding and/or speech motor processes. Group differences in changes in movement variability within and between sessions suggest reduced practice and retention in AWS. Relevance of the combined use of both behavioral and kinematic measures to interpret the nature of the skill acquisition deficit in persons who stutter is discussed. Educational objectives: At the end of this activity the reader will be able to: (a) summarize the process of skill acquisition; (b) discuss the literature on skill acquisition deficits in adults who stutter, (c) summarize the differences between AWS and control participants in speech accuracy and speech kinematics with short-term practice and retention of nonwords, (d) discuss potential research directions in the area of skill acquisition in AWS.

Stuttering is a developmental speech disorder characterized by the presence of repetition, prolongations, and blocks in speech. Several theories have been proposed to account for stuttering as a deficit in sensorimotor processes (Kalvaram, 2001, Max et al., 2004, Neilson and Neilson, 1991 and Smith and Kelly, 1997). The investigation of such processes has been undertaken using both speech and non-speech tasks and the findings allude to a deficit in skill acquisition (Bauerly and De Nil, 2011, Namasivayam and Van Lieshout, 2008, Smith et al., 2010 and Smits-Bandstra et al., 2006), although some mixed findings have been reported. The aim of the present study is to investigate skill acquisition abilities in the short-term in adults who stutter (AWS) using nonword repetition, which requires the encoding, assembly, and execution of previously known or unknown speech sound combinations and the constituent motor gestures. 1. Motor skill acquisition: theory Motor skill acquisition refers to the process by which movements produced alone or in a sequence come to be performed effortlessly through repeated practice and interactions with the environment (Willingham, 1998). Of the different stages involved in skill acquisition, studies in the speech motor literature have focused on practice (defined as improvements in performance within a single session) and retention (defined as performance of the practiced skill between sessions without any practice in the intervening duration between sessions) in typical and atypical (e.g., stuttering) populations (e.g., Behrman et al., 2000, Namasivayam and Van Lieshout, 2008, Smith et al., 2010 and Smits-Bandstra et al., 2006). A theoretical notion that has received some attention in the motor literature is the neuromotor noise hypothesis according to which changes with motor learning are the consequences of reducing variability in neural command signals ( Kleim et al., 2002, Kleim et al., 2003 and Newell et al., 2003). Studies of speech and non-speech motor learning suggest that both children and adults exhibit higher levels of neural noise during the acquisition of new movement sequences, evident as higher movement variability ( Green et al., 2002, Smith and Zelaznik, 2004, Takahashi et al., 2002, Vangalen et al., 1993, Walsh et al., 2006 and Yan et al., 2000). With maturation and practice, increasing neuronal synchronization results in reduced neuromotor noise levels, thereby facilitating motor coordination through the formation of optimal movement synergies. Thus, movement coordination is achieved through a neurophysiologically mediated motor learning processs ( Harris and Wolpert, 1998 and Smith and Zelaznik, 2004).

Present findings support the presence of a speech skill acquisition deficit in AWS. Group differences in both speech accuracy and speech kinematics were evident with nonword repetition. Furthermore, manipulating nonword length, PC, and complexity resulted in a higher error rate in AWS that was not reported in all of the earlier studies. Reduction in speech errors across sessions (as a result of retention) was comparable in the two groups. The movement variability data demonstrated that AWS showed limited practice and retention effects on inter-articulatory coordination even for shorter and simpler nonwords where the groups were comparable in speech errors. Some cautions are raised in interpreting the present findings. First, fewer AWS than control participants contributed to some of the nonword categories. Second, only five trials were used to assess speech motor practice and learning effects. We justify this based on other studies that have used a similar number of trials and reported such effects with nonwords varying in length, PC, and complexity in both typically fluent individuals and in AWS (Sasisekaran et al., 2010, Smith et al., 2010 and Walsh et al., 2006). Furthermore, using the same nonwords as those in this study, Sasisekaran and Weisberg (2013) reported similar findings in speech accuracy and movement variability in a group of monolingual and bilingual speakers. Finally, the present study investigated short-term skill learning in AWS using a limited set of nonwords, and the groups could not be compared in speech kinematics at all levels of nonword length and complexity. This necessitates further in-depth investigations of speech skill acquisition with extended practice and longer time to retention in AWS with a larger set of nonwords.