Lingual Speech Motor Control Assessed by a Novel Visuomotor Tracking Paradigm
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Like other types of human motor control, speech production is thought to be accomplished through the process of receiving sensory feedback and continually refining predictive feedforward models to achieve the desired articulatory movement. Visuomotor tracking (VMT) has been an influential paradigm used to test this motor control theory. VMT tasks examine articulatory movement by requiring participants to follow external signals visually presented on a screen. By varying the predictability of the signal, information about processes involved in speech motor planning and execution can be gained. Previous studies of healthy adults have suggested that when tracking predictable frequencies, an internal model of the target movement is formed to guide accurate movement. When the signal is unpredictable, no model can be formed, and feedback information is used to detect errors and aid in tracking. Research from this line of work has also suggested that the underlying basis of apraxia of speech, a speech motor disorder, is due to a deficit in feedforward processing. Speech VMT studies have focused on the lips and jaw (external articulators); therefore, little is known about the tracking capabilities of the tongue, the primary articulator for speech. Although it could be the case that tongue motor control uniformly resembles that of other articulators, its biomechanically unique properties (i.e., a muscular hydrostat) and braced position during speech suggest it may not share the same motor control properties. In the present study, tongue motor control was assessed using a novel VMT paradigm based on an electromagnetic articulography system (Opti-Speech). In a first experiment, ten healthy young adults (mean age = 28.8 years) used their tongue tip to track a virtual intra-oral moving target that varied in conditions of predictability, frequency (0.4, 0.6, 0.8 Hz), and direction (vertical, horizontal, lateral). These conditions tested feedforward/feedback control, speed-accuracy tradeoff, and speech-like versus non-speech-like properties, respectively. In a second experiment, another group of ten healthy young adults (mean age = 21.0 years) participated in a similar tracking experiment assessing whether synchronous tongue-jaw patterns extend to cases of the tongue moving in isolation. In both experiments, tracking accuracy was measured by computing correlation coefficients, amplitude ratios, and phase differences. Experiment 1 demonstrated significantly higher accuracy in the predictable condition than the unpredictable condition, providing support for the notion of an internal model guiding expected movement. In addition, a speed-accuracy tradeoff was found, with significantly higher accuracy for the slowest frequency (0.4 Hz) compared with the fastest (0.8 Hz). Amplitude ratio data revealed significantly higher accuracy in the lateral direction when compared to the vertical direction, suggesting a difference in control of movement for speech-like (vertical) versus non-speech-like (lateral) directions. Results from Experiment 2 corroborated that basic motor control principles noted in Experiment 1 are also found for movement of the tongue alone. Taken together, the findings suggest that tongue motor control shares similar properties with the limbs and previously studied speech articulators. Results serve as a basis for expanded investigations into visual feedback and feedforward deficit theories.