PREDICTIVE TIMING AS A CONTROLLER OF FLUENCY DISORDERS
This new project explores the hypothesis that a domain-general capacity – predictive timing, rooted in the motor system – underlies both dysfluent reading and speech. It is inspired by the key insight that fluent behaviors share a fundamental reliance on the need to schedule events in time, be they movements of the effector systems (e.g., eyes, vocal tract) or the execution of higher-order procedural rules for sequencing and integrating incoming information. Thus, behaviors that appear quite removed from the motor system (e.g., discourse comprehension), in fact rely critically on its predictive timing component to fluently coordinate lexical, syntactic, semantic, referential, and other information that must be integrated as a discourse unfolds.
Predictive timing can be indexed behaviorally by asynchronous manual tapping. Such tapping has also been associated with poor reading ability. In pilot work, we examined correlations between tapping and eye movements in a sample of low ability readers (N=47). We observed significant positive correlations between inter-tap intervals (mean and SD) and key eye movement indices: first fixation duration (rs = .26 - .4), gaze duration (rs = .33 - .47), total reading times (rs = .33 - .47). Notably, these correlations were not present in a separate sample of proficient readers (N=135). In addition, while there was no relation between tapping and regressive eye movements in proficient readers, in less proficient readers correlations ranged from r = .31 - .39. Neurologically, predictive timing has been associated with disordered beta oscillations. Consistent with the tapping data, our initial work has found a tight relationship between Beta power and eye-movements during reading (symbol-only task in the figure).
This project offers a comprehensive account of how a single domain-general disruption in the motor system manifests as different types of dysfluency (stuttering, reading disability). For individuals who stutter, the effects of internal timing deficits are known to include unstable speech production and difficulty with lexical-semantic, syntactic and phonological processing. If the two types of disfluency share a common etiology, deficits in reading fluency would also be expected in stutterers, however this has not yet been systematically studied; this is one of the specific aims of this project. In addition, the project is novel in examining predictive timing in both low-level effector coordination during speech and reading, and in sequencing of higher level syntactic and discourse units, which are crucial for accurate comprehension. The expected outcome is a functional account of how domain general timing deficits rooted in the brain's motor system lead to domain-specific dysfluencies of speech and reading. A deeper understanding of the motor system's contribution to fluency across domains has the potential to advance neurobiological models of speech and natural text reading and transform remediation through highlighting heretofore neglected links between fluent motor planning and serial information processing.