Exploring the impact of speaking rate on reaction time: Cognitive load or motor synchronization?

Background: Understanding cognitive load during speech modification is critical in speech therapy, where optimizing it can enhance outcomes. Reaction time (RT) for a visual secondary task increases for clear speech, which typically involves a slower speaking rate, compared to habitual speech, potentially reflecting cognitive load or motor synchronization. This study aimed to determine which factor—cognitive load or motor synchronization—primarily affected RT changes during speech modification. Methods: Six healthy female speakers of American English (ages 19–21) participated. They counted numbers at three speaking rates—slow, habitual, and fast—while performing a visual RT task, pressing a key when a square appeared on a screen. We hypothesized that if RT reflects motor synchronization, RT would vary systematically with speaking rate, showing differences across all three conditions. Results: Mean RTs were 0.47 s (Slow), 0.44 s (Habitual), and 0.51 s (Fast). Speaking rate significantly affected RT, F(2, 2665) = 17.50, p < .001, with RTs longer for Fast compared to both Slow (p = .001) and Habitual (p <.001). Slow and Habitual rates did not differ (p = .120). Conclusion: These preliminary findings suggest cognitive load, not motor synchronization, drives RT differences, with Fast speech imposing the greatest demands.