Differential contribution of cognitive and psychomotor functions to the age-related slowing of speech production.

Healthy elderly adults are slower to initiate and produce speech. However, the sources of the slowing remain poorly understood. The present study evaluates to which extent psychomotor and cognitive changes influence age-related slowing in speech generation. Four verbal tasks varying in degree of difficulty and cognitive demands were used to evaluate 30 young and 30 healthy elderly. Speed of word production was measured by reaction times and pronunciation durations. Stroop test and Digits backwards were used as cognitive predictors while the Purdue Pegboard and Finger Tapping were used as psychomotor predictors. The relative contribution of cognitive and psychomotor functioning was evaluated by hierarchical regression analyses and based on the processing speed hypothesis. Results showed that Vocabulary and psychomotor execution significantly explained a portion of the variance in RTs depending on type of verbal task. These variables explained 36% of the total variance in reading, 26% in naming, 31% in phonemic fluency and 47% in semantic fluency. Also, Vocabulary and psychomotor functions strongly predicted pronunciation speed. Conversely, tests related to executive functions and working memory were not significant predictors. These data demonstrate the importance of the interplay between Vocabulary and psychomotor decline on speed of language production among healthy elderly adults.