Asymmetry in a verbal-manual task combination: habitual vs controlled verbal production.

Interference between verbal production and left- and right-hand tapping was studied in male right-handed subjects in two experiments. The hypothesis was studied that controlled rather than automatic verbal production causes the interference with right-hand performance. Experiment 1 compared the repeated production of one verbal number response with the production of a random sequence of numbers. Experiment 2 studied one additional condition: counting from one to 20. The results showed that repetition and counting do not selectively interfere with right-hand tapping, while the randomization of numbers does. These results support the controlled-processing hypothesis of interference with right-hand performance.