REACTION-TIME LATENCIES OF EYE AND HAND MOVEMENTS IN SINGLE-TASK AND DUAL-TASK CONDITIONS

The goal of this study was to investigate whether ocular and hand moto r systems operate independently or whether they share processes. Using dual-task methodology, reaction time (RT) latencies of saccadic eye a nd hand motor responses were measured. In experiment 1, the hand and e ye motor systems produced rapid, aimed pointing movements to a visual target, which could occur either to the left or right of a central fix ation point. Results showed that RT latencies of the eye response were slower in the dual-task condition than in the single-task condition, whereas the RT latencies of the hand response were virtually the same in both conditions. This interference effect indicated that the ocular and manual motor systems are not operating independently when initiat ing saccadic eye and goal-directed hand movements. Experiment 2 employ ed the same experimental paradigm as experiment 1, except for one impo rtant modification. Instead of a goal-directed hand movement to the ta rget stimulus, subjects had to make a button-press response with eithe r the index or middle finger of the right hand dependent upon whether the stimulus occurred to the right or left of the control fixation poi nt. The aim of experiment 2 was to investigate the issue whether the o bserved interference effect in experiment 1 was specific or non-specif ic (e.g. overhead costs due to coordinating any two responses). The fi nding that saccadic eye movements and button-press responses in the du al-task condition could be initiated without delay relative to the sin gle-task conditions, supports the specific interference interpretation .