Eye-hand-coordination in man: a reaction time study.

Reaction time measurements were used to test the possibility that an eye and a reach movement both aimed at the same visual target share a common final motor command. This hypothesis predicts highly correlated pairs of reaction times on a trial by trial basis. The experiments were based on the earlier observation that long saccadic reaction times (above 200 ms) with a large scatter (150-300 ms) are obtained if the central fixation point remained visible through out a trial (overlap paradigm), whereas extremely short saccadic reaction times (around 120 ms) with a small scatter (100-150 ms) occur if the fixation point was turned off some time (200 ms) before the target appeared (gap paradigm). In the overlap paradigm high correlations (coefficients up to 0.95) between saccadic and reach reaction times were obtained and the corresponding linear regression lines had a slope of almost one. In the gap paradigm, however, correlations were almost absent even though the mean reach reaction times were considerably decreased. These observations clearly contradict the "common command" hypothesis, but can be explained by the assumption that the execution of the reach movement depends on the completion of the preparation of the saccade but not vice versa ("one way synchronization" hypothesis).