The effect of directional compatibility on the response latencies of ocular and manual movements.

Visuomotor coordination is essential for the successful performance of everyday activities, and it could be affected by the directional compatibility between ocular and manual movements. Many tasks, such as driving or operating devices in the workplace, require a variety of coordination patterns with different levels of compatibility between the eyes and the hand. For example, the movement of the eyes and the arm can be coupled when both effectors point towards the same direction whereas in other tasks the movement of the eyes and the arm can be dissociated, for instance when a peripheral object is foveated while a button press response is executed concurrently. The objective of this study was to examine the latency of ocular and manual movements in tasks characterized by variations in directional compatibility. Four tasks were used to manipulate compatibility: 1. point and look at a peripheral stimulus (POINT AND LOOK)--high directional compatibility; 2. point to a peripheral stimulus while fixating in the center (POINT AND FIXATE)--low directional compatibility; 3. press a button while looking at a peripheral stimulus (PRESS AND LOOK)--low directional compatibility; and 4. press a button while fixating in the center (PRESS AND FIXATE)--no directional motor requirement. We hypothesized that the latency of (1) manual and (2) ocular responses would be faster in the task with high directional compatibility compared with the tasks with low compatibility or the task with no directional motor component. Ten healthy participants executed pointing and pressing movements with and without concurrent eye movement to randomly presented visual stimuli. In agreement with the first hypothesis, results showed that in a task with high directional compatibility, manual responses were initiated significantly faster when compared with the tasks with low compatibility or a task with no directional motor component: 1. pointing while looking was initiated 22 ms faster on average than pointing while fixating; 2. pointing while looking was initiated 91 ms faster than pressing accompanied by an eye movement; and 3. pointing while looking was initiated 102 ms faster than pressing while fixating. The second hypothesis was partially supported by data which showed that eye movements directed toward peripheral stimuli were initiated significantly more slowly (30 ms on average) when accompanied by pressing in comparison with the latency of eye movements in the high-compatibility task. In contrast with the hypothesis, eye movements that were accompanied by pointing were not initiated faster than those in a task which required looking without pointing. In summary, these data suggest that directional compatibility is an important aspect of motor control. The effects of directional compatibility are discussed in a conceptual framework that considers the neurophysiological substrates that might be involved in mediating these effects.