Abstract
During rapid, goal directed hand movements, eye and hand position are usually yoked. The saccade typically leads the hand. Visual and proprioceptive feedback can also be used to update the movement online. However, it is not yet clear what effect the new, high resolution image of the target location gathered by this first saccade has on online control of the hand and eye-hand coordination. If this lately acquired visual information significantly modulates performance, it would have significant implications for theories of eye-hand coordination. We investigated the impact of the visibility of the goal at different times during the reach on endpoint precision and accuracy. If visual information about the target gathered by the first saccade is used to update a movement online, endpoint precision and accuracy should decrease if target visibility decreases late in the movement. Target contrast can significantly affect visual localization thresholds. In our experiment, we varied target contrast and duration in the reach to manipulate the quality of the visual information gathered by the first saccade. The target could appear at one of 8 different locations, each 8 degrees eccentric to initial fixation. In Experiment 1, participants pointed to targets of varying contrast and varying duration. We measured pointing precision, accuracy and movement time. Contrast significantly affected pointing precision. Pointing accuracy for low contrast targets was significantly better at longer target durations. In Experiment 2, participants pointed to a target that either reduced or increased its contrast either early or late in the reach. Low contrast targets resulted in longer movement times. The results demonstrate that target contrast significantly impacts on pointing performance, and suggests that aggregation of information can affect rate of movement, perhaps as a corollary of Fitts' law. We will discuss the implications of these findings for theories of eye-hand coordination.