Performance in rapid, sequential visually-guided pointing movements

Purpose: We investigated how directional change, distance, and reward affected the speed and accuracy of rapid pointing movements to multiple targets. Method: On each trial subjects saw two targets presented on a horizontal LCD touch screen. They attempted to hit both targets in a specified order within 400 ms after they initiated movement. Successful hits resulted in monetary rewards. Subjects were first trained to perform the task (640 trials) without reward. In the experimental session, subjects were rewarded for hits. Four subjects participated, three naïve. We varied direction: the two movements required a change of 0, 90, 180, or 270 degrees in direction; distance: The two movement distances were equal (7.5cm) or the second was increased by a factor of 1.375; and reward structure: In one reward condition, both targets had the same value. In the other reward condition, the second target was worth 5 times more than the first target. Subjects performed 16 blocks (4 directions x 2 distances x 2 rewards) of 50 trials each for a total of 800 experimental trials per subject. Results: When distance and reward were equal, all subjects spent considerably more time transiting to the first target (mean 148±15ms) than to the second (mean 106±9ms) and the likelihood of hitting the first was correspondingly higher (87±1%) than for the second (63±2%). When the distance to the second target was greater subjects spent only 13±2ms more on the second movement. When reward was unequal, subjects spent more time in transiting from first to second target but with only a slight increase in probability (7.6%) of hitting the second. We found no significant influence of hitting one target on the probability of hitting the other. This lack of interaction and the overall tendency to favor the first target were unexpected and will be discussed.