MECHANISMS UNDERLYING THE GENERATION OF AVERAGED MODIFIED TRAJECTORIES

In this work we have studied what mechanisms might possibly underlie a rm trajectory modification when reaching toward visual targets. The do uble-step target displacement paradigm was used with inter-stimulus in tervals (ISIs) in the range of 10-300 ms. For short ISIs, a high perce ntage of the movements were found to be initially directed in between the first and second target locations (averaged trajectories). The ini tial direction of motion was found to depend on the target configurati on, and on D: the time difference between the presentation of the seco nd stimulus and movement onset. To account for the kinematic features of the averaged trajectories two modification schemes were compared: t he superposition scheme and the abort-replan scheme. According to the superposition scheme, the modified trajectories result from the vector ial addition of two elemental motions: one for moving between the init ial hand position and an intermediate location, and a second one for m oving between that intermediate location and the final target. Accordi ng to the abort-replan scheme, the initial plan for moving toward the intermediate location is aborted and smoothly replaced by a new plan f or moving from the hand position at the time the trajectory is modifie d to the final target location. In both tested schemes we hypothesized that due to the quick displacement of the stimulus, the internally sp ecified intermediate goal might be influenced by both stimuli and may be different from the location of the first stimulus. It was found tha t the statistically most successful model in accounting for the measur ed data is based on the superposition scheme. It is suggested that sup erposition of simple independent elemental motions might be a general principle for the generation of modified motions, which allows for eff icient, parallel planning. For increasing values of D the inferred loc ations of the intermediate targets were found to gradually shift from the first toward the second target locations along a path that curved toward the initial hand position. These inferred locations show a stro ng resemblance to the intermediate locations of saccades generated in a similar double-step paradigm. These similarities in the specificatio n of target locations used in the generation of eye and hand movements may serve to simplify visuomotor integration.