On-line visual control of aiming movements?

Two experiments are reported which addressed the flexibility of visuo-motor processing by manipulating the availability of visual information while executing a discrete aiming movement. The flexibility of visuo-motor processing was tested by unexpectedly changing the proportion of the movement trajectory that visual feedback was present. Visual feedback was manipulated for a short (0.30), medium (0.60) or long (0.90) proportion of the trajectory within a block of trials. Each of these three proportions of vision occlusion (Experiment 1) or visual disclusion (Experiment 2) during the initial trajectory was examined. Within a visual condition, one of the three visual feedback proportions occurred with a high probability (p = 0.72), whereas the remaining two proportions each occurred with a low probability (p = 0.14). The results clearly indicated that spatial accuracy was determined by the actual vision period, independent of its probability of occurrence. The data are consistent with a model of continuous on-line control of movement execution.