Perseveration and contrast effects in grasping.

In order to assess sequential effects in grasping a disc, grip aperture was measured as a function of whether the previous disc was smaller or larger than the current target. In Experiment 1, a biphasic sequential effect was found over the course of the reach: Early in the movement, a contrast effect was observed in which grip aperture was wider following a smaller target; later in the movement, a perseveration effect was observed in which grip aperture was smaller following a smaller target. In Experiment 2, the target was accompanied by context discs that were larger and smaller than the range of target sizes. In this case, there was no contrast effect, and a perseveration effect was observed over the course of the movement trajectory. In a third experiment, a sequential contrast effect was found when subjects did not grasp the disc but merely estimated its size. Our interpretation is that there are two mechanisms producing sequential effects: a perceptual contrast effect in which the target appears larger following a smaller disc, and a motor perseveration effect in which subjects tend to reuse similar motor control parameters from trial to trial. These effects were overlaid in Experiment 1, producing the observed biphasic response. However, in Experiment 2, the context eliminated sequential perceptual contrast, and grip aperture only showed an effect of perseveration. In Experiment 3, only the perceptual effect was found because subjects did not need to grasp the disc.