Elsevier

Neuropsychologia

Volume 47, Issue 6, May 2009, Pages 1578-1584
Neuropsychologia

Perseveration and contrast effects in grasping

https://doi.org/10.1016/j.neuropsychologia.2008.12.032Get rights and content

Abstract

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.

Section snippets

Experiment 1

In the first experiment, we asked subjects to reach out and grasp a plastic disc in front of them. The stimuli varied in size from 26 to 34 mm in diameter in 2 mm steps, and the trials were arranged so that the immediately preceding trial used a disc that was either 2 mm larger or 2 mm smaller than the current stimulus. The empirical question was whether the size of the disc on the preceding trial would have an effect on grip aperture on the current trial.

Experiment 2

In Experiment 2, we evaluated our interpretation of the biphasic effect as consisting of two independent, overlaid trends. If this analysis is correct, one should be able to affect one mechanism but not the other. In particular, our prediction was that if the size-contrast effect were eliminated, one should observe a perseveration effect across the duration of the movement. The manipulation we used to eliminate the size-contrast effect was to provide a consistent context for the target on every

Experiment 3

In Experiment 3, we assessed whether the sequential contrast effect on perception that we hypothesized in Experiment 1 would be obtained if subjects did not grasp the disc but merely estimated its size. If the contrast effect we observed in that experiment was due to perceptual factors, the same result should be found for perceptual estimation under similar conditions.

General discussion

At a general level, the present results demonstrate the importance of memory in motor control. Indeed, both the size-contrast effect and the perseveration effect involve memory in some form. The size-contrast effect observed in Experiment 1 can be explained by assuming that perceptual estimates of the target's size depend in part on a comparison with the size of the target on previous trials. The perseveration effect entails perhaps a reuse or retrieval of aspects of the action performed

Acknowledgements

The research was supported by a Natural Sciences and Engineering Research Council research grant to the first author. The authors wish to thank Henry Li and Victoria Richards for assistance in running the experiments.

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