Perseveration and contrast effects in grasping
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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