The context dependence of grasping movements: an evaluation of possible reasons.

Laboratory experiments typically examine grasping movements as isolated motor acts executed for their own sake. In real life, however, grasping is often part of complex and meaningful movement sequences. We have shown before that grasping characteristics differ substantially between these two behavioral contexts and that these differences can be reduced to five orthogonal factors. Now we evaluate the role of focused attention, movement speed and/or embedding in other behavior as possible causes for the observed context differences. Subjects grasped in three variants, which deviated from our standard laboratory condition in one of the above three ways: In one, subjects' attention was withdrawn from grasping by a concurrent visual memory task; in a second, subjects were pre-trained to grasp as slowly as in our standard everyday-like condition; and in the third variant, grasping was part of a more complex behavioral sequence. Grasping kinematics, grip forces and eye movements were registered across 20 repetitions of each variant, the outcome was normalized with respect to our standard laboratory and standard everyday-like conditions, and the normalized data were reduced to the underlying orthogonal factors. We found that the three variants of the laboratory condition had a non-uniform effect on grasping: decreasing the difference to the standard everyday-like condition for some factors, increasing it for others and leaving it unchanged for yet others. We interpret this finding as evidence that none of the three variants successfully reduced the difference between standard laboratory and standard everyday-like context; differences between contexts are therefore probably related to factors other than focusing of attention, movement speed and embedding in other behavior.