Psychologically Distinct Classes of Motor Behavior Inferred from Individual Differences: Evidence from a Sequential Stacking Task

ABSTRACT

A number of studies have demonstrated regularities in how individuals select and perform single object manipulations, but little work has been concerned with the manipulation of multiple objects. To this end, the authors asked participants to stack a set of linearly spaced containers onto various goal locations. Our aim was to determine whether participants adopted specific strategies to complete this task. We focused on whether the distance between the objects, the goal location of the objects, or both, determined the classes of movement sequences that individuals used to perform the task. The results showed that some individuals tended to use one hand for lifting and releasing the containers whereas other individuals tended to use both hands for lifting and releasing the containers. Those participants who tended to use one hand varied which hand was used according to the goal location of the containers but not the distance between containers. The emergence of these individual differences provides a new basis for inferring psychologically distinct classes of motor behavior.

Keywords:

• grasping
• individual differences
• object manipulation
• planning
• reaching

ACKNOWLEDGMENTS

Jewels D. Rhode's involvement in this research was made possible by her participation in the Pennsylvania State University Summer Research Opportunity Program (SROP), July and August, 2009. SROP is a program designed to encourage research participation by minority students. Rebecca Roberts and Jennifer Wartsky helped with data collection.

Notes

1. The greater preponderance of two-hand releases than two-hand lifts seen in Figure 4 is not inconsistent with the fact that in Table 1, the number of participants lifting with both hands more than zero times was the same as the number of participants releasing with both hands more than zero times. Table 1 concerned number of participants carrying out actions a varying number of times. Figure 4 concerned the mean number of times actions were carried out (averaged over participants and so making the standard error bars meaningful).

2. A reviewer asked whether we might also need the following two rules: (a) Don't turn after picking an object and before releasing it, and (b) Don't pass an object from one hand to the other. We did not specifically code the data for the number of times participants turned, so we cannot yet confirm or disconfirm the need for Rule 3. Rule 4, in our view, can be seen as being implied by Rule 1. If a participants passed an object from one hand into another, in violation of Rule 4, he or she would also be in violation of Rule 1 because using the hand farthest from the goal location would require him or her to reach farther than using the hand closest to the goal location. It is possible, of course, that some participants actually relied on rules that are implied by other rules. The most important point, perhaps, is that a debate like this can help our field embark on a quest for the most powerful rules for action selection. Such rules can be implicitly represented rather than explicitly represented in the nervous system.