Abstract # 204:

Scheduled for Saturday, June 23, 2007 11:15 AM-11:30 AM: Session 20 (North Main Hall F/G) Oral Presentation

When to Use End-State-Comfort: The Effect of Handle Orientation on Grip Selection in Rhesus Monkeys (Macaca mulatta)

E. L. Nelson1 and M. A. Novak1,2
1University of Massachusetts Amherst, Neuroscience and Behavior Graduate Program, Amherst, MA 01003, USA, 2Psychology Department, University of Massachusetts Amherst, Amherst, MA 01003, USA
     Prior research has shown that monkeys differ in the way they grip a handle depending on its direction of movement. Whereas all monkeys used an overhand grip to move a handle downward, three out of five monkeys used an underhand grip to turn a handle upwards. This use of an underhand grip suggests that some monkeys show motor planning and alter the trajectory of their hand. Because the hand begins in an awkward position and ends in a comfortable position once the handle has been rotated, this phenomenon is referred to as “end-state-comfort.” Previously, monkeys learned to open boxes with a handle that either turned upwards or downwards with the handle oriented to the monkey’s preferred hand (right-preferent=4; left-preferent=1). In the current study, monkeys were tested on the same task with the handle oriented to their nonpreferred hand to assess whether motor planning was affected. Data were analyzed with binomial probabilities [a=0.05]. All monkeys continued to use their preferred hand significantly above chance. Two monkeys relied solely on the overhand grip and were not affected by the change in handle orientation. However, two of the three monkeys that had previously turned the up-handle with the underhand grip began using an overhand grip significantly more than chance when the handle orientation was changed. Thus in some monkeys, grip selection is based on feedback and planning and is not the result of a trained response.