Abstract
We have shown that monkeys can learn the sequential movements of reaching and grasping tweezers placed at their front-right with their right hand, bringing the tips of tweezers to a piece of food placed at their front-left, and picking it up with them. In the task, the tweezers were placed with the tips facing left toward the food (9 o’clock). When the tweezers were placed in the reverse direction (3 o’clock), monkeys initially picked them up backwards and brought the reverse end to the food in a vain attempt to grasp the food. They repeated the same action, suggesting that they did not understand that the tip end must be used, and sometimes threw the tweezers away, probably due to accumulated stress. With continued training, monkeys were able to foresee the outcome and aborted the reach movement on the way to the food. Then, suddenly they showed a new strategy of first rotating the tweezers by 90-180 degrees with their fingers, which enabled them to grasp the tweezers properly and pick up food successfully. When the tweezers were placed facing different directions, monkeys developed strategies that were most efficient for the situation, substituting wrist rotation rather than tweezer rotation when appropriate. These findings show that monkeys can understand tweezers as an object used for picking up food and can invent strategies to use them in various situations, demonstrating that monkeys can use tweezers as a “tool”. [Jpn J Physiol 55 Suppl:S180 (2005)]
