Arpeggio and fractionated digit movements used in prehension by rats

Although it is well known that rats can use one limb to reach for and retrieve food, the way that the paw and digits are used to grasp the food has not been described. Using results obtained from a high speed videorecording procedure, the present study describes how small and large pieces of food are located by the paw and grasped with the digits in a reaching task. As the rat pronates a paw over food, the tips of digits 5 through 2 are successively placed and spread over the target area in an arpeggio movement. The target area is then palpated by downward movements of the palm. If food is not present, the paw is withdrawn without grasping. If food is contacted, the food is manipulated and grasped by the digits with grasp patterns that depend upon food size. Large food pellets are contacted with the pad of digit 3 and then grasped between digits 3 and 4. Small food pellets are first contacted with pad of digit 4 and are grasped between digits 4 and 5. Digit 5 can be partially flexed medially so that the food is held in a modified power grip between digits 4 and 5. The results show that rats use a whole paw movement to position the digits in an arpeggio fashion and they use fractionated digit movements to grasp food. The results are discussed in relation to the possible comparative and anatomical significance of the movements.