The reaching reaction in the rat: a part of the digging pattern?
The relationship between acquired lateralized reaching and analogous innate movement patterns was examined in adult rats. In Expt. I, 8 naive rats were trained to reach into a narrow trough-shaped feeder. Analysis of the videorecorded movement showed that the forelimb extension starts 120 ms before the forepaw enters the feeder, lasts 190 ms and can be repeated at 280-ms intervals. Before reaching, the rat assumes a stereotyped posture and inserts its nose into the feeder entrance. Twelve rats used in Expt. II were raised from weaning with limited opportunity to practice manipulative skills. Six of these animals were then trained as in Expt. I. In spite of the lack of experience, they learned the reaching task as efficiently as normal rats. The remaining 6 rats were offered food placed into a wide tunnel-shaped feeder. The animals did not take advantage of the possibility of using both forelimbs, but developed typical lateralized reaching. In Expt. III, 4 rats were trained to retrieve food pellets buried under a layer of sand. The videorecorded digging movement essentially resembled reaching but the two forepaws alternated, the extension amplitude was shorter and the movement was never terminated by grasping. It is concluded that the lateralized reaching develops from modified digging and that the kinship of the two movements accounts for the stereotypy and limited modifiability of the acquired reaction.