Skilled reaching an action pattern: stability in rat (Rattus norvegicus) grasping movements as a function of changing food pellet size.

Many animals in different mammalian orders display skilled forelimb use for prehension, but there has been little study of the rules underlying skilled forelimb use that may provide insight into its neural organization. Inflexibility of movement would imply an underlying fixed neural control characteristic of innate action pattern, whereas flexibility of movement would imply more plastic neural control. In the present study, rat reaching was examined by analyzing movements used to obtain nine different sized food pellets, weighing between 20 and 1000 mg. The prediction was that if the rats' reaching movements were flexible, then systematic modifications in reaching would occur as food pellet size changed; whereas if reaching was inflexible, disruptions should occur for extreme food pellet sizes. Reaching was filmed using normal (30 frames/s) and high-speed (60 frames/s) video-recording procedures. Behavior was scored in terms of successful reaches and, in addition, a new rating scale, derived from Eshkol-Wachman Movement Notation, was used to evaluate the qualitative aspects of movement. Reaching success was stable across smaller food pellet sizes but dropped sharply for larger food pellets sizes. Qualitative analysis of limb movements revealed that animals were unable to change their typical movement sequence to obtain the larger pellets. This result indicates that rat skilled reaching is relatively inflexible, supporting the position that it is produced by a complex, relatively fixed neural circuitry. This finding is discussed in relation to the evolution of skilled reaching, the modifications of skilled reaching that are observed after motor cortex and other nervous system injuries, and comparative differences in reaching in rats and primates.