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 skille d forelimb use that may provide insight into its neural organization. Infle xibility of movement would imply an underlying fixed neural control charact eristic of innate action pattern, whereas flexibility of movement would imp ly more plastic neural control. In the present study, rat reaching was exam ined by analyzing movements used to obtain nine different sized food pellet s, weighing between 20 and 1000 mg. The prediction was that if the rats' re aching movements were flexible, then systematic modifications in reaching w ould occur as food pellet size changed; whereas if reaching was inflexible, disruptions should occur for extreme food pellet sizes. Reaching was filme d using normal (30 frames/s) and high-speed (60 frames/s) video-recording p rocedures. Behavior was scored in terms of successful reaches and, in addit ion, 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 foo d pellets sizes. Qualitative analysis of limb movements revealed that anima ls were unable to change their typical movement sequence to obtain the larg er pellets. This result indicates that rat skilled reaching is relatively i nflexible, supporting the position that it is produced by a complex, relati vely fixed neural circuitry. This finding is discussed in relation to the e volution of skilled reaching, the modifications of skilled reaching that ar e observed after motor cortex and other nervous system injuries, and compar ative differences in reaching in rats and primates. (C) 2000 Elsevier Scien ce B.V. All rights reserved.