HIGH-RESOLUTION ANALYSIS OF FORCE LEARNING IN RATS

In a series of experiments, detailed recordings of motor performance w ere made while rats learned to press a beam with a criterion force in order to earn liquid food reinforcement. The data indicate that differ entially reinforcing specific response forces in a free operant situat ion leads to the orderly development of beam-pressing skills through a sequence of stages. High resolution analyses of beam presses also rev eal the presence of ''sub-movements'' similar to those seen in spatial ly constrained rapid aimed movements (Crossman & Goodeve, 1963/83; Mey er, Abrams, Kornblum, Wright, & Smith, 1988). During the initial stage s of learning when the animals are acquiring the beam-pressing respons e, response force is modulated by response duration which reflects an increase in the number of sub-movements. However, as training proceeds , the number of sub-movements decrease resulting in a decline in respo nse duration and an increase in rate of rise of force which sustains r esponse force at criterion levels. Thus, the force-time trajectories o f force-targeted beam presses change with practice from many sub-movem ents to a singular, ballistic sub-movement which more efficiently sati sfies the force criterion. It is argued here that sub-movements expres s corrections in force output and that the evolution of such sub-movem ents during force learning reflects a change from a feedback to a feed forward mode of force control (Woodworth, 1899).