THE SUCCESSIVE DIFFERENTIATION OF A LEVER DISPLACEMENT RESPONSE.

Maximum displacements of lever presses by rats were recorded under eight successively-smaller reinforcement zones (RZ). The largest RZ included displacements from 3 degrees to 44 degrees ; the smallest, from 24 degrees to 29 degrees . As the RZ decreased, displacement distributions reflected a least-effort tendency: distributions peaked at the lower limit of RZ and most non-reinforced presses fell just below the lower limit. Successive distributions (a) differed significantly in shape, (b) showed reduced variability, and (c) indicated more presses and more presses per reinforcement. Prolonged training under the smallest RZ gave no improvement in performance.