Further observations on overt "mediating" behavior and the discrimination of time.

When the lever-pressing behavior of five rats was maintained by a DRL schedule (reinforcement was scheduled only when a specified waiting time between successive responses was exceeded), collateral behavior developed that apparently served a mediating function. In two cases this behavior did not arise until the experimental environment included pieces of wood that the rats started to nibble. When collateral behavior first appeared, it was always accompanied by an increase in responses spaced far enough apart to earn reinforcement. If collateral behavior was prevented, the number of reinforced responses always decreased. Extinction of lever pressing extinguished the collateral behavior. Adding a limited-hold contingency to the schedule did not extinguish collateral behavior. It appears that the rat can better space its responses appropriately when concurrently performing some overt collateral activity. The amount of this activity apparently comes to serve as a discriminative stimulus. To assume the existence of internal events that serve as discriminative stimuli in temporal discriminations is, at least under some circumstances, unnecessary.