The hyperactive spontaneously hypertensive rat learns to sit still, but not to stop bursts of responses with short interresponse times.

The spontaneously hypertensive rat (SHR) is hyperactive and has been proposed as an animal model of attention-deficit hyperactivity disorder (ADHD). Although ADHD in most cases is treated with central stimulants, behavior therapy has also been used, but with relatively limited success. The purpose of the present study was to investigate suppression of SHR hyperactivity by differentially reinforcing immobility (DRI) using a positive reinforcer. The DRI schedule required that the rat remain immobile in a particular part of an operant chamber, the target, in order to obtain the reinforcer. The time requirement, the DRI value, of these periods was increased progressively. The results showed that time spent on the target increased by increasing DRI value in both hyperactive and control rats. However, the total number of movements, on as well as outside the target, was higher for the hyperactive rats. The behavior grouped into two independent response components. One component consisted of immobility responses with durations less than 1 s, actually bursts of active responses; the other component consisted of immobility responses with durations more or less matching the DRI requirement. The reinforcement schedule modified the long-lasting immobility component in both groups. SHR received more reinforcers than WKY as long as the schedule did not require too long periods of immobility. However, the total number of movements on target was not reduced in SHR; on the contrary, it increased somewhat as the schedule requirements increased. If the behavior of ADHD children consists of two, or more, independent components similar to the ones observed in the present study, the present results may offer an explanation of why behavior therapy has limited success in the treatment of ADHD.