MODIFICATION BY DRUGS OF PERFORMANCE ON SIMPLE SCHEDULES OF POSITIVE REINFORCEMENT

The basic elements of technique for the use of the free operant are by now quite w e l l k n o ~ n . ~ The purpose of this paper is to illustrate the use of the technique for drug studies by showing how 1 particular procedure has been used to start the analysis of the behavioral effects of some drugs. The work described here has been done on pigeons kept close to a constant weight of lietween EO and $0 per cent of the weight a t which they stabilized when fed ad l i b i h m . The pigeons were trained to work as previously described, the arrangement being shown in FIGURE 1. l~ They pecked a t a translucent plastic disk that had variously colored lights behind it. Each peck broke a circuit that enabled it to be recorded and counted. According to a schedule, the pigeon was rewarded (“reinforced”) for pecking with food from a tray that rose so that the food was accessible to the bird for 5 seconds. The distribution of pecks in time, that is, the rate of pecking from time to time, is extremely sensitive to the precise contingencies relating pecks to rewards. For example, if every sixtieth peck is rewarded under the conditions of our experiments, the pigeon comes to peck at high sustained rates. This is a fixed ratio schedule; there is a fixed ratio of reinforcements to pecks. Henceforth this performance will be referred to as “ratio performance.” On the other hand, if a single peck is rewarded when, and only when, a constant interval of time (for example, 15 minutes) has elapsed, there is a period at the beginning of the interval when the bird does not peck a t all, and then there is a fairly smoothly accelerating rate of pecking until the rewarded peck. This is a fixed interval schedule; a fixed interval of time must elapse before a reward can be obtained, and the pattern of pecking engendered by it will be referred to as “interval performance.” It can be arranged that when a light of one color is on, the schedule is fixed ratio, and when a light of a different color is on, the schedule is fixed interval. The bird comes to perform appropriately to each of the schedules according to which light is on. Thus the bird’s performance can be observed on more than 1 schedule during a short period of time without disturbing the animal in any way. This is a “multiple schedule” in the terminology of Ferster and Skinner. They stay adult and in their prime for many years without apparent change. They very rarely show signs of disease and they stand food deprivation for long periods without obvious ill effects. Their food, grain, is very convenient both for use in an automatic magazine and for daily weighing to maintain the birds close to constant weight. Their excellent vision makes use of colored lights as discriminant stimuli possible. These lights are very convenient and make it Departntent of Pharmacology, Harvard Medical School, Boston, Mass.