Several years ago, we began a series of studies which we hoped would allow us to specify some of the important parameters of food reinforcers. Our first aim was the development of a method which would allow investigation of these factors in the individual organism. Hav-ing the benefit of the observations of Jenkins and Clayton (1949) and Guttman (1953), we assumed that rate of response under an interval reinforcement schedule would provide a datum sensitive to differences in reinforcing stimuli. We used pigeons, because we hoped to separate such potentially important factors as amount of food per se and number of pecks required to obtain the food. Our early efforts were uniformly disappointing: birds were run under Fl or VI conditions, with a given amount of food per reinforcement (each amount with an associated key color), until their day-to-day rates were relatively stable;3 the amount of reinforcement was then changed and the process repeated. Comparisons of the final rates achieved with each amount failed to show the expected differences, even though both amount of food and amount of consummatory responding were varied over rather wide ranges. Differences were noted, however, with four types of procedures: (1) The very first minu,tes of a daily session, prior to the delivery of the first one or two reinforcements, frequently produced rates correlated with the amount of food which, in past sessions, had been paired with the present key color. The disciminative stimulus of key color thus seemed to be controlling the response rate at the start of a session. (2) After many sessions in which one amount of food was used on odd-numbered days and a different amount on evennumbered days, rate differences were obtained during an extinction session in which the key colors formerly paired with each amount of food were presented alternately for 2-minute periods.4 (3) Although no differences between two amount-of-reinforcement conditions were observed after stable Fl 2 behavior was reached, a shift to Fl 4 BO 2 produced clearcut differences which then gradually disappeared over four daily 2-hour sessions on this new schedule. (4) A procedure similar to the "probe-stimulus" technique (Ferster & Skinner, 1957) showed response rates during probes that were correlated with the amount of reinforcement previously paired with the stimulus now used as a probe. In this procedure, two amounts of reinforcement, each with a correlated key color, were presented in alternate daily training sessions until day-to-day variability was relatively low. At this point, no differences between average rates for the two reinforcement conditions were seen. On subsequent sessions, the key light associated with one amount of food was inserted for 2-minute periods while the other amount with its key light was used for the remainder of the session. Probing with the larger-amount stimulus yielded an immediate increase in response rate; probing with the smaller-amount light gave an immediate decrease in rate. These probes were repeated three times daily...

Rats were trained to respond to the brighter of two keys. Four animals were trained with food pellets and four with electrical brain stimulation. Each discrimination sequence was initiated when the animal broke a light beam at the rear of the chamber, turning on the key lights and starting a 30-sec reinforcement period. An initial response on the brighter key was immediately reinforced, and further responses on the brighter key were then intermittently reinforced. Any time the dimmer key was pressed, a 30-sec timeout was introduced. During timeout, no response had any programmed consequence. When the reinforcement period or the timeout ended, a new discrimination sequence could be initiated. Daily 1-hr training sessions were conducted, and after seven or eight sessions, all animals were at or near errorless performance levels. The luminance of the brighter key was then systematically reduced, in seven steps, with two 30-min test sessions at each step. Orderly psychometric functions were generated for individual animals. Initial acquisition, once position preferences were broken, was equally rapid for food and for brain-stimulation animals, and the two reinforcement procedures yielded comparable levels of brightness discriminability.Intracranial self-stimulation (ICS) has been increasingly applied to

Rats trained in a discrimination-reversal situation with either food pellets or intracranial stimulation as the reinforcing stimulus were not appreciably different in their first-task performance, but reversal training was slower if the first discrimination was learned with brain stimulation.

Rats were trained and extinguished under conditions that were comparable in all respects except for the reinforcement proc edures. Food 5s responded more frequently during extinction than IC5 animals, whether the training had been with CRF or FR-lO reinforcement schedules.