Abstract:
Pigeons were studied under two-key concurrent variable-interval schedules with food as the reinforcer for responding on one key and brain stimulation as the reinforcer for responding on the other. Food reinforcement was 3-sec access to a hopper containing wheat and brain-stimulation reinforcement was a fixed interval of time in which each peck produced a short period of brain stimulation. Choice was studied under various parameters of food and brain-stimulation reinforcement. When brain-stimulation parameters were kept constant while the rate of food reinforcement availability was varied, it was found that the pigeons allocated responses and time to the two keys according to the rates of reinforcement obtained on the two keys, and the ratio of the values of the two types of reinforcer remained constant (Experiment I). Choice behaviour was further studied with equal rates of reinforcement but with varied brain-stimulation current intensity (Experiment II). The pigeons allocated responses and time to the two keys according to the magnitudes of the reinforcers. This further demonstrated that parameters of two qualitatively different reinforcers can be varied in the same way as, for example, rate, duration, or immediacy of a single reinforcer, without affecting the ratio of the values of the two types of reinforcer. In Experiment III duration of brain-stimulation reinforcement was varied in two ways. When the duration of the brain-stimulation reinforcement interval (total time in which responses could produce brain stimulation) was varied, preference for food over brain stimulation did not change. However, preference for food decreased with an increase in the duration of each response-produced brain stimulation when the reinforcement interval was held constant. These results were interpreted in terms of the pigeons' greater preference for food over brain stimulation and the observation that a pigeon generally obtained only one or two response-produced brain stimulations before taking up a position in front of the food key. The effect of food deprivation on choice was also studied (Experiment IV). Five pigeons with electrodes implanted in striatal areas decreased their preference for food when body weight increased, but the preferences of two birds with hypothalmic electrodes were unaffected by deprivation. The latter birds, however, had a much greater preference for food than for brain stimulation. These results supported the suggestion that there is some relation between self-stimulation behaviour, electrode placement, and degree of deprivation.