Electrical brain stimulation and food reinforcement dissociated by demand elasticity

Rats, living in test chambers, responded on two levers delivering, in one case, two signalled 0.5 sec trains of biophasic square wave brain stimulation or, in the other case, a signalled 45 mg food pellet. Unlike all previous studies, the subjects were allowed 24 hr access with the delivery of reinforcers from both levers scheduled by two equal variable-interval (VI) schedules that set the average inter-reinforcement interval. As the availbility of reinforcers in time was decreased and the mean VI values increased, responding for EBS became dissociated from responding for food. Response rates for food increased in contrast to the usual decrease observed in short sessions under fixed deprivation conditions. Response rates for EBS, at the same time, showed a large decrease as would be expected from data collected in short sessions. This distinction was true even when stimulation intensity was increased by a factor of 2.5. As a result of this difference in response rate functions, food consumption remained constant but the number of self-stimulations delivered decreased. In economic terms, these findings indicate that demand for food was relatively inelastic while demand for brain stimulation was relatively elastic and that the reward of food is different from the reward of lateral-hypothalamic brain stimulation.