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Four experiments replicated the phenomenon that rats responding for intracranial stimulation (ICS) fail to show a conditioned emotional response following fear conditioning. Procedural variations in the experiments indicated that failure to suppress responding was not due to alteration of footshock thresholds, failure to perceive the CS, ICS-bound resistance to distraction, or disruption of associative processes by neural disorganization. The results were compatible with an incentive model of rewarding ICS.

No abstract

Rats were trained to press a lever for hypothalamie intracranial stimulation and then tested on a passive-avoidance task. Half of the animals were exposed to traumatizing inescapable footshock (preshock) prior to passiveavoidance testing and half received no preshock. Half of the animals received a 3-ma. and half a 1.5-ma. punishing shock during passive-avoidance testing. Analysis of variance and a Newman-Keuls test showed that passive avoidance developed only in rats exposed to preshock and the 3-ma. test shock.

Rats were trained to press one lever on a variable interval schedule of reinforcement in order to produce a second lever. Each response on the' second lever produced a single burst of hypothalamic electrical stimulation, and following a fixed number of bursts the second lever was retracted . After stabilization of responding, fear conditioning was superimposed on the leverpressing situation. All of the rats developed complete suppression of responding within a few trials. The schedule of reinforcement then was systematically manipulated to increase either the density or the intensity of reinforcement by increasing the frequency with which the second lever was made available, increasing the number of reinforced responses allowed on the second lever, or increasing the intensity of the brain stimulation. Following each such manipulation, there was a temporary attenuation of suppression followed by the reappearance of suppression. have pointed out, such conditions are not necessarily comparable to the standard operant situation where a rat working for food presses a lever and then performs a chain of approach and consummatory responses prior to ingestion. Pliskoff et al. (1965) and Merrill et al. (1969) proposed that the response of an animal pressing a lever which results in the immediate delivery of ICS may be a consummatory response rather than an instrumental response. Although consummatory responses per se are not necessarily resistant to conditioned suppression, it is possible that the combination of a high incentive and a consummatory-like response might produce resistance to conditioned suppression or passive avoidance. Indeed, Vogel and Spear (1966) found that, while conditioned suppression of licking could be obtained readily in rats trained to lick a tube for a 4% sugar water solution, suppression was attenuated in animals licking for a 32% solution. This is in contrast to the fact that conditioned suppression is readily obtained in rats which have to perform an instrumental leverpressing response in order to obtain a 30% sugar water solution (e.g. , Merrill et al., 1970). Therefore , in an attempt to separate the consummatory and instrumental aspects of responding for ICS, the present study utilized the two-bar schedule described by Pliskoff et al. (1965) . METHOD SubjectsThe subjects were five naive male 90-to 120-day-old LongEvans rats. They were housed individually and allowed free accessto food and water. Surgeryand HistologyBipolar stainless steel electrodes (Plastic Products Co., Roanoke, Virginia) were stereotaxically implanted in all animals while they were under sodium pentobarbital anesthesia (40 mg/kg). The electrodes were placed in the posterior medial forebrain bundle, 3.5 mm posterior to bregma, 1.5 mm lateral 151