The "priming effect" in electrical self-stimulation of the brain is the tendency for self-stimulation performance to weaken and slow down as the time since the last stimulation increases. Collaborative experiments among three laboratories varied the intertrial interval in a runway task in order to measure the effect for 37 electrode placements in 25 rats. The incidence of the effect was 100%, despite wide variation in type and placement of electrodes, type of runway, and type of stimulus wave form. The magnitude of the effect varied greatly. Some rats ran only .5-2.0 see/run more slowly at the longer intertrial interval; whereas others would not perform. The choice of stimulation parameters could influence considerably the magnitude of the effect.
Rats were fixed with chronically indwelling electrodes to provide intracranial stimulation (ICSl ICS for one group stimulated the medial forebrain bundle and was positively reinforcing. ICS for another group stimulated subcortical areas away from the forebrain bundle and yielded forced movement and no reinforcement. After tests for effects of ICS, rats were trained to avoid footshock by climbing from a grid to a shelf that retracted periodically. After avoidance was established, footshock was terminated and rats were prevented from further responding by removal of the shelf. During three different periods of response prevention, the two kinds of ICS and no ICS were imposed, yielding nine treatments conforming to a 3 by 3 factorial design. After treatment, the shelf again was available and rats' perseveration of avoidance tabulated. Rats receiving positively reinforcing ICS during longer response prevention times responded the least.
Recent investigations have found a multiphasic retention function associated with avoidance training (e.g., Holloway & Wansley, 1973a, bl, The present experiment was done to determine if a similar retention function also describes appetitively motivated behavior. Rats were allowed access to preferred solution in the shock compartment of a passive avoidance apparatus. Then either .25 h after the appetitive preexposure or at successive 3-h intervals up to 24 h, the rats were administeredone-trialpassive avoidancetraining consisting of a strong shock presented in the shock compartment. The retention of the appetitive preexposure was determined by its effect on performance of the passive avoidance task as measured 24 hafter the shock trial. The results dernonstrated that the retention function associated with the appetitive preexposure was phasic with an altemation between high and low retention every 12 h. Specifically, retention was higher after the .25·, 9-, 12-, and 24-h intervals than after 3·, 6-, 15-, and 18-h intervals. These results are consistent with prior research on the retention of avoidancetraining.
Rats were run in alleys for water reinforcement with intertrial intervals (ITIs) similar to the intervals typically used in studies of intraoranial reinforcement. When deprived over 15 hr., rats ran faster with very short (7 sec.) ITIs than with longer ITIs (95 sec.). Comparisons with rats described in other studies running for intracranial reinforcement led to the conclusion that, regardless of what kind of reinforcer is used, rats run faster for a reinforcer when ITIs are only seconds long rather than over a minute.
In the first experiment, rats given a 5-min period of preexposure (simple exploration) to a two-compartment box showed poorer passive avoidance of the compartment where they were subsequently shocked than a control group which was not preexposed to the apparatus. The second experiment involved preexposure to sugared milk (8M), flashing light and loud noise (LN), or simply the apparatus (EC). One group received no exposure to the apparatus (NC). Following one shock trial, the
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