Rats given a footshock 2 min after placement in a box subsequently freeze much more in that box than rats given the shock immediately upon placement. This lack of freezing following the immediate-shock is termed the immediate-shock deficit (ISD), and it is presumed to reflect a learning failure. The purpose of my dissertation was to examine possible mechanisms for the ISD and to further characterize one-trial context fear conditioning. Experiment 1a found the ISD data pattern, but freezing was weak in the delay procedure. Experiment 1b used a smaller box and found enhanced freezing only in the delay procedure. Also, for the delay-shock rats, freezing was distributed on the test day such that freezing increased up to about the time that shock was given on the conditioning day and then decreased. Experiments 2a and 2b used the opioid blocker naloxone to see if the ISD was due to a reduced potency of the immediate-shock brought about by opioid analgesia. The results argued no. Also, for the first time, more freezing and defecation was found in immediate rats than in no-shock control rats. This finding was strengthened by the results of Experiment 3 which measured escape, side preference, side and nose crossings, freezing, and defecation. For each measure, the immediate-shock rats behaved more like the delay-shock rats than no-shock rats. The results of Experiments 2a, 2b, and 3 argue that the ISD is not a complete failure of learning, but instead is only a partial reduction in learning. Experiment 4 found that freezing after experiencing an immediate-shock is not an unconditional effect of shock, but instead reflects learning. Experiment 4 also found that rats shocked 2.5, 15, or 405 s after placement in box freeze less than rats shocked 45 or 135 s after placement. These results suggest that the ISD can be subsumed as a specific instance of a more general interstimulus interval (ISI) effect. Experiments 5a, 5b, and 5c, taken together, determined that an immediate-shock, while only weakly conditioning the context in which it was delivered, could condition other cues that enjoy a more favorable ISI with that shock. This result further suggests that the ISD is not due to any reduced shock potency, but due instead to an unfavorable ISI. The parallel between many of the results found here with those seen in more complex but better studied Pavlovian conditioning models argues that the one-trial context fear conditioning preparation can serve as a simple model system for studying Pavlovian conditioning.
| Identifer | oai:union.ndltd.org:UMASS/oai:scholarworks.umass.edu:dissertations-8742 |
| Date | 01 January 1993 |
| Creators | Bevins, Rick Allan |
| Publisher | ScholarWorks@UMass Amherst |
| Source Sets | University of Massachusetts, Amherst |
| Language | English |
| Detected Language | English |
| Type | text |
| Source | Doctoral Dissertations Available from Proquest |
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