Cardiac Activity in Rats during Bar-Press Avoidance and “Freezing” Responses

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Abstract
7 rats were trained to avoid shock by bar-pressing in an instrumental conditioning situation. The US was preceded by a 30-sec. sound signal (CS). The intertrial period was held constant (20 sec). HR was recorded at fixed intervals during 7 sequences (Intertrial-CS-US or Avoidance Response) in each 30-min. daily experimental session. 5 of the 7 initial Ss reached a high frequency of avoidance responses, while the other two failed to reach the fixed criterion adopting rather a freezing response. The animals were then placed in a situation leading to the experimental extinction of the instrumental avoidance response. The results show that, when the response is to be one of avoidance, the HR rises immediately upon CS-onset and remains at that high level until the response is effected. In between avoidance responses the base HR level is relatively high. on the other hand, in the case of a “freezing” non-avoidance response, the HR also rises upon CS-onset but drops immediately afterward to a level significantly lower than the pre-CS level. The intertrial HR frequency is also significantly lower than that observed between avoidance responses. The pre-CS and during-CS drops in HR become more and more important as the experiment progresses, providing evidence in favor of a learning process.

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