Analysis of response systems in Pavlovian conditioning reveals rapidly versus slowly acquired conditioned responses: Support for two factors, implications for behavior and neurobiology

A review of the literature on aversive Pavlovian (classical) conditioning was conducted to de- termine whether various conditioned responses (CRs) develop at the same or at different rates. Statistical analyses revealed a bimodal distribution of the rates of acquisition of various types of CRs. Rapid acquisition (M = 5.7 trials to first consistent CR) occurs for conditioned suppres- sion and changes in galvanic skin response, blood pressure, respiration, pupil size, and heart rate. Slower acquisition (M= 71.3 trials) characterizes nictitating membrane, eyelid, and flexion CRs. Differences in conditioned stimulus intensity, unconditioned stimulus intensity, intertriaLinter- val, and interstimulus interval between the two groups cannot explain the dichotomy. Studies of concurrent measurement of more than one type of CR further support the "fast/slow" CR dis- tinction. The findings support two-factor theories of Pavlovian conditioning. The results are dis- cussed with regard to various two-factor learning theories, and the implications for behavioral and neurobiological studies of learning are considered. Since the pioneering work of Pavlov (1927), classical conditioning has been used in an enormous array of be- havioral and neurobiological studies. The fact that this research continues unabated is a testament to the power and richness of Pavlovian classical conditioning as a re- search tool. It also indicates that a satisfactory understand- ing of such learning is still sought. Many different behavioral response systems have been studied. A question that arises is, How "equal" are the various systems that have been recorded in Pavlovian con- ditioning? As an example, consider a hypothetical exper- iment in which simultaneous recordings are obtained from all possible response systems. Would conditioned re- sponses (CRs) appear in all systems at the same time? A number of theorists have suggested that the answer to this