A comparison of nonspecific and nictitating membrane conditioned responses: Additional support for two-factor theories

Our review of the Pavlovian defensive conditioning literature (Lennartz & Weinberger, 1992) was presented as support for two-factor theories and demonstrated a bimodal distribution of acquisition rates. “Nonspecific” responses (e.g., heart rate changes or conditioned suppression) condition more rapidly than do “specific” responses (e.g., nictitating membrane). However, Kehoe and Macrae (this issue) have argued that the acquisition rate of nictitating membrane (NM) conditioned responses is inversely related to the number of trials per day, and that our survey included, almost exclusively, NM studies with a relatively large number of daily trials. We reanalyzed the data to eliminate this (unintended) bias. The results show that even with very few trials per day, NM conditioned responses are more slowly acquired than is conditioned suppression. We also address other issues raised by Kehoe and Macrae, such as the distinction between associative systems and peripheral responses, and other investigators’ views on two-factor theories.