Similarities between Pavlovian conditioning in nonhumans and causal judgment by humans suggest that similar processes operate in these situations. Notably absent among the similarities is backward blocking (i.e., retrospective devaluation of a signal due to increased valuation of another signal that was present during training), which has been observed in causal judgment by humans but not in Pavlovian responding by animals. The authors used rats to determine if this difference arises from the target cue being biologically significant in the Pavlovian case but not in causal judgment. They used a sensory preconditioning procedure in Experiments 1 and 2, in which the target cue retained low biological significance during the treatment, and obtained backward blocking. The authors found in Experiment 3 that forward blocking also requires the target cue to be of low biological significance. Thus, low biological significance is a necessary condition for a stimulus to be vulnerable to blocking. In recent years, numerous researchers have remarked on the similarity of the conditions that encourage the acquisition of causal relationships in humans and those that foster

Despite the many demonstrations of blocking in animals, there is still little evidence of blocking with human subjects, which is problematic for general learning and behavior theory. The purpose of this research was to examine blocking with human subjects using a design and behavioral procedure (conditioned suppression) similar to those commonly used in animal research. First, subjects learned an operant task. Later, they were instructed to suppress responding when a visual US was presented. Two Pavlovian acquisition phases and a test phase occurred while the subjects were performing the operant task. In the first Pavlovian phase, CS A predicted the US for the experimental group, but was uncorrelated with the US for the control group. In the second Pavlovian phase, a compound CS AX predicted the US for both groups. At test, CS X was presented to all subjects and suppression ratios were assessed. Experimental subjects suppressed responding in the presence of CS X less than did control subjects, thereby demonstrating a blocking effect. This research, in demonstrating blocking in humans, adds to the known similarities in animal and human behavior. � 1997 Academic Press Since Kamin described the blocking effect in 1968, this effect has been investigated primarily with rats and other nonhuman animals as experimental subjects. In a traditional blocking procedure, there are two acquisition phases. In the first phase, the experimental group is exposed to a conditioned stimulus