A Review of Backward Higher-Order Conditioning: Implications for a Pavlovian Conditioning Analysis of Stimulus Equivalence

Stimulus equivalence (SE) is demonstrated when participants exposed to conditional discrimination training pass tests for reflexivity, symmetry, transitivity, and equivalence (symmetry combined with transitivity). Most theorists attribute the origin of SE to operant processes, but some argue that it results from Pavlovian conditioning. Symmetry is problematic for the latter hypothesis because it seems to require excitatory backward conditioning. However, equivalence tests resemble backward sensory preconditioning (BSP) and backward second-order conditioning (BSOC), two well-established processes. A review of associationistic theories of BSP and BSOC showed that the temporal coding hypothesis (TCH) explains outcomes that other associationistic theories cannot explain (i.e., BSOC and BSP effects after first-order conditioning with delay vs. trace conditioning and forward vs. backward conditioning). The TCH assumes that organisms encode the temporal attributes of stimulus events (e.g., order and interval duration) and this temporal information is integrated across separate phases of training. The TCH seems compatible with a behavioral analysis if direct stimulus control replaces the notion of temporal maps. The TCH perspective does not seem applicable to SE because SE tests are not predictive tasks. This suggests that SE is fundamentally different from BSP and BSOC and a Pavlovian conditioning analysis of SE is inadequate. This conclusion is consistent with previous criticism of a Pavlovian account of SE according to which Pavlovian conditioning cannot be interpreted as stimulus substitution.