Bidirectional conditioning: Revisiting Asratyan’s ‘alternating’ training technique

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.

Functional equivalence classes in rats: Repeated olfactory discrimination reversals and delayed stimulus probes

The simultaneous matching-to-sample procedures that are widely used to study stimulus equivalence in human participants have generally been unsuccessful in animals. However, functional equivalence classes have been demonstrated in pigeons and sea lions using a concurrent repeated reversal discrimination procedure. In this procedure, responding to one set of stimuli is reinforced but responding to a different set is not and the set associated with reinforcement is changed with multiple reversals during the experiment. The experiments reported here were designed to assess whether functional equivalence classes could be demonstrated in rats using similar techniques. Rats were initially trained with two sets of olfactory stimuli (six odors/set). Following many reversals, probe reversal sessions were conducted in which rats were exposed to a subset of the members of each set and, later in the session, the withheld stimuli were introduced. Responding to these delayed probe trials in accord with the reversed contingencies constituted transfer of function. There was some evidence of transfer in Experiment 1, but the effects were relatively weak and variable. Experiment 2 introduced procedural changes and found strong evidence of transfer of function consistent with the formation of functional equivalence classes. These procedures offer a promising strategy to study symbolic behavior in rodents.

Simultaneous learning of directional and non-directional stimulus relations in baboons (Papio papio)

While humans exposed to a sequential stimulus pairing A-B are commonly assumed to form a bidirectional mental relation between A and B, evidence that non-human animals can do so is limited. Careful examination of the animal literature suggests possible improvements in the test procedures used to probe such effects, notably measuring transfer effects on the learning of B-A pairings, rather than direct recall of A upon cuing with B. We developed such an experimental design and tested 20 Guinea baboons (Papio papio). Two pairings of visual shapes were trained (A1-B1, A2-B2) and testing was conducted in a reversed order, either with conserved pairings (B1-A1, B2-A2) or broken ones (B1-A2, B2-A1). We found baboons’ immediate test performance to be above chance level for conserved pairings and below chance level for broken ones. Moreover, baboons needed less trials to learn conserved pairings compared to broken ones. These effects were apparent for both pairings on average, and separately for the best learned pairing. Baboons’ responding on B-A trials was thus influenced by their previous A-B training. Performance level at the onset of testing, however, suggests that baboons did not respond in full accordance with the hypothesis of bidirectionality. To account for these data, we suggest that two competing types of relations were concomitantly encoded: a directional relation between A and B, which retains the sequential order experienced, and a non-directional relation, which retains only the co-occurrence of events, not their temporal order.

Associative symmetry: a divide between humans and nonhumans?

Anthropocentrism can bias scientific conclusions. As a case study, we challenge the 40-year-old associative symmetry dogma, supposed to cognitively set apart humans from other species. Out of 37 human studies surveyed, only three truly demonstrate symmetry, of which only one (on five participants) suggests that symmetry is spontaneously formed.

An update on the search for symmetry in nonhumans

Sidman et al.'s (1982) failure to find evidence for symmetry (bidirectional associations between stimuli) in monkeys and baboons set the stage for decades of work on emergent relations in nonhumans. They attributed the failure to the use of procedures that did not (1) promote stimulus control based on the relation between the sample and correct comparison and (2) reduce control by irrelevant stimulus features. Previous reviews of symmetry in nonhumans indicated that multiple exemplar training and successive matching might encourage appropriate stimulus control. This review examined 16 studies that investigated symmetry in 94 subjects, including pigeons, rats, capuchin monkeys, and baboons. Several studies used alternative training procedures to minimize sources of irrelevant stimulus control, and many combined multiple exemplar training with other procedural modifications. Symmetry was observed in approximately 30% of subjects. Studies that reported the strongest evidence for symmetry used successive matching-to-sample procedures that included training on both symbolic and identity relations, and studies finding mixed evidence employed alternative methods. These studies highlight the challenge in creating training procedures that promote symmetry and the need to assess the underlying sources of control on positive demonstrations.