Janice E. Weaver

Transitive inference in pigeons: Simplified procedures and a test of value transfer theory

Minimal procedures for the demonstration of transitive inference (TI) in animals have involved the training of four simultaneous discriminations: for example, A+B−, B+C−, C+D−, and D+E−, followed by the demonstration of a preference for B over D on test trials. In Experiment 1, we found that TI in pigeons can be found with successive training involving A+B−, B+C−, A+C−, C+D−, D+E−, C+E−, and A+E−. In Experiment 2, we found that demonstration of TI did not require inclusion of experience with the nonadjacent stimulus pairs (A+C−, C+E−, A+E−). Experiment 3 provided a test of value transfer theory (VTT; Fersen, Wynne, Delius, & Staddon, 1991). When pigeons were trained with stimulus pairs that did not permit the transitive ordering of stimuli, but did permit the differential transfer of value (e.g., A+B−, C−E+, C+D−, & A+E−), preference for B over D was still found. Analyses of the relation between direct experiences with reinforced and nonreinforced responding and stimulus preferences on test trials failed to support a reinforcement-history account of TI.

Transitive inference in pigeons: Control for differential value transfer

Transitive inference (TI) effects have been demonstrated in several nonhuman species using a nonverbal version of the task in which A is better than B is represented in a simple simultaneous discrimination, A+B−. Following five-term training (e.g., A+B−, B+C−, C+D−, D+E−), the choice of B over D on test trials is taken as evidence of TI. Recently, differential value transfer from the S+ to the S− in a simultaneous discrimination has been proposed as a noncognitive basis for these effects. Two experiments were conducted to control for differential value transfer. The results suggest that differential value transfer is not the only basis for nonhuman TI performance. An alternative account based on spatial mapping is discussed.

Pigeons group time intervals according to their relative duration.

In the present research, we asked whether pigeons tended to judge time intervals not only in terms of their absolute value but also relative to a duration from which they must be discriminated (i.e., longer or shorter). Pigeons were trained on two independent temporal discriminations. In one discrimination, sample durations of 2 and 8 sec were associated with, for example, red and green hue comparisons, respectively, and in the other discrimination, sample durations of 4 and 16 sec were associated with vertical and horizontal line comparisons, respectively. If pigeons are trained on a temporal discrimination and tested with intermediate durations, the subjective midpoint typically occurs close to the geometric mean of the two trained values. The 4- and 8-sec values were selected to be the geometric mean of the two values in the other discrimination. When a 4-sec test sample was presented with the comparisons from the 2- and 8-sec discrimination, the pigeons preferred the comparison associated with the shorter sample. Similarly, when an 8-sec test sample was presented with the comparisons from the 4-and 16-sec discrimination, the pigeons preferred the comparison associated with the longer sample. Thus, a relative grouping effect was found. That is, durations that should have produced indifferent choice were influenced by their relative durations (shorter than or longer than the alternative) during training.

Presence/Absence-Sample Matching by Pigeons: Divergent Retention Functions May Result From the Similarity of Behavior During the Absence Sample and the Retention Interval

Divergent choose-absence retention functions typically found in pigeons following presence/absence-sample matching have been attributed to the development of a single-code/default coding strategy. However, such effects may result from adventitious differential responding to the samples. In Experiment 1, retention functions were divergent only when differential sample responding could serve as the basis for comparison choice. In Experiment 2, when pecking did not occur during the retention interval, a choose-absence bias was found, but when pecking occurred during the retention interval, a choose-presence bias resulted. In Experiment 3, positive transfer was found when a stimulus associated with the absence of pecking replaced the absence sample but not when a stimulus associated with pecking replaced the presence sample. Thus, presence/absence-sample matching may not encourage the development of a single-code/default coding strategy in pigeons.

Present/absent sample matching in pigeons: Is comparison choice controlled by the sample stimulus or by differential sample responding?

In two experiments involving present/absent sample matching, we tested whether the visual stimuli or differential sample behavior served as the basis for comparison choice. In both experiments, one group (FR/DRO) was required to peck the present sample and to refrain from pecking the absent sample (as typically occurs with fixed duration present/absent samples), and the other group (FR/FR) was required to peck both samples. In Experiment 1, the samples were a black dot on a white field (present) and the white field alone (absent). In Experiment 2, the samples were a yellow hue (present) and a dark response key (absent). In both experiments, divergent retention functions were found only for the FR/DRO group. These results suggest that, in nonhedonic present/absent sample matching, it is the behavior directed toward the present sample, rather than the visual stimulus itself, that serves as the basis for comparison choice.

Value transfer in concurrent-schedule discriminations by pigeons

When pigeons are trained on a discrete-trial simultaneous discrimination, some of the value associated with the positive stimulus appears to transfer to the negative stimulus (Zentall & Sherburne, 1994). Pigeons preferred a negative stimulus that had been discriminated from an always-positive stimulus (S+) over a negative stimulus that had been discriminated from a sometimes-positive stimulus (S±). A very different finding (suggestive of transitivity of preference or contrast) was reported by Belke (1992). On concurrent probe tests of stimuli associated with equal variable interval (VI) schedules but originally trained in alternative concurrent pairs (one with a richer schedule, the other with a poorer schedule—VI 20 sec vs. VI 40 sec and VI 40 sec vs. VI 80 sec), the stimulus originally paired with the poorer schedule was preferred. But Belke’s results may have been obtained because the pigeons had been trained to peck the VI 40 sec paired with the poorer schedule and they had been trained not to peck the VI 40 sec paired with the richer schedule. In the present experiment, we avoided this bias by training pigeons on two concurrent schedules in which the tested stimuli both had been associated with the poorer schedule of the pair [A(VI 20 sec) vs. B(VI 80 sec) and C(VI 40 sec) vs. D(VI 80 sec)]. Evidence for value transfer was demonstrated when on probe trials pigeons preferred B over D.

Symmetry training in pigeons can produce functional equivalences

Functional stimulus equivalence has been demonstrated using a transfer of training design with matching-to-sample training in which two sample stimuli are associated with the same comparison stimulus (A-B, C-B; many-to-one matching). Equivalence is shown by training a new association (A-D) and demonstrating the presence of an emergent relation (C-D). In the present experiment, we show that symmetry training, in which a bidirectional association is trained between two stimuli (A-B, B-A, using successive stimulus presentations followed by reinforcement), can also produce functional equivalence using a transfer of training design (i.e., train B-C, test A-C). The results suggest that training pigeons in the substitutability of two stimuli may be sufficient to produce functional stimulus equivalence between them. The results also have implications for the development of an emergent transitive relation, because training on A-B and B-C relations results in the emergence of an untrained A-C relation, if B-A training also is provided.

Simultaneous Discrimination Learning in Pigeons: Value of S - Affects the Relative Value of its Associated S+

In a simple simultaneous discrimination involving a positive stimulus (S+) and a negative stimulus (S−), it has been hypothesized that positive value can transfer from the S+ to the S− (thus increasing the relative value of the S−) and also that negative value can transfer from the S− to the S+ (thus diminishing the relative value of the S+ Fersen, Wynne, Delius, & Staddon, 1991). Evidence for positive value transfer has been reported in pigeons (e.g. Zentall & Sherburne, 1994). The purpose of the present experiments was to determine, in a simultaneous discrimination, whether the − diminishes the value of the S+ or the Sis contrasted with the S+ (thus enhancing the value of the S+). In two experiments, we found evidence for contrast, rather than value transfer, attributable to simultaneous discrimination training. Thus, not only does the S+ appear to enhance the value of the S−, but the S− appears to enhance rather than reduce the value of the S+.