Associative symmetry and stimulus-class formation by pigeons: the role of non-reinforced baseline relations

Abstract

Two experiments tested the assumption of Urcuioli's (2008) theory of pigeons' equivalence-class formation that consistent non-reinforcement of certain stimulus combinations in successive matching juxtaposed with consistent reinforcement of other combinations generates stimulus classes containing the elements of the reinforced combinations. In Experiment 1, pigeons were concurrently trained on symbolic (AB) and two identity (AA and BB) successive tasks in which half of all identity trials ended in non-reinforcement but all AB trials were reinforced, contingent upon either responding or not responding to the comparisons. Subsequent symmetry (BA) probe trials showed evidence of symmetry in one of four pigeons. In Experiment 2, pigeons learned three pair-comparison tasks in which left versus right spatial choices were reinforced after the various sample-comparison combinations comprising AB, AA, and BB conditional discriminations. Non-differentially reinforced BA probe trials following acquisition showed some indication of symmetrical choice responding. The overall results contradict the theoretical predictions derived from Urcuioli (2008) and those from Experiment 2 challenge other stimulus-class analyses as well.

Figure 1

Top panel: The stimulus classes hypothesized to develop from the reinforced sample-comparison successive matching sequences in hue-form symbolic matching, hue identity matching, and form identity matching. Ellipses highlight common class members. Bottom panel: Two 4-member stimulus classes hypothesized to arise from the merger of the stimulus classes shown in the top panel via their common elements. Arrows indicate sample-comparison sequences to which pigeons should preferentially respond in a symmetry test assuming reinforced red sample – triangle comparison and green sample – horizontal sequences in training. R = red, G = green, T = triangle, H = horizontal, 1 = first ordinal position within a matching trial, 2 = second ordinal position within a matching trial.

Figure 2

Comparison-response rates (pecks/s ± 1 SEM) on symbolic matching baseline trials (open circles) and non-reinforced symmetry probe trials (filled circles) averaged over the eight test sessions for each pigeon in Experiment 1 that met the acquisition criterion. Positive = symbolic baseline relations for which responding to the comparisons was reinforced and their symmetrical test relations. Negative = symbolic baseline relations for which not-responding to the comparisons was reinforced and their symmetrical test relations. Note the different ranges of comparison-response rates across pigeons that are identified individually in the bottom left corner of each panel.

Figure 3

Average number of training sessions to reach 80% correct or better matching accuracy (± 1 SEM) on each component pair-comparison task in Experiment 2 for the five pigeons that met the acquisition criteria.

Figure 4

The percentage of symmetry-consistent choices for each pigeon meeting the acquisition criteria in Experiment 2 averaged over all test sessions. Stars indicate performances significantly above chance (indicated by the dashed line).