This apparently led to response side biases controlled by these lateral stimuli. The analysis revealed an overall location imbalance of the comparison stimuli in relation to the reinforcement sites. An explanation for this behavior was sought in further tests and in a detailed post-hoc analysis of the training configuration. A test with novel configurations, planned to demonstrate associative transitivity between equivalent stimuli, revealed instead a different but consistent behavior. Six visual patterns in 16 configurations were presented in an operant chamber where reinforcement was delivered next to the correct keys. Pigeons were conditioned with a symbolic matching-to-sample paradigm. The results agree with the assumption that both humans and pigeons encode stimulus inequality structures with parallel processing neural networks rather than with a sequentially processing algorithm. The time it took the subjects to learn the tasks as well as the final choice latencies and/or error rates increased with the number of deviating inequalities. For a second and third group, the reinforcement allocations of one or three, respectively, of the stimulus pairs deviated from such ordering. For one group, the reward/punishment allocations within the pairs agreed with a linear hierarchy. Groups of subjects of each species were taught to discriminate all 10 pairwise combinations of 5 stimuli with an operant conditioning method. The experiments reported here compare the efficiency with which humans and pigeons process sets of stimulus pairs embodying different inequality structures. The possibility that an artfactual cue may have inadvertently accentuated this capability in an earlier own experiment is considered.Ĭarmesin and Schwegler (1994) have determined theoretically that a linear hierarchical stimulus structure can be encoded by a parallel network of minimal complexity. A smaller-scale experiment using slide-projected unequivocal symmetric and asymmetric patterns yielded results compatible with the supposition that pigeons are capable of a symmetry–asymmetry categorization. Additionally one of the studies involved patterns of inconsistent symmetry at global and local levels. This circumstance turns out to constrain conclusions drawn by earlier symmetry–asymmetry studies that used computer-generated patterns displayed on cathode ray tube monitors as these suffered from pictorial distortions. Attention is drawn to pigeons’ comparatively superior visual flicker resolution and superior visual linear acuity by reporting results of two ad-hoc experiments. This note looks into the reasons why earlier reports may have arrived at differing conclusions about pigeons’ capacity to categorize bilaterally symmetric and asymmetric visual patterns.
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