Unexpected discrimination strategy used by pigeons

Pigeons were conditioned with a symbolic matching-to-sample paradigm. Six visual patterns in 16 configurations were presented in an operant chamber where reinforcement was delivered next to the correct keys. A test with novel configurations, planned to demonstrate associative transitivity between equivalent stimuli, revealed instead a different but consistent behavior. An explanation for this behavior was sought in further tests and in a detailed post-hoc analysis of the training configuration. The analysis revealed an overall location imbalance of the comparison stimuli in relation to the reinforcement sites. This apparently led to response side biases controlled by these lateral stimuli. The results of additional tests accorded with this hypothesis. The directing effect was unequivocal when both lateral stimuli biased a response in the same direction. When the directing effects by these stimuli were opposite, the animals only solved the task when they could resort to a rote learning of individual configurations. The characteristics of the rules that were identified are contrasted with those that have previously been thought to operate during conditional discrimination learning.