Same/different discrimination learning with trial-unique stimuli.

A long-standing issue in same/different discrimination learning concerns the possible role of individual stimulus memory through repeated presentation. The aim of eliminating any effect of repetition prompted us to devise a new method for generating trial-unique stimuli. These stimuli were arrays of 16 mosaics, each containing 16 cells, which could be filled with 16 possible luminance levels. In Experiment 1, we successfully trained 4 pigeons with these trial-unique stimuli in a two-alternative forced choice same/different discrimination task to 80% correct-choice performance. We later conducted two tests that explored the nature of this discrimination and suggested that pigeons compared the mosaics in the arrays on the basis of their spatial configurations, not on the basis of lower level perceptual properties. In Experiment 2, college students responded similarly to the same sequence of training and testing. Our results suggest that pigeons and people may use similar mechanisms in relational discrimination learning.