Fig 4.

Comparison pecks/sec (± 1 SEM) on the baseline arbitrary matching trials (open circles) and nonreinforced probe trials (filled circles) averaged over the eight form–form (B–B) test sessions in Experiment 2 for the 4 Group TRANS pigeons that did not exhibit emergent A–A matching in Experiment 1. Positive  =  reinforced arbitrary baseline trials and test trials on which the form comparison matched the preceding form sample. Negative  =  nonreinforced arbitrary baseline trials and test trials on which the form comparison did not match the preceding form sample. Note that ordinates differ across pigeons.