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Comparison pecks/sec (± 1 SEM) on hue-identity baseline trials (open circles) and nonreinforced hue–hue probe trials (filled circles) averaged over the eight test sessions for each Group GI pigeon in Experiment 2. Matching = trials on which the comparison matched the preceding sample. Nonmatching = trials on which the comparison did not match the preceding sample. Note that the ordinate for 2 of the pigeons (GI1 and GI2) differs from the other 2.
