Fig. 7.

Comparison–response rates in pecks/sec (± 1 SEM) on the symbolic matching baseline trials (open circles) and the nonreinforced symmetry probe trials (filled circles) averaged over the eight test sessions for each Group HO pigeon. Positive = reinforced symbolic baseline trials and test trials in which the samples and comparisons of the reinforced baseline trials were reversed. Negative = nonreinforced symbolic baseline trials and test trials on which the samples and comparisons of the nonreinforced baseline trials were reversed.