Average percentage of correct choices (± 1 standard error of the mean) across 12 pigeons on arbitrary (AB) matching-to-sample baseline trials with left- and right-key samples, arbitrary (AB) matching-to-sample trials with novel-location (center-key) samples, and symmetry (BA) test trials with center-key samples. Adapted with permission from “Stimulus control topographies and tests of symmetry in pigeons”, by K. M. Lionello-DeNolf and P. J. Urcuioli, 2002, Journal of the Experimental Analysis of Behavior, 78, pp. 472–473. ©2002 by the Society for the Experimental Analysis of Behavior.