The processing of positional information in a two-item sequence limits the emergence of symmetry in baboons (Papio papio), but not in humans (Homo sapiens).

When trained to associate Stimulus A to Stimulus B, humans can derive the untrained symmetrical B to A relation while nonhuman animals have much more difficulties. Urcuioli (2008, Journal of the Experimental Analysis of Behavior, 90, 257--282; 2015, Conductal, 3, 4--25) proposed that the apparent difficulty of animals in symmetry testing reflects their double encoding of the information on the stimuli (identity and relation) and their positional (i.e., spatial and temporal/ordinal) characteristics. This comparative study tested the emergence of symmetry in humans and baboons in a task in which the position of the stimuli was manipulated independently of their relation. Humans and baboons initially learned to associate pairs of visual shapes on a touch screen in a specific order. Three pairs of (A-B, C-D, and E-F) stimuli were used in training. After training, the two species were tested with the B-A, F-C, and E-D pairs. The B-A pairs preserved the association initially learned with A-B but reversed the positional information relative to training. The F-C pair neither preserved the association nor the positional information of the training pairs, and positional information were the only cues preserved in the E-D pair. Humans showed a response time advantage for B-A, suggesting symmetry, but also for E-D, suggesting that they also process positional information. In baboons, the advantage was found only for E-D, suggesting that they only process positional information. These results confirm that the processing of stimulus pairs differ between nonhuman animals to humans.