Asymmetries in the production of self-directed behavior by chimpanzees and gorillas during a computerized cognitive test.

Self-directed behaviors (SDBs) are a commonly used behavioral indicator of arousal in nonhuman primates. Experimental manipulations, designed to increase arousal and uncertainty, have been used to elicit SDB production in primates. Beyond measuring rates of SDB production, researchers have also recorded their lateralized production by primates, thought to reflect laterality of hemispheric brain control and response to emotion. Although a handful of such studies exist, all have been conducted with chimpanzees. Expanding on this line of inquiry, we tested both chimpanzees (N = 3) and gorillas (N = 3) in a serial learning task presented on a touchscreen interface that incorporated both EASY (two-item list) and HARD (four-item list) versions of the task. Although SDB production by the apes did not differ across the two levels of task complexity, both species produced higher rates of SDB when they made an error, regardless of task difficulty. Furthermore, the apes made more SDB with the left hand-directed to the right side of their body (contralateral SDB) and left side of their body (ipsilateral SDB)-when they made an incorrect response. There was no difference in the rate of SDB produced with the right hand across correct compared to incorrect trials. The apes' responses reflect previous reports that show humans are quicker at selecting negative emotional stimuli when using their left, compared to their right, hand (the reverse is true for positive stimuli). However, previous work has shown that chimpanzees are more likely to produce (contralateral) SDB with their right hand when aroused and so we discuss our results in relation to these findings and consider how they relate to the 'right hemisphere' and 'valence' models of emotional processing in apes.