Social and nonsocial stimuli alter the performance of bonobos during a pictorial emotional Stroop task.

The emotional Stroop task is a paradigm commonly applied in human studies to investigate how emotionally laden stimuli interfere with cognitive processes. Recent modifications of this task have enabled researchers to study similar Stroop effects in zoo-housed primates. Across three experiments using a pictorial emotional Stroop task, we investigated if the attention of bonobos was influenced by social (facial expressions during play, conflict, and neutral events) and nonsocial stimuli (a preferred food item, predator, and flower). Four bonobos successfully learned to complete the task on a touchscreen. First, we tested the bonobos on a standard color-interference Stroop task and found that they made more errors in color-congruent trials. Second, we included facial expressions of unknown conspecifics and found that it took the bonobos longer to select targets with play facial expressions compared to neutral expressions. Last, we included objects and found that the negative, positive and neutral objects altered performance. Our findings show that the cognitive processes of bonobos are influenced by both relevant social and nonsocial stimuli. Specifically, play faces interfered with the bonobos' attention suggesting that these facial expressions form a salient stimulus within bonobo society. Nonsocial stimuli also altered accuracy and reaction times during the task which may be explained by their evolutionary relevance. Our results help us to better understand the (socio-)emotional competencies of bonobos and how they respond to external stimuli. Future studies can further examine how a wider range of biologically relevant stimuli interfere with attentional processes in bonobos.