Visual and auditory conditional position discrimination in chimpanzees (Pan troglodytes). 
Chimpanzee cognition has been studied predominantly through the visual modality, and much less through the auditory modality.  The aim of this study was to explore possible differences in chimpanzees' processing of visual and auditory stimuli.  We developed a new conditional position discrimination (CPD) task requiring the association between a stimulus (from either the auditory or the visual modality), and a spatial position (left or right).  The stimuli consisted of the face and voice of two individuals well known to the subjects (one chimpanzee and one human).  Six chimpanzees participated in both the visual and the auditory conditions.  We found contrasting results between the two conditions: the subjects acquired the CPD more easily in the visual than in the auditory condition.  This supports previous findings on the difficulties encountered by chimpanzees in learning tasks involving auditory stimuli.  Our experiments also revealed individual differences: the chimpanzee with the most extensive experience in symbolic visual matching tasks showed good performance in both conditions.  In contrast, the chimpanzee expert in an auditory-visual intermodal matching task showed no sign of learning in either condition.  Future work should focus on finding the most appropriate procedure for exploring chimpanzees' auditory-visual cognitive skills.