Mirror-induced self-directed behaviors in rhesus monkeys after visual-somatosensory training

Mirror self-recognition is a hallmark of higher intelligence in humans. Most children recognize themselves in the mirror by 2 years of age. In contrast to human and some great apes, monkeys have consistently failed the standard mark test for mirror self-recognition in all previous studies. Here, we show that rhesus monkeys could acquire mirror-induced self-directed behaviors resembling mirror self-recognition following training with visual-somatosensory association. Monkeys were trained on a monkey chair in front of a mirror to touch a light spot on their faces produced by a laser light that elicited an irritant sensation. After 2-5 weeks of training, monkeys had learned to touch a face area marked by a non-irritant light spot or odorless dye in front of a mirror and by a virtual face mark on the mirroring video image on a video screen. Furthermore, in the home cage, five out of seven trained monkeys showed typical mirror-induced self-directed behaviors, such as touching the mark on the face or ear and then looking at and/or smelling their fingers, as well as spontaneously using the mirror to explore normally unseen body parts. Four control monkeys of a similar age that went through mirror habituation but had no training of visual-somatosensory association did not pass any mark tests and did not exhibit mirror-induced self-directed behaviors. These results shed light on the origin of mirror self-recognition and suggest a new approach to studying its neural mechanism.