Bryant, C. E., Rupniak, N. M. J., Iversen, S. D. 1988. Effects of different environmental enrichment devices on cage stereotypies and autoaggression in captive cynomolgus monkeys. Journal of Medical Primatology 17(5), 257-269.

Autoaggression and stereotypies in individually housed cynomolgus monkeys were compared in a standard primate cage and an enriched playpen environment. Stereotypy and autoaggression were markedly reduced in the playpen, but reappeared on return to the home cage. Some of the various activities available in the playpen but not others engaged the animals' attention.