Brent, L., Stone, A. M. 1993. Long-term use of enrichment for paired and singly caged chimpanzees. American Journal of Primatology 31, 301 (Abstract).

Chimpanzees used televisions, balls, and mirrors for 0.27-1.53% of the observation time after several years of exposure to the enrichment items. Television and ball use were significantly higher than mirror use.