De Monte M., Le Pape, G. 1997. Behavioural effects of cage enrichment in single-caged adult cats. Animal Welfare 6, 53-66.

A loss of interest in objects [tennis ball suspended 12 cm above the floor; 12 cm diameter x 40 cm long wooden log hooked against the wall] over time was observed. On the fifth day after the introduction, [single-caged] cats spent only 3 per cent of their time using the log, and 10 per cent using the ball.