Recently, Redford (2010) found that monkeys seemed to exert metacognitive control in a category-learning paradigm. Specifically, they selected more trials to view as the difficulty of the category-learning task increased. However, category-learning difficulty was determined by manipulating the family resemblance across the to-be-learned exemplars. Although this effectively influenced the learning difficulty, difficulty was confounded with novelty. For instance, a weak family resemblance made category learning difficult, but also increased the amount of perceptual change from trial to trial. The current research rules out novelty in favor of difficulty by manipulating the number of dots involved in the dot distortions while controlling the amount of perceptual change.
Redford, Joshua S.
"Humans and Monkeys Exert Metacognitive Control Based on Learning Difficulty in a Perceptual Categorization Task,"
The Psychological Record: Vol. 60
, Article 2.
Available at: http://opensiuc.lib.siu.edu/tpr/vol60/iss4/2