A learning curve for brightness discrimination in one budgerigar.

<p>Two stimulus pairs were used in training, one pair with a 0.38 (solid line with filled circles) and one pair with 0.18 Michelson contrast (dashed line with filled squares). In each training session, the stimulus pairs were presented 10 times each in a mixed random order. This bird qualified for testing by fulfilling the criterion of having correct choice frequencies of at least 80% for both stimulus pairs in two consecutive sessions (session 6 and 7 in this example). All birds completed training within 7 sessions.