Attention shift and errorless reversal learning by the California sea lion.

Repeated combinations of a previously well-learned size clue preference with the previously negative form cue followed by gradual reduction of the size cue resulted in nearly errorless performance by two sea lion. on a series of nine form-discriminination reversals. Systematic insertion of probe trials, during which the size cue was not present, revealed that attention was primarily focused on the size dimension during early stages of training, when size differences were large, and then was gradually shifted to the form dimension as a function of the increasing difficulty of the size discrimination.