Ambiguous stimuli and delayed matching to sample

Four pigeons learned a delayed matching task in which the sample and choice stimuli were line orientations. After performance had reached a high level of accuracy the procedure was altered so that only one of the line choice stimuli was presented on each trial and a uniform white field was presented on the other key. In the first experiment the position of the white key alternated between left and right in successive sessions. Three of the four subjects suffered no loss in accuracy when the white key was on one side but responded only slightly better than chance when it was on the other. A second experiment showed that this position effect was not eliminated by keeping the white key in the same position for a number of sessions and, for three of the four subjects, was not affected by changing the color from white to red.