Duration ratio discrimination in pigeons: a criterion-setting analysis.

Pigeons received trials beginning with a sequence of two colors (blue-->yellow) on the center key of a three-key array. The colors lasted different lengths of time. At the end of the sequence pigeons chose between two keys based on a criterial ratio of the temporal sequence. One choice was reinforced if the time ratio was less than the criterion and the alternate choice was reinforced if the time ratio was greater than the criterion. The criterial ratios (first to second duration) were 1:1, 1.5:1, and 3:1. The same set of intervals was used for the different criterion ratios, producing a balanced distribution of time ratios for the 1.5:1 condition, and unbalanced distributions for the 1:1 and 3:1 conditions. That is, for the 1.5:1 condition half of the duration pairs were less than the criterion and half were greater. However, for the 1:1 and 3:1 conditions, more duration pairs were less than (3:1) or greater than (1:1) the criterion. Accuracy was similar across criterion ratios, but response bias was influenced by the asymmetries of time ratios in the 1:1 and 3:1 conditions. When these asymmetries were controlled, the response biases were reduced or eliminated. These results indicate that pigeons are flexible in establishing a criterion for discriminating duration ratios, unlike humans, who are less flexible and are bound to categorical distinctions in the discrimination of duration ratios.