The hierarchical representation of three-item sequences

Pigeons were trained to discriminate one sequence of three colors from other sequences made from the same three colors. At each stage of acquisition, a regression analysis estimated the degree to which the birds' performance was controlled by each of seven hierarchically organized units. Three units represented single elements acting individually (Stimulus 1, Stimulus 2, and Stimulus 3), three represented pairs of stimuli (i.e., the combination of Stimuli 1 and 2, the combination of Stimuli 2 and 3, and the combination of Stimuli 1 and 3), and one represented the triplet of all three stimuli. Responding was initially controlled by the third stimulus in the sequence, but eventually came under the control of higher order units representing combinations of stimuli. These results indicate that pigeons are capable of using coherent, hierarchical representations of sequence information. They also argue against a number of list-processing schemes, such as a retrospective trace-strength discrimination scheme and a prospective conditional sequential discrimination scheme.