Stepwise learning of compound multidimensional visual stimuli by sorting out the dimensions of which they are composed in pigeons.

Pigeons (Columba livia) were trained on a stage-wise go/no-go visual discrimination task. Sixteen compound stimuli were created from all the possible combinations of two stimulus values from four separable visual dimensions: shape (circle/square), size (large/small), line orientation (horizontal/vertical), and brightness (dark/light). Starting with 1 S + and 1 S - that differed in all four-dimensional values, in our later steps, we added S - stimuli one by one, sharing at first 1, then 2, and then 3 dimensions with S + by sorting them out. When the pigeons had clearly shown attending to each of four dimensions, we presented all 16 stimuli. In this last stage, the pigeons correctly rejected most of the S - stimuli despite seeing them for the first time. Thus, to discriminate 16 unique multidimensional stimuli, it was not necessary to learn all of them as compound stimuli in such an approach. However, the 4 learnt dimensions did not give fully comprehensive information about all the new and unique compound stimuli presented in the last stage. Mistakes were associated with similarity to S + and with the order of dimensional learning. Most pigeons made mistakes in the discrimination of S - stimuli that shared 3 (some shared two) dimensions with S + . The knowledge of the first-learned dimension of compound stimuli was less reliable than the dimensions learned in the last stage.