Effects of natural categories on pictorial short term memory in pigeons.

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Four pigeons were trained in a same/different task with successively presented pairs of slide pictures. On same trials, a slide identical to that of the previous trial was presented for 10 sec while extinction was in effect. On different trials, keypeckings to a slide different from that of the previous trial were reinforced at irregular intervals averaging 6 sec. The slides were a wide variety of 120 pictures such as mountains, builidings, animals, and miscellaneous objects. In Experiment 1, intertrial interval was lengthened after completion of the training. Although per-formance decreased as a function of retention interval, the pigeons retained relatively good performance even at the longest in terval of 50 sec, longer than with physical stimuli such as color or line orientation. In Experiment 2, the slides were rotated on same trials, and the subjects were tested with normally oriented pictures, left-right reversals, top-bottom reversals, and left-right and top-bottom reversals. The subjects were tested with 90 novel pictures and then with 90 familiar pictures. The subjects performed less accurately with reversed novel pictures, and top-bottom reversal more adversely affected performance than left-right reversal. The subjects maintained accurate performance with familiar ones and reversals of familiar picture had no effect on performance. Therefore, relatively long memory was in effect for pigeons' slide identification, and rotation of familiar pictures did not change the information necessary for slide identification. The subjects may have used the information of a slide as a whole, rather than small parts of picture, for slide identification.