A study of primate perceptual completion in a comparative and developmental perspective.

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1)Perceptual completion processes of pigeons and rhesus macaques were experimentally analyzed in a comparative perspective. The subjects were trained to classify the absolute length of a single black horizontal bar at the center of the CRT display with a touch sensor on it into "long" or "short" ; they actually touched either of the two response location at the bottom of the display. After they mastered this task, a large gray box was placed next to the target bar. The gap between the target bar and the gray box remained constant in the baseline training. After the subject performed well in this situation, they were tested how the proportion of "long" and "short" responses were affected by the size of the gap in probe trials in which all the responses were nondifferentially reinforced. The monkey consistently reported "long" more often than baseline when the gap was 0. That is, they overestimated the length of the target bar when it touch to the gray box. This suggests that the monkeys perceptually completed the "hidden" part of the target bar. On the other hand, pigeons showed a similar tendency neither in this situation nor the two experiments that followed. There seems to be a big difference in the prosses of perceptual completion in pigeons and nonhuman primates. 2)The spatiotemporal boundary formation (Shipley & Kellman, 1994) was studied in the chimpanzee. Two chimpanzees were shown random dots on the touchsensitive monitor. The dots within an imaginary figure had a color different from the others out of it. The subjects matched 5 different imaginary figures to solid ones. When the imaginary figure noved, the subjects' matching accuracy was higher than when it stayd still. Like humans, chimpanzees may integrate the spatial and temporal information to perceive invisible contours.