Comparative study of figure perception by nonhuman and human primates

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Perception of geometric figures by nonhuman primates were studied in terms of : a) perception of the Ponzo illusion and b) visual search performances. For the study of illusory perception, rhesus monkeys and chimpanzees were trained to discriminate absolute length of horizontai bars on the computer display. Touching one of the two report locations was reinforced when the bar was longer than a predetermined length and touching the other location was reinforced when the bar was shorter than that. A variety of context figures were superimposed on the bar. When the converging lines were superimposed, the proportion of the "long" reports by both subject species was increased as the target bar approached the apex of the context lines. It was demonstrated that both species perceived the Ponzo illusion. The increase of perspectives of context figures did not enhance the illusion. The gap between the target bar and the context figures were suggested as an important determinant of this illusion but figural contexts above and below the target bar also affected the amount of this illusion. Japanese monkeys were tested for their visual search performances. The subjects had to touch the odd target figure among number of distracter figures. Two major findings were obtained : a) oblique bars were easily detected among vertical bars but the latter figures were difficult to find among the former figures. Thus search asymmetry was found. b) visual search was slow when the target was defined by the conjunction of inclination and the texture of the bars. These findings were similar to human performances.