Two adult chimpanzees (Pan troglodytes) were trained to discriminate a biological motion pattern (chimpanzee quadrupedal walking) from a random motion pattern using the visual search task. One of the chimpanzees acquired this task and showed clear search asymmetry for biological motion patterns (Experiment 1). The subject exhibited faster and flat response times when the target was the random pattern and the distractors were normal walking patterns, whereas she showed slower and increasing response times as a function of display size when the target was the normal pattern. Experiment 2 confirmed that this discrimination was not due to the synchronized movements of the distractors. To examine what features caused the search asymmetries, the subject was given a further task where she had to discriminate the original patterns (normal and random) from their scrambled patterns. In these scrambled patterns the movements of each point remained unchanged but the positions or starting frames were randomized (Experiment 3). The subject again showed search asymmetries in response times (faster response times for the scrambled than for the original target) both for the normal and random patterns. Thus, search asymmetries observed in the present study were not specific to the biological motion patterns. These results are discussed in relation to deviations from familiarity and from the perspective of movements specific to biological motions.

Key words: biological motions, visual search, search asymmetry, chimpanzees