Michael J. Beran, Ph.D.
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September 1, 2021



Beran, M. J. (2004). Chimpanzees (Pan troglodytes) respond to nonvisible sets after one-by-one addition and removal of items. Journal of Comparative Psychology, 118, 25-36.

Two chimpanzees made numerousness judgments of nonvisible sets of items. In Experiment 1, 1 to 10 items were dropped, one-at-a-time, into one opaque cup, and then an additional 1 to 10 items were dropped, one-at-a-time, into another opaque cup. The chimpanzeesí performances were high and were more dependent on factors indicative of an analog magnitude mechanism for representation of set size than on a mechanism such as an object file. In Experiment 2, a third visible set was made available after the sequential presentation of the first two sets. The chimpanzees again performed at high levels in selecting the largest of the three sets. In Experiment 3, one of the two initially presented sets was reduced in number by the sequential removal of 1, 2, or 3 items. Both chimpanzees performed above chance levels for the removal of one item but not for the removal of more than one item.

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