title: Two-item same/different discrimination in rhesus monkeys (Macaca mulatta).
abstract: Almost all nonhuman animals can recognize when one item is the same as another item. It is less clear whether nonhuman animals possess abstract concepts of "same" and "different" that can be divorced from perceptual similarity. Pigeons and monkeys show inconsistent performance, and often surprising difficulty, in laboratory tests of same/different learning that involve only two items. Previous results from tests using multi-item arrays suggest that nonhumans compute sameness along a continuous scale of perceptual variability, which would explain the difficulty of making two-item same/different judgments. Here, we provide evidence that rhesus monkeys can learn a two-item same/different discrimination similar to those on which monkeys and pigeons have previously failed. Monkeys' performance transferred to novel stimuli and was not affected by perceptual variations in stimulus size, rotation, view, or luminance. Success without the use of multi-item arrays, and the lack of effect of perceptual variability, suggests a computation of sameness that is more categorical, and perhaps more abstract, than previously thought.
authors.affiliation: Section on the Neurobiology of Learning and Memory, Laboratory of Neuropsychology, National Institute of Mental Health, NIH, Building 49, Room 1B80, 49 Convent Drive MSC 4415, Bethesda, MD, 20892-4415, USA. benjamin.basile@nih.gov.
authors.forename: Benjamin M
authors.initials: BM
authors.lastname: Basile
authors.affiliation: Section on the Neurobiology of Learning and Memory, Laboratory of Neuropsychology, National Institute of Mental Health, NIH, Building 49, Room 1B80, 49 Convent Drive MSC 4415, Bethesda, MD, 20892-4415, USA.
authors.forename: Emily J
authors.initials: EJ
authors.lastname: Moylan
authors.affiliation: Section on the Neurobiology of Learning and Memory, Laboratory of Neuropsychology, National Institute of Mental Health, NIH, Building 49, Room 1B80, 49 Convent Drive MSC 4415, Bethesda, MD, 20892-4415, USA.
authors.forename: David P
authors.initials: DP
authors.lastname: Charles
authors.affiliation: Section on the Neurobiology of Learning and Memory, Laboratory of Neuropsychology, National Institute of Mental Health, NIH, Building 49, Room 1B80, 49 Convent Drive MSC 4415, Bethesda, MD, 20892-4415, USA.
authors.forename: Elisabeth A
authors.initials: EA
authors.lastname: Murray
mesh_terms: D000818:Animals; D002755:Choice Behavior; D003210:Concept Formation; D004193:Discrimination Learning; D008253:Macaca mulatta / Q000523:psychology*; D008297:Male; D010364:Pattern Recognition, Visual; D014163:Transfer, Psychology
keywords: Categorization; Concept learning; Entropy; Perceptual variability