Symbol addition by monkeys provides evidence for normalized quantity coding.

Proceedings of the National Academy of Sciences of the United States of America

PubMedID: 24753600

Livingstone MS, Pettine WW, Srihasam K, Moore B, Morocz IA, Lee D. Symbol addition by monkeys provides evidence for normalized quantity coding. Proc Natl Acad Sci USA. 2014;.
Weber's law can be explained either by a compressive scaling of sensory response with stimulus magnitude or by a proportional scaling of response variability. These two mechanisms can be distinguished by asking how quantities are added or subtracted. We trained Rhesus monkeys to associate 26 distinct symbols with 0-25 drops of reward, and then tested how they combine, or add, symbolically represented reward magnitude. We found that they could combine symbolically represented magnitudes, and they transferred this ability to a novel symbol set, indicating that they were performing a calculation, not just memorizing the value of each combination. The way they combined pairs of symbols indicated neither a linear nor a compressed scale, but rather a dynamically shifting, relative scaling.