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Paper Chase

Symbol addition by monkeys provides evidence for normalized quantity coding.

Proc. Natl. Acad. Sci. U.S.A.. May 6, 2014;111(18):6822-7.
Livingstone MS, Pettine WW, Srihasam K, Moore B, Morocz IA, Lee D.

Department of Neurobiology, Harvard Medical School, Boston, MA 02115.


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.