Symbol Addition By Rhesus Monkeys
For this study, Rhesus monkeys were trained to recognize symbols representing the numbers 1-25, and to select the larger quantity in a pair, by rewarding them with a corresponding number of liquid drops for each choice. They were then given a choice between a set of two numbers versus a single number to see whether they could learn to add. The monkeys did grasp that the larger number in the two-number set was increased by the presence of the second number in the set, though they consistently underestimated the precise amount of the increase.
[Abstract excerpted from original text.]
“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.”
http://www.pnas.org/content/111/18/6822.full