Four juvenile (all male) and six adult (two female) Rhesus macaque monkeys learned to use touchscreens in their home cages to choose quite accurately between pairs of stimuli to select a reward amount (Figures 1C–1F). The two stimuli could be arrays of dots inside a circle or two symbols (Arabic numerals or English letters). Reward amounts corresponded to

the number of dots in a circle or the assigned value of the symbol—numerals Navitoclax concentration 0 through 9 corresponded to 0 to 9 drops, and the letters X Y W C H U T F K L N R M E A J represented 10 through 25 drops. The monkeys were first trained on 0 versus 1, and each new symbol was introduced, in ascending order, only after the monkey’s choice behavior indicated that he or she had learned the value of the preceding symbol. After 1 year of daily training, during a month-long period while no new symbols were introduced, the monkeys were tested on alternate days with symbol pairs or dot pairs exclusively, with values between 0 and the maximum learned symbol (21 for the juveniles and various lower values for each of the adults). Reaction-time histograms (Figure 2A) for adults and juveniles were similar when they chose between dot arrays (peak of a log Bortezomib Gaussian fitted to the distribution = 470 ms for the juveniles; 490 ms for the adults), and reaction times

for juveniles were about the same regardless of whether they chose between symbols (peak = 460 ms) or between dots (470 ms). Adults only, however, were slower specifically when choosing between symbols (peak = 650 ms). One year later, after learning more symbols (up to value 25 for the juveniles and various lower values for each

of the adults), the reaction times of all the monkeys were measured again during another month-long period while no new symbols were introduced. The peak of the fitted reaction time distribution was 490 ms for juveniles using dots, 510 ms for adults using dots, 450 ms for juveniles using symbols, and 630 ms for adults using symbols. Thus the average reaction times were stable, and adults choosing between symbols were slower than adults choosing between dots or juveniles choosing between either symbols or dots. Figure 2B compares the peaks of the fitted reaction heptaminol time distributions between dots and symbols for each monkey over the two testing periods. Reaction times were not significantly different between the two testing periods (t(19) = −1.894, p = 0.08, two-tailed t test) so the reaction time distributions from the two test periods were combined to obtain a single peak time for each monkey for statistical comparisons between adults and juveniles and between dots and symbols. The juveniles responded slightly faster to symbols than to dots, but the difference was not significant (t(6) = −0.99, p = 0.36, two-tailed t test), while the adults showed slower reaction times for symbols than for dots (t(10) = 2.66, p = 0.04, one-tailed t test, corrected for multiple comparisons).