To solve a mathematical problem, such as which of two products is less expensive, we use abstract rules that dictate how to structure, process and evaluate numerical information. New research shows that rhesus monkeys have specific neurons that encode the abstract 'greater than' rule and others that encode the 'less than' rule. This suggests that individual cells in human brains might also encode mathematical rules.

Andreas Nieder and Sylvia Bongard of the University of Tübingen in Germany trained two rhesus monkeys to do a simple rule-based numerical task (Proc. Natl. Acad. Sci. USA published online 19 January 2010; doi:10.1073/pnas.0909180107). This task required the monkeys to compare set sizes (e.g., the number of dots in two different displays) and flexibly switch back and forth between the abstract 'greater than' and 'less than' rules.

Credit: Eric Isselée

For each of the 160 trials, the monkeys were first presented with a sample stimulus that displayed a specific number of dots. After a delay period (delay 1), the monkeys were presented with a cue that indicated whether they should follow the 'greater than' or the 'less than' rule. Following another delay (delay 2), the monkeys were shown a test stimulus that displayed a number of dots. When following the 'greater than' rule, the monkeys had to release a lever if the test display showed more dots than the sample display. For the 'less than' rule, they had to release the lever if the test display had fewer dots than the sample display. The monkeys learned the task and responded correctly most of the time. They also immediately generalized the 'greater than' and 'less than' rules for numbers that had not been previously tested, showing that they understood the basic mathematical principle.

During these tests, Nieder and Bongard recorded the activity of 484 randomly selected single neurons in the prefrontal cortex region of each monkey's brain. They found that during the second half of the delay 2 period (after the monkeys knew the rule, but before they could know how to respond to the test display), 19% of neurons showed activity that varied significantly and exclusively with the cued rule.

These results show that, in rhesus monkeys, single prefrontal cortex neurons represent specific 'greater than' and 'less than' rules and help guide mathematical decisions. The authors explain that these results help “pave the way for a better understanding of the processing of basic mathematical rules in the primate brain.”