Counting and Timing in the Monkey Brain

The capacity to represent time and number is often considered to be a sophisticated, uniquely human facility. However, animals as diverse as rats, pigeons, and monkeys have demonstrated the ability to use numerical and temporal information. We are interested in where and how number and time are encoded by the brain.

To test whether time and number are represented by a common nonverbal system, we measure how monkeys and humans judge stimuli that differ on the basis of these dimensions. We have found that subjects spontaneously classify ambiguous series of light flashes on the basis of both numerical and temporal information, even when given no instruction of the relevant feature.

Clinical cases and imaging studies suggest that parietal cortex is a critical component of the circuits that form the basis of numerical abilities in humans. Because the functional organization of human parietal lobe resembles that found in monkeys, we measure the activity of single neurons in posterior parietal cortex (PPC) while monkeys judge visual stimuli based upon the number of elements presented or the duration of their presentation. The profile of activity of PPC neurons will provide insight into how neurons represent the accumulation of discrete events to encode the abstract properties of number and time.