A group of Duke University scientists have just published an interesting study in the open-access PLoS Biology Journal. They wanted to verify the existence of so-called integrator neurons whose activity reflects the number of items in their receptive field. To get monkeys to pay attention to the quantities, Roitman et al. used a paradigm in which monkeys received a reward for delayed eye movement toward a previously displayed target. During this task, the experimenters also presented the monkeys with a group of dots that represented the size of the reward that they would receive upon successful task completion. The basic design is illustrated below:
Roitman et al. simultaneously measured the activity of single neurons in the monkey’s parietal lobe. They found one subset of neurons in the lateral intraparietal area (LIP) that activated more strongly to larger numbers than to smaller numbers. Moreover, they found another group of neurons in the LIP that activated more strongly to smaller numbers than to larger numbers. Both groups of neurons exhibited an initial increase in activity upon presentation of the number cue, but differences were found in how fast the activity decayed. The exact timecourse can be seen below:
Roitman et al. hypothesize that neurons exhibiting a graded response to numerical quantity provide crucial inputs for cells tuned to cardinal numbers (these cells have already been located in the prefrontal cortex and parietal lobe). In other words the parietal cortex would first estimate quantity and then translate this estimate into an integral value. Alternatively, it is possible that the information flows the opposite direction and general estimates of quantity follow more discrete identification of cardinal numbers. Hopefully future studies will be able to discriminate between these hypotheses.