Contributions of primate prefrontal and posterior parietal cortices to length and numerosity representation

The ability to understand and manipulate quantities ensures the survival of animals and humans alike. The frontoparietal network in primates has been implicated in representing, along with other cognitive abilities, abstract quantity. The respective roles of the prefrontal and parietal areas and the way continuous quantities, as opposed to discrete ones, are represented in this network, however, are unknown. We investigated this issue by simultaneously analyzing recorded single-unit activity in the prefrontal cortex (PFC) and the fundus of the intraparietal sulcus (IPS) of two macaque monkeys while they were engaged in delayed match-to-sample tasks discriminating line length and numerosity. In both areas, we found anatomically intermingled neurons encoding either length, numerosity, or both types of quantities. Even though different sets of neurons coded these quantities, the representation of length and numerosity was similar within the IPS and PFC. Both length and numerosity were coded by tuning functions peaking at the preferred quantity, thus supporting a labeled-line code for continuous and discrete quantity. A comparison of the response characteristics between parietal and frontal areas revealed a larger proportion of IPS neurons representing each quantity type in the early sample phase, in addition to shorter response latencies to quantity for IPS neurons. Moreover, IPS neurons discriminated quantities during the sample phase better than PFC neurons, as quantified by the receiver operating characteristic area. In the memory period, the discharge properties of PFC and IPS neurons were comparable. These single-cell results are in good agreement with functional imaging data from humans and support the notion that representations of continuous and discrete quantities share a frontoparietal substrate, with IPS neurons constituting the putative entry stage of the processing hierarchy.