Neuronal encoding of recognition memory for numerical quantities in macaque intraparietal and prefrontal cortices

The parieto-frontal number network in primates is vital for extracting and memorizing numerical information. However, how neurons in these regions retain abstract numerical categories to recognize target numbers amidst ongoing numerical input is unclear. To explore this, single neurons were recorded from the ventral intraparietal cortex (VIP) and lateral prefrontal cortex (PFC) of two male macaques trained to memorize and recognize target numerosities while viewing sequences of irrelevant numerosities. In the VIP, neuronal selectivity for both target and irrelevant numerosities declined rapidly, making it unable to distinguish relevant from irrelevant quantities. Conversely, PFC neurons maintained selective tuning for target numerosities over time but not for irrelevant ones, enabling the distinction between sought and irrelevant quantities. Match enhancement effects, where firing increased for repeated target numerosities, were observed only in the PFC. In contrast, match suppression effects, involving reduced firing for repeated target numerosities, occurred in both the VIP and PFC. These findings suggest the VIP primarily encodes displayed numerosities, while the PFC is specialized for processing, storing, and recognizing numerical quantities by enhancing familiar numerosities. This highlights the PFC's key role in recognition memory, contrasting with the transient coding observed in the VIP.