Characteristics of Neuronal Persistent Activity in the Lateral Intraparietal Area of the Macaque Posterior Parietal Cortex Associated with Visuospatial Working Memory

Abstract

Working memory (WM) – the cognitive ability to briefly store a small amount of information for goal-directed behaviour – is essential for daily functioning, but its neural substrate is not fully understood. One potential mechanism is the persistent activity observed in neurons within the primate prefrontal and posterior parietal cortex during the retention interval of a WM task. This activity pattern is hypothesized to be mediated by recurrent networks endowed with NMDA receptors. Persistent activity is a principal characteristic of the neurons in the lateral intraparietal (LIP) area of rhesus macaques, which is comprised of distinct ventral and dorsal subdivisions (LIPv and LIPd, respectively). Examining in detail the neuronal activity in these subdivisions could therefore shed light on the neural substrate (e.g., NMDA receptor density) that supports this activity. This study characterized the activity of LIPv and LIPd recorded in four macaques (Macaca mulatta) while they performed a task in which saccadic eye movement is made to a visual stimulus after a mnemonic or visual delay period. Distinctions between putative LIPd and LIPv neuronal activity were most pronounced in the mnemonic (VWM) task relative to the visually-guided task. Neurons in LIPd showed stronger persistent activity that had significantly better spatial tuning, reliability, and fidelity, as well as intrinsic timescale; their saccade-related activity was also larger. LIPd could thus be better suited for VWM, but data variance was high. Overall, differences in structure and connectivity between the two LIP subdivisions does not seem to impact considerably their persistent activity, suggesting that the mechanisms underlying VWM are somewhat robust. Nevertheless, the small differences in signal quality between LIPd and LIPv uncovered in this study could be taken to reveal different demands from the distinct circuits to which they contribute: visual and oculomotor, respectively.