Topographic organization of saccade-related response field properties in the marmoset posterior parietal cortex

Despite various histological, electrophysiological, and imaging studies, the topographic organization of saccade-related activity in the posterior parietal cortex (PPC) has been notoriously difficult to characterize. In part, this is because areas of interest in PPC are often embedded deep in sulci in macaques and humans. Understanding the extent of topographic organization in PPC can provide insights into the computation contributions of PPC. The lissencephalic cortex of the common marmoset offers a unique opportunity to investigate fine-scale topographic organization in PPC. Recordings were obtained from the PPC of two male marmosets performing a visually-guided center-out saccade task with 8 or 36 peripheral targets using multi-channel electrode arrays with 100 μm spacing. By plotting the pattern of saccade direction tuning preferences across all penetrations and cortical depths, uncovered topographic organizational features within PPC were uncovered. Like other primates, multiunits in marmoset PPC tend to prefer saccade targets in the contralateral visual field. The results detail how preference for saccadic direction changes in a systematic manner across cortical distance, such that response units closer in proximity tend to show systematic changes in their tuning preferences. Across cortical distance, the visual field was also systematically encoded but reversals in direction varied across penetrations. The analysis highlights the likelihood of multiple representations of the visual field for saccade-direction preference across PPC. These novel findings suggest a possible functional organization of saccade-related activity in marmoset PPC, giving insights into the computational capacity of the PPC. Significance statement Topographic maps are found across the primate brain, particularly in visual areas. A fundamental feature of these maps is that neurons involved in similar computations are spatially clustered. Moving from early sensory areas to higher-order areas, the degree of topographic organization decreases. This may allow high-order areas more flexibility and to respond to stimuli in a dynamic environment. The extent to which the posterior parietal cortex (PPC) shows topographic organization has been a subject of debate. This study presents the first detailed mapping of saccade direction topography in marmoset monkeys. These results provide novel insights into the topographic organization of the PPC in primates, and provide insights into its computational role in visual behavior.