Place-modulated activity among neurons in the hippocampal formation presents a means to organize contextual information in the service of memory formation and recall1,2. One particular spatial representation, that of grid cells, has been observed in the entorhinal cortex (EC) of rats and bats3,4,5, but has yet to be described in single units in primates. Here we examined spatial representations in the EC of head-fixed monkeys performing a free-viewing visual memory task6,7. Individual neurons were identified in the primate EC that emitted action potentials when the monkey fixated multiple discrete locations in the visual field in each of many sequentially presented complex images. These firing fields possessed spatial periodicity similar to a triangular tiling with a corresponding well-defined hexagonal structure in the spatial autocorrelation. Further, these neurons showed theta-band oscillatory activity and changing spatial scale as a function of distance from the rhinal sulcus, which is consistent with previous findings in rodents4,8,9,10. These spatial representations may provide a framework to anchor the encoding of stimulus content in a complex visual scene. Together, our results provide a direct demonstration of grid cells in the primate and suggest that EC neurons encode space during visual exploration, even without locomotion.
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We thank S. Potter, C. Erickson, J. Manns, M. Meister and K. Dunne for comments on the manuscript, and M. Tompkins and D. Solyst for assistance with experiments. This project was funded by the National Institute of Mental Health, R01MH093807 (E.A.B.), R01MH080007 (E.A.B.), MH082559 (M.J.J.), the National Center for Research Resources P51RR165, and is currently supported by the Office of Research Infrastructure Programs/OD P51OD11132. N.J.K. was supported by the NSF IGERT program (DGE-0333411).
The authors declare no competing financial interests.
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Killian, N., Jutras, M. & Buffalo, E. A map of visual space in the primate entorhinal cortex. Nature 491, 761–764 (2012) doi:10.1038/nature11587
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