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Joshua Julian, Alex Keinath, Giulia Frazzetta, Russell Epstein; Evidence for a grid-like representation of visual space in humans. Journal of Vision 2017;17(10):307. doi: https://doi.org/10.1167/17.10.307.
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Most cortical regions represent visual space retinotopically. However, many behaviors would benefit from a non-retinotopic representation of visual space. Grid cells in the entorhinal cortex (EC) may provide a neural substrate for such a non-retinotopic representation. In freely navigating rodents, grid cells fire when the animal's body occupies a hexagonal lattice of spatial locations along the chamber floor. In head-fixed monkeys, on the other hand, grid cells fire when the animal directs its gaze to a hexagonal lattice of locations on the visible screen (Killian et al., 2012). To determine whether similar scene-based grid responses can be identified in humans, we scanned participants with fMRI while tracking their gaze during an unconstrained visual search task in which they had to find a target letter ('L') among numerous distractors letters ('T's). Building on fMRI methods previously used to identify the grid signal during virtual navigation (Doeller et al., 2010), we used a quadrature filter approach to measure fMRI responses as a function of gaze movement direction. In particular, we first extracted gaze movement directions modulo 60°, thus equating all 6-Fold symmetric gaze movement directions. Then, using half of the fMRI data, we computed the rotation of the gaze movement directions that maximally modulated EC activity. Using this fit rotation angle, we predicted EC activity in the withheld fMRI data. Examination of this independent data confirmed that there was significant modulation of EC activity bilaterally as a function of gaze movement direction. Follow-up analyses confirmed that this modulation only exhibited the 6-fold rotational symmetry characteristic of grid cell firing, and not 4- or 8-fold symmetries. These results mark the first evidence of a grid-like representation of visual space in humans, and suggest that the same mechanisms supporting the cognitive map of navigational space may also support a map of visual space.
Meeting abstract presented at VSS 2017
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