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Rory Sayres, Kalanit Grill-Spector; Retinal position and object category effects in human lateral occipital cortex. Journal of Vision 2008;8(6):82. doi: 10.1167/8.6.82.
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© ARVO (1962-2015); The Authors (2016-present)
Object-selective regions of human cortex, including the lateral occipital complex (LOC), are known to be sensitive to the retinotopic position of object stimuli, as well as the category of object. However, there has been little quantitative measure of the extent, organization or relative magnitude of these effects. Further, the relationship between different functionally-defined cortical regions is unclear: while the LOC is defined by preferential responses to objects over non-object images, the retinotopic maps LO-1 and LO-2 are known to be located in the vicinity. We sought to relate measures of object selectivity and retinotopy with a series of fMRI experiments. We imaged six subjects in a 3T MRI scanner using a standard retinotopic stimuli, as well as block-design experiments in which different object categories were presented at six distinct retinotopic positions. We then examined responses in region LO, a subset of the LOC positioned posterior to hMT+ along the lateral cortical surface.
We found substantial retinotopic modulation by checkerboard wedge and ring stimuli in LO. LO exhibited a modest overlap with LO-1 and LO-2, and retinotopic modulation in LO extended well beyond the boundaries of LO-1 and LO-2. Further, LO showed a pronounced lower visual field bias: more LO voxels represented the lower contralateral visual field during the retinotopic mapping experiment, and the mean LO response was higher to objects presented below fixation than above fixation. Finally, we examined how object category and retinal position affect the distributed response across LO. We found a stronger effect of position than category on the distributed LO response: response patterns to two stimuli were more correlated if the category was the same than the position. These results indicate that retinal position affects BOLD response at least as strongly as category, and these effects may be explained by retinotopic organization in LO.
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