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Marc Himmelberg, Alex Wade; Measuring cortical temporal contrast sensitivity across population receptive field (pRF) eccentricity and sizes using fMRI. Journal of Vision 2018;18(10):251. doi: 10.1167/18.10.251.
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© ARVO (1962-2015); The Authors (2016-present)
The visual system's sensitivity to contrast and temporal frequency (TF) changes across visual space due to differences in retinal cell sensitivity. First, peripheral photoreceptors respond faster, and are therefore more sensitive to rapidly changing input, compared to more foveal photoreceptors. Second, there is an eccentricity-dependent decrease in the ratio of midget:parasol retinal ganglion cells (RGCs), leading to increased sensitivity to TF and contrast in the peripheral visual field. Are these retinal sensitivities maintained within the visual cortex? Here, we use functional magnetic resonance imaging (fMRI) to measure contrast response functions (CRFs) at four TFs, comparing responses at different population receptive field (pRF) eccentricities and sizes in V1, V2, V3, V3a and V4. Nineteen participants completed an event-related fMRI experiment that measured sensitivity to a contrast-reversing sine grating at 20 combinations of TF (1, 5, 10 20Hz) and contrast (1, 4, 8, 16, 64%). pRF maps were collected for each participant to provide eccentricity and pRF size estimates. Within retinotopically defined regions, we partitioned our data into foveal (0.20° - 3°), parafoveal (3° - 6°), and peripheral (6° - 10°) eccentricities, and small and large pRF sizes. We fit responses with hyperbolic ratio functions at each TF to generate CRFs, extracting C50 (contrast sensitivity) and Rmax (maximum response) parameters. In V1-V3a, C50 measurements indicated increased contrast sensitivity at high TF (20Hz) in peripherally tuned voxels. Increased contrast sensitivity was found in larger pRFs in all visual areas. Overall, visual areas were most sensitive to 10Hz flicker, in line with previous literature. Finally, Rmax increased in larger pRFs, independent of TF, in V1 and V2. Our data reflect a cortical representation of the retinal mechanisms that facilitate increased sensitivity to TF and contrast in peripheral vision. These findings may suggest the functional organisation of the visual cortex originates in the organisation of retina.
Meeting abstract presented at VSS 2018
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