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Marc Himmelberg; Measuring temporal contrast sensitivity across the visual field and visual cortex using fMRI. Journal of Vision 2017;17(15):18. doi: 10.1167/17.15.18.
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© ARVO (1962-2015); The Authors (2016-present)
The visual system's sensitivity to both contrast and temporal frequency changes across visual space. In general, the fovea is dominated by cells contributing to the parvocellular (PC) pathway and the ratio of parvo to magnocellular (MC) retinal ganglion cells decreases with increasing eccentricity – leading to higher temporal sensitivity and, potentially, less linear responses to contrast in the periphery. Here, we describe a series of fMRI experiments that examine the relationship between eccentricity, contrast sensitivity, temporal frequency tuning and pRF size in five human visual areas with well-defined eccentricity maps (V1, V2, V3, V4, V3a).
We obtained fMRI population receptive field (pRF) maps for each subject (N=10). In addition, all subjects completed an event-related fMRI experiment that measured responses to circularly-windowed contrast reversing sine gratings at 20 different combinations of temporal frequency (1, 5, 10, 20Hz) and achromatic contrast (1, 4, 8, 16, 64%). We partitioned our data into foveal (0.25° – 3° eccentricity) and peripheral (6°–10° eccentricity) regions, and small pRF (0.25 – 3) and large pRF (4–10) sizes, within retinotopically defined regions. We fit the resulting response surfaces with hyperbolic ratio functions at individual TFs.
Both the shape and sensitivity of the contrast response functions varied as a function of visual area, eccentricity and temporal frequency. Most strikingly, the semisaturation constant of the fits (c50) was significantly reduced in the peripheral compared to foveal eccentricities within V1 at high (20Hz) but not low (1Hz, 4Hz) temporal frequency conditions. Further, we found that Rmax significantly increased for larger pRF sizes compared to smaller pRFs at 5Hz. Further analysis explores the responses in higher visual areas – particularly in areas thought to be dominated by either PC or MC inputs.
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