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Noah C. Benson, Eline R. Kupers, Marisa Carrasco, Jonathan Winawer; Asymmetries around the visual field in human visual cortex. Journal of Vision 2020;20(11):543. doi: https://doi.org/10.1167/jov.20.11.543.
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[BACKGROUND] Visual performance in many tasks, including acuity and contrast sensitivity, declines precipitously with eccentricity. Performance also varies substantially around the visual field at isoeccentric locations: Performance is better on the horizontal than vertical meridian (horizontal-vertical asymmetry, “HVA”) and better on the lower than upper vertical meridian (vertical meridian asymmetry, “VMA”) (Carrasco, Talgar, Cameron, 2001). These two asymmetries decrease gradually from the cardinal meridia (Abrams, Nizam, Carrasco, 2012). Here, we quantified asymmetries around the visual field in terms of surface area in human visual cortex.
[METHODS] We used the Human Connectome Project retinotopy dataset (n=181; Benson et al., 2018) to characterize the cortical HVA and VMA. We computed the percent difference in cortical surface area for V1/V2 regions of interest (ROIs) near the cardinal meridia (upper vs lower, VMA; horizontal vs vertical, HVA). To test the angular specificity of these asymmetries, we defined several wedge-shaped ROIs surrounding each cardinal meridian, ranging in width from narrow to wide (±10° to ±50° of polar angle from the meridia). All ROIs were restricted to 1-6º eccentricity.
[RESULTS] We find that cortical surface area in V1 and V2 is distributed asymmetrically around the visual field with greater area for the lower than the upper visual field (maximum VMA ≈ 46% at ±10°) and for the horizontal than vertical meridia (maximum HVA ≈ 54% at ±20°). The asymmetries are largest for the narrowest wedges and decrease gradually with angular distance from the meridia. This pattern matches the behavioral asymmetries, which also decrease gradually away from the meridia, indicating a close correspondence between psychophysical performance and cortical representations in early visual retinotopic maps.
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