Visual adaptation is more pronounced at the horizontal than vertical meridian

[BACKGROUND] Contrast sensitivity varies with polar angle (i.e., performance fields); it is better along the horizontal than vertical (horizontal-vertical anisotropy, HVA), and along the lower than upper vertical meridian (vertical meridian asymmetry, VMA). Adaptation, which suppresses contrast sensitivity, is more pronounced at peripheral than parafoveal locations. Here we investigate whether contrast adaptation similarly decreases sensitivity at cardinal locations, diminishes or exacerbates the performance fields. [METHODS] Observers performed a 2-AFC orientation discrimination task, with and without adaptation. The adaptor was a 5-cpd horizontally- or vertically-oriented Gabor patch (100% contrast) presented at the left or right horizontal meridian, and upper- or lower-vertical meridian at 8° eccentricity. Observers determined whether a 5-cpd Gabor patch (4° diameter) was tilted clockwise or counterclockwise compared to a horizontal (Exp.1) or vertical (Exp.2) reference. The contrast of the target was titrated for 75% accuracy with 4 randomized independent staircases at each location. [RESULTS] In the non-adapted condition, contrast threshold was higher at the vertical than horizontal meridian, and at the upper than lower meridian, consistent with the typical HVA and VMA. Critically, in both experiments, the adaptation effect [(adapted - non-adapted threshold) / (adapted + non-adapted threshold)] was more pronounced (contrast sensitivity was more suppressed) at the horizontal than vertical meridian, and there was no difference between the upper and lower meridian. [CONCLUSION] This study reveals that the extent of contrast adaptation varies around the visual field, with stronger adaptation effects at the horizontal than vertical meridian, regardless of the stimulus orientation. This effect may be due to the larger cortical area corresponding to the horizontal than vertical meridian. This difference in the adaptation effect diminishes performance asymmetries, rendering visibility to be more homogenous around the visual field.