Visual adaptation stronger at horizontal than vertical meridian: Linking performance with V1 cortical surface area
Visual adaptation, a mechanism that conserves bioenergetic resources by reducing energy expenditure on repetitive stimuli, leads to decreased sensitivity for similar features (e.g., orientation and spatial frequency). In human adults, visual performance declines with eccentricity and varies around polar angle for many visual dimensions and tasks: Performance is superior along the horizontal than the vertical meridian (horizontal-vertical anisotropy, HVA), and along the lower than the upper vertical meridian (vertical meridian asymmetry, VMA)(Carrasco et al., 2001). However, it remains unknown whether visual adaptation differs around polar angle. In this study, we investigated adaptation effects at the fovea and perifovea across the four cardinal locations, for horizontal and vertical adaptor and target orientations, with stimulus size adjusted as per a cortical magnification factor (Rovamo & Virsu, 1979). We measured contrast thresholds at each location separately for adaptation and non-adaptation conditions. Results confirmed the expected HVA and VMA effects in non-adapted conditions and showed they are stronger for horizontal than vertical orientations. They also revealed that, for both orientations, adaptation effects are stronger along the horizontal than the vertical meridian, which in turn is stronger than at the fovea. Furthermore, for both orientations, individual's adaptation effects at the perifoveal locations positively correlated with their cortical surface area of V1. The association of a stronger adaptation effect with larger V1 surface area suggests a more pronounced conservation of bioenergetic resources along the horizontal than the vertical meridian. Visual adaptation alleviates the HVA in contrast sensitivity, promoting a more homogeneous perception around the visual field.