Spatial scaling of central and peripheral contrast-sensitivity functions.

Contrast sensitivity was measured as a function of spatial frequency at various eccentricities in the nasal visual field. Eccentricity influenced resolution more for vertical gratings than for horizontal ones, demonstrating a nasal field anisotropy. When grating apertures and spatial frequencies were varied together, by changing viewing distance, peripheral and central contrast-sensitivity functions could not be entirely superimposed by scaling the spatial-frequency stimulus parameter. However, when gratings apertures were designed to project a standard-sized patch on a hypothetical vertical surface at an angle of 9 deg to the line of sight, the superimposition of central and peripheral contrast-sensitivity functions improved. Central and peripheral contrast-sensitivity functions can be superimposed by describing the grating stimulus with reference to this putative surface in terms of cycles per surface distance. The surface-scaling approach leaves a minor effect of eccentricity on contrast sensitivity for vertical gratings but readily accommodates the vertical-horizontal anisotropy in grating resolution. These findings suggest a novel functional role for gradients in the spatial sampling of the visual field, in relation to the internal representation of the structure and geometry of visible surfaces.