Spatial pattern is one of the primary effective elements of visual stimulation. Pattern vision begins with the ability to sense variations over space in the intensity of the light image. Development of models of this ability has therefore been, and continues to be, a goal of much of vision research. Early treatments of spatial sensitivity emphasized the role of summation within a fixed area, exemplified in such formulations as Ricco's Law (Graham & Margaria,
1935), and resolution, exemplified in acuity measurements (Shlaer,
1937). Introduction of the contrast sensitivity function (Campbell & Robson,
1968) lead to a somewhat more general model embodied in a spatial filter (Campbell, Carpenter, & Levinson,
1969), and later developments led to the idea of multiple spatial filters (Blakemore & Campbell,
1969). Separately, there have been advances in our understanding of how sensitivity varies with eccentricity (Robson & Graham,
1981), orientation (Berkley, Kitterle, & Watkins,
1975; Campbell, Kulikowski, & Levinson,
1966), and pattern size (Robson & Graham,
1981).