The goal of human visual processing is to generate functional information from the natural environment, which casts retinal images that are quite unlike sine-wave grating stimuli. The relevance of the grating-based approach to understanding visual processing has recently been challenged (Olshausen & Field,
2005) and is currently the subject of axiomatic debate (Felsen & Dan,
2005; Rust & Movshon,
2005). Natural images have characteristic and broad distributions of spatial frequency (Bex, Dakin, & Mareschal,
2005; Bex & Makous,
2002; Billock,
1996; Burton & Moorhead,
1987; Field,
1987; Hancock, Baddeley, & Smith,
1992; Ruderman,
1994; Tolhurst, Tadmor, & Chao,
1992; van der Schaaf & van Hateren,
1996; van Hateren & van der Schaaf,
1998), orientation (Betsch, Einhäuser, Körding, & König,
2004; Coppola, Purves, McCoy, & Purves,
1998; Hancock et al.,
1992; Hansen, Essock, Zheng, & DeFord,
2003; Keil & Cristóbal,
2000; Switkes, Mayer, & Sloan,
1978; van der Schaaf & van Hateren,
1996), and temporal frequency (Bex et al.,
2005; Billock, de Guzman, & Kelso,
2001; Dong & Atick,
1995; van Hateren,
1997). A single natural image also contains a wide range of luminances and contrasts (Balboa & Grzywacz,
2000,
2003; Frazor & Geisler,
2006; Mante, Frazor, Bonin, Geisler, & Carandini,
2005; Ruderman & Bialek,
1994).