Psychophysical and physiological studies have revealed that the visual system analyzes motion signals in several different processes. The process operating at the first stage is believed to detect local motion energies (Adelson & Bergen,
1985; Watson & Ahumada,
1985), independently of actual displacements (Adelson & Bergen,
1985; Anstis & Rogers,
1986; Shioiri & Cavanagh,
1990). As in the detection process of luminance contrast (Blakemore & Sutton,
1969) or disparity changes (Shioiri, Hatori, Yaguchi, & Kubo,
1994; Yang & Blake,
1991), the motion detectors at the first stage are suggested to have several channels with differences in spatial frequency tuning (Anderson & Burr,
1985; Ashida & Osaka,
1994; Bex, Verstraten, & Mareschal,
1996; Cameron, Baker, & Boulton,
1992; Thompson,
1998). In contrast, qualitative differences have also been suggested in motion analyses in different pathways or different stages: short/long range motion (Braddick,
1974), passive/active motion (Cavanagh,
1992), local/global motion (Bex, Metha, & Makous,
1999; Cavanagh & Favreau,
1980; Morrone, Burr, & Vaina,
1995; Snowden & Milne,
1997), component/pattern motion (Adelson & Movshon,
1982; Nishida,
1993), first/second order motion (Cavanagh & Mather,
1989; Nishida & Sato,
1995) and so on.