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Introduction
Optic flow, the complex pattern of motion generated by a moving observer, provides rich perceptual information about the direction and speed of an observer's motion, and the layout of surfaces in the environment (Gibson, 1966, 1979). There has been considerable progress in measuring perceptual sensitivity to optic flow (Andersen & Dyre, 1989; Stoffregen et al., 1987; Warren et al., 1988; Atchley & Andersen, 1998; Stoffregen & Riccio, 1990; Warren & Mestre, 1991; Lappe & Rauschecker, 1994; Crowell & Banks, 1993, 1996; Burr et al., 1998; Morrone et al., 1999, 2000; Rushton et al., 1999; Hanada & Ejima, 2000; Burr & Santoro, 2001b ; Habak et al., 2002; Baumberger & Fluckiger, 2004; Frenz & Lappe, 2005). Computational analysis has shown that any optic flow pattern can be decomposed into a combination of elementary vector fields including expansion, rotation, translation (horizontal and vertical), and deformation (Koenderink, 1986). Whether this sort of strict decomposition applies to biological systems has not yet been resolved. However, psychophysical evidence suggests that adult observers readily distinguish between expansion/contraction, rotation, and translation (Morrone et al., 1995; Burr et al., 1998, 2001; Morrone et al., 1999; Meese & Harris, 2001; Burr & Santoro, 2001b ; Habak et al., 2002; Meese & Anderson, 2002).
There is also a substantial literature describing the response characteristics of brain regions sensitive to optic flow in both monkey (Duffy & Wurtz, 1991b ; Graziano et al., 1994; Lagae et al., 1994; Duffy & Wurtz, 1995; Orban et al., 1995; Andersen et al., 1996; Bradley et al., 1996; Duffy, 1998; Heuer & Britten, 2004; Xiao et al., 2006) and adult human (de Jong et al., 1994; Morrone et al., 2000; Dukelow et al., 2001; Braddick et al., 2001; Kourtzi et al., 2001; Peuskens et al., 2001; Ptito et al., 2001; Huk et al., 2002; Wunderlich et al., 2002; Seiffert et al., 2003; Goossens et al., 2006; Smith et al., 2006). The earliest investigations of optic flow processing in the primate revealed the existence of neurons tuned for a variety of flow patterns in dorsal portion of the middle superior temporal area (MSTd) (Tanaka et al., 1989; Tanaka & Saito, 1989; Duffy & Wurtz, 1991a , 1991b ; Duffy, 1998; Duffy & Wurtz, 1995, 1997