Content area
Full text
Introduction
We can perceive motion as a result of either the movement of luminance-defined spatial structures in a visual stimulus, or the movement of higher-order spatial structures, for example contrast envelopes or texture borders, (Badcock & Derrington, 1985; Derrington & Badcock, 1985; Chubb & Sperling, 1988). The two types of motion are called, respectively, first-order motion and second-order motion, (Cavanagh & Mather, 1989). First-order motion information abounds in our moving natural environment, often accompanied by second-order motion information. Second-order motion information can be used to break camouflage and evaluate movement when first-order motion information is unreliable or even absent.
There is substantial disagreement in the scientific literature whether first-order motion and second-order motion are processed separately. Motion is processed at multiple and distinct stages within the visual system (Zeki, 1990; Movshon & Newsome, 1996) and this have additionally confused the issue. In this study we address the question of whether first-order motion processing in V1, V2, and the third visual complex is influenced by an additional second-order motion signal.
Derrington and Badcock (1985) found that the discrimination of the direction of motion of first-order and second-order stimuli were different. Sensitivity to a moving second-order stimulus was lower and the necessary temporal resolution of the second-order stimulus was also lower than that for a moving first-order stimulus. The authors also found that second-order motion stimuli do not elicit a motion after-effect and proposed that the human visual system uses different mechanisms to process first- and second-order motion (Badcock & Derrington, 1985; Derrington & Badcock, 1985). Many other psychophysical studies have been used to argue that processing systems for first-order motion and second-order motion are largely independent (Harris & Smith, 1992; Mather & West, 1993; Ledgeway & Smith, 1994; Lu & Sperling, 1995; Nishida et al., 1997; Scott-Samuel & Georgeson, 1999). Separate processing of first-order motion and second-order motion has also received some support from physiological (Zhou & Baker, 1993, 1994; Mareschal & Baker, 1998a ) and neurological studies (Vaina & Cowey, 1996; Greenlee & Smith, 1997).
Psychophysical, physiological, and neurological evidence have been used to propose models in which first-order motion is processed in a separate cortical area to second-order motion (Wilson et al., 1992; Vaina & Cowey, 1996; Clifford & Vaina, 1999). Wilson et...