The ability to accurately identify the motion in an image is a critical property of the visual system and one that has attracted a great deal of research interest over the past 30 years or so. However, despite the wealth of data collected and the extent of the confluence of that data, there is still uncertainty regarding how we detect the motion of the simplest luminance edge, let alone the more complex patterns employed in much of the recent motion psychophysics (Cropper & Wuerger,
2005; Derrington, Allen, & Delicato,
2004). What the evidence does suggest is that the motion detection system is a strongly hierarchical process, and the initial signal specific to the motion subsystem is related to the direction of the motion of an edge (Lennie & Movshon,
2005). The edge is usually coded by a first-order modulation of the image statistics, and the directional signal relating to any one-dimensional edge is only accurate to within ±90 degrees; a property known as the “aperture problem” (Marr & Ullman,
1981). This relatively simple “seed” is progressively built into a signal that ends up as a remarkably complex and powerful contributor to the overall percept of the visual scene, revealing not only the motion in the input but also the depth and, in some cases, the form (Warren,
2004). This hierarchy of signal development is seen both in the behavioral (Wilson, Ferrera, & Yo,
1992) and in the neurophysiological (Duffy,
2004) data and in turn dictates the way in which we define and describe the stimuli that we use.
An example of this is the description of a visual stimulus in terms of its first-order and second-order spatial statistics and of the consideration of each spatial dimension (
x and
y) independently. Thus, there has been an argument presented within the motion literature for independent pathways in the system that deal with the first-order and second-order components of the pattern independently (e.g., Badcock & Derrington,
1985; Badcock & Khuu,
2001; Edwards & Badcock,
1995; Ledgeway & Smith,
1994; Lu & Sperling,
1995,
2001; Wilson et al.,
1992). Furthermore, it is thought that the early cortical pathways are principally one-dimensional in their sensitivity; a two-dimensional percept being recovered from those one-dimensional components, as is the overall spatial structure.
Counter to this approach, there is, however, some evidence to suggest that these nominal pathways may not be so separate, and that both a two-dimensional representation and a composite spatiotemporal representation may be generated in a more coherent and integrated manner (Geisler,
1999; Johnston, McOwan, & Buxton,
1992). In particular, it has recently been shown that there are strong interactions between form and motion cues in the image; two components previously considered to be quite separate early on in the visual process. Specifically, the orientation properties of elements within a stimulus have a profound impact on the perceived direction of the first-order spatial profile (Badcock, McKendrick, & Ma-Wyatt,
2003; Nishida & Johnston,
1999; Ross,
2004; Ross, Badcock, & Hayes,
2000), and the motion of a pattern also impacts upon its perceived spatial position (Nishida & Johnston,
1999; Whitney & Cavanagh,
2000,
2003).
Both these results suggest that spatial properties such as form and position interact with motion, and Geisler's (
1999) model proposes that this is at an early stage when both are coded by luminance modulation. In the context of the hierarchical and parallel approach to motion processing, we were interested to determine whether first-order and second-order spatial signals also interacted within the motion system.
We have addressed this issue by examining the interaction between first-order spatial form and the motion of a second-order profile using two-dimensional plaid stimuli that have independently defined first-order orientation content and second-order motion directional signals. This allows us to examine the effect of the former on the perceived direction of the latter.