Optic flow is the physical pattern of optical motion that is normally generated when an observer moves relative to objects in the environment (Gibson,
1950). Optic flow provides a valuable source of information for visually guided behavior because it generates retinal motion that provides information about the speed and direction of self-motion. Previous research has demonstrated the capacity of the visual system to compute optic flow (e.g., Badcock & Khuu,
2001; Edwards & Badcock,
1993; Khuu & Badcock,
2002), and neural mechanisms have been identified in the visual cortex that selectively process the retinal motion it generates (e.g., Duffy & Wurtz,
1991). This retinal motion is sufficient to support the subjective sensation of self-motion; powerful illusions of self-motion can occur (e.g., linear vection) when stationary observers view radially expanding optic flow simulating self-motion in depth. Many studies have shown that linear vection in depth can be increased by adding simulated sinusoidal changes in translational (or
linear) head position to this radial flow, despite the ensuing visual-vestibular conflicts (see Palmisano, Allison, Kim, & Bonato,
2011 for a review). These sinusoidal translations in horizontal or vertical viewpoint increase motion parallax—the perspective gradient in optic flow velocities that scales inversely with distance to the observer. It has been noted that motion parallax could increase vection by enhancing the perception of scene layout (Kim & Palmisano,
2010; Nakamura,
2010; Nawrot,
2003; Palmisano et al.,
2011; Palmisano, Kim, & Freeman,
2012) or increasing the perceived speed of self-motion (Kim & Palmisano,
2008). However, a recent study found that vection increases can also occur when oscillatory horizontal
angular viewpoint rotations are added to radial flow, which does not increase motion parallax (Kim, Palmisano, & Bonato,
2012). Here, we explore parameters of optic flow that may account for these increases in vection.