Vection depends on perceived surface properties

Optic flow provides important information for the perception of self-motion and can be generated by both diffuse and specular reflectance. Previous self-motion research using virtual environments has primarily considered the properties of diffuse optic flow, but not of specular flow. We used graphical simulations to examine the extent to which visually induced self-motion (vection) is robust against the variations in optic flow generated by different surface optics. We found that specular flow alone was capable of generating vection that was equivalent in strength to that generated by diffuse flow (Exp. 1). To test whether this specularly induced vection depends on midlevel visual processing, we measured vection strengths under conditions in which the luminance polarity of specular highlights was inverted. We found that inverting the luminance of specular reflections impaired vection strength, as compared with the vection generated by conditions with ecologically correct diffuse and/or specular flow (Exp. 2). We also found these variations in vection strength were correlated with the perceived relief heights of the surfaces depicted in the image sequences. These findings together suggest that vection can be induced by pure specular flow and that it requires processing beyond the computation of retinal motion velocities-most likely, processes involved in the recovery of 3-D surface shape.