Figure 1.

Quantitative example to illustrate stimulus construction. Positive values represent CW movement, negative values ACW. a, GVS induces a CW vestibular head roll percept of say 2.5° (R_vest = 2.5). VOR acts to stabilize the image: when the vestibular system signals CW head roll, which normally causes ACW retinal motion, the eyes rotate ACW giving compensatory CW retinal motion, say 0.5° (R_{ret_vor} = 0.5), which signals ACW head roll of −0.5° (R_{head_vor} = −0.5), reducing the roll percept to 2.0° (R_perc = 2.0). Note that real CW head motion would make the retinal image rotate ACW on the retina but in the case of GVS, the retinal image is static (ignoring VOR), so you think the world (image) must be rotating CW. b, To cancel such a perceived rotation (Nulled condition), the dot patch must be rotated in the opposite direction (R_{screen_null} = −2.0). This results in a nulled perceived roll (R_perc = 0) and a retinal image rotation (R_{ret_null}) of −1.5°, the sum of R_{screen_null} and R_{ret_vor}. c, In the Control condition, retinal motion is required that is equal and opposite to the Nulled condition (R_{ret_ctrl} = 1.5). The screen motion required to achieve this in the presence of VOR (R_{ret_vor} = 0.5) is 1° (R_{screen_ctrl} = 1.0).