Optic flow is generated by observer motion relative to stationary objects, by movement of objects relative to a stationary observer, and by combinations of those situations. To determine the relative contributions of object and self motion to the observed optic flow, the nervous system can use vestibular signals. An object's speed relative to earth is given by the difference between its speed relative to the head and the head's speed relative to the earth. The variance of the difference is the sum of the component variances: σ²_obj=σ²_vis+σ²_vest. In contrast, if observers estimate self-motion from optic flow and vestibular signals, and assume a stationary visual scene, visual and vestibular estimates may be combined in a weighted average to yield more precise self-motion estimates: σ²_self=(σ²_visσ²_vest)/(σ²_vis+σ²_vest). So depending on whether the subject reports object motion or self-motion, the two-modality variance is predicted to be respectively higher or lower than the component variances. To test these predictions, we measured speed-discrimination thresholds for fore-aft translations and roll rotations. There were two single-modality conditions, Visual and Vestibular, and two multi-modality conditions, Self-motion and Object-motion. In the Visual, Vestibular, and Self-motion conditions, observers indicated if the movement was faster or slower than a standard. In the Object-motion condition, observers indicated if the object appeared to move with or against the self-motion. Experiments were conducted on a rotating chair and translating motion platform. The stereoscopic projection system was mounted on the apparatus. Stimuli were random-dot planes that rotated clockwise or anti-clockwise or translated forwards or backwards. In the translation conditions, multi-modal object-motion thresholds were, as predicted, higher than single-modality thresholds, and multi-modal self-motion thresholds were, as predicted, generally lower than single-modality thresholds. Results from the rotation conditions were less clear. Possible causes of differing results for translations and rotations will be discussed.