Head movement imposes the additional burdens on the visual system of maintaining visual-acuity and determining the origin of retinal image motion (i.e. self-motion versus object-motion). Although maintaining visual acuity during self-motion is effected by minimising retinal slip via the brainstem vestibular-ocular reflex, higher-order visuo-vestibular mechanisms also contribute. Disambiguating self-motion versus object-motion also invokes higher-order mechanisms and a cortical visuo-vestibular reciprocal antagonism is propounded. Hence one prediction is of a vestibular modulation of visual cortical excitability and indirect measures have variously suggested none, focal or global effects of activation or suppression in human visual cortex. Using transcranial magnetic stimulation-induced phosphenes to probe cortical excitability, we observed decreased V5/MT excitability versus increased Early Visual Cortex (EVC) excitability, during vestibular activation. In order to exclude non-specific effects (e.g. arousal) on cortical excitability, response specificity was assessed using information theory, specifically response entropy. Vestibular activation significantly modulated phosphene response entropy for V5/MT but not EVC, implying a specific vestibular effect on V5/MT responses. This is the first demonstration that vestibular activation modulates human visual cortex excitability. Furthermore, using information theory, not previously used in phosphene response analysis, we could distinguish between a specific vestibular modulation of V5/MT excitability from a non-specific effect at EVC.