Neuroimaging can address activity across the entire brain in relation to cognition, but is typically correlative rather than causal. Brain stimulation can target a local brain area causally, but without revealing the entire network affected. Combining brain stimulation with concurrent neuroimaging allows a new causal approach to how interplay between extended networks of brain regions can support cognition. Brain stimulation does not affect only the targeted local region but also activity in remote interconnected regions. These remote effects depend on cognitive factors (e.g. task-condition), revealing dynamic changes in interplay between brain areas. We illustrate this with examples from top-down modulation of visual cortex, response-competition, inter-hemispheric rivalry and motor tasks; but the new approach should be applicable to many domains of cognition.