Do people spontaneously form visual mental images when understanding language, and if so how truly visual are these representations? We test whether processing linguistic descriptions of motion produces sufficiently vivid mental images to cause direction-selective motion adaptation in the visual system (i.e., cause a motion aftereffect illusion). We tested for motion aftereffects (MAEs) following explicit motion imagery, and after processing literal or metaphorical motion language. Intentionally imagining motion produces an aftereffect in the overall sample with some participants showing a greater aftereffect than others. We then find that participants who show the strongest imagined motion aftereffects also show aftereffects in the natural course of processing motion language (without instructions to imagine). Individuals who do not show strong motion aftereffects as a result of imagining motion also do not show them from processing motion language. However, the aftereffect from language gained strength as people were exposed to more and more of a motion story. For the last two story installments (out of 4), understanding motion language produced reliable MAEs across the entire sample. The results demonstrate that processing language can spontaneously create sufficiently vivid mental images to produce direction-selective adaptation in the visual system. The timecourse of adaptation suggests that individuals may differ in how efficiently they recruit visual mechanisms in the service of language understanding. Further, the results reveal an intriguing link between the vividness of mental imagery and the nature of the processes and representations involved in language understanding.