There is considerable interest in the cultivation of student graphic literacy among educators and researchers, especially in the sciences. Previous research, however, has shown that many students manifest difficulties in using diagrammatic representations. One explanation that has been proposed to account for these difficulties is that certain forms of diagrams may be more demanding of mental effort to use. Although there had been behavioral evidence to support this explanation, neurophysiological evidence had not confirmed it. Here, we provide evidence, from electroencephalogram recordings of brain activity focusing on the P3b component, that more abstract diagrams (such as graphs) demand greater mental resources to process compared to pictures and text--even when amount of detail has been accounted for. Our results provide initial neurophysiological evidence for the impact of cognitive cost on student learning behaviors and outcomes. Consideration of this finding is important in pedagogical design and, more broadly, in media communication.