We investigated how two different reading tasks, namely reading to memorize [Read & Memorize (RM)] and reading to decide whether a text was relevant to a given topic [Read & Decide (RD)], modulated both eye movements (EM) and brain activity. To this end, we set up an ecological paradigm using the eye fixation-related potentials (EFRP) technique, in which participants freely moved their eyes to process short paragraphs, while their electroencephalography (EEG) activity was recorded in synchronization with their EM. A general linear model was used to estimate at best EFRP, taking account of the overlap between adjacent potentials, and more precisely with the potential elicited at text onset, as well as saccadic potentials. Our results showed that EM patterns were top-down modulated by different task demands. More interestingly, in both tasks, we observed slow-wave potentials that gradually increased across the first eye fixations. These slow waves were larger in the RD task than in the RM task, specifically over the left hemisphere. These results suggest that the decision-making process during reading in the RD task engendered a greater memory load in working memory than that generated in a classic reading task. The significance of these findings is discussed in the light of recent theories and models of working memory processing.