This study investigates the effect on student performance in drawing their attention to relevant task variables, focusing on accuracy of responses and reaction times. We chose this methodology in order to better understand how such interventions affect the reasoning process. The study employs a geometry task in which the irrelevant salient variable (area) interferes with the reasoning process related to perimeter comparison. We compared eighth graders' performances in a pretest and a posttest, with or without intervention. The posttest results showed that raising students' awareness of the relevant task variable activates effortful and time-consuming control mechanisms which help them overcome the interference of the irrelevant variable. The paper discusses the educational importance of helping students to attend selectively to relevant information in order to overcome interference, thus promoting logical reasoning.