As the multiplication problems in their test became more difficult, pupil dilations increased steadily. More recently, Knoblich, Ohlsson, and Raney (2001) and Jones (2003) found that, in a difficult task, mean fixation duration increased significantly. They explained that participants need impasse stages (long gazes without making any moves) for cognitive processes searching for solutions; therefore, the more difficult a task is, the longer the fixation time has to be.