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.
In conclusion, there is in general a relationship between task, cognitive processes, and eye movement mechanisms: depending on tasks, cognitive processes will change, and eye movements will follow and reveal the change of cognitive processes. These findings suggest that test design may considerably influence the results of an eye-tracking test, in the fields of both consumer science and psychology.