Multifractality and non-Gaussianity of eye fixation duration time series in reading Persian texts

There is growing evidence that cognitive processes may have fractal structures as a signature of complexity. It is an ongoing topic of research to study the class of complexity, and how it may differ as a function of cognitive variables. Herein, we explore the eye movement trajectories generated during reading different Persian texts. Features of eye movement trajectories were recorded during reading Persian texts using an eye tracker device. We show that fixation durations, as the main components of eye movements reflecting cognitive processing exhibits multifractal behavior. This indicates that multiple exponents are needed to capture the neural and cognitive processes involved in decoding symbols to derive meaning. We test whether multifractal behavior varies as a function of two different stimuli including fonts, and familiarity of the text for readers during four different reading procedures concerning silent and aloud reading and goal-oriented versus non-goal-oriented reading. We find that, while mean fixation duration is affected by some of these factors, the multifractal pattern in time series of eye fixation durations did not change significantly. Our results suggest that multifractal dynamics may be intrinsic to the reading processes, and the lognormal distribution properly describes the statistics of the fixation duration time series for all the stimuli in our reading experiments.