M.I. Trifonov - Dr.Sc. (Eng.), Leading Research Scientist, I.M. Sechenov Institute of Evolutionary Physiology and Biochemistry of the Russian Academy of Sciences (Saint-Petersburg). E-mail: mtrifonov@mail.ru V.L. Efimova - Ph.D. (Ped.), Speech Therapist, OOO «Logoprognoz», Saint-Petersburg, Russia. E-mail: prefish@ya.ru O.I. Efimov - Child Neurologist, Director of MPK «Prognoz», Saint-Petersburg, Russia. E-mail: prefish@ya.ru V.P. Rozhkov - Ph.D. (Biol.), Leading Research Scientist, I.M. Sechenov Institute of Evolutionary Physiology and Biochemistry of the Russian Academy of Sciences (Saint-Petersburg). E-mail: vlrozhkov@mail.ru

Early identification and interventions are extremely important in children with dyslexia as well as most neurodevelopmental disorders such as attention deficit hyperactivity disorder. In our opinion quantitative eye movement measurements including saccade amplitude, number and mean duration of gaze fixations during viewing of various visual tests and fixational eye movements (fixational noise) amplitude might be useful in solving this problem. It is well-known that a scanpaths (or gaze trajectories) differ among children but are strictly individual. Any deviation from what is regarded as normal for a certain age may serve as indicator for nervous system disorders. A healthy child should be characterized by dynamically stable gaze tra-jectory in the sense of N.A.Bernstein. The latter means that any individual gaze trajectory at given cycle may not coincide ex-actly with any trajectory at previous cycles, but should be close to some extent to the farthest of them. Getting more visual practice one can achieve a scanpath pattern as compact as possible. Dynamical stability in our case may imply the stability of one dimensional probability density function (pdf) of Euclidean distance between randomly chosen pairs of fixation points on the trajectory and reasonable trajectory fuzziness as such. In this work we present the pdf estimations of saccade amplitude and fixational noise amplitude calculating only for pairs of successive samples that were recorded for 28 school-age children (6 to 14 years) reading a specific text aloud and following the discrete star movement in one minute videotest. Some of them have had neurodevelopmental disorders and learning problems. Eye movement recordings were made using infrared eye-tracking system LC Technologies Eyegaze Analysis System, supplied with NYAN 2.0XT software. As a result we obtained the empirical amplitude pdfs for saccades and fixational noise that are clustered into four (two unimodal and two bimodal) and five (all unimodal) groups respectively. Any empirical amplitude pdf for fixational noise is quite good fitted by the same hyper-gamma function but with a specific shape and scale parameters relevant to the subject and visual test. It was found that healthy children are characterized by lower relative value of fixational noise amplitude sample mean to saccade amplitude sample mean while the higher value of this ratio correspond to children with learning problems. We proposed the rather simple two-parametric model, based on using ratio of these sample means and ratio of average distances moved by fixational eye movements and saccades that allows to discriminate healthy children from having learning disabilities accompanied by neurological disorders. To identify the concrete disorder the modification of the proposed model is needed.

 

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