Dyslexic children are confronted with unstable binocular fixation while reading

Abstract

Reading requires three-dimensional motor control: saccades bring the eyes from left to right, fixating word after word; and oblique saccades bring the eyes to the next line of the text. The angle of vergence of the two optic axes should be adjusted to the depth of the book or screen and--most importantly--should be maintained in a sustained manner during saccades and fixations. Maintenance of vergence is important as it is a prerequisite for a single clear image of each word to be projected onto the fovea of the eyes. Deficits in the binocular control of saccades and of vergence in dyslexics have been reported previously but only for tasks using single targets. This study examines saccades and vergence control during real text reading. Thirteen dyslexic and seven non-dyslexic children read the French text "L'Allouette" in two viewing distances (40 cm vs. 100 cm), while binocular eye movements were measured with the Chronos Eye-tracking system. We found that the binocular yoking of reading saccades was poor in dyslexic children (relative to non-dyslexics) resulting in vergence errors; their disconjugate drift during fixations was not correlated with the disconjugacy during their saccades, causing considerable variability of vergence angle from fixation to fixation. Due to such poor oculomotor adjustments during reading, the overall fixation disparity was larger for dyslexic children, putting larger demand on their sensory fusion processes. Moreover, for dyslexics the standard deviation of fixation disparity was larger particularly when reading at near distance. We conclude that besides documented phoneme processing disorders, visual/ocular motor imperfections may exist in dyslexics that lead to fixation instability and thus, to instability of the letters or words during reading; such instability may perturb fusional processes and might--in part--complicate letter/word identification.

Conflict of interest statement

Competing Interests: The authors have declared that no competing intersts exist.

Figures

Figure 1. We selected a sequence of two saccades from the eye movement measurements to illustrate the placement of the markers.

In (a) the version signal ((right eye+left eye)/2; min arc) is shown and two saccades can be detected easily; the saccade start was marked by an B and the end by an E. Further, the end of a fixation period was marked by an F and this end was defined as 10 ms before the next saccade started. All markers from the version signal were transferred into the vergence signal as well. In (b) the vergence signal (left eye – right eye; min arc) is shown. Additionally, for each fixation period the minimum fixation disparity was marked by an M; the interval [E F] in the vergence signal was also used to calculate the standard deviation of fixation disparity during this fixation period.

Figure 2. The average conjugate drift during fixations (deg) as a function of the saccade disconjugacy (deg) for non-dyslexic (a) and dyslexic (b) children.

Figure 3. Histogram of the standard deviation (SD in deg) of fixation disparity measured while the children read the text at (a) close reading distance (40 cm) and at (b) far reading distance (100 cm); the plots show the data for non-dyslexic (dots) and dyslexic children (triangles), respectively.

Figure 4. The different plots show the standard deviation of fixation disparity as a function of horizontal fixation position (deg), i.e., as a function of the horizontal position within the text at which the children looked at.

The two lefthand plots show data for the close reading while the two righthand plots show data for the far reading. Upper plots (a & b) are those for non-dyslexic children (N = 7) while the lower plots (c & d) show data of the dyslexic children (N = 13).

Figure 5. The different plots show the standard deviation of fixation disparity as a function of vertical fixation position (deg), i.e., as a function of the vertical position within the text at which the children looked at.

The two lefthand plots show data for the close reading while the two righthand plots show data for the far reading. Upper plots (a & b) are those for non-dyslexic children (N = 7) while the lower plots (c & d) show data of the dyslexic children (N = 13).

Figure 6. The different plots show the standard deviation of fixation disparity as a function of horizontal fixation position (deg), i.e., as a function of the horizontal position within the painting which the children looked at.

The two lefthand plots show data for a close viewing distance while the two righthand plots show data for the far viewing distance, respectively. Upper plots (a & b) are those for non-dyslexic children (N = 2) while the lower plots (c & d) show data of the dyslexic children (N = 3).