Reading performance is not affected by a prism induced increase of horizontal and vertical vergence demand

Abstract

Purpose: Dyslexia is the most common developmental reading disorder that affects language skills. Latent strabismus (heterophoria) has been suspected to be causally involved. Even though phoria correction in dyslexic children is commonly applied, the evidence in support of a benefit is poor. In order to provide experimental evidence on this issue, we simulated phoria in healthy readers by modifying the vergence tone required to maintain binocular alignment.

Methods: Vergence tone was altered with prisms that were placed in front of one eye in 16 healthy subjects to induce exophoria, esophoria, or vertical phoria. Subjects were to read one paragraph for each condition, from which reading speed was determined. Text comprehension was tested with a forced multiple choice test. Eye movements were recorded during reading and subsequently analyzed for saccadic amplitudes, saccades per 10 letters, percentage of regressive (backward) saccades, average fixation duration, first fixation duration on a word, and gaze duration.

Results: Acute change of horizontal and vertical vergence tone does neither significantly affect reading performance nor reading associated eye movements.

Conclusion: Prisms in healthy subjects fail to induce a significant change of reading performance. This finding is not compatible with a role of phoria in dyslexia. Our results contrast the proposal for correcting small angle heterophorias in dyslexic children.

Keywords: dyslexia; phoria; reading; strabismus; vergence.

FIGURE 1

Reading performance with prism induced changes of the vergence tone. Decreased convergence tone, such as is experienced in esophoria, and increased convergence tone, as is experienced in exophoria was induced by placing prisms base medially and base laterally, respectively. Boxplots (A) show reading speed (letters/s) for normal view and each prism condition. The boxes contain the mean and 50% of the data, and the whiskers indicate 10/90% of the data. Mean reading speed of each subject reveals less intrasubject variability than between subject variability (B; gray = subjects, black = mean of all subjects). Inset figure shows mean variability of mean reading speed within subjects and between subjects. Bar chart showing the mean comprehension score (% correct answers) ± standard error of the mean for normal view and each prism condition (C).

FIGURE 2

Eye movements during reading with prisms. Boxplots for number of saccades per 10 letters (A), proportion of regressive saccades (B), and average fixation duration (C). The boxes contain the mean and 50% of the data, and the whiskers indicate 10/90% of the data.