Eye movements and reading speed in macular disease: The shrinking perceptual span hypothesis requires and is supported by a mediation analysis

Aurélie Calabrèse, Jean Baptiste Bernard, Géraldine Faure, Louis Hoffart, Eric Castet

Research output: Contribution to journalArticlepeer-review

21 Scopus citations

Abstract

Purpose. Reading speed of patients with central field loss (CFL) correlates with the size of saccades (measured in letters per forward saccade [L/FS]). We assessed whether this effect is mediated by the total number of fixations, by the average fixation duration, or by a mixture of both. Methods. We measured eye movements (with a video eye tracker) of 35 AMD and 4 Stargardt patients (better eye decimal acuity from 0.08-0.3) while they monocularly read single-line French sentences continuously displayed on a screen. All patients had a dense scotoma covering the fovea, as assessed with MP1 microperimetry, and therefore used eccentric viewing. Results were analyzed with regression-based mediation analysis, a modeling framework that informs on the underlying factors by which an independent variable affects a dependent variable. Results. Reading speed and average fixation duration are negatively correlated, a result that was not observed in prior studies with CFL patients. This effect of fixation duration on reading speed is still significant when partialling out the effect of the total number of fixations (slope: -0.75, P < 0.001). Despite this large effect of fixation duration, mediation analysis shows that the effect of L/FS on reading speed is fully mediated by the total number of fixations (effect size: 0.96; CI [0.82, 1.12]) and not by fixation duration (effect size: 0.02; CI [-0.11, 0.14]). Conclusions. Results are consistent with the shrinking perceptual span hypothesis: reading speed decreases with the average number of letters traversed on each forward saccade, an effect fully mediated by the total number of fixations.

Original languageEnglish (US)
Pages (from-to)3638-3645
Number of pages8
JournalInvestigative Ophthalmology and Visual Science
Volume55
Issue number6
DOIs
StatePublished - May 15 2014

Keywords

  • Central field loss
  • Fixation duration
  • Mediation model
  • Reading speed
  • Visual span

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