Eye movements and reading speed in macular disease: the shrinking perceptual span hypothesis requires and is supported by a mediation analysis.
OBJECTIVE
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.
Minnesota Laboratory for Low-Vision Research, University of Minnesota, Minneapolis, United States.
Aurélie
A
Calabrèse
Aix Marseille University, CNRS, LPC UMR 7290, Marseille, France.
Jean-Baptiste
JB
Bernard
Low Vision Clinic, University Hospital of La Timone, Marseille, France.
Géraldine
G
Faure
Department of Ophthalmology, University Hospital of La Timone, Marseille, France.
Louis
L
Hoffart
Aix Marseille University, CNRS, LPC UMR 7290, Marseille, France.
Eric
E
Castet
D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D053483:Eye Movement Measurements; D005133:Eye Movements / Q000502:physiology*; D005260:Female; D005500:Follow-Up Studies; D006801:Humans; D008268:Macular Degeneration / Q000150:complications / Q000175:diagnosis / Q000503:physiopathology*; D008297:Male; D008875:Middle Aged; D010775:Photic Stimulation; D011932:Reading; D012607:Scotoma / Q000175:diagnosis / Q000209:etiology / Q000503:physiopathology*; D014792:Visual Acuity / Q000502:physiology*; D014794:Visual Fields / Q000502:physiology*; D014796:Visual Perception / Q000502:physiology*
central field loss; fixation duration; mediation model; reading speed; visual span