Participants were required to read paragraphs of text that were selected excerpts taken from the “Oxford First Encyclopedia” (Oxford University Press, Oxford, UK) so that there was no difficulty with comprehension of the text.
11 Each paragraph was designed to give the same layout consisting of approximately the same number of characters with spaces (mean characters with spaces ± SD = 300.3 ± 8.3, mean number of words = 60.5 ± 3.2, mean words/line = 5.4 ± 0.3).
The paragraphs were generated as Microsoft PowerPoint (Microsoft Corporation, Redmond, WA) slides using font sizes selected to correspond to the following logMAR equivalent sizes: 1.0, 0.8, 0.7, 0.6, 0.5, 0.4, 0.3, and 0.2 logMAR optotype equivalents. Three paragraphs were created for each font size, of which one was chosen using a random number generator created using a Microsoft Excel Template. The logMAR equivalent print sizes were estimated from the height of a lowercase “x.” The corresponding logMAR size was based on the following equation as described in the MNREAD acuity charts
12 :
Left justified text without splitting of words was presented using a monospaced font (Courier New font) and the text was centered on the screen.
The text was presented as black letters (luminance = 0.45 cd/m2) on a white background (luminance = 12.0 cd/m2, letter contrast = 96.25%) using a video projector (Epson EMP-703, 1024 × 768 resolution; Epson, Tokyo, Japan) projecting onto a rear projection screen (1.75 m width and 1.17 m height). To avoid pixilation effects, two throw distances were used for the projector using a smaller distance for the smaller print sizes, with luminances of the fonts and background matched using the projector settings. The order of presentation of fonts was randomized for each projector throw distance using the random number generator in the Microsoft Excel Template mentioned above (smaller throw distance for 0.7, 0.6, 0.5, 0.4, 0.3, and 0.2 logMAR font sizes, and longer throw distance for 1.0 and 0.8 logMAR font sizes). Participants were seated at a distance of 2 m in front of the stimulus display screen with the head stabilized using a forehead and chin rest. Primary position of gaze corresponded to the screen center.
Participants were instructed to read the text silently. Silent reading was used as the chin rest limits jaw movements and speaking can introduce artifacts in eye movement data by causing vibration of the head mounted eye tracker. After reading each paragraph, participants were presented with a single word and asked to decide whether it was relevant to the previously read text, with 18 questions in total. All subjects answered the questions correctly, demonstrating accurate comprehension. Binocular viewing, amblyopic eye viewing, and dominant eye viewing were performed in random order. During monocular reading, the contralateral eye was occluded using a black opaque occluder. The test duration was approximately 1 hour.