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Jean-Baptiste Bernard, Susana T L Chung; The optimal letterstroke-width for reading in people with central field loss. Invest. Ophthalmol. Vis. Sci. 2014;55(13):2152.
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© ARVO (1962-2015); The Authors (2016-present)
Patients with central field loss (CFL) often prefer boldface print over normal print for reading. However, little is known about how reading speed is influenced by the letterstroke boldness of font. In this study, we examined the reliance of reading speed on letterstroke-width for people with CFL.
Five observers with CFL participated in the study (age:58-84, logMAR acuity:0.52-1.1). Oral reading speed was measured binocularly using the rapid serial visual presentation (RSVP) paradigm in which words from short sentences were presented one at a time, each for a fixed word exposure duration. Text was rendered in Courier at six levels of boldness, defined as the stroke-width normalized to that of the standard Courier font: 0.27, 0.72, 1, 1.48, 1.89 and 3.04× the standard. Print sizes used were 0.8× and 1.4× the critical print size (smallest print size that can be read at the maximum reading speed), determined using the standard boldness. Reading speed was defined based on the RSVP word exposure duration that yielded 80% of words read correctly. For comparison, reading speed was also obtained at 10° eccentricity in the lower field for six normally sighted observers (age:17-24).
Averaged across observers with CFL, the plot of reading speed vs. boldness demonstrated an inverted U-shape function, for both print sizes tested. Peak reading speed was obtained at boldness levels of 1× and 1.48× (no difference in reading speed between the two). Reading speed dropped for all other boldness levels. For the thinnest (0.27× boldness) and the thickest (3.04× boldness) letter-strokes, reading speed was 30% and 50% lower than the peak reading speed, respectively. However, the reliance of reading speed on boldness was different in the normal periphery — reading speed for letter-strokes thinner than the standard did not show a drop in reading speed from the peak value. The effect of boldness on reading speed was virtually identical between the two print sizes for both groups of observers.
Contrary to the preference of most patients, bolder print did not yield faster reading speed than for standard boldness, for people with CFL or in the normal periphery. In fact, excessive increase in stroke boldness may even impair reading speed. The difference in the tolerance of reading speed to thinner letter-strokes suggests fundamental differences in the visual processing for people with CFL and the normal periphery.
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