Amblyopic reading is crowded

Abstract

We measure acuity, crowding, and reading in amblyopic observers to answer four questions. (1) Is reading with the amblyopic eye impaired because of larger required letter size (i.e., worse acuity) or larger required spacing (i.e., worse crowding)? The size or spacing required to read at top speed is called "critical". For each eye of seven amblyopic observers and the preferred eyes of two normal observers, we measure reading rate as a function of the center-to-center spacing of the letters in central and peripheral vision. From these results, we estimate the critical spacing for reading. We also measured traditional acuity for an isolated letter and the critical spacing for identifying a letter among other letters, which is the classic measure of crowding. For both normals and amblyopes, in both central and peripheral vision, we find that the critical spacing for reading equals the critical spacing for crowding. The identical critical spacings, and very different critical sizes, show that crowding, not acuity, limits reading. (2) Does amblyopia affect peripheral reading? No. We find that amblyopes read normally with their amblyopic eye except that abnormal crowding in the fovea prevents them from reading fine print. (3) Is the normal periphery a good model for the amblyopic fovea? No. Reading centrally, the amblyopic eye has an abnormally large critical spacing but reads all larger spacings at normal rates. This is unlike the normal periphery, in which both critical spacing and maximum reading rate are severely impaired relative to the normal fovea. (4) Can the uncrowded-span theory of reading rate explain amblyopic reading? Yes. The case of amblyopia shows that crowding limits reading solely by determining the uncrowded span: the number of characters that are not crowded. Characters are uncrowded if and only if their spacing is more than critical. The text spacing may be uniform, but the observer's critical spacing increases with distance from fixation, so the uncrowded span extends out to where the spacing is critical. Amblyopes have normal critical spacing in the periphery, so, when the uncrowded span extends into the periphery, it has normal extent, which predicts our finding that reading rate is normal too. This confirms the theory that reading rate is determined by the width of the uncrowded span, independent of the critical spacing within the span. The uncrowded-span model of normal reading fits the amblyopic results well, with a roughly fivefold increase in the critical spacing at fixation. Thus, the entire amblyopic reading deficit is accounted for by crowding.

Perceptual learning leads to long lasting visual improvement in patients with central vision loss

“…Therefore, it is important to establish whether PL of collinear configurations transfers to the letter recognition task (eccentric VA), this is because there is a relationships between contrast detection and letter recognition (Chung, Legge, & Tjan, 2002;Chung, Mansfield, & Legge, 1998;Legge, Rubin, Pelli, & Schleske, 1985;Levi, Song, & Pelli, 2007;Majaj, Pelli, Kurshan, & Palomares, 2002;Patching & Jordan, 2005;Salomon & Pelli, 1994). Transfer of PL to eccentric VA is shown in Fig.…”

The Effect of Font Size on Reading Performance in Strabismic Amblyopia: An Eye Movement Investigation

“…The nonamblyopic eye of strabismic amblyopes has been shown to exhibit sensory and ocular motor abnormalities, such as unsteady and eccentric fixations, 15 reduced contrast sensitivity 16,17 and Vernier acuity, 18 and increased crowding. 6,19 The most obvious mechanism leading to deterioration in reading speed with reducing font size is the phenomenon of crowding. Levi et al 6 have suggested that the amblyopic deficits observed during reading mainly are due to the increased crowding effect in amblyopic fovea, and speculate that reading rates in amblyopia can be predicted using the ''uncrowded span theory.…”

“…6,19 The most obvious mechanism leading to deterioration in reading speed with reducing font size is the phenomenon of crowding. Levi et al 6 have suggested that the amblyopic deficits observed during reading mainly are due to the increased crowding effect in amblyopic fovea, and speculate that reading rates in amblyopia can be predicted using the ''uncrowded span theory.…”

“…6 Crowding is a phenomenon in which a letter or symbol that is recognized easily on its own becomes unrecognizable if surrounded by other letters or symbols. Crowding becomes greater for smaller font sizes with more narrow spacing of letters.…”

Amblyopia and Real-World Visuomotor Tasks

“…These findings imply that the strabismic subjects produced more saccades at the planning/programming stage that 'over-shot' the text elements required to obtain word meaning, so necessitating a corrective eye movement in the reverse direction, with slower visual processing at fixation underlying the extraction of meaning for comprehension. Partly for these reasons, Kanonidou et al (2010) suggested that these problems were likely related to abnormalities of the strabismic visual system, such as suppression scotomas when viewing with both eyes and increased crowding effects when viewing monocularly with either eye (e.g., Levi et al, 2007), rather than to a primary motor system deficit. Because their oculomotor problems were relatively subtle compared to their marked reductions in MRS, Kanonidou et al (2010) further concluded that they represented just one of several causative factors of the functional reading impairments.…”