Visual Acuity Is Not the Best at the Preferred Retinal Locus in People with Macular Disease.

SIGNIFICANCE: Little is known about how the preferred retinal locus (PRL) develops in patients with macular disease. We found that acuity is worse at the PRL than at other retinal locations around the scotoma, suggesting that the selection of the PRL location is unlikely to be based on optimizing acuity. PURPOSE: Following the onset of bilateral macular disease, most patients adopt a retinal location outside the central scotoma, the PRL, as their new retinal location for visual tasks. Very little information is known about how the location of a PRL is chosen. In this study, we tested the hypothesis that the selection of the location for a PRL is based on optimizing visual acuity, which predicts that acuity is the best at the PRL, compared with other retinal locations. METHODS: Using a scanning laser ophthalmoscope that allowed us to position visual targets at precise retinal locations, we measured acuity psychophysically using a four-orientation Tumbling-E presented at the PRL and at multiple (ranged between 23 and 36 across observers) locations around the scotoma for five observers with bilateral macular disease. RESULTS: For all five observers, the acuity at the PRL was never the best among all testing locations. Instead, acuities were better at 15 to 86% of the testing locations other than the PRL, with the best acuity being 17 to 58% better than that at the PRL. The locations with better acuities did not cluster around the PRL and did not necessarily lie at the same distance from the fovea or the PRL. CONCLUSIONS: Our finding that acuity is worse at the PRL than at other locations around the scotoma implies that the selection of the PRL location is unlikely to be based on optimizing acuity.

The Role of External Features in Face Recognition with Central Vision Loss.

PURPOSE: We evaluated how the performance of recognizing familiar face images depends on the internal (eyebrows, eyes, nose, mouth) and external face features (chin, outline of face, hairline) in individuals with central vision loss. METHODS: In experiment 1, we measured eye movements for four observers with central vision loss to determine whether they fixated more often on the internal or the external features of face images while attempting to recognize the images. We then measured the accuracy for recognizing face images that contained only the internal, only the external, or both internal and external features (experiment 2) and for hybrid images where the internal and external features came from two different source images (experiment 3) for five observers with central vision loss and four age-matched control observers. RESULTS: When recognizing familiar face images, approximately 40% of the fixations of observers with central vision loss was centered on the external features of faces. The recognition accuracy was higher for images containing only external features (66.8 ± 3.3% correct) than for images containing only internal features (35.8 ± 15.0%), a finding contradicting that of control observers. For hybrid face images, observers with central vision loss responded more accurately to the external features (50.4 ± 17.8%) than to the internal features (9.3 ± 4.9%), whereas control observers did not show the same bias toward responding to the external features. CONCLUSIONS: Contrary to people with normal vision who rely more on the internal features of face images for recognizing familiar faces, individuals with central vision loss show a higher dependence on using external features of face images.

The dependence of crowding on flanker complexity and target-flanker similarity.

We examined the effects of the spatial complexity of flankers and target-flanker similarity on the performance of identifying crowded letters. On each trial, observers identified the middle character of random strings of three characters ("trigrams") briefly presented at 10° below fixation. We tested the 26 lowercase letters of the Times Roman and Courier fonts, a set of 79 characters (letters and non-letters) of the Times Roman font, and the uppercase letters of two highly complex ornamental fonts, Edwardian and Aristocrat. Spatial complexity of characters was quantified by the length of the morphological skeleton of each character, and target-flanker similarity was defined based on a psychometric similarity matrix. Our results showed that (1) letter identification error rate increases with flanker complexity up to a certain value, beyond which error rate becomes independent of flanker complexity; (2) the increase of error rate is slower for high-complexity target letters; (3) error rate increases with target-flanker similarity; and (4) mislocation error rate increases with target-flanker similarity. These findings, combined with the current understanding of the faulty feature integration account of crowding, provide some constraints of how the feature integration process could cause perceptual errors.

The optimal use of non-optimal letter information in foveal and parafoveal word recognition.

Letters and words across the visual field can be difficult to identify due to limiting visual factors such as acuity, crowding and position uncertainty. Here, we show that when human readers identify words presented at foveal and para-foveal locations, they act like theoretical observers making optimal use of letter identity and letter position information independently extracted from each letter after an unavoidable and non-optimal letter recognition guess. The novelty of our approach is that we carefully considered foveal and parafoveal letter identity and position uncertainties by measuring crowded letter recognition performance in five subjects without any word context influence. Based on these behavioral measures, lexical access was simulated for each subject by an observer making optimal use of each subject's uncertainties. This free-parameter model was able to predict individual behavioral recognition rates of words presented at different positions across the visual field. Importantly, the model was also able to predict individual mislocation and identity letter errors made during behavioral word recognition. These results reinforce the view that human readers recognize foveal and parafoveal words by parts (the word letters) in a first stage, independently of word context. They also suggest a second step where letter identity and position uncertainties are generated based on letter first guesses and positions. During the third lexical access stage, identity and position uncertainties from each letter look remarkably combined together through an optimal word recognition decision process.

Feature contingencies when reading letter strings.

Many models posit the use of distinctive spatial features to recognize letters of the alphabet, a fundamental component of reading. It has also been hypothesized that when letters are in close proximity, visual crowding may cause features to mislocalize between nearby letters, causing identification errors. Here, we took a data-driven approach to investigate these aspects of textual processing. Using data collected from subjects identifying each letter in thousands of lower-case letter trigrams presented in the peripheral visual field, we found characteristic error patterns in the results suggestive of the use of particular spatial features. Distinctive features were seldom entirely missed, and we found evidence for errors due to doubling, masking, and migration of features. Dependencies both amongst neighboring letters and in the responses revealed the contingent nature of processing letter strings, challenging the most basic models of reading that ignore either crowding or featural decomposition.

Page mode reading with simulated scotomas: oculo-motor patterns.

This study investigated the relationship between reading speed and oculo-motor parameters when normally sighted observers had to read single sentences with an artificial macular scotoma. Using multiple regression analysis, our main result shows that two significant predictors, number of saccades per sentence followed by average fixation duration, account for 94% of reading speed variance: reading speed decreases when number of saccades and fixation duration increase. The number of letters per forward saccade (L/FS), which was measured directly in contrast to previous studies, is not a significant predictor. The results suggest that, independently of the size of saccades, some or all portions of a sentence are temporally integrated across an increasing number of fixations as reading speed is reduced.

Bolder print does not increase reading speed in people with central vision loss.

Patients with central vision loss are often advised by low vision rehabilitation professionals to read bolder print to ameliorate their reading difficulties. Is boldface print really effective in improving reading performance for people with central vision loss? In this study, we evaluated how reading speed depends on the stroke-width of text in people with central vision loss. Ten participants with long-standing central vision loss read aloud single, short sentences presented on a computer monitor, one word at a time, using rapid serial visual presentation (RSVP). Reading speed was calculated based on the RSVP word exposure duration that yielded 80% of words read correctly. Text was rendered in Courier and at six boldness levels, defined as the width of the letter-strokes normalized to that of the standard Courier font: 0.27, 0.72, 1, 1.48, 1.89 and 3.04× the standard. Reading speed was measured for two print sizes - 0.8× and 1.4× the critical print size (the smallest print size that can be read at the maximum reading speed). For all participants and both print sizes, reading speeds were essentially the same for text with stroke-width boldness ranging from 0.72 to 1.89× the standard, and were significantly lower for the thinnest and the boldest print. Most importantly, reading speed was not higher for bolder print than for the standard one. Despite the clinical wisdom that patients with central vision loss might benefit from bolder print, print with stroke-widths larger than the standard does not significantly improve reading speed for participants with central vision loss.

Page mode reading with simulated scotomas: a modest effect of interline spacing on reading speed.

Crowding is thought to be one potent limiting factor of reading in peripheral vision. While several studies investigated how crowding between horizontally adjacent letters or words can influence eccentric reading, little attention has been paid to the influence of vertically adjacent lines of text. The goal of this study was to examine the dependence of page mode reading performance (speed and accuracy) on interline spacing. A gaze-contingent visual display was used to simulate a visual central scotoma while normally sighted observers read meaningful French sentences following MNREAD principles. The sensitivity of this new material to low-level factors was confirmed by showing strong effects of perceptual learning, print size and scotoma size on reading performance. In contrast, reading speed was only slightly modulated by interline spacing even for the largest range tested: a 26% gain for a 178% increase in spacing. This modest effect sharply contrasts with the dramatic influence of vertical word spacing found in a recent RSVP study. This discrepancy suggests either that vertical crowding is minimized when reading meaningful sentences, or that the interaction between crowding and other factors such as attention and/or visuo-motor control is dependent on the paradigm used to assess reading speed (page vs. RSVP mode).

A New Font, Specifically Designed for Peripheral Vision, Improves Peripheral Letter and Word Recognition, but Not Eye-Mediated Reading Performance.

Reading speed is dramatically reduced when readers cannot use their central vision. This is because low visual acuity and crowding negatively impact letter recognition in the periphery. In this study, we designed a new font (referred to as the Eido font) in order to reduce inter-letter similarity and consequently to increase peripheral letter recognition performance. We tested this font by running five experiments that compared the Eido font with the standard Courier font. Letter spacing and x-height were identical for the two monospaced fonts. Six normally-sighted subjects used exclusively their peripheral vision to run two aloud reading tasks (with eye movements), a letter recognition task (without eye movements), a word recognition task (without eye movements) and a lexical decision task. Results show that reading speed was not significantly different between the Eido and the Courier font when subjects had to read single sentences with a round simulated gaze-contingent central scotoma (10° diameter). In contrast, Eido significantly decreased perceptual errors in peripheral crowded letter recognition (-30% errors on average for letters briefly presented at 6° eccentricity) and in peripheral word recognition (-32% errors on average for words briefly presented at 6° eccentricity).

Clustering of Eye Fixations: A New Oculomotor Determinant of Reading Speed in Maculopathy.

PURPOSE: To describe and quantify a largely unnoticed oculomotor pattern that often occurs when patients with central field loss (CFL) read continuous text: Horizontal distribution of eye fixations dramatically varies across sentences and often reveals clusters. Also to statistically analyze the effect of this new factor on reading speed while controlling for the effect of saccadic amplitude (measured in letters per forward saccade, L/FS), an established oculomotor effect. METHODS: Quantification of nonuniformity of eye fixations (NUF factor) was based on statistical analysis of the curvature of fixation distributions. Linear mixed-effects analyses were performed to predict reading speed from oculomotor factors based on eye movements of 34 AMD and 4 Stargardt patients (better eye decimal acuity from 0.08 to 0.3). Single-line French sentences were read aloud by these patients, who all had a dense scotoma covering the fovea as assessed with MP1 microperimetry. RESULTS: Nonuniformity of fixations is a strong determinant of reading speed (-0.76 log units; 95% confidence interval [CI] [-0.86, -0.66]). This effect is not confounded with the effect of L/FS. The per sentence proportion of trials with clustering is predicted by the frequency of occurrence of the lowest-frequency word in each sentence. CONCLUSIONS: The NUF factor is a new oculomotor predictor of reading speed. This effect is independent of the effect of L/FS. Reading performance, as well as motivation to read, might be enhanced if new visual aids or automatic text simplification were used to reduce the occurrence of fixation clustering.

Role of syllable segmentation processes in peripheral word recognition.

Previous studies of foveal visual word recognition provide evidence for a low-level syllable decomposition mechanism occurring during the recognition of a word. We investigated if such a decomposition mechanism also exists in peripheral word recognition. Single words were visually presented to subjects in the peripheral field using a 6° square gaze-contingent simulated central scotoma. In the first experiment, words were either unicolor or had their adjacent syllables segmented with two different colors (color/syllable congruent condition). Reaction times for correct word identification were measured for the two different conditions and for two different print sizes. Results show a significant decrease in reaction time for the color/syllable congruent condition compared with the unicolor condition. A second experiment suggests that this effect is specific to syllable decomposition and results from strategic, presumably involving attentional factors, rather than stimulus-driven control.

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

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.

The effect of letter-stroke boldness on reading speed in central and peripheral vision.

People with central vision loss often prefer boldface print over normal print for reading. However, little is known about how reading speed is influenced by the letter-stroke boldness of font. In this study, we examined the reliance of reading speed on stroke boldness, and determined whether this reliance differs between the normal central and peripheral vision. Reading speed was measured using the rapid serial visual presentation paradigm, where observers with normal vision read aloud short single sentences presented on a computer monitor, one word at a time. 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. Testings were conducted at the fovea and 10° in the inferior visual field. 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). At the fovea, reading speed was invariant for the middle four levels of boldness, but dropped by 23.3% for the least and the most bold text. At 10° eccentricity, reading speed was virtually the same for all boldness <1, but showed a poorer tolerance to bolder text, dropping by 21.5% for 1.89× boldness and 51% for the most bold (3.04×) text. These results could not be accounted for by the changes in print size or the RMS contrast of text associated with changes in stroke boldness. Our results suggest that contrary to the popular belief, reading speed does not benefit from bold text in the normal fovea and periphery. Excessive increase in stroke boldness may even impair reading speed, especially in the periphery.

Can reading-specific training stimuli improve the effect of perceptual learning on peripheral reading speed?

In a previous study, Chung, Legge, and Cheung (2004) showed that training using repeated presentation of trigrams (sequences of three random letters) resulted in an increase in the size of the visual span (number of letters recognized in a glance) and reading speed in the normal periphery. In this study, we asked whether we could optimize the benefit of trigram training on reading speed by using trigrams more specific to the reading task (i.e., trigrams frequently used in the English language) and presenting them according to their frequencies of occurrence in normal English usage and observers' performance. Averaged across seven observers, our training paradigm (4 days of training) increased the size of the visual span by 6.44 bits, with an accompanied 63.6% increase in the maximum reading speed, compared with the values before training. However, these benefits were not statistically different from those of Chung, Legge, and Cheung (2004) using a random-trigram training paradigm. Our findings confirm the possibility of increasing the size of the visual span and reading speed in the normal periphery with perceptual learning, and suggest that the benefits of training on letter recognition and maximum reading speed may not be linked to the types of letter strings presented during training.

Wet versus dry age-related macular degeneration in patients with central field loss: different effects on maximum reading speed.

PURPOSE: To describe new, efficient predictors of maximum reading speed (MRS) in age-related macular degeneration (AMD) patients with central field loss. Type of AMD (wet versus dry) was scrutinized, because this factor seems to offer a promising model of differential visual adaptation induced by different temporal courses of disease progression. METHODS: Linear mixed-effects (LME) analyses were performed on a dataset initially collected to assess the effect of interline spacing on MRS. MRS was measured with MNread-like French sentences in 89 eyes (64 dry and 25 wet) of 61 patients with AMD. Microperimetry examination was performed on each eye. The eyes were included only if they had a dense macular scotoma including the fovea, to ensure that patients used eccentric viewing. RESULTS: Analyses show the unique contributions--after adjustment for the effects of other factors--of three new factors: (1) MRS was higher for wet than for dry AMD eyes; (2) an advantage of similar amplitude was found for phakic eyes compared with pseudophakic eyes; and (3) MRS decreased when distance between fixation preferred retinal locus (PRL) and fovea increased. In addition, the instantaneous slope of the relationship between scotoma area and MRS was much shallower than reported in two other studies. CONCLUSIONS: The four effects improve the ability to predict MRS reliably for AMD patients. The wet/dry difference is a major finding that may result from the different time courses of the two types of disease, thus involving different types of visuomotor and attentional adaptation processes.

Small effect of interline spacing on maximal reading speed in low-vision patients with central field loss irrespective of scotoma size.

PURPOSE: It has been suggested that crowding, the adverse low-level effect due to the proximity of adjacent stimuli, explains slow reading in low-vision patients with absolute macular scotomas. According to this hypothesis, crowding in the vertical dimension should be released by increasing the vertical spacing between lines of text. However, studies with different experimental paradigms and only a few observers have given discrepant results on this question. The purpose of this study was to investigate this issue with a large number of patients whose macular function was carefully assessed. METHODS: MP1 microperimetry examination was performed for each low-vision patient. Only eyes with an absolute macular scotoma and no foveal sparing (61 patients with AMD, 90 eyes; four patients with Stargardt disease, eight eyes) were included. Maximal reading speed was assessed for each eye with French sentences designed on the MNREAD test principles. RESULTS: The effect of interline spacing on maximal reading speed (MRS) was significant although small; average MRS increased by 7.1 words/min from standard to double interline spacing. The effect was weak irrespective of PRL distance from the fovea and scotoma area and regardless of whether an eccentric island of functional vision was present within the scotoma. CONCLUSIONS: Increasing interline spacing is advisable only for very slow readers (<20 words/min) who want to read a few words (spot reading). Vertical crowding does not seem to be a major determinant of maximal reading speed for patients with central scotomas.