Research

Previous studies have shown that the spatial extent of crowding in peripheral vision is reduced when a target letter and its flanking letters have opposite contrast polarity. We have examined if this reduction in crowding leads to improved reading performance. We compared the spatial extent of crowding, visual-span profiles (plots of letter-recognition accuracy versus letter position), and reading speeds at 10° lower field, using white letters, black letters, or alternating mixtures of white and black letters. Consistent with previous studies, the spatial extent of crowding was reduced when the target and flanking letters had opposite contrast polarity. However, using alternating mixed contrast polarity did not lead to improvements in visual-span profiles or reading speed.

The authors describe 3 human spatial navigation experiments that investigate how limitations of perception, memory, uncertainty, and decision strategy affect human spatial navigation performance. To better understand the effect of these variables on human navigation performance, the authors developed an ideal-navigator model for indoor navigation whose optimizing algorithm uses a partially observable Markov decision process. The model minimizes the number of actions (translations and rotations) required to move from an unknown starting state to a specific goal state in indoor environments that have perceptual ambiguity. The authors compared the model's performance with that of the human observer to measure human navigation efficiency. Experiment 1 investigated the effect of increasing the layout size on spatial way-finding efficiency and found that participants' efficiencies decreased as layout size increased. The authors investigated whether this reduction in navigation efficiency was due to visual perception (Experiment 2), memory, spatial updating strategy, or decision strategy (Experiment 3).
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This article details a study which predicted that across a wide range of print sizes dyslexic reading would follow the same curve shape as skilled reading, with constant reading rates across large print sizes and a sharp decline in reading rates below a critical print size. It also predicted that dyslexic readers would require larger critical print sizes to attain their maximum reading speeds, following the letter position coding deficit hypothesis. Reading speed was measured across twelve print sizes ranging from Snellen equivalents of 20/12 to 20/200 letter sizes for a group of dyslexic readers in Grades 2 to 4 (aged 7 to 10 years), and for non-dyslexic readers in Grades 1 to 3 (aged 6 to 8 years). The groups were equated for word reading ability. Results confirmed that reading rate-by-print size curves followed the same two-limbed shape for dyslexic and non-dyslexic readers. Dyslexic reading curves showed higher critical print sizes and shallower reading rate-by-print size slopes below the critical print size, consistent with the hypothesis of a letter-position coding deficit. Non-dyslexic reading curves also showed a decrease of critical print size with age. A developmental lag model of dyslexic reading does not account for the results, since the regression of critical print size on maximum reading rate differed between groups.
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The integration of visual, lexical, and oculomotor information is a critical part of reading. Mr. Chips is an ideal-observer model that combines these sources of information optimally to read simple texts in the minimum number of saccades. This model provides a computational framework for interpreting human reading saccades in both normal and low vision. The purpose of this paper is to report performance of the model for conditions emulating reading with normal vision — a visual span of 9, multiplicative saccade noise with a standard deviation of 30%, and texts based on three full-length children's books. Comparison of fixation locations by humans and Mr. Chips revealed: 1) that both exhibit very similar word skipping behavior; 2) both show initial fixations near the center of words, but with a systematic difference suggestive of an asymmetry in the human visual span; and 3) differences in the pattern of refixations within words that may uncover non-optimal lexical inference by human readers. A human context effect — 30% difference in mean saccade size between continuous text and random sequences of words — was very similar to the 25% effect for the model associated with a corresponding difference in the predictability of text words. Overall, our findings show that many of the complicated aspects of human reading saccades can be explained concisely by early information-processing constraints.
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Our goal is to link spatial and temporal properties of letter recognition to reading speed for text viewed centrally or in peripheral vision. We propose that the size of the visual span — the number of letters recognizable in a glance — imposes a fundamental limit on reading speed, and that shrinkage of the visual span in peripheral vision accounts for slower peripheral reading. In Experiment 1, we estimated the size of the visual span in the lower visual field by measuring RSVP (rapid serial visual presentation) reading times as a function of word length. The size of the visual span decreased from at least 10 letters in central vision to 1.7 letters at 15 deg, in good agreement with the corresponding reduction of reading speed measured by Chung, Mansfield & Legge (1998). In Exp. 2, we measured letter recognition for trigrams (random strings of three letters) as a function of their position on horizontal lines passing through fixation (central vision) or displaced downward into the lower visual field (5, 10 and 20 deg). We also varied trigram presentation time. We used these data to construct visual-span profiles of letter accuracy vs. letter position. These profiles were used as input to a parameter-free model whose output was RSVP reading speed. A version of this model containing a simple lexical-matching rule accounted for RSVP reading speed in central vision. Failure of this version of the model in peripheral vision indicated that people make use of lexical inference to support peripheral reading. We conclude that spatiotemporal characteristics of the visual span limit reading speed in central vision, and that shrinkage of the visual span results in slower reading in peripheral vision.
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In his Letter to the Editor, Skottun (2001) re-plots data from O'Brien, Mansfield, and Legge (2000) to illustrate the claim that decreasing print size maximizes the difference in reading speed between individuals with dyslexia and controls. This is an intriguing and suggestive observation, but we believe a more definitive study is required to justify this claim.

The goal of our study was to investigate the effect of contrast on reading speed in groups of readers with and without dyslexia in order to test the hypothesis that reducing contrast aids dyslexic performance (e.g. Williams, May, Solomon, & Zhou, 1995 ). Our findings indicated that contrast affected dyslexic and non-dyslexic reading similarly, implying similar contrast coding in both groups. We observed this result across several variables, including age of the participants (children and adults), contextual constraint (sentences and random word strings), mode of performance (oral and silent reading), and print size (0.2, 0.8 and 2.0 degree characters). Our purpose for testing at different print sizes was to relate our results to previous findings of spatial-frequency-selective deficits in dyslexic contrast sensitivity.

While our data show a potential group by printsize effect, we are unwilling to draw a firm conclusion. Only a small number of subjects were tested across all print sizes (two dyslexic and two control). This was because we were primarily interested in dyslexic versus non-dyslexic effects of contrast at the three print sizes, and not the effect of printsize per se. We did not perform a statistical analysis of the apparent group by printsize effect, as suggested by one reviewer, because most subjects were tested at a single print size.

An investigation focusing on the effect of print size in dyslexia requires an adequate sample size tested across a range of print sizes in a repeated measures design. Such a study could yield detailed reading-speed-by-printsize curves (cf. Mansfield, Legge, & Bane, 1996 ). Typically, these curves are flat for a range of large print sizes, turn down at a `critical print size' (i.e. the smallest print size yielding maximum reading speed), and drop off rapidly as the reading acuity limit is approached. We would expect maximum reading speed to be lower for individuals with dyslexia. But to further examine Skottun's claim, it would be informative to determine if dyslexic curves have the normal two-limbed shape, and whether their critical print sizes and reading acuities are within the normal range.

To our knowledge, the only other study that addresses character size effects in dyslexia is that of Cornelissen, Bradley, Fowler, and Stein (1991) . They showed that reading errors decreased with large font size (24-point versus 9- and 12-point Helvetica) for reading disabled children who had poor binocular control. The authors concluded that straining the visual system caused reading errors in these children. Corneilissen et al.'s (1991) findings and Skottun's observations point to print size as a variable that might establish a causal link between vision and dyslexia. Since our study had insufficient repeated measures data, it cannot be taken as definitive support for this causal link. In our view, an appropriately designed study is desirable.

References

P. Cornelissen, L. Bradley, S. Fowler and J. Stein (1991). What children see affects how they read. Developmental Medicine and Child Neurology 33 (1991), pp. 755762.

J.S. Mansfield, G.E. Legge and M.C. Bane. (1996). Psychophysics of reading XV. Font effects in normal and low vision. Investigative Ophthalmology and Visual Science 37 (1996), pp. 14921501.

B.A. O'Brien, J.S. Mansfield and G.E. Legge. (2000). The effect of contrast on reading speed in dyslexia. Vision Research 40 (2000), pp. 19211935.

M.C. Williams, J.G. May, R. Soloman and H. Zhou. (1995). The effects of spatial filtering and contrast reduction on visual search times in good and poor readers. Vision Research 35 (1995), pp. 285–291.

Contrast coding has been reported to differ between dyslexic and normal readers. Dyslexic readers require higher levels of contrast to detect sinewave gratings for certain spatio-temporal conditions, and dyslexic readers show faster visual search at low contrast. We investigated whether these differences in early contrast coding generalize to reading performance by measuring reading speed as a function of text contrast for dyslexic children and adults and for age-matched controls. Contrast affected reading performance of dyslexic and normal readers similarly. For both groups, reading speed was relatively constant between 100% and 2% contrast, and decreased rapidly below 2% contrast. This pattern of results held true for both children and adults, for text with and without sentence context, across a range of character sizes, and for reading aloud and reading silently. We conclude that earlier findings of group differences in contrast effects on grating detection or visual search tasks do not generalize to reading.

Braille is an important form of written communication for many visually disabled people. Perceptual factors limiting Braille reading speed are poorly understood, partly because of the lack of accurate, standardized testing methods. In this paper, we describe a new, standardized method for measuring Braille reading speed, adapted from the MNREAD test for print reading speed. The key principles include the use of well-characterized text samples composed of simple vocabulary, presented in a standard spatial layout, and equated for length in standard-length words.

We used the MNREAD test to study Braille reading speed in 44 experienced Braille readers. The median reading speed was 104 standard-length words per minute (SLwpm) Grade 2 reading speed was faster than Grade 1, but the difference can be accounted for by the difference in the number of characters.

A comparison of Braille and print reading speed shows that when conditions are matched and the methods of measurement are the same, the characteristics of reading speed are qualitatively and quantitatively similar.
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Reading in peripheral vision is slow and requires large print, posing substantial difficulty for patients with central scotomata. The purpose of this study was to evaluate the effect of print size on reading speed at different eccentricities in normal peripheral vision. We hypothesized that reading speeds should remain invariant with eccentricity, as long as the print is appropriately scaled in size--the scaling hypothesis. The scaling hypothesis predicts that log-log plots of reading speed versus print size exhibit the same shape at all eccentricities, but shift along the print-size axis. Six normal observers read aloud single sentences (approximately 11 words in length) presented on a computer monitor, one word at a time, using rapid serial visual presentation (RSVP). We measured reading speeds (based on RSVP exposure durations yielding 80% correct) for eight print sizes at each of six retinal eccentricities, from 0 (foveal) to 20 deg in the inferior visual field. Consistent with the scaling hypothesis, plots of reading speed versus print size had the same shape at different eccentricities: reading speed increased with print size, up to a critical print size and was then constant at a maximum reading speed for larger print sizes. Also consistent with the scaling hypothesis, the plots shifted horizontally such that average values of the critical print size increased from 0.16 deg (fovea) to 2.22 deg (20 deg peripheral). Inconsistent with the scaling hypothesis, the plots also exhibited vertical shifts so that average values of the maximum reading speed decreased from 807 w.p.m. (fovea) to 135 w.p.m. (20 deg peripheral). Because the maximum reading speed is not invariant with eccentricity even when the print size was scaled, we reject the scaling hypothesis and conclude that print size is not the only factor limiting maximum reading speed in normal peripheral vision.
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Can people with different forms of low vision use motion parallax to improve depth judgments? We used a staircase method to compare depth thresholds using motion parallax and static viewing. We tested eighteen normal-vision subjects with a range of simulated deficits in acuity, contrast sensitivity, and simulated peripheral-field loss, and ten low-vision subjects with a wide range of acuity, contrast sensitivity, and field loss. Subjects viewed three vertical cylinders monocularly and indicated which one was at a different depth from the other two. For motion-parallax trials, observers moved their heads (in a viewing assembly on rollers) from side to side over a range of 6–12 cm. For static trials, the viewing assembly was fixed in place. Normal-vision subjects' depth thresholds with motion parallax were significantly smaller than those with static viewing by an average factor of 1.95 (p %lt; 0.05) across all levels of acuity and contrast. For low-vision observers, the depth thresholds exhibited large individual differences; however, the motion-parallax thresholds were smaller than the static thresholds by an average factor of 2.05 (p < 0.01). These findings indicate that motion parallax can provide useful depth information for people with low vision.

How is a single visual direction assigned to a binocular feature for which the left and right eyes are signaling different directions? According to geometrical principles, binocular visual direction is the average of the visual directions measured from the left and right eyes. Contrary to this prediction, we have found that the relative visual direction between two Gabor targets presented at different stereoscopic depths could be manipulated by varying the contrast ratio between the left and right images. This finding is consistent with a new model in which the relative alignment of depth features is determined from a maximum-likelihood combination of the direction signals from the left and right eyes. In a second experiment we provide support for this model, showing that the magnitude of the contrast-dependent bias in visual direction is predicted by the uncertainty for spatial localization in the left and right images. Lastly we show that visual direction and stereopsis have different dependencies on interocular contrast differences, suggesting that the computation of stereo depth and visual direction are mediated via different mechanisms.

Purpose: Little is known about the effect of font on low-vision reading. In this study, the authors measured the influence of font in reading with normal and low vision.

Methods: Reading acuity, maximum reading speed, and critical print size (the smallest print that can be read with maximum speed) were measured in 50 normal subjects and 42 subjects with low vision. Data were collected using versions of the MNREAD Acuity Chart printed with the Times (proportionally spaced) and Courier (fixed-width) fonts.

Results: Reading acuity scores obtained with Courier were better than those obtained with Times for both normal (mean difference, 0.05 logMAR, p < 0.001) and subjects with low vision (0.09 logMAR, p < 0.001). Similarly, critical print sizes measured with Courier were smaller than those measured with Times (mean difference, 0.06 logMAR for normal subjects and subjects with low vision, p < 0.002). Maximum reading speeds for normal subjects were 5% faster with Times than with Courier (p < 0.001), but for subjects with low vision, maximum reading speeds were 10% slower with Times than with Courier (p < 0.05). For print smaller than the critical print size, the reading speeds of normal subjects and subjects with low vision were substantially slower (by as much as 50%) for Times than for Courier.

Conclusion: There are small, but significant, advantages of Courier over Times in reading acuity, critical print size, and reading speed for subjects with low vision. For normal subjects, the differences are slighter, with an advantage in reading speed for Times. However, for print sizes close to the acuity limit, choice of font could make a significant difference in both normal and low-vision reading performance.
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A masking paradigm was used to evaluate the orientation selectivity of the mechanisms mediating human stereopsis. Two experienced and eleven naive observers viewed stereograms, spatially filtered to contain contrast energy with Gaussian passbands in spatial frequency and orientation. Using forced-choice procedures we measured contrast thresholds for stereopsis in the presence of oriented masking patterns. Our results show that the masking of stereopsis consists of two components: one is orientation dependent, the other is non-oriented and has greatest amplitude at low spatial frequencies. Contrary to an earlier study, these results imply that stereo mechanisms may have similar orientation tuning to mechanisms mediating contrast detection.