Reading with central field loss: number of letters masked is more important than the size of the mask in degrees

Effects of visual span on Chinese reading performance in normal peripheral vision

The current study examined the relationships among temporal processing speed, spatial visual span and Chinese character reading speed in normal central and peripheral vision. Maximum reading speed (MRS) and critical print size (CPS) of 26 native Chinese readers (13 young and 13 older adults) were determined at three visual field locations: central vision, 10° left and 10° below fixation using a rapid serial visual presentation (RSVP) task. Temporal processing speed was measured using trigrams of randomly selected Chinese characters presented at a range of exposure durations, while spatial visual span was measured using trigrams presented at different spatial positions. It was found that shorter temporal processing speed and larger spatial visual span were associated with faster MRS at the central and inferior visual field locations, but not at the left of fixation location. As expected, reading and visual span metrics were better in central vision compared to both peripheral locations. In addition, reading, temporal processing, and spatial visual span metrics were better in the young than older subjects (except for similar temporal processing speed at two peripheral locations). The results for central and inferior presentation locations support the hypothesis that temporal processing speed and spatial visual span were associated with Chinese character reading speed. Surprisingly, no correlation was observed for the 10° left of the fixation location, suggesting that the factors affecting reading speed might differ for inferior and lateral peripheral viewing locations.

Visual function: the problem with eccentricity

Age-related macular degeneration (AMD) is the leading cause of blindness in developed countries. With an ageing population, the prevalence of such a condition has resulted in a large proportion of the population relying on peripheral vision to undertake activities of daily living. Peripheral vision is not a scaled-down version of the fovea, simply requiring larger print or increased contrast for detection of objects or reading text. Even when print size is scaled and eye movements are minimised, the peripheral retina cannot perform at the level of the foveal region. Understanding how and why reading performance is limited as a function of eccentricity has important implications for how we approach rehabilitation of patients with central visual loss. This brief review of the extensive literature on reading with peripheral vision and the research aimed at better reading rehabilitation for low vision patients focuses on why many of the problems associated with the reduced reading capability of peripheral vision cannot be completely solved with magnification, reducing eye movements or modifying print.

Simulation of a central scotoma using contact lenses with an opaque centre

PURPOSE

This study evaluated the feasibility of using soft contact lenses (CLs) with an opaque centre to induce absolute central scotomas that move with the eye. We examined the geometrical optics prediction that scotoma size will vary with the size of the CL's opaque centre and with ocular pupil size. We also tested the hypothesis that high environmental light levels will ensure that the ocular pupil will remain small enough, even with opaque centre CLs, to generate absolute scotomas representative of those experienced by patients with age-related macular disease.

METHODS

Using an Octopus 900 Perimeter ( www.Haag-Streit.com), kinetic visual fields (VFs) were measured in five normally-sighted subjects using a V4e Goldmann target with CLs that had central opaque areas with diameters of 2.8, 3.0, and 3.2 mm. To control pupil size, VFs were measured with background perimeter bowl luminances of 10, 585, and 1155 cd m^-2 . Subjects attempted to (i) fixate the bowl centre; and (ii) place the scotoma edge at the bowl fixation target (eccentric viewing).

RESULTS

As predicted, there was a direct relationship between scotoma size and both luminance level and diameter of the opacity. Mean scotoma diameters were 0°, 17.6° and 22°, for the low, medium and high bowl luminances, respectively. Scotoma size was determined primarily by the difference between the diameters of CL opacity and the entrance pupil of the eye and the axial separation between them, and between-subject differences in pupil diameters contributed most to the between-subject variability in scotoma diameter at each light level (SD: 6.01°). Scotoma displacement during eccentric fixation confirmed the gaze-contingent characteristics of this experimental model.

CONCLUSION

It is possible to induce a gaze-contingent absolute scotoma and hence mimic central vision loss using centrally-opaque CLs provided that the CL opacity is larger than the entrance pupil of the eye. This simulation tool will, therefore, be ineffective at low environmental light levels (as shown previously) if the entrance pupil of the eye is larger than the CL opacity.

The Impact of Language Opacity and Proficiency on Reading Strategies in Bilinguals: An Eye Movement Study

Reading strategies vary across languages according to orthographic depth – the complexity of the grapheme in relation to phoneme conversion rules – notably at the level of eye movement patterns. We recently demonstrated that a group of early bilinguals, who learned both languages equally under the age of seven, presented a first fixation location (FFL) closer to the beginning of words when reading in German as compared with French. Since German is known to be orthographically more transparent than French, this suggested that different strategies were being engaged depending on the orthographic depth of the used language. Opaque languages induce a global reading strategy, and transparent languages force a local/serial strategy. Thus, pseudo-words were processed using a local strategy in both languages, suggesting that the link between word forms and their lexical representation may also play a role in selecting a specific strategy. In order to test whether corresponding effects appear in late bilinguals with low proficiency in their second language (L2), we present a new study in which we recorded eye movements while two groups of late German–French and French–German bilinguals read aloud isolated French and German words and pseudo-words. Since, a transparent reading strategy is local and serial, with a high number of fixations per stimuli, and the level of the bilingual participants’ L2 is low, the impact of language opacity should be observed in L1. We therefore predicted a global reading strategy if the bilinguals’ L1 was French (FFL close to the middle of the stimuli with fewer fixations per stimuli) and a local and serial reading strategy if it was German. Thus, the L2 of each group, as well as pseudo-words, should also require a local and serial reading strategy. Our results confirmed these hypotheses, suggesting that global word processing is only achieved by bilinguals with an opaque L1 when reading in an opaque language; the low level in the L2 gives way to a local and serial reading strategy. These findings stress the fact that reading behavior is influenced not only by the linguistic mode but also by top–down factors, such as readers’ proficiency.

Effect of pattern complexity on the visual span for Chinese and alphabet characters

The visual span for reading is the number of letters that can be recognized without moving the eyes and is hypothesized to impose a sensory limitation on reading speed. Factors affecting the size of the visual span have been studied using alphabet letters. There may be common constraints applying to recognition of other scripts. The aim of this study was to extend the concept of the visual span to Chinese characters and to examine the effect of the greater complexity of these characters. We measured visual spans for Chinese characters and alphabet letters in the central vision of bilingual subjects. Perimetric complexity was used as a metric to quantify the pattern complexity of binary character images. The visual span tests were conducted with four sets of stimuli differing in complexity--lowercase alphabet letters and three groups of Chinese characters. We found that the size of visual spans decreased with increasing complexity, ranging from 10.5 characters for alphabet letters to 4.5 characters for the most complex Chinese characters studied. A decomposition analysis revealed that crowding was the dominant factor limiting the size of the visual span, and the amount of crowding increased with complexity. Errors in the spatial arrangement of characters (mislocations) had a secondary effect. We conclude that pattern complexity has a major effect on the size of the visual span, mediated in large part by crowding. Measuring the visual span for Chinese characters is likely to have high relevance to understanding visual constraints on Chinese reading performance.

Reading with a filtered fovea: the influence of visual quality at the point of fixation during reading

Reading relies critically on processing text in foveal vision during brief fixational pauses, and high-quality visual input from foveal text is fundamental to theories of reading. However, the quality of visual input from foveal text that is actually functional for reading and the effects of this input on reading performance are unclear. To investigate these issues, a moving, gaze-contingent foveal filtering technique was developed to display areas of text within foveal vision that provided only coarse, medium, or fine scale visual input during each fixational pause during reading. Normal reading times were unaffected when foveal text up to three characters wide at the point of fixation provided any one visual input (coarse, medium, or fine). Wider areas of coarse visual input lengthened reading times, but reading still occurred, and normal reading times were completely unaffected when only medium or fine visual input extended across the entire fovea. Further analyses revealed that each visual input had no effect on the number of fixations made when normal text was read, that adjusting fixation durations helped preserve reading efficiency for different visual inputs, and that each visual input had virtually no effect on normal saccades. These findings indicate that, despite the resolving power of foveal vision and the emphasis placed on high-quality foveal visual input by theories of reading, normal reading functions with similar success using a range of restricted visual inputs from foveal text, even at the point of fixation. Some implications of these findings for theories of reading are discussed.

Gaze-contingent simulation of retinopathy: some potential pitfalls and remedies

Many important results in visual neuroscience rely on the use of gaze-contingent retinal stabilization techniques. Our work focuses on the important fraction of these studies that is concerned with the retinal stabilization of visual filters that degrade some specific portions of the visual field. For instance, macular scotomas, often induced by age related macular degeneration, can be simulated by continuously displaying a gaze-contingent mask in the center of the visual field. The gaze-contingent rules used in most of these studies imply only a very minimal processing of ocular data. By analyzing the relationship between gaze and scotoma locations for different oculo-motor patterns, we show that such a minimal processing might have adverse perceptual and oculomotor consequences due mainly to two potential problems: (a) a transient blink-induced motion of the scotoma while gaze is static, and (b) the intrusion of post-saccadic slow eye movements. We have developed new gaze-contingent rules to solve these two problems. We have also suggested simple ways of tackling two unrecognized problems that are a potential source of mismatch between gaze and scotoma locations. Overall, the present work should help design, describe and test the paradigms used to simulate retinopathy with gaze-contingent displays.

Enhancing visual performance for people with central vision loss

People with central vision loss must use peripheral vision for visual tasks. It is well known that performance for almost all spatial tasks is worse in the normal periphery than in the normal fovea. The primary goal of my ongoing research is to understand the limiting factors and the potential for enhancing vision for people with central vision loss. Here I review my previous work related to understanding the limiting factors on reading, a task that is the primary complaint of many patients with age-related macular degeneration, the leading cause of visual impairment in the elderly. I also review my work related to enhancing visual functions in the normal periphery and how it may be applied to people with central vision loss.

The 35th Sir Frederick Bartlett Lecture: Eye movements and attention in reading, scene perception, and visual search

Eye movements are now widely used to investigate cognitive processes during reading, scene perception, and visual search. In this article, research on the following topics is reviewed with respect to reading: (a) the perceptual span (or span of effective vision), (b) preview benefit, (c) eye movement control, and (d) models of eye movements. Related issues with respect to eye movements during scene perception and visual search are also reviewed. It is argued that research on eye movements during reading has been somewhat advanced over research on eye movements in scene perception and visual search and that some of the paradigms developed to study reading should be more widely adopted in the study of scene perception and visual search. Research dealing with “real-world” tasks and research utilizing the visual-world paradigm are also briefly discussed.

Does context or color improve object recognition in patients with low vision?

Most studies on people with age-related macular degeneration (AMD) have been focused on investigations of low-level processes with simple stimuli like gratings, letters, and in perception of isolated faces or objects. We investigated the ability of people with low vision to analyze more complex stimuli like photographs of natural scenes. Fifteen participants with AMD and low vision (acuity on the better eye <20/200) and 11 normally sighted age-matched controls took part in the study. They were presented with photographs of either colored or achromatic gray level scenes in one condition and with photographs of natural scenes versus isolated objects extracted from these scenes in another condition. The photographs were centrally displayed for 300 ms. In both conditions, observers were instructed to press a key when they saw a predefined target (a face or an animal). The target was present in half of the trials. Color facilitated performance in people with low vision, while equivalent performance was found for colored and achromatic pictures in normally sighted participants. Isolated objects were categorized more accurately than objects in scenes in people with low vision. No difference was found for normally sighted observers. The results suggest that spatial properties that facilitate image segmentation (e.g., color and reduced crowding) help object perception in people with low vision.

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.

Recognition of facial emotion in low vision: a flexible usage of facial features

Age-related macular degeneration (AMD) is a major cause of visual impairment in people older than 50 years in Western countries, affecting essential tasks such as reading and face recognition. Here we investigated the mechanisms underlying the deficit in recognition of facial expressions in an AMD population with low vision. Pictures of faces displaying different emotions with the mouth open or closed were centrally displayed for 300 ms. Participants with AMD with low acuity (mean 20/200) and normally sighted age-matched controls performed one of two emotion tasks: detecting whether a face had an expression or not (expressive/non expressive (EXNEX) task) or categorizing the facial emotion as happy, angry, or neutral (categorization of expression (CATEX) task). Previous research has shown that healthy observers are mainly using high spatial frequencies in an EXNEX task while performance at a CATEX task was preferentially based on low spatial frequencies. Due to impaired processing of high spatial frequencies in central vision, we expected and observed that AMD participants failed at deciding whether a face was expressive or not but categorized normally the emotion of the face (e.g., happy, angry, neutral). Moreover, we observed that AMD participants mostly identified emotions using the lower part of the face (mouth). Accuracy did not differ between the two tasks for normally sighted observers. The results indicate that AMD participants are able to identify facial emotion but must base their decision mainly on the low spatial frequencies, as they lack the perception of finer details.

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).

The relationship between word length and threshold character size in patients with central scotoma and eccentric fixation

BACKGROUND

Understanding limitations on text reading with eccentric fixation is of major concern in low vision research. Our objective was to determine, in patients with a central scotoma, whether threshold character size is similar for different word lengths and paragraphed texts.

METHODS

In 19 patients, we retrospectively analyzed the relationship between minimum readable character size for isolated words and text. Isolated letters, two, five, and ten-letter words and a paragraphed text were presented randomly through a scanning laser ophthalmoscope in eight different character sizes.

RESULTS

Threshold character size varied according to the text stimulus (p<0.05). Threshold character sizes for single letters and two-letter words were matched (p>0.99), as were those for five-letter words, ten-letter words, and paragraphed text (p>0.99). Threshold character size for single letters and two-letter words was significantly lower than that measured with other text stimuli.

DISCUSSION

Reading performance is influenced by a variety of factors such as crowding, contextual effects, visual span, degree of oculomotor adaptation needed, and frequency of a defined word. Globally, when reading with a central scotoma, it appears that within word characteristics have more impact than inter-word parameters on threshold character size.

Eye movements and reading in macular disease: Further support for the shrinking perceptual span hypothesis

Reduced perceptual span is one factor which limits reading speed in patients with macular disease. This study measured the perceptual span and the number of saccades to locate a target in 18 patients with macular disease and seven control subjects on two occasions separated by up to 12 months. Perceptual span changed by up to two letters. Changes in perceptual span were significantly related to changes in reading speed (r2 = 0.43, p < 0.005), and were independent of changes in the number of saccades used to observe a target (r2 = 0.003, p = 0.62). These findings have important implications for the development of training programmes for patients with macular disease.

The role of attention in binding shape to color

Pictures of easily-identifiable objects with novel colors (e.g. a blue frog) or of forms with arbitrary colors (e.g. a green triangle) were presented briefly at 10.6 degrees eccentricity. Stimuli had strong outlines and vivid fill colors (red, green, yellow, blue, or purple). The same pictures were repeated once in each block of 30 trials for 6, 9, or 12 blocks, and recognition was probed after each block. Shapes were acquired quickly, within 3-4 blocks, whether attention was focused on the pictures or split to a demanding foveal task. Color-shape acquisition was also fast with focused attention, but stabilized at a low level with split attention. Delaying the foveal task restored color-shape acquisition. We suggest that attention facilitates the creation and maintenance of novel color-shape bindings in the visual periphery; without attention, binding is less effective.

Is word recognition different in central and peripheral vision?

Peripheral vision plays an important role in normal reading, but its role becomes larger for visually impaired people with central-field loss. This experiment studied whether lexical processing differs in central and peripheral vision through the analysis of word-frequency effects in lexical decisions. We asked two main questions: (1) Do central and peripheral vision differ in the time course of lexical processing? and (2) do central and peripheral vision differ in the quality of lexical processing? To address the first question, we examined the time course of frequency effects in central and peripheral vision over a range from 25 to 500 ms. We found that significant frequency effects occurred for the shortest exposures, 25-50 ms, in central vision, whereas significant frequency effects did not occur in peripheral vision until 100 ms. To address the second question, we used word-frequency effects as a marker for the nature of lexical processing. We compared frequency effects in central and peripheral vision for data within matched ranges of percent accuracy (0-20%, 20-40%, 40-60%, 60-80%, and 80-100%). We found that there was no difference in the pattern of frequency effects in central and peripheral vision at equivalent performance levels. We conclude that lexical processing is slower in peripheral vision, but the quality of lexical processing is similar in central and peripheral vision.

Low-vision patients with age-related maculopathy read RSVP faster when word duration varies according to word length

Normally sighted younger and elder subjects as well as subjects with central visual field loss (CFL) from age-related maculopathy read rapid serial visual presentation (RSVP) text with words presented at a constant rate and at three different rates where word presentation duration varied according to word length. The elder subjects reading sentences foveally read fastest when word duration was constant. The younger group reading random words peripherally read faster at a variable word duration rate. The subjects with CFL read sentences an average of 33% faster when the presentation rate varied with word length. There was a trend for slow readers with CFL to benefit more than fast readers with CFL. We conclude that varying word duration based on word length in rapid serial visual presentation reading would improve reading rates for low-vision patients with CFL.

Psychophysics of reading. XX. Linking letter recognition to reading speed in central and peripheral vision

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 degrees eccentricity, in good agreement with the corresponding reduction of reading speed measured by Chung and coworkers (Chung, S. T. L., Mansfield, J. S., & Legge, G. E. (1998). Psychophysics of reading. XVIII. The effect of print size on reading speed in normal peripheral vision. Vision Research, 38, 2949-2962). 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 degrees ). We also varied trigram presentation time. We used these data to construct visual-span profiles of letter accuracy versus 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 rely more on lexical inference to support peripheral reading. We conclude that spatiotemporal characteristics of the visual span limit RSVP reading speed in central vision, and that shrinkage of the visual span results in slower reading in peripheral vision.

The effects of simulated cataract on reading with normal vision and simulated central scotoma

Reading rates are slower for persons with low vision than for normally-sighted persons. This study investigated the change in reading performance and reading eye movements when we simulated the two most common causes of low vision--central field loss and cataract--and their combination (scotoma + cataract). Three subjects read sentences with each of these simulated impairments at five different letter sizes. They required larger letters to read with the cataract or scotoma than they did with normal vision, and larger still to read with scotoma + cataract; the change in eye movements relative to normal vision was similar across conditions. When reading large letters (1.61 degrees), the cataract had almost no effect, while the scotoma and scotoma + cataract reduced reading rate for two of the subjects. The cataract had a greater impact on performance relative to normal vision for these same two subjects, while for the third subject the cataract had a greater impact with the scotoma in place. Cataract extraction tends to be postponed in patients with central field loss because it is not perceived to be beneficial. The findings from this study, as well as others, suggest that patients with central field loss would benefit from cataract extraction.

Eye movements and the span of the effective stimulus in visual search

The span of the effective stimulus during visual search through an unstructured alphanumeric array was investigated by using eye-contingent-display changes while the subjects searched for a target letter. In one condition, a window exposing the search array moved in synchrony with the subjects' eye movements, and the size of the window was varied. Performance reached asymptotic levels when the window was 5 degrees. In another condition, a foveal mask moved in synchrony with each eye movement, and the size of the mask was varied. The foveal mask conditions were much more detrimental to search behavior than the window conditions, indicating the importance of foveal vision during search. The size of the array also influenced performance, but performance reached asymptote for all array sizes tested at the same window size, and the effect of the foveal mask was the same for all array sizes. The results indicate that both acuity and difficulty of the search task influenced the span of the effective stimulus during visual search.