Low vision reading with sequential word presentation

Reading speed does not benefit from increased line spacing in AMD patients

PURPOSE

Crowding, the adverse spatial interaction due to the proximity of adjacent targets, has been suggested as an explanation for slow reading in peripheral vision. Previously, we showed that increased line spacing, which presumably reduces crowding between adjacent lines of text, improved reading speed in the normal periphery (Chung, Optom Vis Sci 2004;81:525-35). The purpose of this study was to examine whether or not individuals with age-related macular degeneration (AMD) would benefit from increased line spacing for reading.

METHODS

Experiment 1: Eight subjects with AMD read aloud 100-word passages rendered at five line spacings: the standard single spacing, 1.5x, 2x, 3x, and 4x the standard spacing. Print sizes were 1x and 2x of the critical print size. Reading time and number of reading errors for each passage were measured to compute the reading speed. Experiment 2: Four subjects with AMD read aloud sequences of six 4-letter words, presented on a computer monitor using the rapid serial visual presentation (RSVP) paradigm. Target words were presented singly, or flanked above and below by two other words that changed in synchrony with the target word, at various vertical word separations. Print size was 2x the critical print size. Reading speed was calculated based on the RSVP exposure duration that yielded 80% of the words read correctly.

RESULTS

Averaged across subjects, reading speeds for passages were virtually constant for the range of line spacings tested. For sequences of unrelated words, reading speeds were also virtually constant for the range of vertical word separations tested, except at the smallest (standard) separation at which reading speed was lower.

CONCLUSIONS

Contrary to the previous finding that reading speed improved in normal peripheral vision, increased line spacing in passages, or increased vertical separation between words in RSVP, did not lead to improved reading speed in people with AMD.

Relationship between visual span and reading performance in age-related macular degeneration

PURPOSE

Visual-span profiles are plots of letter-recognition accuracy as a function of letter position left and right of the point of fixation. Legge, Mansfield, and Chung [Legge, G. E., Mansfield, J. S., & Chung, S. T. L. (2001). Psychophysics of reading-XX. Linking letter recognition to reading speed in central and peripheral vision. Vision Research, 41(6), 725-743] proposed that reduced size of the visual span is a spatial factor limiting reading speed in patients with age-related macular degeneration (AMD). We have recently shown that a temporal property of letter recognition--the exposure time required for a high level of accuracy--is also a factor limiting reading speed in AMD [Cheong, A. M. Y., Legge, G. E., Lawrence, M. G., Cheung, S. H., & Ruff, M. (2007). Relationship between slow visual processing and reading speed in people with macular degeneration. Vision Research, 47, 2943-2965]. We measured the visual-span profiles of AMD subjects and assessed the relationship of the spatial and temporal properties of these profiles to reading speed.

METHODS

Thirteen AMD subjects and 11 age-matched normals were tested. Visual-span profiles were measured by using the trigram letter-recognition method described by Legge et al. (2001). Each individual's temporal threshold for letter recognition (80% accuracy criterion) was used as the exposure time for measuring the visual-span profile. Size of the visual span was computed as the area under the profile in bits of information transmitted. The information transfer rate in bits per second was defined as the visual-span size in bits divided by the exposure time in sec.

RESULTS

AMD visual-span sizes were substantially smaller (median of 23.9 bits) than normal visual-span sizes in central vision (median of 40.8 bits, p<.01). For the nine AMD subjects with eccentric fixation, the visual-span sizes (median of 20.6 bits) were also significantly smaller than visual spans of normal controls at 10 degrees below fixation in peripheral vision (median of 29.0 bits, p=.01). Information transfer rate for the AMD subjects (median of 29.5 bits/s) was significantly slower than that for the age-matched normals at both central and peripheral vision (median of 411.7 and 290.5 bits/s respectively, ps<.01). Information transfer rates were more strongly correlated with reading speed than the size of the visual span, and explained 36% of the variance in AMD reading speed.

CONCLUSION

Both visual-span size and information transfer rate were significantly impaired in the AMD subjects compared with age-matched normals. Information transfer rate, representing the combined effects of a reduced visual span and slower temporal processing of letters, was a better predictor of reading speed in AMD subjects than was the size of the visual span.

Learning to identify crowded letters: does it improve reading speed?

Crowding, the difficulty in identifying a letter embedded in other letters, has been suggested as an explanation for slow reading in peripheral vision. In this study, we asked whether crowding in peripheral vision can be reduced through training on identifying crowded letters, and if so, whether these changes will lead to improved peripheral reading speed. We measured the spatial extent of crowding, and reading speeds for a range of print sizes at 10 degrees inferior visual field before and after training. Following training, averaged letter identification performance improved by 88% at the trained (the closest) letter separation. The improvement transferred to other untrained separations such that the spatial extent of crowding decreased by 38%. However, averaged maximum reading speed improved by a mere 7.2%. These findings demonstrated that crowding in peripheral vision could be reduced through training. Unfortunately, the reduction in the crowding effect did not lead to improved peripheral reading speed.

Relationship between slow visual processing and reading speed in people with macular degeneration

PURPOSE

People with macular degeneration (MD) often read slowly even with adequate magnification to compensate for acuity loss. Oculomotor deficits may affect reading in MD, but cannot fully explain the substantial reduction in reading speed. Central-field loss (CFL) is often a consequence of macular degeneration, necessitating the use of peripheral vision for reading. We hypothesized that slower temporal processing of visual patterns in peripheral vision is a factor contributing to slow reading performance in MD patients.

METHODS

Fifteen subjects with MD, including 12 with CFL, and five age-matched control subjects were recruited. Maximum reading speed and critical print size were measured with rapid serial visual presentation (RSVP). Temporal processing speed was studied by measuring letter-recognition accuracy for strings of three randomly selected letters centered at fixation for a range of exposure times. Temporal threshold was defined as the exposure time yielding 80% recognition accuracy for the central letter.

RESULTS

Temporal thresholds for the MD subjects ranged from 159 to 5881 ms, much longer than values for age-matched controls in central vision (13 ms, p<0.01). The mean temporal threshold for the 11 MD subjects who used eccentric fixation (1555.8 +/- 1708.4 ms) was much longer than the mean temporal threshold (97.0 +/- 34.2 ms, p<0.01) for the age-matched controls at 10 degrees in the lower visual field. Individual temporal thresholds accounted for 30% of the variance in reading speed (p<0.05).

CONCLUSION

The significant association between increased temporal threshold for letter recognition and reduced reading speed is consistent with the hypothesis that slower visual processing of letter recognition is one of the factors limiting reading speed in MD subjects.

Letter case and text legibility in normal and low vision

It is thought by cognitive scientists and typographers alike, that lower-case text is more legible than upper-case. Yet lower-case letters are, on average, smaller in height and width than upper-case characters, which suggests an upper-case advantage. Using a single unaltered font and all upper-, all lower-, and mixed-case text, we assessed size thresholds for words and random strings, and reading speeds for text with normal and visually impaired participants. Lower-case thresholds were roughly 0.1 log unit higher than upper. Reading speeds were higher for upper- than for mixed-case text at sizes twice acuity size; at larger sizes, the upper-case advantage disappeared. Results suggest that upper-case is more legible than the other case styles, especially for visually-impaired readers, because smaller letter sizes can be used than with the other case styles, with no diminution of legibility.

The case for the visual span as a sensory bottleneck in reading

The visual span for reading is the number of letters, arranged horizontally as in text, that can be recognized reliably without moving the eyes. The visual-span hypothesis states that the size of the visual span is an important factor that limits reading speed. From this hypothesis, we predict that changes in reading speed as a function of character size or contrast are determined by corresponding changes in the size of the visual span. We tested this prediction in two experiments in which we measured the size of the visual span and reading speed on groups of five subjects as a function of either character size or character contrast. We used a "trigram method" for characterizing the visual span as a profile of letter-recognition accuracy as a function of distance left and right of the midline (G. E. Legge, J. S. Mansfield, & S. T. L. Chung, 2001). The area under this profile was taken as an operational measure of the size of the visual span. Reading speed was measured with the Rapid Serial Visual Presentation (RSVP) method. We found that the size of the visual span and reading speed showed the same qualitative dependence on character size and contrast, reached maximum values at the same critical points, and exhibited high correlations at the level of individual subjects. Additional analysis of data from four studies provides evidence for an invariant relationship between the size of the visual span and RSVP reading speed; an increase in the visual span by one letter is associated with a 39% increase in reading speed. Our results confirm the visual-span hypothesis and provide a theoretical framework for understanding the impact of stimulus attributes, such as contrast and character size, on reading speed. Evidence for the visual span as a determinant of reading speed implies the existence of a bottom-up, sensory limitation on reading, distinct from attentional, motor, or linguistic influences.

Acuity, crowding, reading and fixation stability

People with age-related macular disease frequently experience reading difficulty that could be attributed to poor acuity, elevated crowding or unstable fixation associated with peripheral visual field dependence. We examine how the size, location, spacing and instability of retinal images affect the visibility of letters and words at different eccentricities. Fixation instability was simulated in normally sighted observers by randomly jittering single or crowded letters or words along a circular arc of fixed eccentricity. Visual performance was assessed at different levels of instability with forced choice measurements of acuity, crowding and reading speed in a rapid serial visual presentation paradigm. In the periphery: (1) acuity declined; (2) crowding increased for acuity- and eccentricity-corrected targets; and (3), the rate of reading fell with acuity-, crowding- and eccentricity-corrected targets. Acuity and crowding were unaffected by even high levels of image instability. However, reading speed decreased with image instability, even though the visibility of the component letters was unaffected. The results show that reading performance cannot be standardised across the visual field by correcting the size, spacing and eccentricity of letters or words. The results suggest that unstable fixation may contribute to reading difficulties in people with low vision and therefore that rehabilitation may benefit from fixation training.

Beating the beat: reading can be faster than the frequency of eye movements in persons with congenital nystagmus

PURPOSE

Rapid serial visual presentation (RSVP) of text has been reported to foster higher reading rates than presentation in a continuous text (CT) format, possibly because scanning eye movements are minimized. We investigated how this might be relevant for persons with congenital nystagmus (CN).

METHODS

We evaluated whether reading rates differ in persons with CN for RSVP versus CT presentation of single sentences under otherwise similar conditions. In a second experiment, we presented unrelated words to observers with CN in RSVP format while measuring their eye movements to determine whether reading can occur during the high-velocity, nonfoveating periods of the CN wave form. Both sentences and random words were selected from the MNRead corpus and displayed at 2x, 4x, or 8x the threshold word size on a 21-inch computer monitor.

RESULTS

Subjects with CN have virtually equivalent maximum reading speeds of 449 and 448 words per minute, respectively, for RSVP and CT presentation of sentences. Typically, reading rates were faster than the frequency of CN, which suggests that subjects could read during the nonfoveating periods of the nystagmus waveform. This finding was confirmed using random words that, unlike those in sentences, cannot be inferred from contextual cues. Examination of eye movements recorded during reading indicated that random words are read correctly with 47% to 65% accuracy (depending on word size) during the nonfoveating periods of the CN waveform.

CONCLUSION

A clinical implication of these results is that reading performance in persons with CN should be facilitated by large text sizes that remain legible during a greater fraction of the CN waveform.

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.

Serifs and font legibility

Using lower-case fonts varying only in serif size (0%, 5%, and 10% cap height), we assessed legibility using size thresholds and reading speed. Five percentage serif fonts were slightly more legible than sans serif, but the average inter-letter spacing increase that serifs themselves impose, predicts greater enhancement than we observed. RSVP and continuous reading speeds showed no effect of serifs. When text is small or distant, serifs may, then, produce a tiny legibility increase due to the concomitant increase in spacing. However, our data exhibited no difference in legibility between typefaces that differ only in the presence or absence of serifs.

Computer-based measurement of letter and word acuity

Determining causes of poor reading ability is an important step in trying to ameliorate reading performance in low-vision patients. One important parameter is word acuity. The principal aim of the current study is to develop a method to reliably measure acuities for isolated lowercase letters and words of differing length that can be used to test low-vision patients. Using isolated stimuli means that testing is relatively free of potential crowding and/or distracting attentional effects from surrounding words, it is unambiguous which stimulus subjects are trying to read and response times can be recorded for each stimulus. Across a series of experiments, subjects with normal vision were asked to read isolated lowercase single letters and lowercase words of 4, 7 and 10 letters, in separate tests. Acuities for uppercase Sloan letters were also measured to provide a reference, as they are commonly used to measure visual acuity. Each test was based upon the design principles and scoring procedures used in the Bailey-Lovie and ETDRS charts. Acuities for uppercase Sloan letters were found to be equivalent whether measured using ETDRS charts or the computer-based method. Measurement of acuities for lowercase single letters and lowercase words of 4, 7 and 10 letters had a reliability that was no worse than acuities for uppercase Sloan letters. Lowercase word acuities were essentially independent of word length. Acuities for single lowercase letters and lowercase words were slightly better than uppercase Sloan letters acuity. Optimal processing of lowercase single letters and 4-, 7- and 10-letter words occurred at character sizes that were at least 0.2-0.40 log MAR above acuity threshold, i.e. between 1.5 and 3 times threshold acuity for that particular stimulus. In general, critical character sizes appear similar across word lengths as progressive increases or decreases in these values were not observed as a function of the number of letters in the stimulus. We conclude that a computer-based method of stimulus presentation can be used to obtain highly repeatable measures of acuity for lowercase single letters and lowercase words in normal vision.

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.

Rapid categorization of foveal and extrafoveal natural images: associated ERPs and effects of lateralization

Humans are fast and accurate at performing an animal categorization task with natural photographs briefly flashed centrally. Here, this central categorization task is compared to a three position task in which photographs could appear randomly either centrally, or at 3.6 degrees eccentricity (right or left) of the fixation point. A mild behavioral impairment was found with peripheral stimuli with no evidence in support of hemispheric superiority; but enlarging the window of spatial attention to three possible stimuli locations had no behavioral cost on the processing of central images. Performance in the central categorization task has been associated with a large difference between the potentials evoked to target and non-target correct trials, starting about 150 ms after stimulus onset on frontal sites. Present results show that this activity originates within extrastriate visual cortices and probably reflects perceptual stimuli differences processed within areas involved in object recognition. Latencies, slopes, and peak amplitudes of this differential activity were invariant to stimulus position and attentional load. Stimulus location uncertainty and lateralization did not affect speed of visual processing.

Preferred retinal loci and macular scotoma characteristics in patients with age-related macular degeneration

Many patients with macular scotomas due to age-related macular degeneration do not perceive black spots in the visual field where the scotomas are located. Rather, they describe objects as "vanishing," "jumping out of nowhere" or "having blurry parts," or a combination of features. In addition, when the macular scotoma affects the fovea, the visual system uses 1 or more preferred retinal loci (PRLs) as a "pseudofovea" to perform visual tasks. Visual function testing with the scanning laser ophthalmoscope has provided a wealth of information regarding how patients perceive the visual world and how the oculomotor system directs eye movements. This article describes 2 specific functions of the oculomotor system, fixation stability and refixation precision, with data collected from normally sighted people and patients with visual field loss. The implications of the characteristics of PRLs and macular scotomas for clinical testing are discussed.

Short-Term In-Office Practice Improves Reading Performance with Stand Magnifiers for People with AMD

PURPOSE

People with low vision often use optical low vision aids to assist reading. There have been numerous training programs recommended to train people using magnifiers for reading. However, most of the programs are time consuming and labor intensive. In this study, we investigated the effects of home-based large print reading practice on reading performance when stand magnifiers (STM's) are first prescribed.

METHODS

Thirty-two subjects with age-related macular degeneration (AMD) and with minimal experience in using magnifiers for reading were recruited. They were divided into three groups: control, practice 1 (P1), and practice 2 (P2). Before the prescription of STM's, all the subjects were given the same amount of in-office practice with the STM (weeks 0 to 2). In addition, in these 2 weeks, P1 and P2 subjects were given large print books to read daily at home. P2 subjects were required to read the large print books through a reduced field of view. The control group subjects received no additional reading practice. Reading rates with and without STM's on passages of text were assessed for all the subjects regularly for 20 weeks.

RESULTS

There were no significant differences between the control, P1, and P2 groups in the increase in reading rate with STM (p = 0.29). At week 0, reading rate for small print with STM was significantly slower than reading rate on the equivalent-sized large print (p = 0.004); however, as time went on, reading rate with STM's increased significantly (p = 0.02). After 2 weeks of in-office magnifier practice and repeated measures of reading rate with STM, reading rate with STM had improved such that it was not significantly different from reading rate on large print (p = 0.11).

CONCLUSION

Supervised, short-term, in-office practice with the magnifier was effective in improving magnifier reading performance to achieve maximum reading rate. Additional large print reading practice did not result in any greater improvement in reading rate than in-office magnifier practice alone.

Reading Speed Benefits from Increased Vertical Word Spacing in Normal Peripheral Vision

PURPOSE

Crowding, the adverse spatial interaction due to proximity of adjacent targets, has been suggested as an explanation for slow reading in peripheral vision. The purposes of this study were to (1) demonstrate that crowding exists at the word level and (2) examine whether or not reading speed in central and peripheral vision can be enhanced with increased vertical word spacing.

METHODS

Five normal observers read aloud sequences of six unrelated four-letter words presented on a computer monitor, one word at a time, using rapid serial visual presentation (RSVP). Reading speeds were calculated based on the RSVP exposure durations yielding 80% correct. Testing was conducted at the fovea and at 5 degrees and 10 degrees in the inferior visual field. Critical print size (CPS) for each observer and at each eccentricity was first determined by measuring reading speeds for four print sizes using unflanked words. We then presented words at 0.8x or 1.4x CPS, with each target word flanked by two other words, one above and one below the target word. Reading speeds were determined for vertical word spacings (baseline-to-baseline separation between two vertically separated words) ranging from 0.8x to 2x the standard single-spacing, as well as the unflanked condition.

RESULTS

At the fovea, reading speed increased with vertical word spacing up to about 1.2x to 1.5x the standard spacing and remained constant and similar to the unflanked reading speed at larger vertical word spacings. In the periphery, reading speed also increased with vertical word spacing, but it remained below the unflanked reading speed for all spacings tested. At 2x the standard spacing, peripheral reading speed was still about 25% lower than the unflanked reading speed for both eccentricities and print sizes. Results from a control experiment showed that the greater reliance of peripheral reading speed on vertical word spacing was also found in the right visual field.

CONCLUSIONS

Increased vertical word spacing, which presumably decreases the adverse effect of crowding between adjacent lines of text, benefits reading speed. This benefit is greater in peripheral than central vision.

Letter-recognition and reading speed in peripheral vision benefit from perceptual learning

Visual-span profiles are plots of letter-recognition accuracy as a function of letter position left or right of the midline. Previously, we have shown that contraction of these profiles in peripheral vision can account for slow reading speed in peripheral vision. In this study, we asked two questions: (1) can we modify visual-span profiles through training on letter-recognition, and if so, (2) are these changes accompanied by changes in reading speed? Eighteen normally sighted observers were randomly assigned to one of three groups: training at 10 degrees in the upper visual field, training at 10 degrees in the lower visual field and a no-training control group. We compared observers' characteristics of reading (maximum reading speed and critical print size) and visual-span profiles (peak amplitude and bits of information transmitted) before and after training, and at trained and untrained retinal locations (10 degrees upper and lower visual fields). Reading speeds were measured for six print sizes at each retinal location, using the rapid serial visual presentation paradigm. Visual-span profiles were measured using a trigram letter-recognition task, for a letter size equivalent to 1.4x the critical print size for reading. Training consisted of the repeated measurement of 20 visual-span profiles (over four consecutive days) in either the upper or lower visual field. We also tracked the changes in performance in a sub-group of observers for up to three months following training. We found that the visual-span profiles can be expanded (bits of information transmitted increased by 6 bits) through training with a letter-recognition task, and that there is an accompanying increase (41%) in the maximum reading speed. These improvements transferred, to a large extent, from the trained to an untrained retinal location, and were retained, to a large extent, for at least three months following training. Our results are consistent with the view that the visual span is a bottleneck on reading speed, but a bottleneck that can be increased with practice.

Preferred Retinal Locus and Reading Rate with Four Dynamic Text Presentation Formats

BACKGROUND

Electronic display devices hold the potential to improve access to written material by people with low vision. For those with central field loss, the optimal form of electronic text presentation may vary according to the location of the preferred retinal locus, but this has never been investigated. In this study, we examined the relationship between preferred retinal locus location and reading rate for four electronic display formats (rapid serial visual presentation, horizontal scroll, vertical scroll, and page).

METHODS

Short sentences were presented in each format to 35 low-vision (most with central field loss) and 14 age-matched control subjects. Subjects read aloud to determine maximum oral reading rate and read silently to determine preferred silent reading rate.

RESULTS

With the exception of page format, maximum oral reading rates were faster than silent preferred reading rates for both groups of subjects. For the low-vision group, there was no significant difference in maximum oral reading rates between the four display formats; and when reading at a preferred silent rate, page format was faster than the other three formats. Though page format was read more quickly, half of the low-vision subjects preferred the horizontal-scroll format. Contrary to our predictions, there was no significant effect of preferred retinal locus location (vertical vs. lateral) on reading rate and no significant interaction between preferred retinal locus location and display format.

CONCLUSIONS

The differences between maximum oral and preferred silent reading rates and the lack of a relationship between reading rates and preferred display format reinforce the importance of patient preference in the evaluation and selection of a device or display format during rehabilitation.

Chapter 65 The speed of visual cognition

Intracranial electrophysiological recordings in primates showed repeatedly that neurons in several cortical areas are activated very early after visual stimulus presentation, practically at the same time (or even before) the activation of primary sensory neurons. Even neurons at the highest hierarchical levels of the visual system are activated in less than 100 ms. These findings challenge the classical interpretation of human evoked potential (EP) data that assume that the first, "exogenous", EP components from 50 to 150 ms reflect the initial volley of sensory activation in the striate and extrastriate visual cortex and are not yet influenced by cognitive task demands. Indeed, several recent EP studies using analysis methods that go beyond the classical approach of defining "components" at certain scalp positions indicate that highly complex stimulus features can influence EP responses within the first 100 ms. This indicates that sophisticated cognitive processing is much faster than previously thought and opens new perspectives with respect to the role of both, bottom-up as well as top-down mechanisms in visual processing.

The shape and size of crowding for moving targets

Our ability to identify alphanumeric characters can be impaired by the presence of nearby features, especially when the target is presented in the peripheral visual field, a phenomenon is known as crowding. We measured the effects of motion on acuity and on the spatial extent of crowding. In line with many previous studies, acuity decreased and crowding increased with eccentricity. Acuity also decreased for moving targets, but the absolute size of crowding zones remained relatively invariant of speed at each eccentricity. The two-dimensional shape of crowding zones was measured with a single flanking element on each side of the target. Crowding zones were elongated radially about central vision, relative to tangential zones, and were also asymmetrical: a more peripheral flanking element crowded more effectively than a more foveal one; and a flanking element that moved ahead of the target crowded more effectively than one that trailed behind it. These results reveal asymmetrical space-time dependent regions of visual integration that are radially organised about central vision.

Modified astigmatism dial diagram for locating eccentric fixation in patients with central scotoma

PURPOSE

Patients with central scotoma often develop eccentric fixation on a preferred retinal locus (PRL). Identifying the PRL is one of the first steps in low vision rehabilitation training. We present our evaluation of a simple test designed to locate the eccentric fixation in eyes with central scotoma.

METHODS

This was a prospective case series of consecutive patients with age-related macular degeneration and bilateral central scotoma. A numeral was added in the center of an astigmatism dial diagram. After one eye was patched, patients with central scotoma were asked to fixate the dial and describe it, then to look at the 12 o'clock position, and then around the clock. The eccentricity at which the central numeral was best seen was compared with the one determined by scanning laser ophthalmoscopy (SLO). The modified astigmatism dial test and SLO were done independently by two masked investigators. The results of the two methods were expressed in clock hour positions and were considered to be in agreement when they did not differ by more than one hour.

RESULTS

Nine consecutive patients (18 eyes) with severe age-related macular degeneration and bilateral central scotoma were tested. The six women and three men ranged in age from 61 to 86 years (mean 75.8 years). The pattern test correlated with SLO findings in 12 (66%) of the 18 eyes. When considering the best-seeing eye of each patient, results showed agreement in eight (88%) out of nine cases.

CONCLUSIONS

The modified astigmatism dial test appears useful for establishing the location of the eccentric fixation in the best-seeing eye of patients with bilateral central scotoma, allowing visual rehabilitation training to be started without delay.

Spatial-frequency characteristics of letter identification in central and peripheral vision

Spatial-frequency characteristics of letter identification are much better understood in the fovea than in the periphery. The purpose of this study was to compare the spatial-frequency characteristics of letter identification in central and peripheral vision. We measured contrast thresholds for identifying single, Times-Roman lower-case letters that were spatially band-pass filtered. Each of the 26 letters was digitally filtered with a set of nine cosine log filters, with peak object spatial frequencies ranging from 0.63 to 10 c/letter, in half-octave steps. Bandwidth of the filters was 1 octave. Three observers with normal vision were each tested monocularly at the fovea, and at 5 degrees and 10 degrees in the inferior visual field. Letter sizes were 0.2, 0.4 and 0.6 log units larger than high contrast, unfiltered acuity letters. Plots of contrast sensitivity for letter identification vs. frequency of the band-pass filters exhibit spatial tuning. In general, the spatial-frequency characteristics of letter identification are fundamentally identical between central and peripheral vision. These characteristics include the scaling of the peak frequency of the spatial-tuning functions with letter size and the bandwidth of the tuning functions. The only difference between the fovea and the periphery is that for the same physical letter size, peak sensitivity of the spatial-tuning functions occurs at a higher retinal frequency at the fovea than in the periphery. To test whether or not the contrast sensitivity function (CSF) can account for the differences in the spatial-frequency characteristics of letter identification between central and peripheral vision, we incorporated a human CSF into an ideal-observer model, and tested the performance of this ideal-observer on the same letter identification task used with the human observers. Data from this CSF-ideal-observer resemble closely those of human observers, suggesting that the spatial-frequency characteristics of human letter identification can be accounted for by the CSF and the letter-identity information, without invoking selection among narrow-band spatial-frequency channels.

Spatial-frequency and contrast properties of crowding

Crowding, the difficulty in recognizing a letter flanked by other letters, has been explained as a lateral masking effect. The purpose of this study was to examine the spatial-frequency and contrast dependencies of crowding, and to compare them with the properties of pattern masking. In experiment 1, we measured contrast thresholds for identifying the middle letters in strings of three randomly chosen lower-case letters (trigrams), for a range of letter spacings. Letters were digitally filtered using a set of bandpass filters, with peak object spatial frequencies ranging from 0.63 to 10 c/letter. Bandwidth of the filters was 1 octave. Frequencies of the target and flanking letters were the same, or differed by up to 2 octaves. Contrast of the flanking letters was fixed at the maximum value. Testing was conducted at the fovea and 5 degrees eccentricity. We found that crowding exhibits spatial-tuning functions like masking, but with generally broader bandwidths than those for masking. The spatial extent of crowding was found to be about 0.5 deg at the fovea and 2 deg at 5 degrees eccentricity, independent of target letter frequency. In experiment 2, we measured the contrast thresholds for identifying the middle target letters in trigrams for a range of flanking letter contrasts at 5 degrees eccentricity. At low flanker contrast, crowding does not show a facilitatory region, unlike pattern masking. At high flanker contrast, threshold rises with contrast with an exponent of 0.13-0.3, lower than corresponding exponents for pattern masking. In experiment 3, we varied the contrast ratio between the flanking letters and the target letters, and found that the magnitude of crowding increases monotonically with contrast ratio. This finding contradicts a prediction based on a grouping explanation for crowding. Our results are consistent with the postulation that crowding and masking may share the same first stage linear filtering process, and perhaps a similar second-stage process, with the additional property that the second-stage process in crowding pools information over a spatial extent that varies with eccentricity.

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.

Prescribing near magnification for low vision patients

Most patients with low vision request help with reading. Despite improvements in the assessment of visual functions, determining the appropriate magnification for reading still often involves a trial and error approach. Recent research has shown that with accurate and systematic assessments of vision, the required magnification can be predicted but this magnification needs to be much higher than has been previously recommended. This paper presents a systematic approach to enable practitioners to determine the power of a near addition or simple magnifier needed to assist patients with mild or moderate low vision who may present seeking help for reading. Guidelines for appropriate referral to low vision services are also provided.

Elicited sequential presentation for low vision reading

This paper reports on a variant of the rapid serial visual presentation (RSVP) technique for low vision reading called elicited sequential presentation (ESP). In both techniques, words are presented sequentially at a constant screen location, but with ESP, the reader elicits presentation of each new word by means of a button press, rather than (as with RSVP) being presented with it automatically at fixed intervals. An experiment comparing reading speeds using, ESP, RSVP and a conventional closed-circuit television (CCTV) reading aid showed that for 15 slow readers who were customary CCTV users with low vision, ESP is superior to RSVP and yields reading speeds averaging 47% faster than RSVP--about the same as CCTV reading speed. The log of the ratio of ESP to RSVP reading speeds was significantly negatively correlated with the log of RSVP reading speed, showing that slower readers benefit more than faster readers; regression predicted no benefit for readers who read with RSVP at 133 wpm or greater. Finally, word length and word presentation duration chosen by subjects reading with ESP were significantly correlated, suggesting that part of the benefit of ESP is due to reader's ability to allocate time based on word length and difficulty.

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

When the center of a readers, visual field is blocked from view, reading rates decline and eye movement patterns change. This is true whether the central visual field is blocked artificially (i.e. a mask) or through disease (e.g. a retinal scotoma due to macular degeneration). In past studies, when mask size was defined in terms of the number of letters masked from view, reading rates declined sharply as number of letters masked increased. Patients with larger central scotomas (in degrees of visual angle) also read slower. We sought to determine whether number of letters masked or size of the mask in degrees is the predominant factor affecting reading rates and eye movement behavior. By matching number of letters masked across several mask sizes (and compensating for reduced acuity in the periphery), we found that number of letters masked is the more important factor until mask size is quite large (> or = -7.5 degrees) and number of letters masked from view is more than seven.

Psychophysics of reading. XVIII. The effect of print size on reading speed in normal peripheral vision

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.

High and low luminance letters, acuity reserve, and font effects on reading speed

We compared reading speed with two fonts, Dutch (serif) and Swiss (sans serif). Text was displayed on a computer monitor, white letters on black, with the RSVP method. Luminance of the letters was either 146.0 or 0.146 cd m-2. Lower-case x-height of the fonts was approximately 5.5 times as large as letter acuity. At the high luminance, there was no difference between reading rates. There was a significant advantage for the Swiss font at the low luminance. The acuity reserve for Swiss was higher than for Dutch at the low luminance, which may account for the difference in reading speeds.

Simulated cataract does not reduce the benefit of RSVP

Normally sighted younger and older (mean age 71 years) observers read sentences and random lists of words from a rapid serial visual presentation (RSVP) display and a scroll display using their normal vision and through two levels of cataract simulators. Unlike patients with central field loss (CFL), there was no decrease in the benefit of RSVP with reduced vision due to the cataract simulators. However, the usefulness of sentence-level context was reduced as visual acuity was reduced. In addition, older readers did not benefit as much from RSVP as younger readers, and many in the older group were unable to read using the more severe cataract simulators from either display format. From these data we conclude that the benefits of RSVP are not reduced with reduced acuity and contrast sensitivity, and that there are age-related changes in reading rates from dynamic text displays not related to acuity.

Psychophysics of reading--XVI. The visual span in normal and low vision

The visual span in reading is the number of characters that can be recognized at a glance. The shrinking visual span hypothesis attributes reading deficits in low vision, and slow reading in normal vision at low contrast, to a reduction in the visual span. This hypothesis predicts that reading time (msec/word) becomes increasingly dependent on word length as text contrast decreases. We tested and confirmed this prediction using the rapid serial visual presentation (RSVP) method. Estimates of the visual span ranged from about 10 characters for high-contrast text to less than two characters for low-contrast text. Eye-movement recordings showed that longer reading times at low contrast are partitioned about equally between prolonged fixation times and an increased number of saccades (presumably related to a reduced visual span). RSVP measurements for six out of seven low-vision subjects revealed a strong dependence of reading time on word length, as expected from reduced visual spans.

A comparison of word recognition and reading performance in foveal and peripheral vision

Word recognition thresholds and rapid serial visual presentation (RSVP) reading rates for both unrelated words and meaningful sentences were examined across the visual field. Both word recognition thresholds and RSVP reading rates for unrelated words can be equated across the visual field by an increase in size scale. RSVP reading rates for meaningful sentences cannot be equated across the visual field, with the fovea demonstrating a qualitative superiority over the periphery, irrespective of scale. The results suggest that the early visual machinery which underlies word recognition is the same across the visual field apart from a change in scale, whilst the periphery is qualitatively inferior to the fovea at interpreting sentences with meaning.

Detection and identification of mirror-image letter pairs in central and peripheral vision

Reading performance is poorer in the peripheral than in the central visual field, even after size-scaling to compensate for differences in visual acuity at the different eccentricities. Since several studies have indicated that the peripheral retina is deficient with respect to spatial phase discrimination, we compared the psychometric functions for detection (D) and identification (I) of size-scaled, mirror-symmetric letters (i.e. letters differing in the phase spectra of their odd symmetric components) at three inferior field eccentricities (0, 4, and 7.5 deg) using a two-alternative, temporal, forced-choice procedure and retinal image stabilization to control retinal locus. Each subject's data were fit with Weibull functions and tested for goodness-of-fit under several hypotheses. This analysis revealed that while the psychometric functions were of constant shape across eccentricity for the respective tasks, they showed statistically significant variations in the D/I threshold ratios. However, these variations were so small that poorer reading outside the fovea is unlikely to be due to reduced letter discriminability that might occur secondary to a loss of peripheral field phase sensitivity.

Scrolled and rapid serial visual presentation texts are read at similar rates by the visually impaired

Visually impaired observers read dynamically displayed text faster than text displayed in a normal page view. The goal of this study was to compare reading rates from two dynamic-presentation methods that have been proposed to facilitate reading from computer-based displays. Prior research has shown that both normally sighted and low-vision observers read text displayed to the same location, one word at a time [known as rapid serial visual presentation (RSVP)], faster than a page of text. A similar comparison with text scrolled continuously across the screen also shows faster reading for low-vision patients, but the relative change from a standard page view is substantially less (15% faster for the scroll display versus 80% faster for RSVP). In this study we directly compared these techniques. For those with normal vision, reading from the RSVP display was 1.3 times faster than reading from the scroll display [t(9) = 3.32, P = 0.009]. Although the difference in reading rates for the visually impaired group did not reach statistical significance, as a group they read 13% slower from the RSVP than from the scroll display.