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Visual function Battista, Kalloniatis and Metha
OPTOMETRY INVITED REVIEW
Visual function: the problem with eccentricity
Clin Exp Optom 2005; 88: 5: 313–321 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.
Josephine Battista* BSc (Hons) Michael Kalloniatis† PhD MScOptom Andrew Metha* PhD BScOptom BSc * Department of Optometry and Vision Sciences, The University of Melbourne, Parkville, Australia † Department of Optometry and Vision Science, University of Auckland, Auckland, New Zealand Submitted: 5 August 2005 Revised: 8 September 2005 Accepted for publication: 11 September 2005
Key words: eccentricity, periphery, reading
Is peripheral visual function simply a scaled down version of central vision? The answer is no. The apparently poorer performance of the peripheral retina, despite the possibility of increased letter size or contrast, is problematic for patients who have lost their central vision and must rely on their peripheral vision for activities of daily living. Approximately 50 per cent of patients presenting to low vision clinics have lost their central vision.1 Visual impairment is more common with increasing age. Among elderly people, macular alteration is common and reading difficulties result as a consequence of loss of resolution. Although other definitions exist, low vision can be defined functionally as
the inability to read newspaper print at a normal reading distance of 40 cm, with the best refractive correction.2 Low vision can result from defects in the eye’s optics, the retina or other parts of the visual system and therefore encompasses a great variety of pathologies. According to statistics from the Royal National Institute for the Blind, in 1996, there were approximately 1.1 million blind or partially sighted people in the United Kingdom. Of these, 82 per cent were aged 65 years or older. The major causes of visual impairment included age-related macular degeneration (AMD) and cataract.3-5 Statistics from the Blue Mountain Eye Study 6 and the Melbourne Visual Impairment Project7,8
indicate that in 2004, approximately 480,300 Australians had low vision (visual acuity less than 6/12), including 50,600 with blindness (visual acuity less than 6/60). Almost half of all cases of blindness were caused by AMD.6-8 It is predicted that the number of Australians with low vision and blindness will double by 2024.9 AMD is a degenerative disorder that progressively affects the macular region of the retina, often resulting in an irreversible central scotoma. In this common retinal problem of the aging eye, the macular area and fovea become compromised due to pigment epithelial degeneration, drusen formation and leakage of fluid behind the fovea. A photograph of a
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normal retina as observed during ophthalmoscopy is presented on the left of Figure 1. The macula is a highly specialised area of the retina that provides fine discrimination. It has a high cone density and is located approximately four millimetres temporal to the optic disc. At the centre of the macula is the fovea, the area of the retina that provides the best visual resolution.10 An ocular fundus with AMD is shown on the right of Figure 1, where drusen can be observed. Central visual loss eventually results from the degeneration of the foveal cones.11 Peripheral vision is required for patients with such a disorder to perform everyday tasks, such as reading. However, reading is particularly difficult for patients with AMD because peripheral reading performance is reduced in comparison to that of the fovea.2 Visual field abnormalities such as relative or absolute scotomata also present reading difficulties because an abnormal field of effective vision requires an adapted eye movement strategy.12 Surveys show that retaining the ability to read is regarded as the most important concern for patients seeking visual rehabilitation.13-15 As central visual loss is currently irreversible, research on how to better rehabilitate patients with AMD is aimed at determining why peripheral visual acuity and reading performance are reduced in comparison to the fovea and how to make the best use of the remaining usable field. Visual acuity provides an estimate of the smallest characters that can be read; however maximum reading speed usually occurs for characters much larger than the acuity limit.16 While widely used in the clinic, visual acuity measurements do not provide a predictive measure of reading performance.12,17,18 Therefore, while visual acuity is important, it is critical to understand the process of reading and its specific limitations. READING The normal reading process can be described as a language code picked up through the visual or tactile system and then processed further; a procedure
Figure 1. Left: The posterior pole of the ocular fundus of a normal eye. Right: Ocular fundus of a patient with early AMD showing drusen and retinal pigment epithelial disturbance. Photographs kindly provided by Associate Professor Algis Vingrys.
involving a number of different human activities.12 To understand the complex process of reading, Legein and Bouma12 attempted to discern the separate subprocesses involved and these activities were distinguished as involving the optics of the eye and visual sensory input, the control of eye movements, word recognition in a single eye pause (or eye fixation) and the integration of text information over consecutive eye pauses. Following these subprocesses, many other language processes occur that involve higher cognitive aspects of comprehension and memory. For fluent reading, all these processes are very important.12 When reading a horizontal line of text, there are three regions of the retinal image that need to be considered: the fovea, parafovea and the periphery. The fovea extends two degrees across the point of fixation. Around the fixation point, the parafovea extends 10 degrees and the remaining region is the periphery.19 Like visual acuity, which is highest in the fovea and decreases towards the periphery, word recognition decreases dramatically when presented outside the fovea and at increasing distances from central fixation.20 Visual rehabilitation for patients with AMD often involves the prescription of either optical or electronic magnification. While magnification compensates for loss of visual acuity, reading speed is still impaired with a reading rate of less than 50
words per minute for low vision patients with a central scotoma (within five degrees).2 This is significantly lower than the 80 words per minute reading rate considered to be minimally ‘fluent’ 21 and far slower than the median normal reading rate using central vision (150 words per minute).2 A wide variation in peak reading rates exists among low-vision observers. Most of the variance can be accounted for by two major distinctions. These include intact central fields versus central-field loss, and cloudy versus clear ocular media. Observers with central-field loss have very low peak reading rates (median 25 words per minute), while peak reading rates for observers with intact central fields are at least 90 words per minute (median 130 words per minute).2 To read, most low vision patients require magnification (which is usually prescribed on the basis of their acuity) and read best with enlarged print. On average, four times the acuity limit is the optimum letter size.2 More magnification is required for patients with a central scotoma or advanced macular degeneration, compared to those whose central fields are intact and the required magnification is usually a greater multiple of their acuity.2 A possible explanation for the reduced reading speed of patients with central visual loss may be associated with problems of oculomotor control.14,22,23 Studies exam-
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ining eye movements when reading text with a simulated macular scotoma show multiple fixations and small saccades within words, resulting in a disorganised eye movement pattern, even when the text is magnified.24-26 Eye movements have important implications for reading speed, as is demonstrated in studies aimed at simplifying the task of reading for low vision patients, by employing the rapid serial visual presentation (RSVP) paradigm. For this mode of presentation, words are presented one at a time in a fixed location. If the words fit within the visual span27 (the number of letters that can be recognised in a single fixation—described in more detail below), then eye movements are not required to read the text. Normallysighted observers can read approximately four times faster with RSVP in comparison to reading static text.1,28 Low vision patients with intact central visual fields can double their reading speed using RSVP, however, low vision patients with a macular scotoma show the least improvement, reading on average 40 per cent faster with RSVP. This result indicates that peripheral reading is still reduced despite minimising eye movements.1 This small improvement in reading speed for patients who use their peripheral vision for reading, despite reducing oculomotor control, demonstrates that eye movements alone cannot account for reduced reading speed in peripheral vision. It is not yet fully understood why peripheral reading performance is reduced in comparison to the fovea or how to overcome this impasse for patients who are forced to use their peripheral vision for this task. The following sections review studies examining the many possible explanations for the differences in performance of visual functions in central and peripheral vision. DIMENSIONS OF THE RETINAL MOSAIC It is difficult to estimate the distribution of rod and cone photoreceptors across the human retina due to the large variability between eyes. In the fovea, there may be as much as a three-fold range in maximum
cone density of young adult human eyes.29 Various studies have estimated that approximately four to six million cones29-31 and approximately 120 million rods29,30 tile the average human retina. Rods and cones differ not only in their shapes and numbers but also in their geographical distribution over the retina. The peak density of cones is at the fovea, with an approximate density of between 120,000 and 200,000/mm 2. 29,30,32-35 This density decreases to approximately 5,000/mm2 at eccentricities greater than 10 degrees.29,30 Cones are found all the way to the periphery even though their density is reduced away from the fovea, however, rods are absent in the central foveola, with no rods found at eccentricities smaller than 0.5 degrees.29,30 The number of rods increases rapidly beyond this distance and they are most densely packed at approximately 20 degrees from the fovea, reaching a peak density of approximately 170,000 rods/ mm 2. 29,30 The density of rods then decreases regularly and always remains much higher than the density of cones. It has been suggested that photoreceptors are lost in AMD, with rods dying in older eyes without evidence of overt retinal pigment epithelial disease.36 For those susceptible to AMD, the retinal pigment epithelium becomes dysfunctional. Secondarily, rod loss continues and cones begin to degenerate. Eventually, only degenerate cones remain and all photoreceptors may ultimately disappear.36 These findings are consistent with functional and clinical studies. Psychophysical examinations of rod vulnerability in AMD have demonstrated the greatest severity at two to four degrees from the fovea, with this deficit decreasing with increasing eccentricity.37 It is well known that the human eye’s resolving power is greatest in the fovea and falls off rapidly with increasing angular distance from fixation. A possible explanation for this reduced visual acuity with eccentricity may be the reduced density of cones and the changes in synaptic organisation with increasing eccentricity from the fovea. Various studies, including those by Polyak31 and Jones and Higgins,38 examined the role of cone density in lim-
iting visual acuity. While the results of these early studies were potentially limited by the eye’s optical performance, it was demonstrated that for Landolt C targets presented at various distances from the fovea, maximum acuity occurred at the centre of the retina with a notable reduction in visual acuity at a distance of only two degrees from the fovea, reducing rapidly until approximately 10 to 12 degrees. As cone density declines in a similar manner, cone density and therefore the retinal mosaic have been suggested as playing a major role in limiting visual acuity. More sophisticated studies have bypassed the optics of the eye to determine exclusively neural limits and showed that at eccentricities of less than two degrees, resolution closely correlates with Østerberg’s30 theoretical limits for a mosaic of receptors. At eccentricities greater than two degrees, visual acuity is worse than that predicted by cone spacing. This indicates that post-receptoral retinal elements limit visual acuity and that the limits of visual acuity cannot be determined simply by only the fineness of the retinal photoreceptor mosaic.39 To further investigate whether reduced reading speed with peripheral vision is related to the differences between rodand cone-mediated vision, Chaparro and Young 40 investigated cone- and rodmediated reading by comparing reading speeds obtained when using targets equated for detectability by rods and cones. The speed of rod-mediated reading was reduced in comparison to conemediated reading. As the targets were equally detectable by either photoreceptor, this finding suggests that the differences in visual sensitivity of the cone and rod visual systems cannot account for the differences in reading speed between central and peripheral vision. In other words, it is unlikely that the intrinsic differences between rods and cones alone can explain why reading speed is reduced with peripheral vision. CORTICAL MAGNIFICATION In a classic study by Weymouth41 examining how visual performance for various
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Figure 3. A schematic representation of reading speed in central and peripheral vision as a function of print size, as measured using the RSVP paradigm (modified after Chung, Mansfield and Legge47). For both central and peripheral vision, reading speed increases with larger print size until a critical print size is reached (CPS), after which maximum reading speed (MRS) has been reached. The different print sizes at which CPS is reached for central and peripheral vision reflect the different resolution limits for the two different eccentricities. However, the key difference is that the MRS is lower for the peripheral retina, despite increased print size. The peripher y cannot provide reading speed comparable to the fovea.
tissue (mm) representing one degree of visual angle in the primary visual cortex.45 When the stimulus size is scaled in proportion to the inverse of the linear cone density at each eccentricity, the retinal periphery performs as well as central vision for a variety of resolution tasks. The inverse of the cortical magnification factor M-1, which is the number of degrees of visual space per millimetre of cortex, rises approximately linearly with eccentricity.43 It has been suggested that the degradation of some visual functions (vernier thresholds, word acuity thresholds) is limited by the cortical magnification factor, while others (grating acuity, single letter acuity) are limited by the change in retinal elements (ganglion cell density) as a function of eccentricity.43,46-48 These differences in the rate of degradation with eccentricity are presented schematically in Figure 2,
where it can be observed that vernier acuity declines three to four times faster with eccentricity than does high-contrast grating acuity. Subsequently, it has been suggested that visual function thresholds that require higher brain functions (beyond the primary visual cortex) fall more rapidly with retinal eccentricity, suggesting that the decline of visual functions with increasing eccentricity is representative of the complexity of visual information processing of the periphery.43 PRINT SIZE It has been suggested that the differences between central and peripheral reading speed may be due to the increased convergence of cone-photoreceptors on one ganglion cell in the periphery. This increase in convergence in the periphery
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Figure 2. A schematic representation of the ratio of peripheral to foveal thresholds for high-contrast word recognition thresholds, vernier acuity and grating acuity. The E2 value is the eccentricity at which foveal threshold has doubled (horizontal broken line). The eccentricity for the word and vernier acuity is almost identical (~0.7 degrees), whereas for grating acuity (similar to single letter acuity), the eccentricity at which foveal threshold has doubled is at ~2.5 degrees (shown by the vertical arrows). The figure is modified after Abdelnour and Kalloniatis.48
visual functions varies with eccentricity, many visual functions were found to degrade approximately linearly with eccentricity and different visual functions declined at different rates with increasing distance from the fovea. This rate of falloff can be characterised for many visual functions by the parameter E2, which represents the eccentricity at which the foveal threshold has doubled.42,43 Westheimer 44 and Levi, Klein and Aitsebaomo43 examined the results obtained by Weymouth41 in the light of the ‘cortical magnification’ or ‘scaling factor’. Cortical magnification refers to the distortion of the brain’s topographic map in the primary visual cortex, as the amount of cortical tissue devoted to the fovea far exceeds the amount devoted to the periphery. The cortical magnification or scaling factor refers to the linear extent of brain
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explains why some spatial tasks result in similar visual performance at different retinal eccentricities provided that the stimuli are scaled in size to equate with the coverage of ganglion cells. The spatial contrast sensitivity function is an example because it remains shape-invariant at various retinal eccentricities with the appropriate scaling of the size of the stimulus.49,50 Following this idea, it is plausible to question whether reading performance can be equated by scaling print size in peripheral vision.47 Despite enlargement of letter size to compensate for decreased acuity with eccentric viewing, peripheral reading speed in normal observers does not approach that achieved using the fovea.2,18,21,47,51 Examination of the effect of print size on reading speed in normal peripheral vision using the RSVP presentation paradigm by Chung, Mansfield and Legge47 showed that reading speed increased with print size up to a ‘critical print size’ (CPS), beyond which reading speed remained at a plateau level, termed the ‘maximum reading speed’ (MRS) as shown schematically in Figure 3. The average values for maximum reading speed decreased from the fovea to the periphery, with larger print sizes required to achieve maximum reading speed in peripheral vision in comparison to central vision. The rate of change in reading speed as a function of print size remained invariant in central and peripheral vision and even when print size was not the limiting factor, maximum reading speeds were still lower in peripheral vision compared to central vision. In other words, we cannot simply scale print size for the periphery to achieve reading speeds attainable in the fovea. CROWDING ‘Crowding’, also known as ‘lateral masking’, refers to the decreased visibility of a visual target in the presence of nearby objects. In relation to reading, this refers to the spatial interaction between adjacent letters within the same word, with adjacent letters obstructing each other’s recognition via the effect of crowding. It has been suggested that crowding is a major factor contributing to reduced reading speed in
peripheral vision because even when targets are scaled in size, the spatial extent46,52,53 and magnitude52,54 of the interaction are still greater in peripheral vision than in central vision. It is well established that resolution deteriorates more rapidly with eccentricity for test targets surrounded by flanking stimuli.46,52,53 Thus, it is suggested that the effect of crowding is greater in the periphery than in the fovea.46,53,55 In other words, peripheral targets appear relatively more crowded and less easily recognised than central targets when letter separation is kept constant and eccentricity is varied. Bouma56 found that if the adjacent letters were separated by a distance equal to half the retinal eccentricity, then the recognition of peripherally presented letters flanked by adjacent letters could reach that of unflanked letters. Hence, the crowding phenomenon has limited spatial extent. Today, most printed material uses variable or proportional spacing or pitch because it is visually appealing. With this type of spacing, the amount of horizontal space that a character occupies depends on the width of the individual character, so that the letter i takes up much less space than the letter w. For fixed-width spacing, all characters occupy the same amount of horizontal space, that of the widest character, which is usually the uppercase W. Despite the aesthetics of proportionally-spaced fonts, Arditi, Knoblauch and Grunwald57 showed that reading speed is fastest when fixed-width fonts are used for both central and peripheral vision. Chung 55 suggested that if crowding among individual letters contributes to reduced reading speed in peripheral vision, then a solution may be to eliminate or minimise crowding by increasing the spacing between adjacent letters. She found that increasing the letter spacing beyond the standard size did not improve reading speed in central or peripheral vision and suggested that this may be because the increased letter spacing disrupted the word shape or word form information. In addition, the increased spacing would have reduced the visual span.
THE VISUAL SPAN When we read, we recognise only a few letters with each glance. The visual span was defined by O’Regan58,59 as the region (or distance) around the point of fixation within which characters of a given size can be resolved. The extent of the visual span is indicated by the retinal eccentricity at which letters (in text) can no longer be recognised. This limit is influenced by the decline in visual acuity with increasing distance from the fovea and also by the effects of crowding between adjacent letters.56 The linear scaling laws that apply to peripheral letter acuity and crowding60 suggest that the size of the visual span is approximately constant when measured in letter spaces over a moderate range of angular character size and as a consequence, more letters cannot be squeezed into the visual span by using smaller print. Various studies have estimated the visual span in normal central vision to be approximately 10 letters, measured over a range of print sizes subtending between approximately 0.3 and one degree.19,58,59,61 In contrast, visual span estimates for patients with low vision range from normal values to less than one character.61 Due to the decreasing visual acuity from the fovea to the periphery, larger print sizes are required for peripheral viewing. As a result, words presented in central vision will fit comfortably within the limits of the visual span, as smaller print sizes will be required for correct recognition. At eccentric locations away from the fovea, the letters of words with greater lengths may fall outside the visual span because larger print sizes are required for correct recognition. The ‘shrinking visual span hypothesis’ as proposed by Legge, Mansfield and Chung 62 states that the number of letters recognised on each glance shrinks in peripheral vision. As a result, more time is required to recognise words, the lengths of which exceed the size of the visual span, as two or more glances are necessary. The visual span estimates from Legge, Mansfield and Chung62 decreased from 10 letters in central vision to 1.7 letters at 15 degrees eccentricity. This reduction in the size of the visual span with
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increasing eccentricity parallels reductions in reading speed. In other words, the ability to process several letters in parallel deteriorates in peripheral vision and this smaller visual span is considered a fundamental limit to reading in peripheral vision, especially due to the high magnification required by low vision patients. Therefore, further research examining the visual span in peripheral vision may have important implications for patients with central visual loss. WORD LENGTH In reading materials, we encounter words with different lengths and this is important to consider when examining word recognition in central and peripheral vision. It is important to investigate the effects of word length because of its role in the word recognition and reading processes. The length of the word influences saccades, with more saccades required when word length information is removed. 19,63-66 Word length experiments show that perceived length directly influences word recognition responses, suggesting that information regarding the length of the word may contribute to word recognition providing a separate cue in the recognition process.66 In fact, word length information aids word recognition, possibly by adding information about word shape or form. Increased word length also hinders word recognition. This may be because longer words require more eye movements (because their letters do not fit within the visual span), thus limiting reading speed. A simple demonstration of the difficulties associated with reading words of longer length (the letters of which fall outside the limits of the visual span) with peripheral vision is presented in Figure 4. While maintaining fixation on the ‘x’ above the corresponding letters or words, the individual letters are always easier to recognise than the 12-letter words. This is particularly clear with increasing distance from fixation, even with larger print size. As mentioned previously, distance single letter acuity does not correlate with reading acuity, especially for low vision
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Figure 4. A demonstration of the effects of the difference in resolution of single letters versus words at different retinal eccentricities. The angular subtense of each letter is identical for the corresponding letter and word, however, the word is more difficult to read than the single letter. The difference is likely to be related to the reduced visual span in the periphery.
patients,17,18 so from a clinical point of view, it is not possible to predict reading performance from distance visual acuity measurements alone. This has prompted the development of new tests of reading acuity, for example the Bailey-Lovie word reading chart67 and the MNREAD (a Minnesota reading test) Acuity chart.68 However, the question of how to measure reading acuity effectively still remains. Future studies may be directed at incorporating systematic changes in word length in near visual acuity charts, rather than single letters alone. Investigating the effect of word length in the periphery may have important implications in predicting reading performance and thus designing visual rehabilitation programs for patients with central visual loss. In this review, we have presented evidence showing that peripheral reading performance does not reach that of central vision, even when print size is scaled for the periphery or when eye movements are minimised. A summary of peripheral visual function for reading, single letter and word resolution is provided in Table 1. Also, it has been mentioned that the visual span is a fundamental limit to peripheral reading, particularly due to the high mag-
nification required by low vision patients and words of longer length. With less magnification, more letters would fit within the visual span and reading performance would improve. Perhaps training may be the answer to overcoming this problem, if the size of print required for reading can be reduced with practice. A question that may arise before investigating training effects is whether perceptual learning would be possible for reading, as training is not observed for all visual functions. PERCEPTUAL LEARNING Perceptual learning involves an improvement in the ability to discriminate simple sensory attributes, such as pitch, texture, visual acuity et cetera that does not involve adaptations of the primary sensory endorgans. Much of the interest generated by studies of cortical plasticity stems from its potential relevance to learning.69 Profound changes in cortical functional architecture that follow retinal lesions may represent the mechanisms of recovery after lesions of the central nervous system. They may also reveal adaptive mechanisms associated with normal processes such as perceptual learning. The current view is that
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Peripheral visual function Reading
Slower reading rate (with scaled print size) Increased critical print size (CPS) Reduced maximum reading speed (MRS) attainable (with scaled print size and reduced eye movements) Reduced eye movement control Greater effect of crowding Reduced visual span: central vision ~10 letters peripheral vision ~1-2 letters
Single letter resolution
Limited by changes in retinal elements: Threshold declines slowly with increasing eccentricity Eccentricity at which foveal threshold has doubled: E2 ~ 2.5
Word resolution
Limited by the cortical magnification factor: Threshold declines rapidly with increasing eccentricity Eccentricity at which foveal threshold has doubled: E2 ~ 0.68
Table 1. Summary of peripheral visual function for reading, single letter and word resolution
speed improved. These improvements transferred to the untrained retinal location and were retained for three months after training. The authors suggested that this confirmed that the visual span is a bottleneck in peripheral reading but that this can be improved with training. Future studies aimed at determining how we can better rehabilitate reading for patients with AMD may be focused on overcoming the problems associated with a reduced visual span by examining the effects of training on reading with peripheral vision. From a clinical perspective, research in this field may trigger the incorporation of training patients with central visual loss to read with peripheral vision in their visual rehabilitation programs. ACKNOWLEDGEMENTS
the likely cause for learning is a sustained and lasting change that has synaptic plasticity as its basis.69 Research on perceptual learning in peripheral vision has shown that for an adult with normal vision, spatial visual functions including orientation discrimination, bisection and vernier acuity can be improved by a factor as much as three.70,71 The learning effects in peripheral hyperacuity are well established, however, research on learning effects on visual acuity and reading in the retinal periphery is lacking and it remains unclear whether visual acuity or reading performance in the periphery can be improved with training. Beard, Levi and Reich72 demonstrated that both physiological and cognitive processes contribute to the improvement with practice and Westheimer73 showed that learning effects or training can take place in stereoscopic, orientation, vernier, bisection and time discriminations but not in resolution and Landolt C acuities. It has been suggested that failure to improve with training is not a function of the observer or the retinal location but is specific to the resolution task. In other words, the tasks for which learning does not occur are considered to have a process which
is of a more primitive kind, more robust and closer to sensory origins.73 For example, resolution tasks such as grating acuity have a strong retinal basis and depend on the spacing of retinal elements, while other localisation tasks of the hyperacuity kind have an element of cortical processing.43 Following this idea, if word acuity thresholds follow the cortical magnification factor,48 such as vernier acuity, which can be trained,73 then it is plausible to propose that practice will also improve word recognition thresholds. While controversy surrounds the notion that simple peripheral resolution performance can be improved with training,73 recent studies have demonstrated trainingrelated improvements in peripheral reading performance,74,75 Chung, Legge and Cheung 75 specifically examined whether visual span profiles (plot of letter recognition accuracy as a function of letter positions left or right of the midline) could be modified through repeated training on a letter recognition task in peripheral vision. The results from this study showed that training with a letter recognition task at an eccentric location led to changes in the visual span profiles. Letter recognition accuracy increased and maximum reading
Supported in part by the Robert G Leitl Trust Professorship held by Michael Kalloniatis. We would like to sincerely thank Associate Professor Algis Vingrys for providing the fundus photographs presented in this manuscript. REFERENCES 1. Rubin GS. Vision rehabilitation for patients with age-related macular degeneration. Eye 2001; 15: 430-435. 2. Legge GE, Rubin GS, Pelli DG, Schleske MM. Psychophysics of reading. II. Low vision. Vision Res 1985; 25: 253-265. 3. Dana MR, Tielsch JM, Enger C, Joyce E, Santoli JM, Taylor HR. Visual impairment in a rural Appalachian community: prevalence and causes. JAMA 1990; 264: 24002405. 4. Rahmani B, Tielsch JM, Katz J, Gottsch J, Quigley H, Javitt J, Sommer A. The causespecific prevalence of visual impairment in an urban population: the Baltimore Eye Survey. Ophthalmology 1996; 103: 1721-1726. 5. Royal National Institute for the Blind (RNIB). Office of National Statistics mid1996 population estimates: estimates for 1996 of visually impaired people and the number of people registered as blind and partially sighted as at 31 March 1997 in the United Kingdom. http://www.rnib.org.uk/ wesupply/fctsheet/authuk. 6. Attebo K, Mitchell P, Smith W. Visual acuity and the causes of visual loss in Australia. The Blue Mountains Eye Study. Ophthalmology 1996; 103: 357-364. 7. Livingston PM, Carson CA, Stanislavsky YL,
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Corresponding author: Dr Andrew Metha Department of Optometry and Vision Sciences The University of Melbourne Parkville VIC 3010 AUSTRALIA E-mail:
[email protected]
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