In everyday life, many people experience discomfort when looking at repetitive striped patterns, as when ironing striped shirts or using escalators (Wilkins, 1995). The intensity of the effect varies according to the parameters of the pattern and individual susceptibility. For susceptible people, these patterns induce eyestrain; headache; and illusions of colour, shape and motion. The symptoms of discomfort (visual stress) and visual perceptual distortions have been given the name ‘patterned glare’ (Wilkins and Nimmo-Smith, 1984) and subsequently ‘pattern glare’ (Evans and Drasdo, 1991).
Pattern glare is typically tested with square wave gratings. To elicit maximum pattern glare, the gratings should have the following properties: a spatial frequency of about 3 cycles per degree (cpd), even width and spacing (duty cycle 50%), high contrast and be viewed binocularly (Wilkins et al., 1984; Wilkins, 1995).
Reading and pattern glare
Wilkins and Nimmo-Smith (1984, 1987) investigated whether the pattern of stripes formed by text was sufficient to induce pattern glare and visual discomfort in susceptible subjects. They showed that the contrast, spatial frequency and duty cycle of text were all within the range required to produce visual discomfort, and they proposed that text could produce pattern glare. Wilkins et al. (1984) showed that the most unpleasant patterns give rise to the most illusions and that people who experienced more illusions suffered from more headaches. When asked to describe the effects they experienced, participants reported: colours, diamond shaped lattice, shimmer, blurring, dazzle, glare, bending, flashing, blobs and flickering (Wilkins et al., 1984). The above effects are thought to be due to cortical hyper-excitability and the patterns need to be specific to induce symptoms (Wilkins et al., 1984; Wilkins, 2003).
Reading performance has been the subject of further studies on pattern glare. One study used reading-like visual tasks and showed that sensitive subjects took significantly longer because of the pattern formed by the lines on the page (Conlon et al., 1998). This pattern glare can cause visual discomfort during reading (Wilkins and Nimmo-Smith, 1984) and may account for the benefit from coloured filters (Evans et al., 1994, 1995, 1996, 2002) in a condition that has been called Meares–Irlen syndrome (Evans, 1997) or visual stress (Wilkins, 1995). This condition is described hereafter with the acronym MISVIS (Meares–Irlen syndrome/visual stress).
A more recent study showed that a group with high visual discomfort was less efficient at both conscious and automatic attention tasks regardless of the background pattern (Conlon and Humphreys, 2001). This indicates that the reduced reading efficiency is probably not just a result of an increased level of global interference of patterns, such as from lines of text (Conlon and Humphreys, 2001). It had previously been suggested that the high visual discomfort group had reduced efficiency of the parvocellular system (Conlon et al., 2001). In the Conlon and Humphreys (2001) study, it was argued that poorer performance could be the result of an overload in the magnocellular system, resulting in reduced efficiency of the attentional spotlight or alternatively less efficient spatial attention via the parvocellular system. Evans (2001) suggested a model that links pattern glare in some cases of dyslexia with a deficit of visual attention and indirectly with a magnocellular deficit.
Migraine and pattern glare
As long ago as 1934, it was noted anecdotally that a small number of migraine sufferers could have migraine episodes triggered by glare from bright light or patterns. For example, driving past railings or looking at a flickering flame could precipitate an attack (Turville, 1934). More recently, a strong correlation has been found between migraine and pattern glare (see Harle and Evans, 2004; for review) and some patients with migraine find that precision tinted lenses reduce the frequency of their attacks (Wilkins et al., 2002), probably because of pattern glare (Evans et al., 2002).
Schoolchildren with migraine have been found to lose more than double the number of schooldays to illness compared with their counterparts without migraine (Abu-Arefeh and Russell, 1994). On days when adults are going to have headaches, and up to 24 h before, susceptibility to pattern glare is increased (Nulty et al., 1987). Where the headaches are on one side the distortions tend to be greater in one visual hemifield (Wilkins et al., 1984) and in migraine sufferers the distortions tend to be on the same side as the aura (Khalil, 1991). A recent controlled study of optometric function in visually-sensitive migraineurs found pattern glare to be the strongest optometric correlate of migraine (Evans et al., 2002).
Epilepsy and pattern glare
A series of experiments (Wilkins et al., 1984) pointed to neurological processes in common between epilepsy and pattern glare. These researchers demonstrated that the spatial properties of patterns that elicit epileptiform electroencephalographic abnormalities in a group of people with photosensitive epilepsy are similar to the spatial properties that make patterns uncomfortable for people who do not suffer from epilepsy.
Four per cent of patients with epilepsy suffer from visually induced seizures caused, for example, by flicker or steadily illuminated patterns such as stripes (Wilkins, 1995). Experiments in 1975 and 1979 by Wilkins showed that striped patterns can provoke epileptiform EEG activity (Wilkins, 1995). Meldrum and Wilkins (1984) hypothesised that the photosensitive epileptiform abnormalities are because of a minor failure of GABAergic cortical inhibition and so these findings support a relationship between migraine headache and cortical dysinhibition noted elsewhere in the literature (Palmer et al., 2000).
Aetiology of pattern glare
The cells of the visual cortex are organised into columns that are responsive to gratings of a specific orientation and spatial frequency. Therefore, gratings should produce a concentrated excitation within the nerve network (in a few cortical columns having appropriate orientation sensitivity) compromising the shared inhibitory processes. The inhibitory interneurons are shared between columns so the synthesis and reuptake of the inhibitory neurotransmitter may be insufficient to meet demand under conditions of strong excitation. Patterns of stripes may therefore cause a high level of cortical stimulation leading to a breakdown in cortical inhibition. If the discharge does not spread but remains local, the neural excitation could be responsible for visual perceptual distortions without being sufficient to induce electrical activity measurable at the scalp (Meldrum and Wilkins, 1984).
It has been shown that some cortical cells respond more to gratings than edges or bars and that the linearity of the contour is more important than the total number of contours, so that stripes are more epileptogenic (Meldrum and Wilkins, 1984) and more likely to cause pattern glare (Wilkins et al., 1984) than checks.
Further evidence that the illusions associated with pattern glare are triggered in the cortex is provided by the observation that the effect is greater under binocular than monocular conditions (Wilkins et al., 1984). The input from the two eyes remains separated until it reaches the cortex. The cortex is arranged functionally into orientation specific and ocular dominance columns that overlap. The input from the two eyes is combined through interneurones whose neurotransmitter is GABA (Kandel et al., 1995). Monocular occlusion can be used as a treatment for photosensitive epilepsy because it has been shown to reduce the photosensitive response to flicker (Wilkins, 1995). Monocular occlusion may help some dyslexic children (Evans, 2001), possibly because of the fact that pattern glare is reduced by covering one eye (Wilkins et al., 1984).
Alternative explanations for pattern glare
The evidence reviewed above all points to a cortical mechanism for pattern glare (Wilkins et al., 1984). Less likely non-cortical explanations for pattern glare include the small eye movements that occur during fixation and also accommodative fluctuations. If these mechanisms were correct then less specific tasks would lead to pattern glare. An investigation into the effect of spatial frequency on accommodation using sinusoidally modulated vertical gratings of 1.67, 5 and 15 cpd on pre-presbyopic subjects found that an over-accommodation occurred with the highest spatial frequency and that accommodation was most stable at 5 cpd (Ward, 1987). It has also been suggested that flicker at higher spatial frequencies may result from involuntary eye movements (Wade, 1977). However, later work by Wilkins and Neary (1991) found that coloured lenses, which reduce pattern glare (Evans et al., 2002), had idiosyncratic effects on ocular motor balance and their results suggested it was unlikely that accommodation had a role. This conclusion was supported by Evans et al. (2002), and Simmers et al. (2001) found increased accommodative micro-fluctuations in MISVIS but concluded that these were most likely a correlate rather than a cause.
Methods for reducing pattern glare
The literature reviewed above establishes that pattern glare can be a significant factor in people’s lives, but how can pattern glare from text be minimised? Changing the spacing of the lines, the length of the lines (Wilkins and Nimmo-Smith, 1987) or the number of lines visible at one time can reduce the striped effect produced by text (Wilkins and Nimmo-Smith, 1984). One way of doing this is to use a typoscope. This is simply a card with a rectangular slot positioned so that only a few lines of text are visible at one time.
A controlled study using reduced contrast reading material (half of the text was printed on medium grey paper to do this) concluded that the score for the learning difficulty students was 10% higher on the pages with reduced contrast compared with the pages of full contrast (Giddings and Carmean, 1989).
One controlled study assessing the benefit of using the DEX frame (a combination of a typoscope, magnifier and coloured filter) for children with specific learning disability found no benefit compared with the control group (Taylor et al., 1992). However, although MISVIS is correlated with dyslexia, most people with dyslexia do not have MISVIS (Kriss and Evans, 2005; White et al., 2006) and studies of MISVIS in dyslexic populations will therefore suffer from reduced statistical power.
It has also been suggested that the use of the Visual Tracking Magnifier (VTM) can help children with learning difficulties. This is a magnifier of up to 1.5× magnification. Either side of a central clear strip (approximately 0.7 cm wide) are patterned areas intended to reduce distortions from the surrounding text. There are two designs and the widest is 11.5 cm long. The magnifier is placed flat on the text and moved along the lines. A major disadvantage of this magnifier is the small field of view and we have not been able to find any randomised controlled trials of this intervention. However, the use of magnifiers is known to reduce reading speed in fully sighted people (Bowers, 2000).
Pattern glare seems to be the mechanism, or one of the mechanisms, underlying the visual stress that is a core feature of MISVIS. MISVIS is defined as symptoms, on viewing text, of asthenopia and visual perceptual distortions that are alleviated by using individually prescribed coloured filters (Evans, 1997, 1999). This condition, the use of coloured filters and the relationship with pattern glare will now be described in more detail.
Meares–Irlen syndrome, coloured filters and pattern glare
Meares (1980) first described the symptoms of MISVIS. She noted that some children’s reading abilities were influenced by the characteristics of their reading material and that their reading could be improved by changing the size of the print, the contrast or using coloured paper. Irlen (1991) called this Scotopic Sensitivity/Irlen Syndrome and described this as a visuo-perceptual syndrome in which sufferers experience eyestrain and perceptual illusions with reading which could be alleviated with the use of coloured filters.
A study of 20 patients wearing Irlen coloured glasses found that coloured lenses reduced discomfort when looking at gratings (Wilkins and Neary, 1991). Several theories have been proposed to explain why the coloured lenses helped, including pattern glare (Wilkins and Neary, 1991). Binocular uncoordination or accommodative dysfunction are sometimes associated with MISVIS and, although these problems are not usually the explanation for the benefit from coloured filters, people with MISVIS need careful optometric evaluation and differential diagnosis (Wilkins and Neary, 1991; Evans et al., 1995, 1996; Scott et al., 2002; Evans, 2005).
The symptoms of pattern glare and MISVIS are very similar (Evans and Drasdo, 1991; Wilkins et al., 1991). Patients who benefit from coloured overlays or spectacles often experience pattern glare (Evans et al., 1995, 1996). In matched control group studies, pattern glare has been found to be the strongest correlate of a benefit from coloured filters in people with reading difficulties (Evans et al., 1995) and migraine (Evans et al., 2002). The aetiology of MISVIS is not known for sure, but it seems very likely that pattern glare from cortical hyperexcitability is at the core of the condition (Wilkins and Neary, 1991; Evans, 2001; Wilkins, 2003). Coloured lenses have also been found to be beneficial in photosensitive epilepsy again suggesting some common neurological link with MISVIS and, therefore, pattern glare (Wilkins et al., 1999).
In conclusion, precision tinted lenses seem to be an effective intervention for MISVIS (Evans, 2001). Although pattern glare appears to be a correlate of reading difficulties (e.g. dyslexia; Kriss and Evans, 2005), migraine (Harle and Evans, 2004), epilepsy (Wilkins et al., 1999) and autism (Ludlow et al., 2006) only a proportion of people with these conditions are likely to have MISVIS and therefore to benefit from precision tinted lenses. When the symptoms are centred on text based activities then coloured overlays can be used to screen for the benefit from colour (Wilkins, 1994, 2002). But there are some patients with migraine whose symptoms of visual stress are not specific to text, occurring, for example, more generally under fluorescent lighting or in the office. It was felt that it would be useful to develop a tool that would help to identify these cases. As reviewed above, pattern glare is a strong correlate of MISVIS, and is probably a manifestation of the underlying aetiology of cortical hyperexcitability. In research studies, pattern glare testing had been found to be a simple procedure and anecdotal comments of those who had used these research tests suggested that a test of pattern glare for use by eyecare and possibly other professionals might be useful.
The Wilkins and Evans Pattern Glare Test
The Wilkins and Evans Pattern Glare Test was published in 2001 to allow practitioners to assess pattern glare (Figure 1). The test is based on the literature on pattern glare and the instructions give guidance on how to interpret the results (Wilkins and Evans, 2001). However, neither the effectiveness of the test nor the precise test norms have yet been established.
Figure 1. The Pattern Glare Test. The actual test is larger than illustrated. Courtesy of i.O.O. Marketing Ltd, London.
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The test is designed to induce visual perceptual distortions in susceptible patients. There are three high-contrast grating patterns, each with a duty cycle of 50%, which are viewed binocularly. Each grating pattern subtends an angle of 13.63° at the eye when viewed at the testing distance of 40 cm. The orientation of the gratings is horizontal so that it mimics text. For each grating, patients are asked to report which, if any, of the following visual perceptual distortions are perceived: colours, bending of lines, blurring of lines, shimmering/flickering, fading, shadowy shapes, others. For each pattern, the number of these distortions is summed to give a Pattern Glare Score (maximum 7), in line with previous research (Wilkins et al., 1984; Conlon et al., 1999).
Pattern 1 is a control with a low spatial frequency (0.5 cpd) that is expected to trigger relatively few distortions, which are not likely to be associated with headaches and eye strain (Wilkins et al., 1984). The main purpose of pattern 1 is to detect patients who are highly suggestible and might respond ‘yes’ to almost any question about visual perceptual distortions. As revealed in the literature reviewed above, the spatial frequency (3 cpd) of Pattern 2 will maximally elicit pattern glare and individuals who complain of many symptoms of visual stress in everyday life should report more distortions in response to Pattern 2 compared with gratings that have higher (Pattern 3) and lower (Pattern 1) spatial frequencies. Pattern 3 is another form of control, with a higher spatial frequency grating (12 cpd) and would be expected to elicit less distortion than Pattern 2. Distortions from Pattern 3 would be expected to be of a different nature to those from Pattern 2, reflecting a greater contribution from optical as opposed to neurological factors (Conlon et al., 2001). According to the work of Conlon et al. (2001), individuals with relatively low visual discomfort would be expected to report more distortions in response to Pattern 3 (12 cpd) than Pattern 2, although they may report fewer distortions overall. The anticipated pattern of results in patients with pattern glare compared with the normal result is summarised in Table 1, which is reproduced from the test manual (Wilkins and Evans, 2001).
Table 1. Possible relationship between the Pattern Glare Test results and the degree of visual discomfort likely to be experienced in everyday life
|Visual discomfort in everyday life||Discomfort and distortions in response to|
|Pattern 1 (0.5 cpd)||Pattern 2 (3 cpd)||Pattern 3 (12 cpd)|
In summary, pattern glare would be suggested by a patient having a high score with Pattern 2 (3 cpd) and/or a score with Pattern 2 which is higher than the score with Pattern 3. It is not clear which of these results is the best indicator of pattern glare so both were considered in our analyses.
The common practice of describing square-wave gratings as having a specific spatial frequency is an over-simplification. The quoted spatial frequencies are the fundamental spatial frequency, but a Fourier analysis would also reveal the presence of higher spatial frequency components from the sharply defined edges of the square-wave gratings. Nonetheless, in Fourier terms overwhelmingly the most powerful component will be the fundamental spatial frequency of each grating as described above.