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Whereas in the past patients with visual stress have been identified
on the basis simply of subjective symptoms, patients with migraine
aura have a well-recognised neurological diagnosis. This finding will
make it possible in future to undertake double-masked studies of the
effects of colour on a well-defined group of patients.
Coloured overlays
The above review concerns the effects of coloured light and
coloured lenses. The effects of coloured lenses are the same as those
of coloured lighting when the effects of inter-reflections between
surfaces are minimised, as they are in the colorimeter.
These studies have been complemented by many studies, with large
numbers of participants, using coloured overlays – coloured plastic
sheets placed upon a page of text, see Chapter 6. The overlays provide
a surface colour for the central visual field, one among many coloured
surfaces. In contrast, coloured lenses tint the entire visual field. This
means that the extent to which a person adapts to colour will not be
the same for overlays and lenses. The neurological effects of the two
interventions will not be the same, which may explain why there is little
relationship between the colour optimal in lenses and that optimal in
overlays23, see Chapter 9.
As explained above, colour adaptation means that it is possible to
mask the colour used in coloured lenses, but the colour of overlays is
obvious, and cannot be masked. This means that, in research studies,
the best that can be done to control for placebo effects is to provide
clear, minimally tinted or grey overlays with a cover story. When this is
done, and the control interventions are designed, with a cover story, to
generate a placebo effect, there is generally little effect of the control
intervention on reading speed.24 Nevertheless overlays of a colour
Vision, Reading Difficulties and Visual Stress
200
chosen for clarity and comfort are more effective than those chosen
on the basis of colour preference.
Because of the difficulty in controlling placebo effects, research
using coloured overlays to investigate visual stress will only be briefly
summarised.
Do coloured overlays work?
The literature on the effects of coloured overlays on reading speed
is now quite extensive, and has been systematically reviewed in two
papers, each with different conclusions.
1 2
One review by Griffiths2
included all the studies of the effects of colour on reading, many of
which sampled individuals with dyslexia, most of whom would not have
had visual stress. Visual stress occurs only in a minority of individuals
with dyslexia, and the chances of showing any benefits of colour in a
cohort of unselected individuals with dyslexia is therefore small.
It is straightforward and rapid to test children with coloured
overlays and over 1,500 children have now been included in research
with overlays.
24-29
The early literature was reviewed by Wilkins in
2003,24 and more recent literature by Vilhena in 2020.29
There are several considerations regarding the use of overlays for
reading, and studies that fail to account for these factors are unlikely
to produce useful results. These considerations are discussed below.
1. Breadth of choice.
Unless there is a sufficient choice of overlay colour, individuals are
often unable to find a colour that increases viewing comfort and helps
them read. The five overlays initially introduced by one supplier
(Crossbow) were shown to be insufficient in number. In a study by
Smith & Wilkins30 48 children in a local authority primary school who
Chapter 7 Do coloured filters work?
201
reported symptoms of visual stress were individually tested with both
the Intuitive Overlays and the five coloured overlays originally supplied
by Crossbow Ltd in the form of Reading Rulers. The tests were given in
random order, balanced across pupils. There were no differences
between the two systems of overlays as regards children’s preference
or the reduction in symptoms of visual stress. There was, however, a
significant increase in reading speed on the Wilkins Rate of Reading
Test with the Intuitive Overlays and no such increase with the Reading
Rulers. This difference was attributed to the limited choice of colour
with the Reading Rulers rather than their size because when the
Intuitive Overlays were cut to the size of the Reading Rulers there was
no difference in reading speed with the large and small overlays.
2. Time spent reading.
With conventional text the benefits of a coloured overlay can be
observed only when the reader has been reading for several minutes
and begun to tire. This was demonstrated originally by Tyrrell et al.31
who asked children to read passages from their schoolbooks for 15
minutes with and without their chosen overlay in a random order
counterbalanced across children. The overlay had no effect on reading
speed initially, but after 10 minutes reading the children who chose an
overlay read more slowly, and the overlay then increased their reading
speed.
3. Size of text
When the text is small and closely spaced, as in the Wilkins Rate of
Reading Test, the benefits of an overlay can be observed immediately.
When the text is large, the benefits of an overlay are less obvious.
Ritchie et al.32 measured reading speed using the test with large text
and failed to find a significant increase in speed with a coloured
Vision, Reading Difficulties and Visual Stress
202
overlay, whereas Henderson et al.33 using small text found a significant
increase in reading rate. This is to be expected, given the effects of text
size on reading speed34 and on the effects of the spatially repetitious
pattern of lines and letter strokes.35
4. Comprehension
Most conventional reading tests were designed to evaluate
scholastic attainment in reading. They usually consist of meaningful
prose. If the test is to be used more than once (for example, with and
without an overlay) it is necessary to generate versions with different
but equivalent prose. It is difficult to match prose so that the effects on
comprehension are equated, and comprehension of the prose
contributes to the variance in measurement of reading rate. The
effects of overlays in increasing reading rate may be modest compared
with the variance in reading speed due to comprehension. There is no
“quick fix” for reading problems, and coloured filters should not be
expected to bring about a rapid improvement in conventional tests of
reading because, even if visual stress is contributing to the reading
difficulty, it will take time to acquire reading skills. Although Henderson
et al.33 found an effect of overlays on the WRRT, they failed to show an
effect with (brief) passages that were read for comprehension. Wilkins
and colleagues, in the original randomised controlled trial of precision
tinted lenses, were obliged to use a conventional test of reading
because the WRRT had not then been invented. The lenses were used
only for one month and it is unsurprising that a benefit could not be
demonstrated on a conventional test of scholastic attainment in
reading.
5. Sampling bias
The selection of participants on the basis that they report beneficial
Chapter 7 Do coloured filters work?
203
effects of colour on reading raises issues of bias. This bias in selection
applies to many of the early studies of overlays. But there are studies
in which no such selection occurred: entire cohorts of optometrically
screened students have been examined with overlays, and these show
similar effects on reading speed.36
6. Reading speed or accuracy
The use of a simple limited vocabulary in the WRRT is quite
deliberate. The speed of reading is usually greater when the reading is
more accurate. There is no speed/accuracy trade-off in performance
with the WRRT. A Catalan version of the Rate of Reading Test was used
in a study by Cardona et al.37 They increased the complexity of the
words towards the end of the passage. This meant that their measure
of reading speed was complicated by variance due to word familiarity.
Perhaps unsurprisingly, they found in this variation from the usual
WRRT that reading accuracy was a better measure than speed in
showing the effects of overlays.
Several of the points listed above (items 2, 3, 4, and 6) are not
specific to research on coloured filters but may also apply to other
research investigating the effect of optometric interventions on
reading. For example, it may not be surprising that a trial of eye
exercises (vision therapy) did not find a significant improvement in
conventional tests of reading performance.38 It would be interesting for
future research of this type to use the Wilkins Rate of Reading Test,39
which has already been shown to be sensitive to several types of
optometric conditions and interventions.
40-47
Summary
There has been a considerable body of research concerning the
treatment of visual stress with coloured filters. The most important
Vision, Reading Difficulties and Visual Stress
204
studies are randomised controlled trials and these studies have been
reviewed in this chapter, together with other pertinent research. There
are a number of literature reviews on the use of coloured filters for
those with reading problems, but two systematic reviews published in
2016 are particularly thorough.
1 2
They reach different conclusions.
One review includes the studies that have selected individuals with
reading difficulty, only a minority of whom are likely to have had visual
stress. The other review includes only studies that have selected
participants using reasonable criteria for visual stress and which used
the Intuitive system of overlays and lenses. The controversy continues
partly because only recently has a consensus started to emerge on
possible diagnostic criteria for visual stress.17 Another exciting recent
finding is that it may now be possible to identify a cohort of patients
with visual stress on the basis of their conventional neurological
diagnosis.21
Overlay studies will always be limited by difficulties in controlling for
placebo effects. The Intuitive Colorimeter facilitates a double-masked
randomised controlled trial (RCT), and this type of research is
important for investigating treatments. The RCT described earlier in
this chapter had limitations and further RCTs are necessary. Ideally,
these should have larger samples, better participant retention, use
contemporary diagnostic criteria,17 and include appropriate outcome
measures.1 The RCT is often held as the only appropriate evidence of
clinical efficacy, but it is not. It is suitable only when other less
expensive trials have identified the relevant variables. Bradford Hill, the
architect of the RCT, stated “any belief that the controlled trial is the
only way would mean not that the pendulum had swung too far but
that it had come right off the hook”. A better understanding of visual
stress requires a wide range of evidence, with a diversity of research
designs that include RCTs, but are not limited to them.
48
Chapter 7 Do coloured filters work?
205
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Chapter 8
How do coloured filters work?
Chapter abstract
The various possible mechanisms whereby coloured filters might
improve symptoms and reading speed are reviewed. The chapter
starts with the straightforward mechanisms that are well-understood
in terms of physics and biology. These include light scatter, retinal
mechanisms, ocular accommodation, binocular vision anomalies,
magnocellular-dorsal (M-D) deficit, and pattern glare/cortical
hyperexcitability. It is concluded that the most likely mechanism is
cortical and involves a hyperexcitability of neural tissue, triggered by a
form of pattern glare.
Light scatter?
The reason objects appear hazy in fog is because of light scatter
from water droplets and this scatter is greater for short-wavelength
light, which is most energetic. This is one reason why yellow filters,
which block short wavelengths, appear to enhance contrast in fog.
1,2
Even in a healthy eye, some light will be scattered. This scatter of light
within the eye might be expected to reduce the contrast of the retinal
image. Tinted lenses that absorb short wavelength light might
therefore be expected to increase the contrast of the retinal image
(whilst reducing its luminance). Each point in the chromaticity diagram
in Figure 8.1 shows the chromaticity of a precision tinted lens in a
consecutive series of 1000 tints. Most tints are green, turquoise or blue
(just as in the tints used historically, see Chapter 6), and the latter two
© The Author(s), under exclusive license to Springer Nature Switzerland AG 2024
A. J. Wilkins and B. J. W. Evans, Vision, Reading Difficulties and Visual Stress,
https://doi.org/10.1007/978-3-031-65568-5_8
209