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reading speed only when the classrooms were lit with fluorescent
lighting.
The move to LED lighting has unfortunately resulted in the provision
of lamps that flicker more than fluorescent lamps ever did. There is not
the same variation in chromaticity, however. The flicker is due to the
design of the electronic driving circuitry and it is only recently that
standards limiting flicker have been introduced.
Does the tint have to be individually specific?
Vision, Reading Difficulties and Visual Stress
230
The evidence that the tint has to be individual and specific comes
from both of the randomised controlled trials of precision tinted
lenses,
62 63
and a study of overlays and lenses.
64
It also comes from a
study, described briefly in the previous chapter, in which patients
had been using tinted lenses for some time took part.
Reading speed was
who
measured repeatedly under light of randomly chosen chromaticity.65
The reading speed was maximum at a particular chromaticity, different
for each patient, but consistent for that individual. The reading speed
decreased with the difference in colour between the individual
optimum and the colour under which the reading speed was
measured, whether that difference reflected a change in hue, in
saturation or in both. However, when the difference in chromaticity
exceeded a certain value (on average, 0.07) the reading speed showed
no further decrease and remained similar to the reading speed under
white light. It was not possible to relate the way in which reading speed
changed with chromaticity to any of the cone receptors. Instead,
central, cortical, mechanisms appeared to be implicated.
The patients in the above study had been using tinted lenses and
were therefore familiar with their colour and the perceptual effect they
produced. Further evidence that the optimal colour is individual and
specific comes from the study by Wilkins et al.66 with patients who had
little or no previous experience with tints. Patients selected their
optimal colour setting twice with independent (masked) examiners,
one examination immediately after the other, using different models
of colorimeters. The difference in chromaticity between the two
settings averaged 0.043 with a standard deviation of 0.027. Reading
speed was faster with light of the chosen colour than with light of a
chromaticity that differed by 0.07.
Why the specificity?
One possible cortical mechanism for the effects of tinted lenses may
relate to the joint coding of colour and orientation recently
demonstrated in V1.53 Given the joint coding, the use of coloured filters
to help with spatial disorders of vision becomes slightly more
comprehensible.
Another possible cortical mechanism relates to cortical
hyperexcitability. In patients with photosensitive epilepsy there is a
hyperexcitability of the visual cortex.67 The excitability can be quite
specific to certain classes of cortical cells, those with a particular
orientation preference, for example. Sometimes these patients can be
treated with coloured glasses (see Chapter 12).
68
From research in
macaque monkeys, it seems likely that in an area (V2) of the human
visual cortex, adjacent neurons respond to similar colours. It is as if a
colour map, like that in Figure 6.4, could be drawn over the surface of
the cortex showing which colours are detected by adjacent neurons.69
It has been argued that precision tints rearrange cortical activity in such
a way as to avoid strong excitation in hyperexcitable regions
(orientation columns).70 The avoidance of strong excitation in
hyperexcitable columns may prevent the spread of excitation, and in
doing so may prevent the inappropriate firing of visual neurons that
gives rise to illusions and distortions.
Chapter 8 How do coloured filters work?
231
The above hypothesis is speculative, and cannot predict which
colours of tint will help which patients. Currently no psychophysical or
physiological correlate of the optimum colour has been found. The
hypothesis does, however, have one strength, which is that it predicts
that precision tints will be of benefit not only in reading difficulties but
also in a range of neurological conditions that are associated with
hyperexcitability of the visual cortex. Such conditions include the
neurological disorders described in Chapter 11.
A link between visual stress and dyslexia?
It was noted above that three separate studies have indicated that
visual stress affects about 20% of people with dyslexia.
23 26 27
Although
all these studies have low numbers of participants, it is reassuring that
all three produce broadly similar estimates.
There are several potential reasons why the two conditions co-
occur. Some of these have been discounted earlier in this chapter. For
example, it seems unlikely that the M-D deficit causes visual stress,
although it was noted above that it seems possible that visual stress
could exacerbate binocular instability, and vice versa.
At the end of Chapter 5, it was noted that the various theories that
once were seen as competing to explain dyslexia are now seen as
complementary. The “additive risk factor model of dyslexia”71 was
highlighted, which may be supported by anatomical and fMRI
findings.72 According to this model, developmental problems may
cause subtle differences in the structure and the function of the brain
in dyslexia, some of which manifest in a given individual and some of
which may contribute to reading difficulties.
An interesting study by Veitch and Miller found that pattern glare is
exacerbated by cognitive load: when individuals concentrate hard their
Vision, Reading Difficulties and Visual Stress
232
sensitivity to pattern glare increases.73 This raises the possibility that at
least a part of the reason why dyslexic children seem to be particularly
likely to report symptoms of pattern glare when viewing text is because
they have to concentrate harder than other children to read.
Another relevant consideration is what might be called the “double
whammy” hypothesis. If a person who has no problem learning to read
starts to experience eyestrain and visual perceptual distortions when
they encounter small text, their degree of interest in the text may help
them to tolerate or ignore these symptoms. In their overall perception
of the page, the meaning in the text may predominate, rendering their
visual stress less noticeable. However, if a person is struggling to gain
meaning from the page because of dyslexia, the symptoms of visual
stress may become the most noticeable impression they gain from the
page. According to this notion, the sum of the parts (dyslexia and visual
stress) is greater than either part alone.
Summary
Various neurological diseases affecting the visual system are
associated with visual stress. We hypothesise that cortical
hyperexcitability is responsible for visual stress and that visual stress
is in its turn responsible for some of the visual aspects of reading
difficulty. It seems likely that visual stress is at least one of the reasons
why some individuals benefit from individually prescribed coloured
filters.
The symptoms of perceptual distortion and visual discomfort that
occur in patients with visual stress can also occur in a wide variety of
neurological conditions. But the symptoms are non-specific: for
example, similar symptoms can result from refractive error, binocular
vision anomalies, and accommodative anomalies, see Chapter 10.
Chapter 10 describes a clinical protocol for investigating visual stress,
Chapter 8 How do coloured filters work?
233
including differential diagnosis.
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Vision, Reading Difficulties and Visual Stress
238
Chapter 9
Management of visual stress
Chapter abstract
In this chapter we describe the various coloured filters that are used
in the assessment and treatment of patients: how many are needed
and why, and what their physical characteristics should be. Other tools
that are used in the investigation of visual stress are also briefly
described, including adaptations to digital displays (computer screens),
coloured lighting, and altering the characteristics of text.
The nature and number of tints for spectacle lenses
Preliminary studies with the Intuitive Colorimeter showed that
although the chromaticity of the light could be critical for visual
discomfort, its spectral power was less so. Thus, a colour of light with
a given spectral power distribution had the same effect on comfort as
when the colour was reproduced in tinted spectacles that matched the
chromaticity under lighting with a very different spectral power
distribution. In the Intuitive Colorimeter system dyes were therefore
selected with two major considerations in mind: (1) they should have
as high a transmission as possible; (2) the transmission should vary
with wavelength as smoothly as possible. Artificial lighting can
sometimes have a very uneven distribution of spectral energy, and a
smoothly varying transmission avoids peaks and troughs in the energy
coinciding with peaks and troughs in the transmission of the lenses,
and the variation in colour that results. We saw in Chapter 7 that
changes in the lighting (fluorescent, incandescent, daylight) do not
© 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_9
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