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Discomfort from periodic patterns
As the contrast energy in an image is increasingly concentrated in
one orientation the image becomes more and more like a grating.
Gratings with a spatial frequency close to 3 cycles per degree visual
angle induce a strong neurological response within the visual system:
(1) they are more readily seen at low contrast than are other spatial
frequencies;33 (2) they interfere with vision, masking low contrast
images;34 (3) they induce a visual evoked potential of relatively high
amplitude;35 (4) they give rise to perceptual distortions and can be
uncomfortable to look at.36 Patterns of this kind are very different from
the images usually found in nature that the visual system evolved to
process efficiently. The cluster of symptoms of perceptual distortion
and discomfort that such periodic visual patterns evoke is sometimes
called pattern glare. Pattern glare can be appreciated by viewing Figure
8.5 shown at the end of this chapter. Caution. Do not look at this
pattern if you have migraine or epilepsy.
The Pattern Glare test37 contains three patterns, one with a spatial
frequency of approximately 3 cycles per degree, similar to that of
Figure 8.5, and two control patterns, one with a spatial frequency of
approximately 0.3 cycles per degree and one with a spatial frequency
of approximately 11 cycles per degree (not now recommended).38 The
perceptual illusions/distortions reported in response to the 3 cycles
per degree pattern are greater for individuals with migraine,36 and in
individuals for whom coloured overlays improve reading speed, see
Figure 8.6.39
Vision, Reading Difficulties and Visual Stress
220
Figure 8.6. Improvement in reading speed with an overlay of the chosen
colour shown as a function of the number of illusions seen in a striped
pattern (symptoms of pattern glare). Reproduced with permission from
Hollis and Allen.39
When individuals undertake visual search tasks, those who report
pattern glare generally perform more poorly, especially when the
search task has spatial properties likely to exacerbate pattern glare.
40
41
Text as periodic patterns
Forty years ago, Olive Meares, a teacher from New Zealand, was one
of the first people to explore the symptoms of visual stress and the
benefit from coloured filters. Considering children trying to read,
Meares noted “Where else in the child’s world does he even meet, let
alone be required to attend closely to and concentrate intently upon,
such an extreme brightness contrast as the pages of a book?”42
The successive lines of printed text approximate a periodic pattern
similar to a grating, as can be appreciated by inspecting the filtered
image in Figure 8.7 (right panel). As described in the previous chapter,
this helps explain why a simple mask that covers the lines above and
Chapter 8 How do coloured filters work?
221
below those being read can improve the clarity of text. Such a mask is
known as a typoscope, and it acts as a spatial filter, reducing the power
in the periodic pattern. The individuals who report improvements in
clarity with a typoscope are generally those who report many
distortions in patterns of stripes.43 Indeed the algorithm by Penacchio
and Wilkins31 which predicted ratings of discomfort from images has
now been applied to text, and predicts the adjustments people make
when selecting fonts and spacing in electronic ebooks. They choose
typographic parameters that make the text more like images from
nature.44
Figure 8.7. Left: Lines of words. Centre: The Fourier amplitude spectrum of
the image (averaged over orientation). Right: The image has been blurred
to show the mid-range spatial frequency components.
There are stripes not only from the lines of text but in the vertical
strokes of letters. Words such as mum are more striped than other
Figure 8.8. The horizontal autocorrelation of the words mum and over.
Vision, Reading Difficulties and Visual Stress
222
words such as over. The stripes can be measured using a mathematical
technique called autocorrelation. Imagine two identical images of a
word reproduced in black ink on overhead transparencies, and placed
in register on the surface of an overhead projector. As one
transparency is moved horizontally across the other, the overall light
transmitted through the combined transparencies decreases initially
but then increases as one letter stroke is superimposed on its
neighbour. The way the light transmitted varies with the relative
horizontal position of the two transparencies resembles the horizontal
autocorrelation function, shown for the words mum and over in Figure
8.8.45
The first peak in this function provides a measure of the similarity
in shape between neighbouring letter strokes within a word. The height
of the initial peak not only predicts the appearance of a word as
“striped” but also, more importantly, predicts the speed with which the
word can be read: words that are “striped” take longer to read, even by
fluent readers.45 Indeed, reading speed is increased when the stripes
are made irregular by compressing the centre of the word, thereby
distorting the font. People read the distorted text more quickly even
though they prefer the undistorted text.45
When you read, the eyes make a series of rapid jerks across the
page called saccades (Chapter 4). Following each saccade, the eyes
have to be re-aligned to reduce the error in alignment (vergence error)
and this takes time (Chapters 3 and 4). It takes longer to establish
minimal vergence error when the word has a high horizontal
autocorrelation than when the autocorrelation is low. It takes longer
because the alignment of the two eyes has then to be more precise.46
The process of re-alignment might take even longer in individuals who
have poor binocular coordination (Chapter 3).
Chapter 8 How do coloured filters work?
223
Neurological basis for the aversion to periodic
patterns
Individuals who see most distortions in periodic patterns are generally
those who experience frequent headaches.36 On days when they have
a headache, they see more illusions, up to 24 hours before headache
onset.48 If the headaches are on one side of the head, the illusions
predominate in one lateral visual field,36 suggesting a neurological
mechanism. Individuals with migraine are particularly susceptible to
the illusions, and can find the patterns very aversive; viewing the
patterns may even induce a migraine attack.
36 49
Figure 8.9. Mean number of illusions (broken lines) and probability of
epileptiform EEG activity in patients with photosensitive epilepsy (solid
lines), shown as a function of various pattern parameters indicated
schematically below the horizontal axes. The shaded bars show the
parameter range for text considered as a striped pattern. Reproduced
under STM Permissions Guidelines from Wilkins and Nimmo-Smith.45
Vision, Reading Difficulties and Visual Stress
224
Patients with migraine are not the only individuals at risk from such
patterns. Many patients with photosensitive epilepsy who are liable to
seizures from flickering light are also liable to seizures from patterns
of stripes. The patterns responsible for seizures have characteristics
similar to those that induce illusions. The broken lines in Figure 8.9
show the number of illusions as a function of line length, spatial
frequency, contrast, pattern size, and duty cycle (the proportion of one
cycle of the pattern occupied by a stripe). The solid lines show the
probability of epileptiform EEG activity in patients with photosensitive
epilepsy, indicating the likelihood of seizures. Note that the functions
are similar. The shaded areas in Figure 8.9, show the parameters of the
stripes formed by the successive lines of printed text, and as can be
seen, the parameters resemble those of aversive periodic patterns.
To summarise, un-natural periodic patterns with mid-range spatial
frequency can provoke a strong neurological response in the visual
cortex, to which patients with migraine and photosensitive epilepsy are
particularly susceptible. Text approximates such patterns, the more so
when it is small and closely spaced.
Physiological mechanisms of pattern glare
Using functional magnetic resonance imaging (fMRI), it is possible to
obtain a signal that is dependent on the use of oxygen by the brain,
known as the blood oxygenation level dependent (BOLD) response.
The BOLD response can be used to indicate the areas of the brain that
are most active in response to certain stimuli. Huang et al.50 measured
the BOLD response in the visual cortex when volunteers viewed
gratings with various spatial frequencies. Patterns with mid-range
spatial frequencies that elicit pattern glare evoked in normal
volunteers a slightly larger BOLD response than patterns with higher
and lower spatial frequencies. Huang et al. compared the response in
Chapter 8 How do coloured filters work?
225
normal volunteers with that in migraineurs. The response at mid-range
spatial frequencies was abnormally high in individuals with migraine,
consistent with their greater susceptibility to pattern glare and
perceptual distortions, noted above, see Figure 8.10.
Figure 8.10. The blood oxygen level dependent (BOLD) signal change in
normal volunteers and patients with migraine when they observed a
grating with square-wave luminance profile, shown as a function of spatial
frequency. Redrawn from Huang et al.50
The elevated response is consistent with other evidence for a
hyperneuronal response in migraine: for example, stimulation of the
brain with a strong magnetic pulse results in flashes (phosphenes) and
does so more readily in individuals with migraine than in others.51 It is
likely that in individuals with migraine the visual cortex is
hyperexcitable in some way:51 four categories of antiepileptic drugs
have been shown to prevent migraine attacks.52
Why does colour help?
Electrophysiological studies of single neurons have generally
supported the textbook model that shape and color are extracted by
Vision, Reading Difficulties and Visual Stress
226
distinct neurons in primate primary visual cortex (V1). However, it has
recently become possible to sample the functional architecture of
thousands of neurons with single-cell resolution using the expression
of a multistate green fluorescent protein (GCaMP6f) and two-photon
calcium imaging. It has become clearly apparent that single neurons
can code for both colour and orientation. Colour and orientation are
jointly organized within V1.53 This raises the possibility that varying the
colour of the visual scene can affect spatial processing directly.
Earlier in this chapter it was shown how the discomfort from a visual
image can be predicted from its spatial properties, in particular its
repetitive spatial structure, as reflected in the Fourier amplitude
spectrum or the autocorrelation. Discomfort also depends upon the
difference in colour in an image. Haigh et al.
54
showed that striped
patterns consisting of alternating colours with the same luminance
could be uncomfortable depending on the magnitude of difference in
the chromaticities of the colours. Figure 6.4 shows the CIE 1976
uniform chromaticity scale diagram. Haigh et al. found that the further
apart the colours of the stripes were in this diagram, the more
uncomfortable the pattern. Not only were the patterns more
uncomfortable, they induced a greater use of oxygen by the brain,
measured with near infrared spectroscopy.
Penacchio et al.55 have shown that similar considerations apply to
more complex images. They asked observers to rate the discomfort
they experienced from images of contemporary (non-
representational) art. They then measured the difference in
chromaticity between neighbouring pixels in the images and averaged
this for all the pixels in each image. The average differences predicted
the rated discomfort from the images. The average difference was
higher for images of art than for images of natural scenes. Once again,
the more un-natural the image the more uncomfortable it was. The
Chapter 8 How do coloured filters work?
227
difference in colour within an image predicted discomfort over and
above that predicted from the spatial aspects of the image (as reflected
in the Fourier transform discussed earlier).
This finding supports the use of coloured filters in reducing visual
discomfort. When the tinted spectacles provide a colour that is strongly
saturated the effect will typically be to reduce the average chromaticity
differences in the scene by restricting the gamut of the available
colours. Although this shows that coloured filters may reduce
discomfort, it does not explain why different individuals benefit from
different colours. There are many possible reasons for the individual
differences. One possibility worth considering is adaptation to
fluorescent lighting.
School lighting
Until recently, the most common form of lighting in schools was
fluorescent. A fluorescent lamp has two heated electrodes at each end
of a glass tube that encloses a gas (often mercury vapour at low
pressure). When a voltage is applied between the two electrodes a
discharge is created in the gas. The electromagnetic energy from the
discharge is partly in the ultraviolet part of the spectrum, but the
ultraviolet component is converted to visible light by a coating of
phosphor on the surface of the tube, creating white light. When the
electrodes are connected directly to the alternating current supply two
discharges occur with each cycle of the supply as the current flows in
one direction then in the other. As a result, the light flickers 100 times
per second (or at 120Hz in North America). Some phosphors exhibit
persistence, that is, they continue to emit light for a short period after
excitation by the gas discharge. The resulting light from the lamp
therefore varies continually not only in luminance but also in spectral
composition.
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228
In the lamps manufactured in the 1990’s the phosphor was often a
halophosphate, which absorbed the ultraviolet light and emitted a
long-wavelength orange light. The lamps varied continually from bright
white to dim orange. Later, shorter persistence television phosphors
were introduced and the spectral composition changed. Although the
flicker is too rapid to be seen (at least by most people), it can readily be
detected by their retinal cells, as demonstrated using the
electroretinogram in normal observers by Berman.56 With all phosphor
types the 100Hz variation in energy captured by the long, and medium
wavelength cones was usually 20-30% and for short wavelength cones
was far greater (see Table 1 in reference 57).
100Hz flicker from fluorescent lighting affects the size of the
movements the eyes make across text58, and 120Hz flicker is known to
impair visual performance and visual comfort.59 In 1989 the flicker was
shown to cause headaches in office workers60 and this encouraged a
move to a more efficient form of fluorescent lighting in which electronic
circuitry was used to create discharges at frequencies greater than
20kHz. Despite these improvements, a survey of school classrooms in
2008 found that 80% of classrooms continued to be lit with 100Hz
lighting, and at lighting levels that were often excessively bright.61
Coloured overlays will reduce glare from the page simply by reducing
the luminance to comfortable levels. They will also reduce the rapid
variation in chromaticity and therefore any effect such variation may
have. Exposure to continuous fluctuation in chromaticity might be
expected to adapt the early colour channels, making the observer less
sensitive to the fluctuation. The individual differences in therapeutic
colour might reflect individual differences in exposure to fluorescent
lighting and differences in the nature of the phosphors used in the
lamps. In India, Srinivasan et al. compared the effect of coloured
overlays in classrooms lit by natural light with that when fluorescent
lighting was used (personal communication). The overlays increased
Chapter 8 How do coloured filters work?
229