Добавил:
Sekretar
kiopkiopkiop18@yandex.ru
t.me/Prokururor I Вовсе не секретарь, но почту проверяю
Опубликованный материал нарушает ваши авторские права? Сообщите нам.
Вуз:
Предмет:
Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_5253_Библиотеки_им_академика_М_И_Перельмана.pdf
colours preferentially absorb long rather than short wavelength light.
Even if scatter of light within the eye is responsible for a reduction in
image contrast, it is not immediately apparent why such a contrast
reduction would contribute to rather than remove the perceptual
instability that precision tints reduce.
Figure 8.1 Chromaticities of 1000 Precision Tints.
Retinal mechanisms?
Individuals who benefit from coloured filters generally have no
retinal abnormality on examination of the back of the eye3. They are
just as likely as anyone else to have a colour vision deficiency, at least
on the basis of conventional clinical tests.3 Subtle impairments of S-
cone function have, however, been discovered in individuals with
migraine4 and migraine is relatively common among individuals who
benefit from coloured filters.5
Vision, Reading Difficulties and Visual Stress
210
Typically, humans have three types of cone receptors in the retina,
known as L, M, and S cones (because they respond to, respectively,
long, medium, and short wavelength light; Chapter 6). Individuals differ
considerably as regards the relative proportion of L and M cones they
possess.6 Although this might appear to offer an explanation for the
individual differences in optimal tint,7 there are indications to the
contrary.8 Individuals with relatively few M cones and those with
relatively few L cones give exactly comparable responses when mixing
red and green light to produce unique yellow.6 Evidently, the visual
system has gain mechanisms that compensate for the differences in L
and M ratios. The gain mechanisms may ultimately explain the
individual differences in optimal tint, but there is little or no evidence
as yet.
The three classes of cone and the rods are not the only
photoreceptors in the eye. A class of melanopsin containing retinal
ganglion cells is intrinsically photoreceptive, detecting the amount of
light energy reaching the retina, and contributing to the pupillary light
reflex.
9 10
The sensitivity of these melanopsin-containing ganglion cells
peaks at 482nm, which might mean that coloured lenses preferentially
transmitting short wavelength light alter the size of the pupil, with
consequences for retinal image quality. Allen et al.11 measured pupil
size in participants with and without visual stress under double masked
conditions. There were no differences in pupil size despite differences
in accommodative lag. (Accommodative lag is a tendency to focus the
eyes less than is optimum for clear vision and is explained in Chapter
3).
Ocular accommodation?
Short wavelength light is focused on a different plane to long-
wavelength light and in all eyes, this causes an optical imperfection
Chapter 8 How do coloured filters work?
211
called chromatic aberration. Coloured tints affect the spectral
composition of light entering the eye, and might therefore be expected
to affect chromatic aberration, and with it, accommodation (Chapter
3). However, research reveals no relationship between the type of
refractive errors (e.g., long-sighted or short-sighted) and the
chromaticity of the chosen lenses,
12 13
and the results of clinical tests of
accommodation do not seem to provide an explanation for visual
stress.
12–14
Nevertheless, abnormally large fluctuations of
accommodation have been recorded in patients who use coloured
lenses.3 Tinted lenses reduce the fluctuations, but so do spectrally
neutral grey lenses of equivalent photopic transmission. By way of
contrast, in six individuals who wore Irlen lenses, Ciuffreda found that
tinted Irlen lenses increased the variability of the accommodative
response.15
In a more rigorous experiment, under double-masked conditions,
Allen et al.
11
compared two groups of young adult participants, one
group with and one without visual stress. There were no significant
differences between the groups in accommodative fluctuations, but
the study did detect differences in accommodative lag (see Chapter 3).
The lag was greater in the group with visual stress, and was reduced
when the stimulus background had the colour of the chosen overlay.
The magnitudes of the lag and of the refractive error were related to
one another, but not, however, to the hue angle of the colour. In other
words, once again there was no indication that the effect of the colour
was such as to reduce the image blur from any residual refractive error.
It is possible that changes in accommodation result from visual stress
rather than the other way around, since the perceptual distortions of
visual stress could interfere with the feedback pathway that controls
accommodation.
Vision, Reading Difficulties and Visual Stress
212
Binocular vision anomalies?
The binocular co-ordination of patients with visual stress
sometimes exhibits subtle anomalies (see Chapter 3), but these are by
no means always found and there are patients with perfectly normal
binocular function who benefit from tints,
12 13 16
and have the
symptoms that are typical of visual stress. The binocular coordination
during reading has yet to be fully explored (see Chapter 3),17 partly
because the vergence error is small and difficult to measure. In any
event, the relationship between binocular instability and benefit from
tints does not appear to be causal. As discussed in Chapter 4, reading
places unusual demands upon the vergence system, which is
responsible for re-aligning the eyes following saccades. For this reason,
it may be anticipated that reading will be slower in individuals with
poor binocular coordination.
Magnocellular-dorsal (M-D) deficit?
The distinction between the characteristics of the magnocellular-
dorsal (M-D) and parvocellular-ventral (P-V) divisions of the visual
system were reviewed in Chapter 5. It is an over-simplification to think
of these systems as distinct. Their functions overlap and there are
numerous inter-connections between the two pathways. There is also
a third pathway, the koniocellular pathway, that is comparatively little
studied, but may turn out to be of considerable importance.
As noted in Chapter 5, children with signs of an M-D deficit are more
likely than other children to make reading errors that are suggestive of
visual confusion, and they are less likely to be aware of the precise
position of letters in a word. It remains to be seen whether the M-D
visual deficit directly causes these visual confusions, whether the M-D
deficit causes binocular instability resulting in the confusions, or
Chapter 8 How do coloured filters work?
213
whether, indeed, both confusions and binocular instability are the
result of some other causal link. It might seem reasonable to suppose
that dyslexic individuals with signs of the M-D deficit tend to be those
with the dyseidectic form of dyslexia, which is characterised by the types
of reading errors that one would expect to result from visual
confusions. Surprisingly, two studies found this is not the case,
although both studies had small sample size.
8 18
Several publications have offered explanations for the
improvements in reading speed with coloured filters in terms of M-D
deficits.
19–21
These explanations have been questioned, because
studies (with one exception22) have failed to show M-D deficits in
children who are subject to perceptual distortion of text and benefit
from coloured filters.
13 23-,25
An in-depth study of 22 individuals with
dyslexia found two individuals with evidence of a M-D deficit, and six
with visual stress. There was no overlap between these two groups.
23
The one study that has found a correlation between visual stress and
M-D function studied an unselected sample (not selected, for example,
for reading difficulties) and considered their findings could be
explained by their M-D test (coherent motion) causing symptoms of
visual stress.22
Among adults26 and children27 selected as poor readers, the
proportion who read more quickly with overlays is greater than in the
general population. According to a contemporary criterion, which was
independently arrived at by three separate studies, visual stress affects
approximately 20% of people with dyslexia.28 It also affects a
substantial proportion of individuals with a range of neurological
deficits reviewed in Chapter 11.
When is vision stressful?
Vision may be stressful when the neurological processes that
Vision, Reading Difficulties and Visual Stress
214
underlie vision are not functioning appropriately. This can occur under
a wide variety of circumstances, for example: (1) when the eyes are
inappropriately aligned (as in some binocular vision anomalies), (2)
when the visual image is such as to provide insufficient or
inappropriate vergence information (as when using 3-D viewers or 3-D
displays), (3) when the visual image is impoverished (as when the
illumination is low or glaringly high), and (4) when the visual image itself
gives rise to a strong neurological response. As will now be described,
some individuals are unusually susceptible to the overly strong
neurological response that results from certain visual stimuli.
At the most basic level, vision can be considered as an ability to
perceive striped patterns, ranging from coarse stripes (low spatial
frequency) to fine stripes (high spatial frequency). The contrast
sensitivity function measures the ability of observers to see faint
stripes, and this ability is greatest at mid-range spatial frequencies
when about three cycles of the pattern occupy one degree subtended
at the eyes.
Fourier analysis of an image is a way of deconstructing the image
into its component spatial frequencies. Consider the image of the
flower in Figure 8.2a. Part of the image in the rectangular section
shown is reproduced below in Figure 8.2b. The luminance profile of this
section is represented as a graph in the top curve in Figure 8.2c. The
profile can be constructed by adding the Fourier components shown in
remaining curves in the figure. Any image can be decomposed into
simpler components that add together to make up the image. The
contrast (amplitude) of the Fourier components decreases as their
spatial frequency increases. In images from nature the amplitude of
the Fourier components is approximately proportional to their
wavelength. In other words, the amplitude is approximately
proportional to the reciprocal of spatial frequency, 1/f. The amplitude
Chapter 8 How do coloured filters work?
215
is shown as a function of spatial frequency in Figure 8.2d and is a
straight line when plotted as a graph with log-log scales. Images from
nature have approximately linear functions with slopes between -0.8
and -1.5.29
Figure 8.2. (a) Image of a flower including a faint horizontal bar showing a
section enlarged in (b). (c) The luminance profile of the enlarged section (top
curve) and its decomposition into Fourier components (curves beneath). (d)
The amplitude of these components expressed as a function of their spatial
frequency (solid curve) and the way in which the components can change
when the image is uncomfortable (broken line).
Fernandez and Wilkins29 obtained ratings of the discomfort that
observers describe when viewing a wide variety of complex images
derived from works of contemporary non-representational art,
photographs of rural and urban scenes and images of filtered noise.
They found that whereas the amplitude spectrum of the comfortable
Vision, Reading Difficulties and Visual Stress
216
images had a linear function similar to the solid line in Figure 8.2d, the
uncomfortable images had a curvilinear function similar to the broken
line in Figure 8.2d, with a relatively greater amplitude at mid-range
spatial frequencies, where the visual system as a whole is most
sensitive. Figure 8.3 left panel, shows for one of their studies of
contemporary art, the ratio of amplitude in images rated as
uncomfortable relative to that for images rated as comfortable. The
ratio is shown as a function of spatial frequency. Uncomfortable
images had a greater Fourier amplitude than comfortable images at
mid-range spatial frequencies close to 3 cycles per degree.
The correlation between discomfort and contrast energy is shown
in the right panel of Figure 8.3 separately for individuals at near and
distant viewing distances. In Figure 8.4 (solid line), the average of
similar functions for four studies is shown. Figure 8.4 (fine line) shows
the contrast sensitivity function for sine-wave gratings of equivalent
subtense. The similarity in spatial frequency tuning suggests that
images are uncomfortable when they have an excess of contrast
energy at those spatial frequencies to which the visual system is
generally most sensitive. These findings have been supported by
Juricevic et al.30 Penacchio and Wilkins31 argued that it was preferable
to take into account the orientation of the contrast energy because
patterns were generally more uncomfortable when the energy was
concentrated in one orientation. They fitted a cone with slope 1/f to the
two-dimensional Fourier transform and showed that the degree of fit
explained more than 25% of the variance in judgments of discomfort –
the more unnatural the image, the greater the discomfort.
Penacchio et al.32 have presented images to a computation model
of the visual cortex originally developed by Li Zhaoping. They found
three clear markers of aversive images: a larger overall activation, a
less sparse response with more “neurons” active, and a more
Chapter 8 How do coloured filters work?
217
unbalanced distribution of activity across spatial orientations.
Figure 8.3. Left: Ratio of the amplitude for uncomfortable and comfortable
images. Right: Correlation between ratings of discomfort and Fourier
amplitude. Reproduced under STM Permissions Guidelines from Fernandez
and Wilkins29.
Figure 8.4. Bold line: Correlation between Fourier amplitude and
discomfort, averaged over four studies. Fine line: contrast sensitivity
Vision, Reading Difficulties and Visual Stress
218
function for gratings of equivalent subtense. Dotted line: rated aversion in
response to a grating with square-wave luminance profile. Broken line:
Chapter 8 How do coloured filters work?
219
Probability of paroxysmal epileptiform EEG activity in patients with
photosensitive epilepsy, in response to a similar grating. Reproduced under
STM Permissions Guidelines from Fernandez and Wilkins29.
COVER THIS PATTERN IF YOU HAVE MIGRAINE OR EPILEPSY.
Figure 8.5. A grating with square-wave luminance profile. From a reading
distance of 40cm the grating has a spatial frequency close to that at which
aversion is maximal.
COVER THIS PATTERN IF YOU HAVE MIGRAINE OR EPILEPSY.