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
The M-D deficit has also been linked with visuospatial attention,
127
which might provide a mechanism or mechanisms by which the M-D
deficit could contribute to reading difficulty. This is discussed below.
Detection of a M-D deficit
The tests commonly used in research to detect a M-D deficit are
summarised at the beginning of this chapter and in Table 5.2. In recent
years, most studies have relied on tests of motion coherence
perception. These tests are not available to most eyecare practitioners.
The M-D deficit may be associated with impaired results on visual
field tests that use the frequency-doubling technique (FDT; e.g. the
Humphrey FDT perimeter),
11 54 74
although these effects seem to be
subtle and unlikely to confound the use of FDT perimetry for the
detection of glaucoma.52 Pammer and Wheatley argued that the FDT
may have potential as a diagnostic tool for dyslexia,11 although their
data showed a considerable overlap in the overall FDT thresholds of
good readers and those of disabled readers. It is interesting, the FDT
result only accounts for about 24% of the variance in motion
coherence,11 which is a measure that has been used more widely to
assess M-D function in the research literature (Table 5.2).
Some authors have assumed they are studying a M-D deficit when
instead they have investigated different functions that are only
hypothesised to be linked to a M-D deficit, such as binocular
coordination.
128
Treatment of a M-D deficit
In recent years, there have been preliminary attempts to research
treatments for a M-D deficit. One small study suggested, ‘temporal
vision therapy’ may bring about an improvement in measures of M-D
Chapter 5 The magnocellular-dorsal (M-D) deficit and associated theories
129
function and reading skills,
129
although there was no control treatment
so this improvement could have resulted from a placebo effect.
Lawton studied 58 dyslexic children, comparing three treatments,
designed to: (1) target timing of auditory and (2) visual pathways and
(3) linguistic word building.
126
The treatment designed to stimulate the
M-D visual pathway involved direction discrimination of moving
patterns and this was associated with improved functions, including
reading. The other treatments were not beneficial, and the results were
taken as supporting the hypothesis that faulty timing in synchronizing
the activity of the M-D system with the parvocellular visual pathway is
a fundamental cause of dyslexia, as opposed to a phonological
deficit.
126
However, some experimental details raise doubts over the
results: there were only six participants in the auditory timing group
and these all originated from one school (unlike the participants in the
other groups); the control condition involved researchers paying less
attention to the participants, and the sample sizes were small.
Therefore, these results are best taken as preliminary.
Gori and colleagues found that a training programme (playing card
games on a computer) designed to improve M-D function, and not
involving any auditory-phonological stimulation, leads to improved
reading skill in dyslexic adults.67 However, the control group received a
treatment (card games) that is likely to have been a poor match in
terms of credibility as a training programme, and therefore the study
may not fully have controlled for placebo effects. The researchers
argued that their findings could not be explained by improved noise
exclusion mechanisms, disconfirming the hypothesis of Sperling.
130
Gori and colleagues do not discount that auditory-phonological
processing deficits are a cause of dyslexia, but consider their research
demonstrates that a M-D deficit is an additional cause of dyslexia.67
Joo and colleagues found that participants with low motion
Vision, Reading Difficulties and Visual Stress
130
sensitivity show an improvement in motion sensitivity with training in
a motor discrimination task, but a similar improvement occurs in
participants who initially had high motion sensitivity.68 They conclude
that the improvement with training reflects attention and task learning
rather than changes in sensory processing. Qian and Bi found that a
visual-motor task improved M-D function, but the lack of any
intervention for their control group means this finding could be
attributable to a placebo effect.
131
Another study assessing treatment
of an M-D deficit was noted earlier in this chapter to have suffered from
significant limitations.
120
In particular, the experimental treatment
appeared to give participants more individualised attention.
Tulloch and Pammer designed a computer game that they
hypothesised tapped M-D function (although they did not include a
conventional test of M-D function).
132
Performance at the computer
game was correlated with reading rate, even after controlling for IQ.
The results were interpreted as supporting the visuo-spatial attention
deficit hypothesis and the further development of portable games,
potentially for treatment.
132
An RCT by Ebrahimi and colleagues evaluated 12 sessions (30-40
minutes two days a week) of a “magnocellular-based visual-motion
training” programme.79 This comprised a coherent dot motion task,
saccadic eye movement training, computerised digit counting, and dot
counting. The control group received the same pattern of sessions
when they played a non-training computerised game. The
experimental treatment was associated with improved visual spatial
attention and reading abilities. It seems likely that the control
treatment, a passive task, may have engendered less of a placebo
effect than the experimental treatment.
The research studies reviewed in this section all have limitations,
leading to the conclusion that there is no strong evidence for the
Chapter 5 The magnocellular-dorsal (M-D) deficit and associated theories
131
efficacy of any of these treatments. In any event, there is still no
overwhelming evidence that a M-D deficit contributes causally to
reading difficulties. Even if there is a causal relationship then it may be
that this is mediated indirectly via some other variable (e.g., binocular
instability or a visuospatial attention deficit; see below) which might
respond better to treatment.
Stein has contended that the functioning of M-D cells is dependent
on one omega 3 long-chain fatty acid, Docosahexaenoic acid (DHA).92
He reviewed research indicating dietary supplements that supply this
fatty acid improve reading and reduce antisocial behaviour.
Visuospatial attention
Background
The perceptual span, visual attention span, or form resolving field
are terms used to refer to the size of the area around the fixation point
within which letters are recognised. This has been described as a
spatial attentional filter and is extended to the right in people who read
text from left to right, and to the left in people who habitually read
languages written in the opposite direction.
133
As described below, the
M-D pathway seems to play an important role in visual spatial attention
and therefore this topic is included in this chapter.
The area of the visual scene to which the patient selectively attends
has been described as the spotlight of selective visual attention.
134 135
The use of the word attention in this spatial context is quite different to
its use in the condition attention deficit hyperactivity disorder (Chapter
1), which relates more to the ability to maintain attention over time.
However, there is also some evidence that dyslexic adults have
prolonged attentional dwell time
136 137
and difficulty rapidly allocating
Vision, Reading Difficulties and Visual Stress
132
attention over time.
138
In a general sense, attention has been described
as the catalyst that links perception with cognition.61
Is a deficit of visuospatial attention associated with reading
difficulty?
For many years, there has been a debate as to whether visuospatial
attention is abnormal in dyslexia,
139 140
with some arguing that
perceptual span does not limit reading performance.
141
A previous
review suggested the span of visual attention may be atypical in
dyslexia.85 There is growing evidence of abnormalities in visuospatial
attention in people with reading problems,
91 135 138 142-155
and this seems
to be similar across the various subtypes of dyslexia.
156
van der Kleij
and colleagues found that visual attention span (together with
phonological awareness and verbal short-term memory) is a predictor
of word reading accuracy in normal readers. These authors stated that
they found no evidence of a causal relationship between visual
attention span and improvement in reading fluency, although it is not
clear how they tested for causality.
152
It is often hypothesised that impaired visuospatial attention in
dyslexia results from a M-D deficit, although studies that make this
assumption typically do not test M-D function directly.
147
One
exception is a study of normally reading adults by Omtzigt and
colleagues.
127
They used colour contrast that impaired M-D activity and
compared weak-luminance conditions that impaired parvo activity. The
results supported the hypothesis that the M-D system is involved in the
identification of flanked letters and is important to the allocation of
attention.
127
An interesting paper by Liu and colleagues in 2022 used fMRI to
assess the regions of the brain activated during phonological
processing and during assessment of visual attention span (VAS) in
Chapter 5 The magnocellular-dorsal (M-D) deficit and associated theories
133
dyslexic and control children.
157
The results are consistent with the
hypothesis of VAS and phonological deficits as two dissociable causes
of reading disability, each with its own neural basis.
The research described above has studied the spatial distribution of
attention. In contrast, McLean and colleagues studied visual attention
over time, and found “no evidence for a prolonged attentional blink in
dyslexia”, despite some of the dyslexic group having signs of attention
deficit disorder.
158
In contrast, Santoni et al. found evidence of atypical
attentional sampling in dyslexia, which the researchers felt was
consistent with sluggish attentional shifting.83
Does a deficit of visuospatial attention cause reading difficulty?
A deficit of visuospatial attention may produce a perceptual
interaction of target and background elements and has been
associated with impaired visual search
135 159
and reading difficulty.
135
A
thorough study by Valdois and colleagues showed a visual attention
span deficit can occur with deficits in phoneme awareness and rapid
autonomised naming: two factors associated with reading skills. Each
of these three skills, visual attention, phoneme awareness and rapid
automatised naming was an independent predictor of reading
fluency.
154
There is some evidence that a visuospatial attention deficit
can be modified with training and that this might help reading.
160
In 1999, Evans proposed a unifying hypothesis that such a
weakened attentional spotlight might cause poor readers to become
more aware of the peripheral detail on the page, resulting in pattern
glare (Chapter 8) and the characteristic symptoms of visual stress.
161
He also suggested, this might impair the fusion lock (see Chapter 3)
resulting in binocular instability.
161
There is some limited experimental
evidence that dyslexic people have particular difficulty suppressing
distracting information in the right visual field.
149
Rima and colleagues
Vision, Reading Difficulties and Visual Stress
134
found that in good readers the visual field to the right of fixation has
an advantage, but not in dyslexics and they hypothesised that this is
related to lack of reading experience.
162
Vidyasagar hypothesised that the M-D system may be important in
controlling the attentional spotlight, although the M-D deficit was not
directly measured.
163 164
Data from Omtzigt and colleagues supports
this hypothesised link between the M-D deficit and the deficit of
visuospatial attention.
127
If this is confirmed, it could challenge Evans’
theory since people with visual stress do not seem to have a M-D
deficit,
60 99-101
at least in most studies.
103
Nonetheless, there is evidence
that visual stress is associated with impaired visual search.
101 165-169
Vidyasagar and Pammer subsequently hypothesised that a deficit of
visuo-spatial attention is the critical deficit in dyslexia, and that the
poor phonological awareness is a consequence of the visuo-spatial
deficit.
170
More recently, the same group proposed a unifying theory
involving an auditory temporal sampling framework (TSF) and the M-D
deficit.6 They speculate that the TSF causes poor sampling of speech
sounds over time, causing the phonological deficit in dyslexia, and
linking to the M-D visual deficit.6
Detection of a deficit of visuospatial attention
There is no widely supported or generally available clinical test for
detecting a deficit of visuospatial attention. It has been suggested that
poor visuo-attentional abilities in dyslexia impair eye movement
patterns during reading,
171
but a more prevalent view is that the
abnormal eye movement patterns are secondary to the language
processing difficulties (Chapter 4).
Treatment of a deficit of visuospatial attention
Training may be able to modify visual selective attention in normal
Chapter 5 The magnocellular-dorsal (M-D) deficit and associated theories
135
subjects
172
and to treat a visuospatial attention deficit.
160
Facoetti and
colleagues evaluated a treatment regimen based on visuospatial
attention and obtained encouraging results, although this preliminary
study was not double-masked.
173
Peters and colleagues carried out a systematic review of dynamic
visuo-attentional interventions.
174
Their starting point was that dyslexia
is associated both with phonological and visuo-attentional deficits.
Their review found 18 relevant studies (620 participants) and they
concluded that initial evidence indicates visuo-attentional
interventions may improve reading for at least two months.
174
As they
noted, further research with larger sample sizes and longer follow-up
is required.
Franceschini and Bertoni, in a small group of dyslexic children,
investigated the effect of commercially available action video games on
phonological decoding speed and phonological short-term memory.
175
These skills were increased only in the children whose video game
scores improved. The authors interpreted this as supporting the causal
role of visual attentional skills, although they did not measure the skills
as such. This study, and another with similar findings,
176
did not include
a control group that received a placebo therapy, so it is possible that
the results are explained by placebo effects. Subsequent research that
compared action video games with non-action video games found that
action games are associated with an improvement in phonological
decoding speed and visual search.
177
The authors attributed this to
improved attentional control, but the research did not measure
attentional control directly.
Luniewska and colleagues in 2018 compared the effects of two
types of video games on reading in dyslexic children: action video
games and non-action video games that were designed to improve
phonological awareness.
178
Both groups exhibited a similar
Vision, Reading Difficulties and Visual Stress
136
improvement in reading, phonological awareness, selective attention,
and rapid naming. However, there was a similar improvement in
reading in a control group who did not receive any video game training,
leading the authors to conclude that the improvement in the video
game groups were attributable to normal reading development or
practice at the reading tests.
Lazzaro and colleagues investigated the effect of transcranial direct
current stimulation (tDCS) of left and right temporo-parietal regions on
reading, phonological skills, visuo-spatial working memory, visuo-
spatial attention, and motion perception in a small group of dyslexic
children and adolescents.77 The researchers compared left
anodal/right cathodal with right anodal/left cathodal stimulation. The
authors claimed that “…when tDCS enhanced left neural excitability
while decreasing right neural excitability” this was associated with
improved reading associated with better motion sensitivity. However,
this result is difficult to interpret without a sham zero current
condition.
Crowding
Visual crowding describes the impaired ability to recognise objects
when presented in clutter. There is evidence that people with dyslexia
require more time to identify targets in crowded scenes and this has
been attributed to an increased crowding effect in dyslexia.
179-183
This
might be linked to the deficit of visuospatial attention, summarised as
“a deficit in visuo-spatial attention that underpins our ability to
selectively attend to individual objects in a cluttered world”.
184
Bertoni
and colleagues provided experimental evidence of a direct link
between attentional mechanisms and crowding.
182
In one study of
adults with dyslexia, a subgroup who were sensitive to crowding read
more quickly when letter, word, and line spacing were increased.
181
Pel
Chapter 5 The magnocellular-dorsal (M-D) deficit and associated theories
137
and colleagues found that a small group of dyslexic adults performed
similarly to controls in the detection of crowded targets, but needed
more time than controls.
183
Aleci and Belcastro (2020) defined “visual dyslexia” as individuals
with reading difficulty who read faster when letter spacing is
increased.
185
(The term visual dyslexia is deprecated in this book; see
Appendix 1.) They found the effects of letter spacing affected only a
minority of people with dyslexia, and suggested several mechanisms
for this effect. Based on their experimental work, the most likely of the
mechanisms they evaluated seems to be a form of asymmetric visual
crowding (spatial relationship anisotropy). Sensory visual stress
(Chapter 8) provide an alternative explanation for the increased
reading speed when text is widely spaced
185
because the striped
pattern that lines of text form will be less prominent when text is widely
spaced.
186
Manning and colleagues speculated that differences in internal
noise could be related to CDM perception and crowding in dyslexia.81
Research by Bertoni and colleagues suggests that crowding may
play a causal role in poor reading.
182
They argued that when children
play action video games this can reduce their sensitivity to visual
crowding and improve spatial attention. Although Bertoni and
colleagues provided some experimental evidence supporting this
hypothesis, small group sizes and the absence of a control treatment
means the results could be explained by placebo effects.
182
Other theories relating to visual perception
Stordy advanced the idea that dyslexia may be associated with poor
dark adaptation owing to abnormalities in visual perception with rods
which might be improved by fatty acid supplementation.
187
A thorough
Vision, Reading Difficulties and Visual Stress
138
investigation did not support this hypothesis.
188