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Figure 4.2. Schematic classification of ocular dominance. Classifications are in normal text in boxes and the text at the bottom,
in italics, provide examples of specific tests. Reproduced with permission from Evans (2001) Dyslexia and Vision. Wiley,
Chichester.
Vision, Reading Difficulties and Visual Stress
88
Is abnormal ocular dominance associated with reading
difficulty?
Annett argues that, depending on the dyslexia sub-type, there may
be an atypical handedness in dyslexia.62 There have been many
theories over the years about possible relationships between the
results of tests of dominance and reading problems.13 Most of these
theories have not stood the test of time, nor indeed the test of
evidence-based practice. In particular, early interest in sighting
dominance and crossed dominance (when the dominant eye and hand
are on opposite sides) in dyslexia proved to be an unproductive area
of research.63
A different measure of ocular dominance, the Dunlop Test, became
popular in some eye clinics in the 1980s and 1990s. The test assesses
a form of motor ocular dominance in which the eyes are forced to
diverge (turn outwards). The patient views a picture of a house, most
of which is seen by both eyes but with two features, each seen only by
one eye. As the person’s eyes are forced to diverge, a point is reached
when the unique feature visible to one eye appears to move, signifying
a fixation disparity (see Chapter 3). When one eye develops a fixation
disparity, the other eye maintains perfect alignment on the target and
is classified as the dominant eye, for this motor task. The test attempts
to measure the stability of motor ocular dominance by repeating the
test 10 times and seeing how often one eye is found to be dominant. If
one eye was consistently dominant for 8 or more out of 10 trials, there
was said to be a fixed referent (dominant) eye.64 However, Dunlop test
results can be unreliable65 and may not be abnormal in dyslexia;
60 66 67
which may explain why this test is now used very rarely, if at all. The
interested reader is referred to previous reviews.
13 68
A study that
Chapter 4 Controversial vision therapies
89
evaluated a range of different tests of ocular dominance found no
relationship between reading performance and dominance stability.60
Stein and colleagues argued that the Dunlop test detects poor
binocular control69 or binocular instability.
70 71
The more reliable tests
for binocular instability that were described in Chapter 3 would seem
more appropriate than the Dunlop test. It was hypothesised that an
abnormal Dunlop test result originated from abnormalities in the
brain’s right posterior parietal lobe,69 although this hypothesis was not
supported by research with a line bisection task,72 which is likely to be
affected by lesions of the right parietal cortex.73
The Dunlop test was originally adopted for dyslexia research
because sighting tests were thought to be inappropriate.
74 75
It was
stressed that the Dunlop test would be expected to give a different
result to tests of sighting dominance. Therefore, it is perhaps surprising
that a recent paper reviewed research on the Dunlop test and decided
to use a test of sighting dominance in its place.76
Can abnormal ocular dominance cause reading difficulty?
There is some evidence that an abnormal response on the Dunlop
test is associated with an increased prevalence of the types of reading
and spelling errors that indicate visual confusions,
70 77 78
perhaps
reflecting binocular instability (see Chapter 3). Whilst such an
association does not prove causality, it is perhaps indirect evidence
that binocular instability may sometimes play a causal role (see
Chapter 3).
Detection of abnormal ocular dominance
Sighting tests of ocular dominance are no longer thought to be
relevant to reading skills.63 As noted above, the Dunlop test is no longer
widely used and more conventional tests for binocular instability
Vision, Reading Difficulties and Visual Stress
90
(Chapter 3) would seem to be more appropriate methods of detecting
binocular instability in children with reading difficulty.66
Treatment of abnormal ocular dominance
It has been argued that an unstable response on the Dunlop test
should be treated with monocular occlusion (a patch worn over one
eye when reading)
64 71
although other research raised doubts over this
approach.65 If the Dunlop test detects binocular instability (albeit
unreliably), occlusion should be helpful. However, the more
conventional treatments for binocular instability discussed in Chapter
3 might be more appropriate.
Behavioural Optometry
Background
Behavioural optometry is commonly practised in the USA, but in the
UK, it is a relatively small controversial sub-speciality of optometry.
Behavioural optometrists provide specialist services to people with
reading difficulties.79 These often involve intensive vision therapy
carried out by trained vision therapists. Some vision therapy
programmes are offered by vision therapists independently of
optometrists.80
The role of behavioural optometry in reading difficulties
One of the underlying assumptions of behavioural optometry is that
a young child’s visual system and visual perception needs to be ready
for learning to read and, if not, should be trained to acquire readiness.
However, an early study questioned whether visual perceptual skills
are essential to academic achievement and noted that much research
on this topic has failed to control for the effect of intelligence (IQ; see
Chapter 4 Controversial vision therapies
91
Chapter 12).81 This view is supported by other authors,
82 83
and it
continues to be the case that studies detecting a correlation between
visual perceptual skills or visual information processing and academic
performance tend to be those that have not controlled for potentially
important variables.84 These may include age, sex, opportunity to
learn, socio-economic background, IQ and ADHD. Also, the results of
tests of visual perception (e.g., visual-motor integration) can be
influenced by basic optometric functions such as visual acuity.85
Behavioural optometrists often carry out extensive testing of visual
perceptual skills, and sometimes of auditory perceptual skills. Typically,
these tests have sub-tests (e.g., visual spatial, visual motor) and each
of these divisions is broken down further (e.g., one sub-system under
visual motor is the extent to which the person confuses right and left
directions), with the sub-systems sometimes being broken down still
further. It seems to be assumed that this reductionist approach reveals
the building blocks of visual perception, as if this occurs in a simple,
“bottom-up”, hierarchical way. However, this notion is not supported
by the evidence, which instead indicates a reciprocal relationship
between perceptual (bottom-up) and cognitive (top-down)
processing.86 An early review that combined several studies (meta-
analysis) concluded, “perceptual-motor training is not effective”.83
Another review, that was more positive, noted that such therapy
should complement and not replace normal educational/reading
interventions.87
A review by Shute in 1991 noted “the evidence that perceptual skills
training really does promote learning is not strong”.88 Shute also noted
that if the relationship between perception and cognition (e.g., reading)
is reciprocal: it is as valid to train the reading as the perception. A major
review on dyslexia in 2009, the Rose report, concluded that there is a
well-established evidence base supporting systematic teaching that
Vision, Reading Difficulties and Visual Stress
92
prioritises phonological skills as an effective intervention for reading
and writing.89 Since training reading, not surprisingly, helps reading
difficulties and also trains low level perceptual skills, this would seem
to be a more effective use of a child’s time than attempting to train the
low level visual skills.
Some behavioural optometrists assess and retrain primitive
reflexes, arguing this helps people with specific learning difficulties.
Such interventions (see later) have also been advocated by other
authors, although there are differences of approach.90
The UK College of Optometrists commissioned a review of
behavioural optometry in 2000, which concluded that it was ‘unlikely
that, at present, behavioural optometry can satisfy evidence-based
scrutiny; indeed there must be concern that groups of optometrists
following idiosyncratic management strategies within areas
traditionally associated with other professions might hinder the
credibility and development of optometry as a whole’.91
The College of Optometrists commissioned an updated review,
published in 2008, which reached similar conclusions: ‘the continued
absence of rigorous scientific evidence to support behavioural
management approaches, and the paucity of controlled trials in
particular, represents a major challenge to the credibility of the theory
and practice of behavioural optometry’.79 This review noted that there
is evidence supporting three claims made by behavioural optometrists:
the treatment of convergence insufficiency with eye exercises, the use
of yoked prisms in neurological patients, and in vision rehabilitation
after brain disease/injury. It is possible that some of the other claims
made by behavioural optometrists will be one day be validated by well-
controlled evidence-based research, but at the moment the present
authors do not find the evidence compelling.
Chapter 4 Controversial vision therapies
93
Balance & cerebellar deficit hypothesis
Background
Over the years, several theories have been proposed suggesting
that dyslexia is associated with balance problems or cerebellar
dysfunction.13 An example of these is the Dore or DDAT regimen, which
was a commercial system involving intensive exercise of bodily
movements and eye movements that became insolvent in 2008.
Are balance or cerebellar abnormalities associated with
reading difficulty?
A few studies with small subject numbers found evidence of
cerebellar abnormalities in dyslexia based on anatomic,92 metabolic,93
brain activation,94 and psychophysical
95 96
data. A large but non-
controlled study suggested that cerebellar abnormalities may be
associated with specific learning difficulties.97 This is at variance with a
detailed series of case studies98 that do not support the hypothesis that
balance or cerebellar deficits are associated with reading difficulty.
Developmental coordination disorder (dyspraxia), which is believed to
be associated with cerebellar dysfunction, can be associated with
dyslexia.99 Bucci and colleagues showed that vergence input is
important in controlling postural stability
100
and it therefore seems
possible that binocular instability could contribute to the balance
problems that have been attributed to cerebellar dysfunction. This
team later speculated that vergence dysfunction could be due to
immaturity in cortical structures and that eye movement anomalies
suggest a deficit in allocating visual attention.
101
Stein notes that the magnocellular system projects strongly to the
cerebellum, controlling its timing.
102
He argues, cerebellar activity “is
much reduced in dyslexics”.
Vision, Reading Difficulties and Visual Stress
94
Can balance or cerebellar dysfunction cause reading difficulty?
Although Ramus and colleagues found partial support for impaired
motor skills in some dyslexic children, these authors did not support
the hypothesis that cerebellar dysfunction is a cause of reading
impairment.
103
Ramus and colleagues note that poor performance in
motor tasks by some dyslexic children is at least partly attributable to
co-morbidity with other developmental disorders, such as ADHD and
developmental co-ordination disorder (dyspraxia).
Treatment of balance or cerebellar dysfunction
A controversial study by Reynolds and colleagues supported the
notion that an exercise programme (DDAT or Dore treatment) to treat
cerebellar function was helpful,
104
but this study was robustly criticised
and largely discredited.
105-109
Another controversial view is that persistent primitive (primary)
reflexes, which may be linked to balance, motor co-ordination,
110
and
ADHD,
111
can disrupt the development of reading skills.
112
An
intervention programme based on replicating the movements
generated by the primary-reflex system during foetal and neonatal life
was found to normalise these reflexes, and was associated with a
significant improvement in reading performance.
112
The sample size in
this study was modest, with 20 participants in each group and this
finding would benefit from replication with larger group sizes. Amongst
practitioners who advocate training primitive reflexes, there is
controversy over the best approach.90
Other controversial interventions
The Cellfield programme is a system involving exercises and tasks
that are hypothesised to have a therapeutic effect on dyslexia. The
Chapter 4 Controversial vision therapies
95
system is available commercially at a variety of treatment centres in
several countries. Despite the programme being developed more than
10 years ago, a literature search by the present authors in February
2024 revealed no randomised controlled trials (RCTs). RCTs are
generally considered to be necessary to indicate that interventions are
more effective than placebos (Chapter 12). For example, a study of the
Cellfield programme by Prideaux and colleagues including no control
group and no control treatment,
113
so any improvements after the
Cellfield treatment could result from the extra time and attention given
to the children. If this was the case, then extra teaching might have
been a better use of their time. One study that did include a control
group was not masked (blinded) and the intervention received by the
control group (a computer game) is unlikely to control for placebo
effects.
114
A review in 2015 also concluded that the available research
evidence on the Cellfield approach is “not sufficient”. This review was
also critical of other training systems, including the Dore Programme
(see above) and Brain Gym (noting it has been described as
pseudoscience).
109
In 2017, a paper by Le Floch and Ropars
115
suggested that reading
in dyslexia may be improved by light that is flickering quite rapidly
(above 70 Hz). Following this paper, special spectacles (Lexilens)
116
or
lamps (Lexilamp) were marketed. However, a report
117
and subsequent
research
118
does not support the claim that flicker helps reading in
dyslexia or individuals who have visual stress.
119
Flicker, even when too
rapid to be readily apparent, is known to interfere with eye movement
control.
120
Vision, Reading Difficulties and Visual Stress
96
pursuit eye movements or atypical ocular dominance are causes of
reading difficulties. The evidence-base is also rather weak for
treatments based on behavioural optometry and training of cerebellar
function, and Cellfield.
References
1. CITT-ART Investigator Group. Treatment of Symptomatic Convergence
Insufficiency in Children Enrolled in the Convergence Insufficiency
Treatment Trial-Attention & Reading Trial: A Randomized Clinical Trial.
Optom Vis Sci. 2019;96(11):825-35. doi: 10.1097/opx.0000000000001443.
2. Scheiman M, Denton C, Borsting E, Kulp M, Cotter S, Mitchell G, et al.
Effect of Vergence/Accommodative Therapy on Reading in Children with
Convergence Insufficiency: A Randomized Clinical Trial CITT-ART
Investigator Group. Optometry and Vision Science. 2019;96(11):836-49.
doi: 10.1097/OPX.0000000000001442.
3. College of Optometrists in Vision Development: CITT-ART Study.
https://www.covd.org/page/cittart (2014). Accessed 23/12/2021 2021.
4. Evans BJW. Pickwell's Binocular Vision Anomalies. Sixth ed. Philadelphia:
Elsevier; 2021.
5. Simmons K. Oculomotor dysfunction: where's the evidence? Optometry
Visual Performance. 2017;5(5):198-202.
6. Vidyasagar TR. Neural underpinnings of dyslexia as a disorder of visuospatial attention. Clin Exp Optom. 2004;87:4-10.
7. Evans BJW, Drasdo N. Review of ophthalmic factors in dyslexia. Ophthal
Physiol Opt. 1990;10:123-32.
8. Taylor EA. Physiological defects and eye discomfort among
schoolchildren. Controlled reading: a correlation of diagnostic, teaching,
and corrective techniques. Chicago: University of Chicago Press; 1937. p.
161-201.
9. Bonifacci P, Tobia V, Sansavini A, Guarini A. Eye-Movements in a Text
Reading Task: A Comparison of Preterm Children, Children with Dyslexia
and Typical Readers. Brain Sci. 2023;13(3). doi:
10.3390/brainsci13030425.
10. Al Dahhan N, Georgiou GK, Hung R, Munoz D, Parrila R, Kirby JR. Eye
movements of university students with and without reading difficulties
during naming speed tasks. Ann Dyslexia. 2014;64(2):137-50. doi:
10.1007/s11881-013-0090-z.
11. Vagge A, Cavanna M, Traverso CE, Lester M. Evaluation of ocular
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doi: 10.1007/s11881-015-0098-7.
Summary
Chapter 4 Controversial vision therapies
97
In summary, there is no good evidence that abnormal saccadic or