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muscles. The term ocular motor function is a better term, although this
describes the actions of all the ocular muscles, including those involved
in vergence and accommodation, as described in Chapter 3.
Are deficits of saccadic or pursuit eye movements associated
with reading difficulty?
Reading involves saccadic eye movements, so it seems plausible
that reading difficulties might result from a problem with these
movements. It has been argued that the saccadic eye movements
during reading are special in terms of their sequential nature and the
memory demands associated with reading.6 It is certainly the case that
rare conditions that profoundly affect saccadic eye movements (e.g.,
early onset nystagmus)4 do interfere with reading, but they also impair
other everyday functions and are not linked to specific reading
difficulties.7 Saccades are fundamental to the way that animals with a
highly differentiated retina use their visual system: they are not unique
to reading.
Controversy has been fuelled by the observation first made more
than 70 years ago8 that when reading text children with reading
difficulties make more fixations, longer fixations and more regressions
than good readers.
9-11
Recent research has also found evidence of
abnormalities in saccadic control when dyslexic children read.12 The
key question is whether this simply reflects the fact that they are
searching for meaning because of their reading difficulty, described
above as “top down”.13 In other words, are abnormal saccades a cause
or an effect of poor reading? This question still has not been
unequivocally answered,
9-11
but the general consensus nowadays
seems to be that the language difficulties in dyslexia have an
immediate influence on eye movements, which may even help in the
diagnosis of dyslexia.14
Vision, Reading Difficulties and Visual Stress
78
Since eye movement parameters vary with the task it may not be
possible to compare studies that have used dissimilar experimental
conditions. For example, a recent systematic review15 combined
studies to obtain metanalyses for fixation duration, but this is
problematic unless the tasks in each experiment are matched.
In the 1980s attempts were made to address whether there is a
fundamental eye movement abnormality in dyslexia by recording eye
movements when children fixated a row of lights that were presented
in sequence. Some studies found differences between children with
dyslexia and those without reading difficulty, but most controlled
studies did not. A review of this research13 noted that most studies
controlled for IQ but did not control for attention deficit hyperactivity
disorder (ADHD), since this was not widely recognised at this time.
ADHD often co-exists with dyslexia and may be a confounding factor in
this type of research.16 It is quite likely that attention will wander during
the boring task of tracking lights, and this might be misinterpreted as
an eye movement anomaly. Poor visuo-attentional abilities may well
impair eye movement patterns when dyslexics read, as Bellochi and
colleagues have pointed out.17
The controversy about saccadic eye movements and dyslexia still
continues, with some recent studies indicating fundamental
abnormalities of saccadic control in dyslexia
18 19
or simulated reading
tasks,20 yet other studies showing no abnormality of eye movements in
dyslexia during saccadic
21-23
or simulated reading
24 25
tasks. Further
support has accumulated for the point made above, that abnormal
saccadic eye movements are secondary to deficits in linguistic
processing.
11 26 27
One study found fixation instability in some dyslexic
individuals and speculated that this could be secondary to binocular
instability.23 It also seems possible this could result from ADHD, as this
study did not specify exclusion criteria for this condition.
Chapter 4 Controversial vision therapies
79
Caldani and colleagues investigated saccadic eye movements
during reading in children with ADHD in isolation or ADHD combined
with dyslexia.28 The group with ADHD without dyslexia performed
similarly to the control group of normal readers. In contrast, the groups
with dyslexia made significantly more backward saccades both when
the dyslexia was combined with ADHD and when it was not. In a
Chinese population Zhou and colleagues found differences between
the saccadic eye movement patterns of co-occurring ADHD and
dyslexia when compared with populations with ADHD or dyslexia in
isolation.29
The growing consensus that apparent abnormalities of saccadic eye
movements in dyslexia are the result of the reading difficulty (top
down) raises the possibility that eye movement analysis by artificial
intelligence or machine learning could be useful in diagnosing dyslexia.
There is limited emerging evidence to support this notion.
30-34
The Developmental Eye Movement (DEM) Test
It is sometimes claimed that a simple digit reading test, the
Developmental Eye Movement Test (DEM; Figure 4.1), assesses
saccadic or “oculomotor” function.35 Of course, reading digits does
involve saccades, but so does watching television or driving a car, and
neither of these are advocated as tests of saccadic function. Convincing
research has compared the DEM test result with objective recordings
of eye movements, revealing that the DEM results do not reflect eye
movement control.
36 37
Poor DEM scores do not mean that reading
difficulties result from poor eye movement control.
38 39
Interestingly,
these recent findings might have been predicted from a thorough
study of eye movements during digit reading, published in 1953.40
Vision, Reading Difficulties and Visual Stress
80
Chapter 4 Controversial vision therapies
81
Figure 4.1. Tests B and C of the Developmental Eye Movement test.
Reproduced with permission from Medland et al.38
It has been hypothesised that saccadic dysfunction could result
from a magnocellular-dorsal (M-D; see Chapter 5) deficit in dyslexia,41
but this hypothesis was not confirmed by Hutzler and colleagues.25
Hari and Renvall suggested that subtle attentional factors may impair
the ability of people with dyslexia to process rapid stimulus sequences,
Vision, Reading Difficulties and Visual Stress
82
but argued that this is unlikely to cause any significant direct effects on
reading.42 Other researchers have also suggested that saccadic
abnormalities during reading (e.g., longer fixation durations) may
result from constraints in the number of letters dyslexic individuals can
process in each fixation.20 Such visuospatial attention is discussed in
Chapter 5. Atypical saccadic eye movements in dyslexia have also been
linked with binocular instability and with the deficiency of visual
attentional processing discussed above.6
Bogacz and colleagues found that smooth pursuit eye movements
may be abnormal in dyslexia, although they did not consider the
possible confounding variable of attention.43 Pursuit eye movements
are not normally involved in reading, so it is not surprising that Judge
and colleagues found that smooth pursuit impairments do not seem
to be a significant cause of poor reading.44 Recent studies have found
that in pursuit eye movement tasks dyslexic children have slower gain
and make more catch-up saccades than normal, which is believed to
reflect a deficiency in the visual attentional processing, discussed at the
end of Chapter 5, and/or an immaturity of brain structures responsible
for pursuit triggering.
45 46
Can deficits of saccadic or pursuit eye movements cause
reading difficulty?
Since there is no strong evidence that saccadic or pursuit eye
movements are abnormal in people with reading difficulty, these
factors are unlikely to be common causes of reading difficulty. As noted
above, a more widely held opinion is that differences in saccades
between good and poor readers are a consequence, not a cause, of the
reading difficulty.
10 38 47 48
In most languages, a series of rightward saccades is followed by a
large leftward saccade that returns the point of gaze to the beginning
Chapter 4 Controversial vision therapies
83
of the next line. Each of these saccades, small and large, is associated
with a vergence (eye alignment) error: it is usually necessary to correct
the vergence of the eyes in the brief interval that follows the saccade
in order to obtain clear vision.49 It is possible that in individuals whose
binocular coordination is poor this vergence correction may take
longer, slowing reading (see Chapter 3). Bucci and colleagues found
evidence of poor binocular coordination of saccades in dyslexic
children.50
Jainta and colleagues have noted that when the material to be read
has repetitive vertical strokes as in the word mum, vergence correction
takes longer, slowing reading. It is possible that by providing
anomalous fusional targets the stripes disrupt vergence correction,
and that the disruption is greater in individuals whose binocular
coordination is already compromised.51 Further research by this team
lead to the suggestion that poor ocular motor control can lead to
instability of letters or words during reading, possibly interacting with
binocular instability.52 Binocular instability seems to impact saccadic
control in some dyslexics, even in non-reading tasks.53
Detection of saccadic or pursuit eye movement deficits
The objective analysis of eye movement parameters requires
special apparatus that is not found in most optometric consulting
rooms and eye clinics. When this equipment is used, it is relatively easy
to obtain recordings, but much more difficult to obtain meaningful
data since several variables have to be controlled, not least the
attention and cooperation of the child. Careful calibration is also
required. This usually involves instructing the patient to look at a series
of spatially distinct point targets, and if their ability to do so is impaired
because of poor attention it is difficult to interpret the data from any
subsequent recordings. The clinician can only be sure that the
Vision, Reading Difficulties and Visual Stress
84
recordings reflect the activity of each eye if the calibration is
undertaken when the observer looks at the calibration target with both
eyes, even though the target can only be seen by the eye being
calibrated (haploscopic conditions).
As noted above, the commonly used Developmental Eye Movement
(DEM) test, has been shown to be invalid as a test of saccadic eye
movements (“oculomotor” skills). A more sophisticated computerised
test, the SeeRite Reading Diagnostic Program, operates on similar
principles,54 but a PubMed literature search (February 2024) reveals no
published comparisons with objective eye movement apparatus.
Treatment of saccadic or pursuit eye movement deficits
Since it is doubtful that saccadic or pursuit eye movements are
abnormal in the vast majority of people with reading difficulties, it
seems unlikely that it would be useful to assess or treat these basic
visual functions.5 Historically, vision therapy to train eye movements
has often been advocated on the basis of the DEM test, but it was noted
earlier in this chapter that the DEM test is not a valid test of eye
movements.
A cross-over study by Solan and colleagues compared the effects of
reading therapy and saccadic eye movement therapy in children with
reading disabilities.55 Both interventions were associated with a similar
improvement in reading skills and saccadic eye movement parameters.
This study raises doubts over whether eye movement training does
actually train eye movements, although the absence of a control group
who received a placebo treatment limits interpretation of the results.
One eye movement study found reading difficulties were associated
with an increased number of fixations and regressions and slower
reading. The same study also found that these variables were
normalised when using blue (but not grey) filters.56 This highlights the
Chapter 4 Controversial vision therapies
85
complex interaction between different variables. It is possible
(although speculation) that these children may have been suffering
from visual stress and that the resultant perceptual instability
contributed to impaired eye movement control. If this is the case, the
use of coloured filters to correct the underlying visual stress would
seem to be a more appropriate treatment than eye movement
exercises. This study only used a very limited range of coloured filters,
so is likely to have underestimated benefits from filters (see Chapters
7-9).
A thought-provoking review by Simmons in 2017 highlights a
conspicuous lack of studies investigating optometric “oculomotor
training”.5 As the review notes, the occurrence of apparent saccadic
anomalies when dyslexic children read may be secondary to their poor
reading, in which case educational interventions are likely to be more
appropriate than training eye movements. However, the same year
Dodick and colleagues published a randomised controlled trial which
found saccadic eye movement training improves reading fluency and
comprehension in children aged 7-8 years (not selected because of
reading difficulties).57 This supported the findings from a smaller pilot
study.58 However, both studies were not masked, the treatment effects
were small, and it is not known whether a similar duration (30 hours)
of reading training would have been more beneficial.
For reasons explained in Chapter 12, evaluations of treatments that
do not include a control group or where a control group is not given
any intervention (e.g. Dalvand et al.59) are not included in this book.
Vision, Reading Difficulties and Visual Stress
86
Ocular dominance & the Dunlop Test
Chapter 4 Controversial vision therapies
87
Background
It seems to be a widely held belief that people with two eyes have a
dominant eye and that this is constant and unchanging. In fact, with
the exception of people who have an abnormal visual system (as a
result of amblyopia, for example), the notion of a fixed dominant eye
is more of a myth than reality. There are many different tests of ocular
dominance (Figure 4.2) and the eye that is dominant in a given person
often varies with the task,60 and indeed from one moment to another.61
For example, some tests present dissimilar images to each eye and see
which eye’s image predominates (retinal rivalry), and other tests force
the eyes to misalign and classify the eye that maintains alignment as
dominant. Even the widely used sighting tests of ocular dominance,
where a person is required to point at an object to determine which
eye they use to “sight”, give different results depending on whether the
person is instructed to keep one or both eyes open and which hand
they use.