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About the authors
Arnold Wilkins is Professor Emeritus at the University of
Essex. He is a Chartered Psychologist, a Fellow of the British
Psychological Society and an Honorary Fellow of the College of
Optometrists. He obtained his doctorate from the University of
Sussex for work with Alan Baddeley on semantic memory, and then
spent two years at the Montreal Neurological Institute with Brenda
Milner studying the effects on memory of temporal lobe resection
for epilepsy. He was asked by Fred Andermann to see a patient with
pattern-sensitive epilepsy and the resulting study changed the
direction of Wilkins’ research career. On returning to the UK to work
at the MRC Applied Psychology Unit in Cambridge he continued his
studies of photosensitive epilepsy with Colin Binnie and these
broadened into a study of visual discomfort from flicker and pattern.
He demonstrated that the rapid flicker from fluorescent lighting
interfered with the control of eye movements and was responsible
for headaches. He showed that the spatially periodic nature of
printed text could also cause discomfort, and that this could be
relieved by coloured filters. This discovery led to a neurological
theory of visual stress, the development of the Intuitive Colorimeter,
the Intuitive Overlays, the Rate of Reading Test and, with Bruce
Evans, the Pattern Glare Test – 275 articles and four books. Wilkins
moved to a chair at the University of Essex in 1997 and is currently
enjoying collaborating with psychologists, engineers, architects,
optometrists, neurologists and typographers.
Bruce Evans is Visiting Professor to City, University of London and
Visiting Professor to London South Bank University. From 1998-2023
he was Director of Research at the Institute of Optometry, where he
is now Senior Research Fellow. He is a Fellow of the College of
Optometrists and holds their higher qualifications of Diploma in
xv
the authors
xvi
About
Contact Lens Practice and Diploma in Orthoptics. He is a Fellow of:
American Academy of Optometry, European Academy of Optometry,
and British Contact Lens Association. He has worked as a practising
optometrist for 40 years. He has been involved in over 90 research
studies, with main areas of research: dyslexia and vision, binocular
vision (orthoptics), myopia control, contact lenses, optometric
referrals, and migraine. Bruce Evans has supervised 17 doctorate
students, all successfully graduated. He has authored over 270
scientific and professional papers, four books on dyslexia and vision,
and six editions of optometric textbooks. He has given over 250
invited lectures and 140 presentations at scientific conferences. In
2021, he was listed in the top 100 of the Global Optometrist Research
Ranking, in 2022 awarded Life Fellow of the College of Optometrists
in recognition of outstanding contribution to the optometric
profession, and in 2023 recipient of the Lifetime Achievement Award
of the Association of Optometrists.
List of abbreviations
prism dioptre
AC/A accommodative convergence to accommodation ratio
ADHD attention deficit hyperactivity disorder
CDM coherent dot motion
CFM coherent form motion
CIE Commission Internationale de l'éclairage (International
Lighting Commission)
CIES convergence insufficiency exophoria syndrome
CITT-ART a large randomised-controlled trial of eye exercises for
convergence insufficiency exophoria syndrome
CSF contrast sensitivity function
cpd cycles per degree subtended at the eye
D dioptres
D or DV distance vision
DEM Developmental Eye Movement Test
DS dioptres spherical
EEG electroencephalogram
FDT frequency doubling technique
fMRI functional magnetic resonance imaging
Hz Hertz
IFS Institute Free-Space Stereogram eye exercises
ipRGC intrinsically photosensitive retinal ganglion cells
xvii
List of abbreviations
xviii
IQ Intelligence quotient
ITC inferior temporal cortex; an area of the brain
JND just noticeable difference
L left
LED light emitting diode
LGN lateral geniculate nucleus
M-D magnocellular-dorsal visual pathway
MEM monocular estimate method
MIS Meares-Irlen syndrome
MISViS Meares-Irlen syndrome/visual stress
MT an area of the cortex of the brain involved in the
processing of motion. Also described as V5.
N or NV near vision
NIRS near infra-red spectroscopy
nm nanometres
PFC pre-frontal cortex; a region of the brain
phoria heterophoria
plano an ophthalmic lens of zero power
PPC posterior parietal cortex; a region of the brain
PubMed an online search engine for research publications
P-V parvocellular-ventral visual pathway
pxs patients
List of abbreviations
xix
R right
RCT randomised controlled trial
RA reading age
RAF Royal Air Force
RD reading difficulty
s second
SASC Specific Learning Difficulties Assessment Standards
Committee
SpLD specific learning difficulties
SSS scotopic sensitivity syndrome (a deprecated term)
tDCS transcranial direct current stimulation
TSF temporal sampling framework
UCS Uniform Chromaticity Scale
V1 an area of the cortex of the brain devoted to early
analysis of vision
V2,V3 other areas of the cortex of the brain devoted to vision
VA visual acuity
VEP visual evoked potential
WRRT Wilkins Rate of Reading Test
XOP exophoria
yrs years
Chapter 1
Introduction
Chapter Abstract
In this chapter the structure of the book is laid out, together with
the underlying rationale. Types of learning difficulty and their relation
to reading difficulties are described. The key clinical visual deficits that
can accompany reading difficulties – binocular instability and visual
stress – are introduced. The associated symptoms and signs are
described.
Introduction to the book
There are two parts to this book. The first part (Chapters 2-5) is a
discussion of the visual factors that may be associated with difficulty
reading and dyslexia. One of these factors, visual stress (which is
alleviated with coloured filters), is covered in detail in the second part
of the book (Chapters 6-12), which also concerns the use of coloured
filters in neurological disorders. There are several reasons why visual
stress is covered in detail. First, it is believed to be a condition with a
neurological basis, which co-occurs not just with reading difficulties but
also with common neurological conditions (e.g., migraine, autism) and
with other rarer, but important conditions (e.g., epilepsy, stroke,
multiple sclerosis). Also, the authors are aware of a need for an up-to-
date review that brings together the research evidence, discusses the
controversy surrounding visual stress, and summarises clinical
guidelines.
The authors’ goal is for this book to be accessible not only to eye
© The Author(s), under exclusive license to Springer Nature Switzerland AG 2024
A. J. Wilkins and B. J. W. Evans, Vision, Reading Difficulties and Visual Stress,
https://doi.org/10.1007/978-3-031-65568-5_1
1
care professionals and education professionals, but also to parents.
Each section includes a lay description and there are frequent
summaries in non-technical language. Scientific evidence is concisely
reviewed with references to relevant literature, for those to wish to
read further.
In modern society most people are literate, and reading has become
one of the most important tasks we do with our eyes. Nevertheless,
references to reading difficulties can be traced back as far as the early
seventeenth century when few of the population could read or needed
to do so.
Reading requires an individual to use their visual skills, phonetic
ability (ability to analyse speech sounds) and knowledge of language to
successfully recognize words and gather meaning from the written
text. Given the complexity of the processes involved, it is unsurprising
that there are many sources of reading difficulty. Visual factors that can
contribute to reading difficulties are discussed, concentrating on
people who do not have eye disease.
There are several eye care professions. Optometry is the largest of
these. The number of optometrists exceeds the sum of all the other
eye care professionals. Most optometrists work in community optical
practices and carry out eye examinations and sight tests. Optometrists
have a statutory duty to diagnose and correct refractive errors (e.g.,
short-sightedness), orthoptic problems (difficulties with eye co-
ordination), and some eye diseases. Some optometrists work in the
hospital eye service and others in research. In community practices,
optometrists work alongside opticians (dispensing opticians), who are
experts on spectacle frames and lenses. Optometrists and
professionally qualified opticians in the UK must be registered with the
General Optical Council and only registered opticians and optometrists
can dispense spectacles to children.
Vision, Reading Difficulties and Visual Stress
2
Ophthalmologists are medical professionals who have specialised
in eyes. In the UK, nearly all ophthalmologists work in the hospital eye
service where they treat eye disease, either with medicines or surgery.
Orthoptists work mainly in the hospital eye service where they
diagnose and treat eye co-ordination problems (e.g., strabismus).
Many orthoptists are also involved in vision screening, which in the UK
typically takes place once, shortly after school entry.1 Fewer than half
of vision screening services in the UK are compliant with the relevant
specifications, often lacking appropriate personnel or tests.2
The visual correlates of reading difficulties are the visual problems
that are more likely to affect people who struggle with reading.
Throughout this book, priority is given to evidence-based research.
Recommendations about undertaking research in this field are given
in Chapter 12. In matched group studies, visual function is compared
in two groups, one group with reading difficulties (e.g., dyslexia) and
the other group a control group with normal reading skills. The two
groups should be matched for confounding factors that could
influence the comparison. For example, the two groups would typically
be selected as having similar age, ratio of boys and girls, and ideally
would be matched for intelligence, opportunity to learn, and possibly
also for the proportion who have attention deficit disorder.
The other research design that may be used to detect correlates is
a cross-sectional study. Typically, a large group of unselected children
would be tested educationally and optometrically. For example, a
whole school year might be invited to participate. The researchers
would carry out educational tests (e.g., measuring reading skills) and
tests of visual function, to see if the children with worse reading tend
to be those with worse performance on a vision test.
The term intervention is used to describe any form of treatment or
therapy, including spectacles, eye exercises (vision training), or
Chapter 1 Introduction
3
coloured filters. An important type of research for evaluating
treatments or interventions are randomised controlled trials (RCTs). In
an RCT (Chapter 12), participants are randomly allocated to two
groups. Typically, one group will receive the intervention of interest
(experimental treatment) and the other group will receive a control
treatment. Both treatments should be designed to be equally
convincing and to give participants the same time and attention. If
possible, both the participants and the researchers should be unaware
whether each child is receiving the real treatment or the control
treatment.
Learning difficulties and reading
The notion that some people are “visual learners” has been
popularised as part of a model of learning styles, called the VAK model.
This is built upon the idea that children possess a dominant way of
learning tied to a singular learning modality, such as a visual, auditory,
or kinaesthetic/tactile learning style. An interesting paper by Sun and
colleagues in 2023 described this notion as a “myth” that can be
unhelpful, because it biases parents and teachers to have false beliefs
about children’s abilities to learn in different ways.3 Of course, we all
use all available senses when we learn, with the possible exception of
people with a profound impairment of vision or hearing.
Learning difficulties (disabilities) can be broadly classified into those
that are developmental and those that are acquired. Developmental
difficulties are much more common than acquired problems, which
typically result from a neurological disorder. Visual stress can
accompany both developmental difficulties and neurological
disorders.
Learning difficulties can also be classified into those that are general
and those that are specific. General learning difficulties affect a wide
Vision, Reading Difficulties and Visual Stress
4
range of skills including general intelligence, as measured with an
intelligence test, and expressed as an IQ. Specific learning difficulties
(SpLD) only affect specific skills and can occur regardless of IQ.4 SpLD
feature in Chapters 1-5 of this book.
Since most learning is mediated by the written word, those
individuals who have reading and spelling problems will struggle with
education, even if they are intelligent and articulate. SpLD persist
throughout life and can have deleterious consequences for careers.5
There are many types of SpLD.6 Some of the main ones are
summarised below.
• Reading and spelling difficulties, dyslexia. The definition of dyslexia
remains controversial,7 but a common definition is a learning
difficulty that primarily affects the skills involved in accurate and fluent
word reading and spelling.5 Characteristic features of dyslexia are
difficulties in phonological awareness (an understanding of the
sound components of language), verbal memory, and verbal
processing speed.5 Recent thinking on the causes of dyslexia is
discussed at the end of Chapter 5.
• Difficulties with writing, dysgraphia. Dysgraphia can manifest as
poor handwriting and trouble putting thoughts on paper.
• Lack of numerical skill, dyscalculia.8 Dyscalculia refers to a wide
range of life-long learning difficulties involving mathematics. There
is no single form of maths disability, and difficulties vary from
person to person and affect people differently in school and
throughout life.
• Difficulty with co-ordination, dyspraxia. This condition is closely
related to conditions described by two other labels, developmental
co-ordination disorder and clumsy child syndrome.
The skills involved in writing, reading, listening, speaking, and
Chapter 1 Introduction
5