Professor Gordon N Dutton MD,
FRCS Ed Hon, FRCOphth
Children’s Vision Consultant
Emeritus Professor of Vision
Science, Glasgow Caledonian University
Honorary Senior Research
Fellow, University of Glasgow
Cerebral visual impairment (CVI) is the commonest
cause of visual impairment in the developed world. A large part of the brain is
devoted to seeing, so it is not surprising that conditions that affect the
structure or function of the brain can interfere with vision [1].
The
front of the brain ‘thinks’ ‘understands’ and ‘behaves’, the middle ‘feels and
moves the body as well as hearing and understanding language’, and the back
‘sees’.
The
picture made by the eyes is sent to the back of the brain where it is sorted
out in different ways:
1. Processing
the picture
First, at the back
of the brain (the occipital lobes) the detail of the picture is sorted
(measured as visual acuity). Seeing colour, handling the number of shades of
grey that can be seen (contrast sensitivity), knowing the extent of the
picture, (the visual fields), and dealing with the speed and direction of
moving images are all processed in this area.
2. Guiding our
movement and seeing lots of things at once
Second, at the top
of the brain at the back (the posterior parietal lobes), there is an amazing
system. The visual information is carried there from the occipital lobes by a
connecting pathway, called the dorsal stream. For every moment of our waking
lives, this area of the brain uses the incoming moving picture to guide our
movements.
Reach
out and pick something up. You ‘know’ that the picture is out in front of you,
but it isn’t really is it? It’s inside your brain! What is amazing is that the
picture out there in front is so compelling that you believe that it is there,
but you are actually moving through the picture in your brain, fully expecting
that it will coincide with the reality of the object you are about to pick up,
which of course it is. This can take a while to get one’s head round, but it
gets even more complicated. The whole process takes place completely
unconsciously!
Some
people who have gone blind in both eyes because of damage to the back of the
brain, which spares the movement seeing centre, are able to accurately move
through the world that they do not consciously see. Functional scans of the
brain show that they do this with their intact posterior parietal lobes. “But
wait a minute” you may be thinking, “I know what I can see; and I chose and
knew what I picked up.” Yes, but the conscious seeing and knowing is performed
separately by the closely interlinked bottom of the brain at the back, the
temporal lobes, which are connected to the occipital lobes by another pathway
called the ventral stream.
3. Recognition
This
is the third part of the brain’s
conscious visual processing system. The temporal lobes serve conscious vision
and recognition.
You
recognise a friend in a group of other people you don’t know. To do this, you
have a memory store of all the people you know. In came the picture of the
group of people. The pictures of all their faces were compared with your memory
store and there was one match, - your friend. What an amazing bit of computing!
This part of the brain also allows you to find your way around without getting
lost.
4. Dealing
with the cluttered scene
To
be able to move through the visual scene, a number of things need to happen.
The unconscious picture needs to be processed very quickly, it then needs to be
held in automatic unconscious working memory for a short time, after which it
needs to be shed. Otherwise the brain would quickly fill up with useless
out-of-date information.
As
many items as possible in the visual scene need to be available at the same
time to let us move through a cluttered environment, and to let us choose where
we go. This is done by the visual reference library in the temporal lobes
giving conscious knowledge of what we are seeing, while the dorsal stream
pathways are providing the RAM in the computer of the brain so that we can
attend to a number of things at once, by linking to the front of the brain
which makes the conscious choices of where to go and what to focus on.
The
two visual analysis systems, the dorsal and ventral streams, are blended so
well that they work together in harmony, but of course we are unaware of all
the unconscious visual workings of the brain, and therefore believe, wrongly,
that we have conscious awareness of everything that our brains are seeing and
handling.
So
how does this information help in finding out whether a child has cerebral
visual impairment?
Any
element of what has been described may not work so well. Even with the right
glasses, children with cerebral visual impairment can have a range of visual
difficulties, unique to themselves, due to any of the processes outlined not
working so well, in any combination or severity. The table below gives a list
of some of the difficulties due to cerebral visual impairment along with
approaches that can be used to help.
Problem
|
Approaches
|
Reduced
clarity of vision.
|
Enlarge text
Double space
text
Present text
in small sections.
Reduce
distractions
Limit
tiredness
|
Colour vision
and contrast sensitivity impairment
|
Bright and
clear educational material and toys.
Distinct
colour boundaries
Good contrast
|
Lack of
vision on one side
|
Tracing of
text with a finger or ruler
Turning text
vertically or obliquely
Appropriate
seat position in classroom
Turning of
head to check the hemianopic side
Careful
guidance around new environments
Training in
crossing roads
Turn plate to
eat food
|
Lack of
vision down below
|
As with
hemianopia
Regularly
looking down to check the ground ahead
Tactile guide
to ground height
|
Impaired
ability to move the eyes
|
Movement of
the head
Enlarging
text
Double spacing
text
Tracing of
text with a finger or ruler
|
Impaired
ability to see movement
|
Television
programs with limited movement
Educational
material with limited movement
Careful
training or guidance in crossing roads.
|
Difficulty
finding a toy in a toy box or an item of clothing in a pile or wardrobe
|
Separate
storage of favourite items
Organised
storage systems
Always store
in same location
Avoid clutter
Colour coding
and labels
|
Difficulty
finding an object on a patterned background.
|
Use plain
carpets, bedspreads and decoration.
|
Difficulty
finding food on a plate
|
Avoid
patterned plates
Avoid sauces/
gravy
Separate food
portions
|
Problems
seeing a distant object
|
Use zoom on video/ digital camera to view
|
Problems
reading
|
Enlarge text
Double space
text
Masking
surrounding text
Computer
programs to present information
|
Difficulty
finding someone in a group.
|
Wear obvious
identifier
Always stand
in same location
Waving
Speak
|
Tendency to
get lost
|
Training in
seeking and identifying landmarks
Visit new
locations at quiet times
|
Problems with
floor boundaries, steps, kerbs and uneven surfaces
|
Avoid
patterned floor surfaces
Bannister
Mark edge of
stairs
Good lighting
Tactile
guides to gage the height of the ground
Approach
obstacles with
“Look- Slow-
Check- Go”
Activities to
improve coordination
|
Inaccurate
visually guided reach
|
Reaching
beyond an object to gather it
Activities to
improve coordination
Occupational
therapy
|
Difficulty
‘seeing’ when talking at the same time
|
Limit
conversation when walking
Identify
obstacles by tactile stimulation
|
Frustration
at being distracted
|
Limit
distraction
Minimise
background clutter
Minimise
background activity.
Quiet table
at school
|
Difficulty
recognising people and photographs
|
Introductions
Training in
identifying voices
Consistent
identifiers worn
Training to
recognise identifiers
|
Difficulty
recognising shapes and objects
|
Training to
identify and recognise identifiers
Training in
tactile recognition
|
Difficulty
reading facial expression
|
Training in
recognising facial expressions
Expression of
mood by tone of voice
Explanation
of mood in words
|
Getting lost
in known places
|
Training in
orientation.
Encouraging
leading
Incorporating
landmarks in Mnemonics /Poems
|
Difficulty in
new environments
|
Training in
orientation.
Encourage
exploration
–Visit at
quiet times
–Hide and
Seek
–Treasure
Hunts
|
Visual
fatigue
Prolonged
visual processing
|
Minimise
clutter
Reduce
distractions
Reduce detail
and complexity
Well earned
breaks
|
Social
problems
|
Good
understanding and support at school
Identify
problems and solutions
Encourage
child to overcome them
Well known
informed peer group
Find
activities child enjoys and can excel in
|
One bitterly cold Friday in December well over a year ago, in Glasgow, Scotland, I was privileged to observe Gordon Dutton doing his characteristically thorough assessments of children at the Vision Clinic in the Royal Hospital for Sick Children, just weeks before he retired from that post. It was an occasion I am not likely to forget as he patiently quizzed parents about the child's history and tested the child's vision with tools he has developed over the years. Gordon has been leading the field of cerebral visual impairment for some years now. He is a prolific writer on the subject and we look forward with great interest to his new book for teachers on CVI. He has been an enormous inspiration to many in recent years, myself included. Many thanks, Gordon, for submitting this article on such a vital and under-resourced topic. - Maurice
Gordon N Dutton
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