Report on the outcomes of a consultation looking
at visual intervention with a student identified as being Dyspraxic and dyslexic.
This particular post is I believe of considerable importance; it is a detailed analysis / deconstruction of the response of one individual to changes in the presentation of text on a computer screen. This could have been any adult but the graphs would have different peaks. About half of the students show a negative response to red reduction rather than a positive response. Other posts will supply more general background to the ideas. The comments on visual span are highly relevant to the emerging research that was reported at the Oxford event.
The student was
referred as part of her Disabled Student’s allowance.
The student was
diagnosed as Dyslexic/Dyspraxic.
The
consultation was to ascertain visually associated intervention to ameliorate her difficulties with text.
Content
- Background
- Focussing issues, Optical correction
- Font size, image size, reading distance orthoptic/convergence issues.
- Reading speed and stamina
- Crowding.
- Visual span
- Screen luminosity and colour,
- Memory issues
- Summary of interventions
- Comparisons of eye movements in default and optimal conditions.
Background information.
The student was first diagnosed with ophthalmic problems
at the age of 7 years. Since then there
has been a progressive change in her prescription which is to be expected.
She has correction for myopia (short sight) and for
astigmatisms in both eyes. The correction for her left eye is greater than for
her right eye.
She has been told that her left eye is suppressed (there is
difficulty processing visual information with data from her left eye. If she
covers her right eye the image is less clear than if she covers her left eye.
When reading for extended periods she often…
- Covers her left eye
- Turns her head sideways (turning towards the right).
- If using a computer, increases the size of the text using the ‘zoom’ facilities on the computer.
- Becomes very tired giving her very short work periods and needing longer and longer rest/recovery times. These work periods are only a few minutes.
- Becomes increasingly, easily distracted.
- Experiences upper body and neck discomfort.
The
consultation concentrated on identifying these issues quantitatively and
identifying strategies to reduce/remove these barriers to studying.
Outcomes
of the Consultation.
Focussing issues,
Optical correction
Using her glasses, which she uses continuously, the
correction for her right eye appears to leave distance vision still too difficult.
This implies that the
correction is too weak for distance vision. A bifocal
correction might be a solution.
The astigmatism correction appears to be correct for both
eyes.
The vision from her left eye is still compromised at far and
near as would be expected with monocular visual suppression.
This asymmetry in
visual performance would give rise to distance judging problems at far and
near. This would give ‘clumsiness’ characteristics at far and near which would mimic
dyspraxia.
There is visual data being collected by the left eye which
would assist in distance judging but ‘at near’, when reading or writing.
There
is evidence from the eyetracking data that the
left eye data is further suppressed leading to increased suppression of
the left eye and increased and fluctuating fixation
disparity between the two eyes. This
is reduced by the head turning but not prevented.
The head turning would also give rise to upper body and neck discomfort as
small movements would occur as a reflex
in trying to overcome the diplopia.
She experiences diplopia
(double vision effects) when reading or viewing near objects. The diplopia is
greater if the object is nearer. The further away the object is the less the
diplopia.
Using the larger fonts the distance from the text increases,
reduces this effect.
(Diplopia occurs when
the two eyes are focussed (fixated) at points too far apart (fixation disparity) so that the visual
system is unable to compute a single perception (image). In all people there is
some disparity and this is part of
efficient vision. But if it is too great then the visual system is incapable of
the computation of a single image. This
is referred to as ‘insufficent fusional
reserves.. and is associated with the idea of ‘convergence insufficiency’.
If the system can intermittently ‘fuse the data’ or the
disparity keeps varying and data from
one eye is not continually suppressed then the person gets a perception of
unstable or wobbling text or the whole visual scene appears to wobble…
Oscillopsia. To reduce this effect some people keep ‘wobbling their heads
subconsciously which can give rise to nausea and neck/upper body/back aches.
****************************************************
Reading speed
Changing the
font size affects her reading speed as shown in the graph below. This will be
in response to a combination of the following effects.
- Changing reading distance.
- Changing crowding effects(ability of the system to compute the edges of the letters)
- Changing the distance for the eyes to travel between words./ changing the demand on the eye muscles.
The first two of these will affect the number of letters
which she can ‘see’( perceive) in each fixation, her visual span. Recent research has shown this to the controlling factor
in reading speed for many people.
( please remember the reading speed
is a measure of phonological output as a response to changing visual input)
Using the bigger font size increases the image size on her retina, this
would reduce crowding effects and allow the processing of more letters at once
(parallel processing). Too big a letter size will move the target fpor the next
saccade too far into the peripheral retina ( away from the fovea) reducing the
accuracy of the saccade, slowing the reading down.
On default (font 12) the
visual span is averaging 1.60 letters. A person with no difficulties will be
processing 10+ letters per fixation
When using her optimal conditions.
There were initially 3.3 characters per fixation . This is more than a 100% improvement.
In the last line, however, the number of fixations was 14 for 79
characters That
is 5.6 letters per fixation.
There is a continual gain in the size of the visual span as
she reads with optimal conditions and this is reflected in the improving
reading rate the more she reads, as in the graph below taken from the
eyetracking data.
We can compare this to changing reading rate when reading in default
conditions in the graph below..
Combining the two graphs makes the
difference in reading performance very clear.
These reading performance graphs reflect the changing visual span as the reading period changes. Visual span can be considered as controlling
reading performance rather than controlled by reading performance. As the visual
system gets ‘stressed’ the visual span decreases to the point where the process
becomes to difficult to make use of the process. This is likely to be a
component of her reading stamina problem.
Memory issues
If a person has a short visual span, then the number of bits of visual data needed to ‘read’ a
sentence will be much greater than for someone with the ‘normal visual span’
.To read and comprehend a sentence
would make a much greater demand for working memory from the ‘central executive’( Alan Baddeley’s
model) leaving reduced resources for
comparison of the concepts intrinsic in the sentence with the concepts in long
term memory. Other /additional memory strategies would be needed. Study time
would need to be greater.
By increasing the visual span, memory difficulties, when reading should
be ameliorated.
The decreasing reading speed in default reading conditions and associated
limited reading stamina consequence, would further limit her total read/study
time.
Reading speed, screen
pixel luminosity
Overall
screen brightness.
There is a relationship for The student between overall screen brightness and reading
performance. This can be seen in the
graph above.
The total amount of light entering her eyes is controlled by her pupil
dilation. This reflex is designed to optimise the rate at which the photons are
captured by the pigment molecules in the cone cells of her retinas; but it is
controlled by ambient lighting intensity. There may be a difference between the
optimal intensity landing on her fovea ( centre of focus of the images on the
retina) and the peripheral retina. We do not know. In her case when font size
has been optimised this is now limiting her reading performance.
158/255 is the brightness used for the rest of the testing..
Changing the green pixel brightness
As the green pixels are dimmed then the rate at which the green pigment
in the green cone cells gets bleached is reduced. This will lead to a change in nerve impulse
generation. Possibly to an increase in crowding effects and reduced visual span
for The student .
Changing the
red pixel brightness
This is completely different to the effect of changing the green component. Although in a way we are really still changing the ratio of red : green stimulation.This ratio is the basis of the colour vision /colour recognition process which must ultimately be based on changing the impulses per second delivering information to the visual cortex and hence the mediator in object edge detection..visual processing.
This graph
shows clearly the mathematical relationship between the ratio of green to red pixel
brightness and the reading performance of The student .
All the
red:green optimisation to this point has been undertaken with the blue value
set at 158 as determined by the initial screen brightness study.
Changing the
blue pixel brightness.
The cone
cells containing the blue sensitive pigment are not found in the centre of the fovea. There is a response to changing the blue
pixel brightness but often very little and there appears to be a change with
use of the background on screen for reading.
There is good research evidence
that the amount of blue light affects the magnocellular system particularly (
see research by John Stein al.). The red/green ratio is likely to be more
associated with the ‘parvocellular system’, the edge detection system; the
system which collects the data to identify the ‘object being looked
at’/receiving attention.
The graph
below shows the effect on one aspect of reading performance (scanning). However, in terms of visual clarity when
using an overlay or reading The student , preferred not to have the blue
reduced. As such a cyan overlay closely mimicking the optimal red green ratio
was provided for her to use with printed material. Looking at the graph about reducing the red,
it must be remembered that if the cyan filter removed too much red then this
‘same colour’ would start to limit her
reading performance, similarly if the cyan did not remove enough red then there
would only be a partial benefit to her.
The computer screen setting will provide the optimal red/ green.
On her
computer screen she has the option of using a low blue ( green looking!) screen or the optimal
red:green screen ( grey/Cyan).
In two
months time a second consultation will determine changes in her visual system’s
need and then precisely coloured prescription glasses mimicking her optimal
screen settings can be provided as an
alternative to overlays or screen colour management.
Summary
The student needs the following interventions to optimise
/maximise her reading performance.
- Printed material where possible printed at font 21.
- Where possible all documents to be provided electronically to enable optimal reading conditions.
- To be able to make use of her cyan overlay whenever appropriate.
- In lectures meetings, to be able to sit to the left of the main centre of visual attention to minimise distractibility.
- At the next consultation the possible provision of optimally tinted prescription glasses .
Comparison of eye movements using default conditions and
optimal conditions.
With default conditions
the distance between the two graphs keeps changing. Whereas with the optimal
conditions it stays more consistent.
If we look at the more
detailed graphs, shorter time periods the difference between the two conditions
is clearer.
Graph showing the detail of saccades
and fixations by both eyes using optimal conditions for a 2 second period for comparison with a 2 second period using default conditions.
The graph shows that both eyes are in
general working together. If this is
compared with the eye movements when reading on default it is easily seen that
the left eye is hardly saccading.
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