What proportion of adults who are dyslexic would respond
to visual intervention?
Data from Bradford University UK
We are about to hear from the Oxford-Kobe symposium which
will include reports on the visual processing aspects of dyslexia as well as phonological
processing.
In the last few years there have been few accepted
studies in the area of visual interventions.
One report that came out was the outcomes by a study by
Jim Gilchrist and others at Bradford University School of Optometry and
Vision Science in the UK...
Gilchrist and his
colleagues have looked at visual stress problems experienced by Dyslexic
undergraduates at Bradford University.
There were 52 dyslexic
students referred to the clinic at Bradford Department of Optometry and vision
Science. They were taken through specific protocols.
The idea was to identify
any visual dysfunctions/ stress that could be an issue for the students and
contribute to their problems with text.
As Vision Scientists, they
wished to distinguish between reading related visual stress
Originating in Occulomotor / optometric / ophthalmic issues.
Visual stress originating in visual processing problems associated with
the pattern or the spatial characteristics of the task, From now on referred to
as PRVS.
Occulomotor / optometric /
ophthalmic .dysfunction
Did
they experience visual stress associated with an optometric dysfunction?
59%
of the students had no evidence of any accommodation or vergence problems.
Vergence
problems are problems which can be measured as possible faults in the way the
muscles maintain stable binocular vision. Usually the intervention is by the
use of prisms or exercises optometric or orthoptic Vision therapy.
This
component was looking at general uncorrected optometric dysfunction. The
vergence dysfunction would have, I assume have been considered at near.
As
you can see around 60% showed no evidence of problems with any evidence from
this study as possibly having any origin in this area.
Did they exhibit indicators of Pattern Related
Visual stress? (The PRVS study did not I
believe include a reading speed/RAN measure which I would argue is a key
indicator.)
Two
thirds showed evidence of Pattern related Visual stress... Colour mediated.
The
next question is whether those experiencing PRVS might also benefit from
optometric intervention.
Of those who experienced PRVS, 31% also
experienced visual stress originating in accommodation (uncorrected focussing
problems) and vergence problems. That is 20% of the students.
69%
of the students experiencing PRVS had no issues at all which would benefit from
optometric intervention. That is 45% of
all the referred students.
A
further 20% of the students showed no signs of any issues associated with
optometric intervention.
15
% of the referred students showed any evidence of any dysfunction, either PRVS
or optometric in origin. This suggests
that their phonological difficulties were totally phonological in origin with
no visual component.
The
statistics above suggest a moderate relationship between accommodation and vergence
dysfunction. This would be expected in terms of the tendency to suppress an eye
which is giving a weaker set of visual data from poor focussing.
Importantly
it suggests that the PRVS dysfunction is NOT an ophthalmic originating issue
but is separate and by implication should be subject to separate intervention.
There is a post on the blog which covers two case studies which attempt to
quantify the benefits from two separate but parallel interventions .
This
would be in agreement with a link to the spatial characteristics of the retina
and visual system.
.
Foveal diameter.
Crowding effects associated
with the centre –surround system for edge detection. The diameter of these
units being associated with the ratio of red to green sensitive cells which
varies from person to person.
Crowding effects associated
with the variation in cone cell diameter between people. Some would suggest an
inverse relationship between the central foveal cone cells and the diameter of
the fovea itself.
Bigger
fovea…
Probably big visual attention span more
characters parallel processed at a time
Reduced foveal crowding
Increased perceptual span...
more characters per fixation.
Longer saccades
Possible
weaknesses in the study
This
study can be looked at critically in terms of what definitions and protocols
were applied
For example…
Were the students seen by
the department representative of the whole body of dyslexic students at
Bradford?
What was the actual
definition of PRVS?
It can be argued that the
symptoms of PRVS are not necessarily perceived by the ‘person’ it could simply
put extra demand on the central executive reducing phonological output in terms
of speed and ‘quality’ (prosody/intonation)
What I do know is that the academics, that ran this study,
are of the highest calibre and as such the data should not be ignored.
What was reported in a lecture by Jim Gilchrist, was also
that many of those who did not get identified as having PRVS, by the selection
criteria, nevertheless did respond positively to the use of colour chosen using
the Intuitive overlay forced choice selection process. Although this process
itself is from a very limited palette.
But no one could understand why.
The idea that several of those not identified as having
(the symptoms) of visual stress benefitted from the use of colour can be used
to question whether the symptoms of PRVS should be the criteria which limit
access to interventions.
If we look instead at the work of Valdois and Facoetti on
visual span and crowding and hypothesise that any restriction on visual
attention span will decrease the phonological output.
Whether that restriction be from
Crowding.
Focussing problems
Edge detection processing
speed
Vergence difficulties...
binocular problems
Reducing working pigment
density in the cone cells.
Increasing the number of letters processed between
saccades
Restriction in visual attention span and reduced edge
detection processing will give rise to the following changes which will further
limit both visual and phonological performance probably the basis of the reading
stamina difficulties virtually universal amongst dyslexic adults.
decrease the lengths of the saccades
Increase the occulomotor muscle tone, decreasing ocular
motility.
increase the demand on working memory ( central
executive)
increase the difficulties of blending phonemes
slow down the development of automaticity
Reduce the cumulative total reading experience.
Binocular
dysfunction
A way of looking at the effects of binocular dysfunction
on visual processing speed would be to compare rapid automatic naming speeds
under the following three conditions.
Binocular
Monocular right eye … left eye occluded
Monocular right eye occluded.
When I have done this with dyslexic individuals, it is
very rare that binocular reading gives rise to greater reading speeds than both
monocular.
Usually either the right eye or the left eye gives the
greatest reading speed.
Binocularly the reading speed is more often the same as
the left or right monocular speed. Sometimes but not often it is slower than
either.
When the computer screen has been optimised, then usually
this is faster than either eye monocularly.
This suggests that the eyes are then acting
synergistically.
Binocularity
and visual processing speed.
The binocular instability hypothesis that seems central
to the magnocellular deficit hypothesis, suggests that by occluding one eye the
reading should be faster, more effective.
But very few dyslexic undergraduates get rid of their
‘visual stresses by occlusion. Those who
can get rid of the visual stress by occlusion, have nearly always been using
this already as a strategy to extend their reading stamina and decrease the
discomfort.
The dyslexic undergraduates (adults) who report
instability of the text usually report it still when viewing text monocularly.
The instability has to be then associated with spatial and temporal visual data
being collected by the viewing eye. This implies then that the dynamics of
image movement on the retina during fixation is important in the edge detection
process. When I was participating in a pilot study of crowding, where the image
was projected for 50ms time periods I was very aware of ‘seeing’ several images
during the exposure as my eye moved extensively within the 50ms period. During
a fixation visual data is being collected for up to 200 mS.
When you look at the binocular eyetraces for these people
there is usually clear evidence that the visual system is already, subliminally
suppressing the data from one eye anyway.
You can see this in some of the other posts for example. http://tinyurl.com/cceec6h
You
can calculate the optimal screen settings for a person, monocularly and then
apply the settings binocularly. This implies that what is being measured is
associated with the biology of a single eye, rather than binocular aspects of
the system.
The
way the eye movements of both eyes are affected and the synergistic reading
performance benefit when reading binocularly could be the basis of further
conjecture about the system that is operating.
As
a final point; when I have looked at reading performance of fluent readers, the
binocular reading performance exceeds monocular performance. In addition when
reading English (left to right) the left eye appears to be landing in front of
the right eye. I have only ever looked at a fluent bilingual person once. That
person was bilingual in Urdu (right to left) and English. Whichever text was being read the left eye
was always in front. This concurs with Gadi Geiger’s work at MIT on visual
attention from a ‘pre-computer age. I had a great discussion in Boston, Mas. on
this issue ‘earlier this century’.
Fixation
disparity
Work
with Arnold Wilkins et al, looking at the reading performance of year 7 (6 in
the USA) and measuring accommodation, orthoptic and PRVS’ showed only one
correlate with default reading performance across the group, fixation
disparity.
The fixation disparity, it
would seem needs to exist to be a fluent reader! If there is no disparity, then the reading
performance is lower, if it is too high the reading performance is lower. There
wo0uld appear to be an optimal fixation disparity for a person.
This would make sense in
terms of the way many dyslexic people, but not all are gifted graphically. There
seems to be an excess of dyslexic students studying architecture.
It may also be one of the reasons
behind why converting text into graphics, by such means as Mind mapping, is
reported as being a particularly useful study aid for dyslexic students.
What the use of a binocular
eyetracker tells us though is that the amount of binocular disparity employed
by a person’s visual system is to some extent task responsive. Again this would concur with Geiger’s
research. The disparity in fluent readers is directional depending on the
brains identification of the reading direction needed by the language.
.
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