Saturday, 27 April 2013

More on unsteady images, reading speed, dyslexia, dyspraxia and working memory

Unsteady images are possibly major contributor to the difficulties in reading development and hence in dyslexia and probably in the ‘development of IQ’. In that if your system is finding it difficult, taking longer before if it manages it, to create a steady image, then that will be putting more  computing demand on your brain for each ‘unit of look’/fixation.
Also in my previous post on unsteady images , is the idea that to ‘see a letter /word we ‘process several distinct images, in the first 50ms ( I am uncertain about how many milliseconds), and then coordinate them to produce a single intra-fixation image.

Also by inference we acdtually process and coordinate spatially several of these intra-fixation images within each fixation , this in a way is the 'perceptual span'.

This would mean that.....

  • the more unstable the fixation of the eye the smaller the amount of visual data collect within the fixation.
  • The more saccades will be needed to acquire spatial data about a visual scene. More computing power, memory, demand from you central executive is needed for a particular visual task.
  • If for example we think of  a multisyllable word  as a visual scene , then the more unstable the fixation the more computing /brain power or memory will be needed to ‘see’ that whole.
I would suggest that this would delay the development of what is referred to as ‘automaticity’ for that word.

We can  consider the Coltheart Dual Route model of reading   (DRC)   

The automatic matching through the left hand route, uses the orthographic data that will originate in the original mapped visual scene of the word in the lexicon of the semantic system. Unstable  fixation will impact on this system, reducing the probability of a word being held in store there.  This would force the more frequent use of the grapheme-phoneme (decode/blend route).

The effect of this would be a slower reading rate, reduced prosody and impact on the time constraint aspects of working memory.  You are more likely to have ‘forgotten’ the beginning of the sentence by the time you have got to the end.

A further consequence of this stress/demand ultimately on the central executive is that there will be reduced attentional resources available and reduced resources for accessing the long term memory constructs which the text being read impacts on.

This will be most severe when the person is actively trying ‘to learn’ new ideas. Or trying to test the validity existing mental constructs against the new information/data/ mental constructs which the text is introducing them to.

Any aspect of the visual system/phonological system which reduces the number characters per fixation is likely to slow down reading and hence negatively impact study reading performance, self image…………….

If we look again at the DRC, there are three obvious points where phonological processing impacts on the model (The black arrows.) and one where the initially the auditory processing  speed will have an impact ( the green arrow). I say this about the auditory processing aspect because,  auditory prompting of a visual image appears to enable more rapid identification of the image. Also the converse, auditory processing in the hearing impaired is enabled by the visual , lip reading.

At a different scale, we can look at the way uncorrected ophthalmic problems can give rise t a similar reduction in the number of characters processed per fixation.

If a person is myopic ( short sighted) and it is not corrected then they will read more slowly from a blackboard /whiteboard than from a book in a  classroom or lecture theatre.  They are more likely to be disciplined for copying, or having poor attention.. I will develop that idea in another post.

If a person is hyperopic (long sighted) then similar problems arise but the other way around. They will have problems, be slower reading close up and become easily distracted or lose attention when close up than in class work from a board.

The often unconsidered problem is with astigmatisms. This is where focussing on a horizontal line may be slower than on a vertical line.  Refocussing will be taking place during a fixation, processing will be slowed down. The more severe to astigmatism the slower. But the person will usually still be able to read at near and at distance.

I have not considered so far the impact of the reality that most people use two eyes when reading. If one eye is different to the other, the visual system ‘can choose’ to suppress one eye when you read. Normally it will be the one which gives you the most characters per fixation (usually best at focussing on the image).

The reality of course is that may mean one eye for reading a book and writing ( you read as you write)  and the other on a  blackboard etc..

It may be that the system finds managing the direction of two eyes at once more difficult than simply managing one eye. It will suppress one eye.
If not then the image will move in an obvious way which you perceive. Oscillopsia as the system switches attention between the two eyes.( close alternate eyes quickly as you read this if you want confirmation of this effect).

If an eye is suppressed for any reason it is of course still functioning and moving around collecting data. Just subliminally! You will not be aware of it.  But of an image on the retina of your ‘roving’ eye moves cross the retina, it is likely that you system will switch attention to that eye and you will suddenly be aware of the image of that eye and lose the image in the eye you were using. 
All these problems will create attention management problems. And we can see here links with the symptoms of ADD and ADHD.

This roving eye is also likely to be pulling the other eye off its intended target, because of the way the movement mechanisms of the two eyes are linked or ‘yoked’ together. This is likely to increase the difficulties in creating the steady images from the reading eye again limiting the number of characters per fixation with the effects of heightened demand on the central executive and resources available for working memory.

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