Tuesday, 12 November 2013

Eye movements for a student with nystagmus reading compared with a fluent reader.

It has been said that Nystagmus is one of the most common forms of visual disability experienced by Schoolchildren. The same would then of course be true of all age groups, since it does not ‘ go away’.

What I have done in this blog is to try to explain and demonstrate how a ‘nystagmus’ actually affects the biology of reading. I have been privileged in my work with undergraduates in the UK; working with and assisting many adults who despite their nystagmus have made it into Higher education. With each one I have had the opportunity to work with them for several hours in my work with OmniRead and before that TintaVision.

I have been able to work with them to reduce the barriers to studying which their disability creates.

All this work is done objectively, using a binocular eyetracker which allows me to compare the actual dynamics of their eye movements as they read to those students with no reading difficulties.

Together we then calculate the conditions which will maximise their reading performance, by careful  adjustment of the parameters  which control the visual system’s ability to collect and transmit visual data as they read.  All the optimisation work is done using the controlled reading environment of a computer screen using the protocols and software developed by OmniRead and before by TintaVision.

Each person needs their own specific conditions to read the most effectively.  When they use these conditions then the way their eyes collect visual data mimics much more closely the way the most fluent readers do so.

Enjoy this posting . Please post comments or ask any questions that will help you further . There are other postings in the blog which put this work into context.
The graph below shows the eye movements of a Higher education student in the UK reading from a computer screen. This is for a period of 14 seconds.
The data was collected using an infra red eye tracker measuring horizontal eye movement at 300Hz. 

A student with a nystagmus ….

1. Collects and transmits a very small amount of visual data per second compared with a fluent reader.

 2. Almost certainly need to use more computational resources making greater demands on their central executive for visual processing than a fluent reader.

3. Collects reducing amounts of visual data per second as the reading time extends.There is a serious stamina problem.

4. Using optimised reading conditions increases the amount of visual data collected and transmitted per second and can improve the quality of the data, thereby probably reducing the demand for resources from the central executive with the major benefits ensuing from this.

5. A person with a nystagmus has difficulty maintaining a fixation.

A fixation is when the eye stops to collect the visual data allowing edge detection. The computation of the data into lines /edges can be converted into visual images matched against visual images retained in long term memory and enable reading.  This is not really ‘ like photography’ as taught in schools but  more like the way the digital data  from a roadside camera can be used to identify a car number plate. Or the way data is used in object recognition in airport baggage security systems.

The best way of seeing  what is going on is to compare the eye movement of a person with a nystagmus with the eye movement of a fluent reader using a binocular eyetracker.

The graph above shows the eye movements of a typical fluent reader. If we look at the graph as sets of stairs, the flat parts of the steps are when the eyes effectively stop moving for a while ,the fixations, to collect visual data to do the actual ‘reading’. The vertical lines are when the eye moves extremely quickly to position the eyes to take the next picture.  These fast movements are called saccades.

The longer vertical lines are the saccades back to the beginning of the next line of text.

There are 9 to 10  fixations during this 2 seconds. I have marked the fixations in green.
During this 2 seconds of reading, the system is not collecting and transmitting visual data for around 10 milliseconds per fixation, during the rapid movements.  

That  is  for around 100 milliseconds  5% of the time.
This pattern of eye movement is really a modified ‘nystagmus’. 

The nystagmus eye movement pattern  can be considered as a ‘primitive eye  visual search mechanism from before a mechanism developed to allow more extended time to collect and analyse visual data in a more detailed way.  This is partly possible by the development of the types of muscle fibres found in the muscles which control the eye movement. I need to write a posting on that !

Let’s now look again at what happens when a person, with a nystagmus is reading. Look at the graphs below.

What you can see is the eyes moving from left to  right ( the wobbly lines moving gradually up the graph) and after 10 seconds a sudden move back to the left of the page.

The left  eye appears to be continually ‘wobbling’. The right  eye sometimes wobbles, sometimes it does not.  After 11 seconds both eyes start to wobble with a much greater amplitude.

During the 10th second the left eye looks like it is reading moving along the line while the right eye wobbles.  There are 5 wobbles during this 10th second.  What is important is that the reading pattern by the system does ‘change’ over time; sometimes the ‘wobble’ is more obvious, sometimes not.

The duration of the ‘slow stages ( data collection and transmission times) is not consistent. Sometimes the left eye and sometimes the right eye appears to be collecting /sending the most data.

The graph below shows the eye movements after 3 seconds of reading. During these two seconds the right eye ‘wobbles’ 7 times. The left eye appears to wobble about 5 times while the right eye appears to go through an extended fixation.


If we compare this to what happens after 11 seconds when the system goes into a more obvious ‘wobble’/nystagmus; in this 2 seconds there are 6 ‘wobbles’.

Most people when reading take three or four pictures per second, so that is effectively the same as the number of ‘wobbles our  student was experiencing.

If we look at the amount of time being spent actually collecting and sending visual data to the ‘brain’, you can see quite clearly that the left and right eye are able to send different amounts of data and that the   two eyes although acting ‘sort of together’ are to some extent out of step, or phase, with each other.
In the first few seconds of reading by the student with the nystagmus….
the ‘green’ (data transmission) time is far less than the 95%  of time for the fluent reader

1. The fast movements are slower than for the fluent reader.

 2. The ‘slow’ stages are very unstable and actually hardly stop at all, so that the ‘computing of  steady images will be more demanding on the  central executive  leaving fewer resources to  make sense of the ideas in the text.

(Please note though that for even for a fluent reader, when you look really carefully at the eyes during fixations, the eyes do not actually stop. There have to be small movements continuously or they stop collecting and sending data; but these are very small movements.)

The graph of the reading after 11 seconds, shows that the ‘slow movement (visual data collection and transmission time) is becoming more restricted.  Increasing the demand on the visual processing system.

 Now consider what happens in terms of vision during the nystagmus eye movements.

There is no data transmission from retina to ‘the brain’ while the eyes are travelling rapidly,during the saccades.  The transmission only takes place during the moments when the eye is ‘stationary’( the fixations) OR during the slow phases of the nystagmus eye movement, as the eye changes direction.
In the graphs for the student with nystagmus above the slowest phases the eye effectively stops. Often it seems to ‘stall’ as if it is being ‘held back’ as if there is a feedback inhibiting the ‘fast movement’ or saccade.

There is a mechanism for ‘fixing’ but the feedback seems very weak and variable.

The following graphs were made using data when the student was reading using optimised conditions.

The first graph shows all the data collected by the binocular eyetracker with a period of about 2 seconds before they saw the text. This shows the ‘typical eye movement of a person with a nystagmus. There is then a period of around 12 seconds of reading ,when the eye movements are much more organised, starting to look much more like those of a fluent reader.  This reading period is followed by 3 seconds when the text has been removed from the computer screen. The eye movements revert to the typical nystagmus ‘style’.

Using the optimised conditions the visual data collection and transmission time ( green  sections) is a far greater proportion of the time.  There are now quite clear ( although still unstable) fixations.  The fast movement phases are ‘faster’ and a much smaller proportion of the reading time.

The student starts to enjoy reading.


No comments:

Post a Comment