Tuesday, 13 November 2012

A possible mechanism for the effect of colour on reading performance

Introduction to a likely mechanism for the colour background settings effect on reading from a computer or the effect of colour on reading performance.

(This is a simplified version and ought to lead to further questions)

Last night I was trying to assist a student on Skype and realised that an attempt at explaining the mechanism might be useful.

When we read the image is focussed on the retina with the centre of attention (the first few letters of the word) centred on the middle of the fovea (the most ‘accurate’ part of the retina)

  1. The fovea only responds to red and green light. 
  2. How well the light from the image of this first group of letters, is focussed on this central patch of cells, will affect how many milliseconds it takes to ‘compute the image’ which will allow us to identify the letters /word.
  3. The eyes are held sort of steady (not totally) for around a third of a second.
  4. While it is being held steady, pigment molecules are capturing bits of light (photons) in each of the cone cells.
  5. The ‘red’ sensitive cone cells are collecting lower frequency (longer wavelength/lower energy) photons than the ‘green’ sensitive cone cells.
  6. Every time a pigment molecule catches a photon it releases an electron.
  7. These electrons are what really give rise to our vision, our reading. These bits of data contribute to the computing the image and ultimately reading.

When a pigment molecule has released an electron, it twists out of shape and is no longer able to catch another photon.

 It can only do that again if it leaves the cone cell, travels into another cell, which actually surrounds the bit of the cone cell where the active pigment is held. In this second cell it has the electron replaced and twists back into the shape which can catch another electron.

This ‘recharged’ pigment molecule then has to travel back to the cone cell it came from and once again catch a photon and allow vision to ‘continue’.

 If the recharging/ retwisting process cannot keep up with the photon catching, then the cone cells will become less good at catching light and will reduce their ability to work. Images will become fuzzier, less clear or take longer to compute. Reading would be slower or not possible and the system will fail.

In normal visual activity,( reading is not normal in terms of evolution!) the eyes wander around the visual scene, in a series of saccades( fast movements)  and fixations ( nearly steady picture taking) allowing changing ratios of red and green light to hit the cones allowing them to keep up, maintain sufficient concentration of working pigment molecules.

In reading on a white background the eyes are constantly bombarded with large amounts of equally intensive ‘red’ and ‘green’ photons.
If the green catching cells or the red catching cells cannot keep up a high enough concentration of working pigment then the system is compromised and will need rest periods to catch up.

In addition to computing the image the data from the cone cells is the basis of the control of the muscles, moving and holding the eyes on target, keeping the eyes working together and landing at the right place for the next ‘picture’. If this system is compromised then the muscles will be instructed incorrectly, land the eyes pointing at the wrong, or conflicting parts of the text. Giving rise to incorrect word identification and changing the perception of word sequences and compromise spelling /word automaticity development

People vary dramatically in the numbers ratio of the red and green cone cells in their foveas and also vary in the concentration of working pigment molecules in each cell.
Changing the ratio of red: green pixel brightness on the computer screen background will directly impact on the efficiency of this system.

It is not possible to work out the correct ‘colour’ using paper. Although you can find out if someone definitely is affected. It is very likely that you will get a false negative.
I will deal with blue cones separately.

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