Neural or Visual Adaptation


Refractive surgeons often talk about neural adaptation being important in respect of vision correction.

So lets clarify what this really means in two different scenarios.

  1. Laser eye surgery than has had a ‘good’ outcome
  2. Laser eye surgery than has had a ‘bad’ outcome (they call this a sub-optimal result)

Scenario 1

In scenario 1, the patient although having 20/20 vision (and NOT suffering from debilitating distortions) is complaining that their vision is not correct. Something is not quite right, its not the same. The sharpness has gone, they are unhappy.

Several months pass by, perhaps a year, and they have now adapted and are fairly happy. What has actually happened is that their brain has gotten use to the new quality of vision, i.e. the way the world now looks.

Things still aren’t as sharp as they used to be, nor has there been any physical change in their sight, all that has happened, is that they have forgotten how sharp and clear their vision was originally.

Remember lasers cannot give you HD vision, they cannot improve upon quality of your vision at all, and it is highly likely you will not get the quality of vision that you had in your glasses. But you can get used to it, and as its now impossible to compare the old vision, there is no way for you to complain about what has changed.

The patient has now adapted to having less sharp vision. However for a number of people, for whom sight is very important, for instance those who watch movies on High Def TV’s or use computers daily, they will notice that certain things do not look the same. For some people, colours do not look as vibrant and they feel that everything is a little less sharp.

Scenario 2

Now lets look at scenario 2, bad outcome in that the patient was left with severly distorted vision. When I say severe, I mean, they are unable to work under artificial lighting without feeling disturbed by what they see, they can’t watch TV, go to the movies, participate in sports (in any non daylight envrionment), generally their life is a living nightmare and it is torturous. It is no wonder so many of these patients are miserable, depressed and even have suicidal ideation.

The first thing you need to realise is that distortions can be measured using wave scans and ray tracing devices. Refractive surgeons use these devices and can quantify your distortions into a magic number called the RMS (Root Mean Square).

In a normal un-operated person, the RMS value is about 0.25 (over the entire scotopic pupil). The higher the number, the worse the vision. For scenario 1, if everything went to plan, its likely their RMS is now higher at 0.3 or 0.4 even, the reason for this, is that refractive surgery induces aberrations (distortions) and therefore this number goes higher.

In scenario 2 however, this value is now grossly high, in some cases up to 2.0 or even 3.0. What the refractive surgeons don’t tell people is that the human brain can only adapt to a level of about 0.45 RMS (of spherical aberration according to Professor Dan Reinstein), this is over the maximum pupil, or at the very least up to the maximum mesopic pupil size. Refractive surgeons do not like to generally admit to their patients that there is a threshold and that once breached, it will bother the patient forever. They simply tell them to keep waiting for nerual adaptation, usually until its too late to sue for negligence (even if you have enough time the informed consent will prevent you from suing is most cases).

You can imagine the threshold like pain, you can tolerate a certain amount of pain in the body, for instance a dull headache or some minor injury, but if the pain is too great, it is overwhelming and you can never get used to it. Perhaps like a migraine or a fatal wound, or child birth.

When you start to look at neural adaptation in these respects you now know that some people who are happy(ish) with their refractive surgery outcomes, will have a lower RMS and those who are not happy and cannot adapt have a higher RMS.

More frustratingly, the wavescans are not generally capable of measuring beyond 7mm, even though many people have 8mm or even 9mm, sometimes even 10mm pupils.

These distortions (and therefore the RMS value) actually get exponentially worse as pupil size increases, so those with larger pupils (more than 6.5mm) are at much higher risk than those with smaller pupils. This is a proven fact, and warnings can be seen on the FDA website and on websites for many of the laser manufacturers – if your refractive surgeon says that pupil size is not an issue, then he/she is contradicting the FDA and manufacturers such as AMO and Alcon, as well as a number of notable opthamologists (James Salz for example).
*Some definitions

  • Photopic – Vision in bright daylight
  • Mesopic – Vision in artifical light, down to quite dim light (like in a movie theatre)
  • Scotopic – Vision in no lighting – A moonless night outdoors in the countryside , with only starlight to see by (pretty dark!)
  • RMS – Root Mean Square (magic value to quantify your visual quality)
  • Spherical aberration – How spherically shaped your cornea (and sometimes lens) is. More spherical aberration, the worse your dim light vision.