The eyes are the window to the brain

Sun-speckled delicate beads of rain were trickling down our living room window when I noticed a vivid rainbow diving into the lake behind our house.

‘Hey Dad, come and see the rainbow!’ I called.
‘Where?’ he looked around, not knowing where to focus.
‘Over there to the left,’ I pointed, ‘Isn’t it stunning?’
He gazed at it for a long time before commenting, ‘Yes, those three colours in the sky are lovely.’
‘Three colours?’ I questioned.
‘Yes – can’t you see three wonderful colours?’

Rainbows consist of a continuous spectrum of colours but the human eye usually perceives seven: red (the outermost band), orange, yellow, green, blue, indigo and violet (the innermost band). What did my father mean by three colours?

‘What three colours do you see?’ I asked.
‘Red, yellow and blue.’
‘What about green?’
‘You mean the grass?’
‘No. I mean can you also see a band of green in the middle of the rainbow?’
‘No. There are only three colours in the rainbow. Red, yellow and blue.’
‘What about orange?’
‘You mean the orange peel someone left on the ground?’

I was taken aback and deeply saddened. Alzheimer’s disease is known to affect visual pathways but exactly what this means in terms of a person’s visual capabilities is not well understood. I quickly searched for images of rainbows on the internet and showed my father a high resolution photo of a rainbow with seven easily distinguishable colours.
‘Dad, take a look at this picture. How many colours do you see?’
‘Three,’ he responded again.
‘How many colours is a rainbow supposed to have?’ I probed.
‘I think three.’

I choked back my tears as I remembered getting frustrated at him for not being able to find his watch among a pile of objects on the table. ‘Your watch is right there in front of you,’ I’d muttered in exasperation. ‘Take a proper look, will you?’ There is nothing quite so acute as the pain of guilt. Maybe as he was fading from me, the world was fading from him.

There is a specific type of Alzheimer’s known as ‘Visual Alzheimer’s’ or ‘Posterior Cortical Atrophy’ (PCA) that selectively affects the visual processing part of the brain, but the symptoms are not what my father was experiencing.

In 2013 neurologist Scott Turner from Georgetown University, Washington, observed that the retina (the light-sensitive inner layer of neurons in the eye) was up to 49% thinner in mice who had Dad’s type of Alzheimer’s than in healthy mice.

Then in August 2017 scientists from the Cedars-Sinai Medical Center in Los Angeles found telltale signs of Alzheimer’s in patients’ eyes. One of the hallmarks of the disease is the presence of two rogue proteins in the brain. One is called ‘beta-amyloid’ and it forms plaques between brain cells. The other is called ‘tau’ and it forms tangles within cells. The breakthrough discovery was that these same two proteins occurred in the eyes of people with Alzheimer’s disease. The researchers are now developing a simple eye scan to diagnose Alzheimer’s disease even before the onset symptoms.

This raises two questions for me.

  1. How are these plaques and tangles distorting a person’s vision? What is Dad not seeing that I assume he is?
  2. If an easy Alzheimer’s eye test becomes available, would I want to know that I was on my way to developing the disease? How would that knowledge change the way I lived my life?

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