How do we see?

You help numerous patients to see every day, but when was the last time you thought about how we see?

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Author: Daniel Hardiman-McCartney MCOptom, Clinical Adviser
Date: 7 March 2016

It is easy to forget that sight is a combination of what our eyes detect and how our brains perceive it. Firstly, what does that mean for optometrists? 
When someone says they cannot see something, is that due to the malfunctioning eye or a deficit in the brain? This is complicated further by the retina being considered an extension of the brain, something that it is all too easy for us to forget when in day-to-day practice. So, since what we think of as our sight is really our perception of what we are seeing, at any one moment our sight is comprised of little bits of data from our eyes being added to a pre-existing image in the brain. 

But where is this pre-existing image derived from?
Everything that we ‘see’ starts with nerve impulses from our eyes but that does not mean that the images we experience in our minds are continuously based entirely on the information that our eyes are taking in. Our perceived image exists only in our brain. A hallucination is an example of how our brain sees what our eyes do not.  

Our whole sensory experience is actually perceived slightly delayed. Due to the fact our audio and visual processing systems work at different speeds, our ability to see and hear our hands clapping together simultaneously is based on careful choreography of our mind to process the information of our individual senses. Hence a delay is required between the action and our perception of it. 

OK, but if it could be the brain or the eyes, how can you know whether it is misinterpretation of what we see, or seeing something that is absolutely not there? 
Well if you define a hallucination as seeing a face instead of a shadow with face-like characteristics, then maybe it is possible to say the brain is not interpreting the information from the eyes correctly. But if you see a face when there is no visual stimulus which could objectively give rise to this interpretation, then it is hard to deny the part that the brain is playing in creating the world you perceive. Of course the brain is always creating the world you perceive, but this is an example of how the eyes are data collectors for the brain rather than objective perceivers of reality. 

So how do we perceive reality?
Well first, what is reality? Is it something that is objectively knowable or not. For example, is it something that is definitely knowable if we could bypass our eyes and our brain having to interpret it? Or is any objective truth impossible to prove because our theories of knowledge can only ever be based on our best guess?  

OK, this is all well and good, but what does this mean for my patient?
It means that the visual reality that the patient experiences, what they ‘see’, is as dependent on their ability to perceive as it is on the functioning of their eyes. 

In the example of visual neglect following a stroke, the brain is physically unable to create an image of the world, despite the eyes feeding the brain all the information it needs to build the whole picture. So for this patient, what are they ‘seeing’? A greater appreciation of that kind of experience can enrich our understanding, and in turn our clinical practice, even to the point of being able to identify the difference between defects of sight and normal natural variation in visual experience.

The much debated dress from last year is the perfect example of the latter, where the ambiguity of the image created two equally plausible ways to perceive the colours of the dress. The colours people saw were not due to a defect in their visual-perceptual system but due to an image so nuanced that it made healthy visual systems equally likely to interpret the same information in two different ways.  

Good try, but is there someone who can explain all of this a bit better? 
Yes there is! Sir Colin Blakemore, Professor of Neurobiology and Philosophy at the University of London, a  world leader in the interaction of the eyes and the brain, will be giving a more detailed overview, as well as talking about the latest research in this area, at Optometry Tomorrow as the Keynote Lecture at 1:30pm on Monday 14th March. You can also listen to his seminal series of Reith lectures on the BBC.

Okay then. Cup of tea?
Yes please. 

Daniel Hardiman-McCartney FCOptom
Clinical Adviser, The College of Optometrists

Daniel graduated from Anglia Ruskin University, where he won the Haag Strait prize for best dissertation. Before joining the College, he was Managing Director of an independent practice in Cambridge and a visiting clinician at Anglia Ruskin University. He has also worked as a senior glaucoma optometrist with Addenbrooke’s Hospital in Cambridge, with Newmedica across East Anglia and as a diabetic retinopathy screening optometrist. Daniel was a member of Cambridgeshire LOC from 2007 to 2015 and a member of the College of Optometrists’ Council from 2009 to 2014, representing its Eastern region.  

He is Clinical Adviser to the College of Optometrists for four days each week, dividing the remainder of his time between primary care practice and glaucoma community clinics. Daniel is a passionate advocate of the profession of optometry, committed to supporting all members of the profession and ensuring patient care is always at the heart of optometry. He was awarded Fellowship by Portfolio in December 2018.

Martin Cordiner
Head of Research, College of Optometrists

Martin graduated with a Masters in Modern History from York University in 2005, having completed his BA there in 2003. Since then he has worked in project management in higher education before joining the College and its fledgling research department in 2009, where he now supports the Director of Research and manages the research team to implement all elements of the College’s Research Strategy. 

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