People often ask how I got interested in the brain; my rhetorical answer is: ‘How can anyone NOT be interested in it?’
Vilayanur S. Ramachandran
I’ve been reading Ramachandran’s book Phantoms in the Brain, and it stated me thinking again about the difference between perception and conception.
I am the only patient I always have available and so I asked myself the question “Where are you blind?”
Not in the eyes. I’ve had a whole battery of tests. Wearing glasses I have 20/20 vision, and the tests didn’t indicate any problems with the eyes themselves. Luckily for me they are fine.
No, the problem is at the back of my head, where the brain processes signals from the right-hand side of the world. I know this conceptually, from studying how the visual system works and from the evidence of the brain scan, which shows the visual cortex has been damaged by a stroke.
But what I perceive is entirely different, and of course it is. How could a visual problem have a subjective feeling of coming from the back of the brain? As you’d expect it actually feels as though it is in the eyes. Well, not in the eyes, in the eye: the right eye to be precise. I have absolutely no sensation whatsoever of there being any problem in the left eye, even though its vision loss is equally severe. Although I know conceptually that both eyes are affected, and can easily verify this with simple “wiggly fingers” tests, it is impossible for me to experience it that way. The problem stubbornly goes on feeling as though it’s all in the right eye.
Below I’ve sketched separately what I am capable of seeing through each of my eyes, objectively speaking. Remember this is not what I perceive, it’s what I know.
In the left eye there is a sharp cut-off near the midline and I can’t see anything to the right of that. The right eye is different. From the midline towards the right a big slice of vision is missing, out to about 45 degrees, but then further right I have recovered a fair degree of vision and can see on the periphery.
The results of visual field tests usually show areas of blindness as black, but I haven’t done that because it’s misleading, since I don’t experience blackness. Instead I’ve marked them “blind + neglect” and “filling in”.
What do I experience, if not blackness? Well it’s completely different between the two eyes, but it’s certainly not blackness. My brain has two distinct strategies for dealing with the problem but paradoxically they both hide the blindness from me.
In the left eye the blind area feels exactly like the region round the back of my head. I can imagine what might be there, but I can’t see it, it doesn’t bother me and I don’t expect to see it, or pay the matter any attention. In fact there isn’t a blind area in my left eye, as far as I am concerned, just somewhere that happens to be out of sight and out of mind (hence the term “neglect”).
In the right eye however there is no neglect of the blind area. There probably can’t be, because my vision resumes beyond it, so my brain needs to work out what to do to bridge the gap and construct a consistent picture of the world. So it fills in, as I’ve described before, papering over the blind spot with an approximation that more or less matches the surrounding colours and textures. For example if I’m looking at a brick wall then I see a continuous brick wall. In fact if there was a big hole in the actual wall the bricklayer in my mind would apparently fill it in for me.
As far as I am concerned, I actually see a continuous brick wall, pretty much as I have always been able to. This is not like imagining something, which is a conceptual and voluntary act. For example I can decide to imagine a dinosaur. I cam imagine it rushing swiftly towards me, seeking me out, like a velociraptror from the kitchen scene in Jurassic Park. You can do the same, and if you have seen the film you can probably conjure up something pretty scary. It sure frightens me.
But though imagination is so strong, it isn’t a substitute for perception. I don’t feel I am really seeing a velociraptor, and what I imagine never seems to be out there in the real world. I can decide not to think more about my velociraptor whenever I please and – there, it’s gone!
But when it comes to my blind area I can’t exercise a choice to see nothing, or to replace brickwork with timber cladding: what I see is not in my control, and it feels just like something outside my head, even though it is coming from the inside.
The reason why the brain can do this is another story, but it does mean that I don’t see a black hole, or an area populated by a phantasmagorical host of hallucinations as I did for about a month after the stroke, and the smooth and seamless view I experience now is much less disturbing than either of those possibilities.
So you might wonder why I claim to “feel” a problem in my right eye, since by brain has deviously hidden the blindness from me? What is the difficulty? Well anything I perceive that has been generated “from the inside” is not really additional information to guide me through the world.
There could be a door in the wall but I won’t be able to see it. There could be something written over the door, but I won’t be able to read it. There might be dangers or obstacles I need to avoid, but I won’t know about them. The picture my brain fills in is not to be trusted, and can be covering up multiple problems.
And of course the filling in is no good for reading. It can’t compensate for words I haven’t actually seen. If I look at text I can only read a relatively small portion that I am focusing on. The blind region is filled in with a kind of mushy impression of text, without any of it being properly discernible. And when you read text written in Western language it goes from left to right and a right hemianopic such as me is always therefore reading into a blind spot on the right.
Overcoming these deficiencies has been relatively easy for me, and I’ve been very lucky, since patients often find the problems of hemianopia very difficult indeed to adjust to. I’ve become accustomed to moving my eyes or head back and forth to make sure I haven’t missed anything as I walk about, or to scan the text if I am reading, and it’s become more or less automatic. It would be very interesting to know exactly what I do and I would love to take part in a study. I believe the reading technique I’ve learned is relatively efficient but of course it’s not really possible to monitor ones own unconscious eye and head movements.
And why does my brain seem to have used two different strategies for reconciling the vision loss with a coherent view of the world? Another quote from Ramachandran:
The brain abhors discrepancies.
I think that given my right eye has an area of blindness sandwiched between two areas of vision the brain will naturally attempt to bridge the gap, just as it does with the normal blind spot we all have, but on a bigger scale.
In the left eye there is no such sandwich, so my brain just writes the unseen region out of the script and it joins everything else I don’t expect to be able to see (such as my ears, say).
What would I have seen if the brain damaged had meant the vision in the right eye was completely split at the mid line, with no vision further right than that? I presume that in that case there would have been no filling in, just an area on the right where things were out of sight. If this is correct then someone with complete homonymous hemianopia and no sparing on the side where the sight is lost will have a very different experience from mine.