Saturday, February 18, 2012

Seeing From Eyes to Mind

Many years ago, I was taught in biology class that, while the eyes register visual stimuli, it is the brain that actually sees. I dutifully learnt the fact, but didn't ever truly believe it, or rather didn't really understand it. Experientially, as someone growing up with gradually worsening myopia (short sight), it seemed that my eyes were doing all the work, and I didn't comprehend how the brain was really involved.

Sure, I understood about binocular vision and depth perception and all that stuff, but it wasn't until I damaged my brain and suffered some visual problems as a result that it became much clearer. I now understand better than ever that the eyes 'merely' process visual information, the brain sees.

To explain how this has worked for me, I'm going to bore you with my current deficits again. As you may have gathered, 'deficit' is the charming circumlocution used to destigmatise phrases like 'brain damage' and 'disability' that nevertheless makes it pretty clear to me that I am less than once I was. While unfortunately I recall what it was like not having deficits, fortunately I also stand to recover the damage done; I prefer to focus on recovery than damage.

Anyway I have two or three deficits to contend with at the moment: double vision (diplopia) and one and a half syndrome, which may be the cause of the third, oscillopsia. My diplopia is diagonal, with the left eye image up and to the left of the right eye image. The oscillopsia I've had in both eyes, and is worse first thing in the morning, or late at night if I'm tired. I'm generally worse in all respects at those two times: Starting the day with a mug of tea seems to help.

One and a half syndrome is sufficiently rare that it causes excitement bordering on glee when experienced stroke neurologists encounter it. I'm often the first case they'll have seen outside of an exam question, and have in fact been asked if I would mind being an exam question (I don't mind at all: I'm game for anything that involves students in neurology and all research studies along the way). The wikipedia article describes the syndrome well, and I won't repeat that here, but you might want to have a read: one and a half syndrome.

What it means from the inside is that my eyesight is kind of screwed for the time being.

The most debilitating effect has been the diagonal diplopia. Soon after my stroke I found that it was practically impossible to operate with both eyes uncovered. My brain could make almost no sense of the competing visual fields offered by my eyes. This was quite peculiar, because although at a higher level I am able to recognise intellectually that I am only holding up a single right hand, my brain cannot rationalise the two hand images into a singular binocular image and insists on 'showing' both. The data one part of my brain is receiving can not be processed into the type of image another part of my brain is used to receiving, so it just gives up and passes on the two superimposed images.

The vertical offset in the diagonal plays havoc with my balance. Although proprioception is the lion's share of balance, there's a considerable visual component as well, and having one eye tell me that the floor is this way, while the other assures me that, no, the floor is that way is a surefire way to keep falling over. My proprioception was shot, too, so all in all the most convenient way to help my brain pick one image over the other and stick to it was to occlude one of my eyes. Since my left eye is the one that doesn't track to the left, and had developed a squint (which I thought was unspeakably ugly and made me more likely to be treated like a fool), I covered the left lens of my glasses with a special frosted covering, and tried to get on with it.

Operating with only one eye is more challenging than I had thought. I had an idea that it would be awkward, but had no idea quite how bad. My undying admiration goes to my friend with a glass eye. I have no idea how she makes everything appear so fluid and easy. Regardless, lack of depth perception has been unpleasant in many ways, and I'm reasonably confident in saying you will have no idea how unpleasant it is until you try it while your balance is already off.

Occluding my left eye made the diplopia manageable, so I can do things like use my laptop and read a book at all (slower, of course, but luckily I can still read), but it highlighted one of the features of one and a half syndrome: nystagmus in my right eye. When I look to the right, I'm able to do so with both eyeballs, but the right eyeball flicks rapidly to the center and back again. To add to this is the experience of oscillopsia: When I focus my gaze on a single point, my eyeballs wobble up and down a little..

These deficits are what has made it very clear that my brain is doing theseeing part. Most of my deficits are in fact neuro-muscular: There's nothing wrong with my eyeballs that wasn't wrong with them beforehand, instead it's the connection between my brain and my left lateral rectus that's been most clearly damaged (the VIth cranial nerve), and my brain's fine control over the whole extra-ocular musculature that has also suffered. 

To form a binocular image, the brain instructs the eyeballs to keep a single object focused on the macula, and combines the data that the two eyes present into a single image with depth This is something we learn as babies: Watch a six-month-old's eyes wander around and the focus sometime. Lacking the muscular control to move my left eye so that it focuses on the same spot as my right (remember the squint I mentioned?) my brain ends up with the double vision. With one eye effectively taken out of the picture, it should have been plain sailing, but it wasn't.

It soon became apparent to me that my right is moving involuntarily. Not much, but if I focus on one word on a page or screen, the motion is obvious to me. Despite this, though, I perceive a largely steady, static world around me. My eye is jiggling around a bit (not enough for truly comic effect, although I do have moments of feeling like Mad Eye Moody), but the world is still. This is the brain seeing. In effect, it's learned to recognise the jiggle as a kind of scanning of the world, and maintains a static image for me to use, because the alternative is chaos.

In effect, my brain is discarding the fuzzy occluded data from my left eye, then applying Occam's Razor to the data it's getting from my right eye, and providing me with that filtered perception. It makes a lot more sense that the world is not constantly jumping up and down, so my eyes' raw data has been subjected to reason at a very low level.

All of which is my own understanding of what's happening in my brain and eyes. Neurologists and ophthalmologists who actually want to talk to stroke patients about their vision are few and far between, and their message has been consistent on both continents: wait four to six months, eye problems with stroke usually clear up on their own. Given how terribly debilitating the visual problems I have are, and how depressing the apparent lack of change in my vision has occasionally been, it's a little surprising and a lot disappointing how little information was available to me. I guess four months doesn't seem that long when you're a clinician looking at a long stream of stroke sufferers ready to start their recovery (or decline), but it has felt like an age to me.

Happily, a few weeks ago I started to suspect that the diploplia was steadily resolving, and that seems to have continued. Like everything else with stroke recovery after the very early stages, it's abominably slow, but I think it's real. The difficulty is that my eyes don't get a lot of time to practice looking at stuff and being wrong and being corrected by my brain: That feedback loop that's been unused since I was a babe in arms needs some exercise. Fortunately there are two times when I take my glasses off, and I'm used to being unable to see (really strong myopia): in the shower and in the pool.

In both cases, I'm usually thinking about something else, so my eyes have free rein to do whatever they want; I put very few demands on them. In the shower, I'm focused on the mechanics of getting clean; in the pool, I'm concentrating mostly on the form of my breast stroke and being as even as possible. Hitting the end of the pool isn't really a problem. So it looks like, with the swimming in particular giving my eyes a bit of a workout, and with the exercise I've been forcing my extra-ocular muscles to take, the double vision is on a path to recovery.

The one and a half syndrome may be with me for good, but the squint is gone, my left eye is tracking in tandem with the right (mostly) now, and itseems to track to the left past the midline. Which is either a positive sign that I may recover from the syndrome, or evidence that the extra-ocular muscles are compensating cunningly. Finally, I have hope (but nothing else) that the oscillopsia is related to the double vision, with both eyes trying to find a common focus, and so far failing. If that's the case, all to the good. If not, well it could have been a whole lot worse.