So, you might be surprised to see that I’m starting this section with a picture of a rally car at maximum speed.
It’s because fast-fluent reading is a really remarkable process. It’s our macula and fovea working to their maximum capacity, to perform a pretty amazing feat. It’s the rally-driving of reading.
It’s easy to forget that our fovea, the only point of our vision which has really fine detail vision, is a really tiny area. As optometrists, we’re really used to a gently-sloped hill of vision as used in visual field analyses.
In contrast, the ‘hill of visual acuity’ is not so much a hill as a pointy skyscraper. VA drops dramatically as soon as you leave the fovea — even the rest of the macula isn’t that great, and outside the macula it’s negligible.
Reading is a Partnership Between Fovea and Macula
When we look at text, we get the impression that it’s all clear, like this:
But in reality, what our eye sees is more like this:
It doesn’t seem that way, because as soon as we switch our attention to another bit of text we shift our fixation to that bit, and we build up an integrated composite image that is full of high detail. It seems strongly counter-intuitive, I know. But try this — keep your fixation on the 7 in the middle of these numbers, and see how well you can read the numbers to either side.
4583 7 2769
Kind of startling, isn’t it? The numbers are really hard to read, especially those in the middle of each group.
Notice also how hard it is not to let you fixation flicker over toward the numbers as you shift your attention towards them — it’s so hardwired, and that’s why eccentric fixation is easier said than done.
Most people think their entire field of vision is clear and detailed, because we all do such a great job of zipping our macula all over the place. When I’m explaining to patients that we only have a tiny area of fine detail, I describe our vision as being like one of those delicious jam biscuits (Americans — I mean cookies) that have a lovely blob of jam in the middle.
Macula disease only affects the jam. But it’s the best bit of the biscuit! It’s reassuring to know that whatever happens, you’ll still always have the rest of the biscuit, and that’s worth preserving (so, keep up the routine eye exams even if you have end-stage AMD). But it’s still really annoying to miss out on the jam.
The job of the macula outside of the foveal area is to see well enough to accurately guide those saccades so the fovea can efficiently build up the detailed composite image. Recognising faces and facial expressions are good examples of this sort of integrative task, and reading is another. In the case of reading, there’s really only one area of the macula that is of critical importance, and that’s the macula immediately to the right of the fovea.
This area (shown in red) is responsible for guiding the saccades along the line of text.
And this is where the rally car comes in. How come those rally car drivers can drive so fast? It’s because they have a co-driver, a navigator sitting right next to them, who is watching further along the road (and also has maps) and is constantly giving the driver instructions like “Just over this rise there is a 30 degree turn to the left… Past this turn it goes straight, you can accelerate… Slow down now, this right turn is followed immediately by a sharp left turn…” With this information feed, the driver can concentrate entirely on the road immediately in front of the car, and push the car to its absolute fastest speed without crashing.
What happens if the navigator suddenly disappears? The driver immediately has to slow down. Without that information about what’s coming up, the driver will crash if they don’t.
Damage to the Right Macular Field Impairs Reading Fluency
As long as the ‘navigator’ part of the macula remains intact, there can be good potential for fluent reading, even with significant acuity loss.
But as soon as that navigator part of the macula is damaged, fluency is badly impaired, even if the fovea and the rest of the macula is still pretty good.
The total loss of the navigator is why a right hemianopia just destroys reading fluency, even when the VA is unaffected.
A left hemianopia leaves the navigator unaffected, so reading fluency is often still quite good. The patient might run into trouble getting to the next line sometimes, but usually they still do quite well. Holding a ruler down the left side of the column (or simply putting their finger there) can help a lot with making sure they saccade far enough to the left to locate the beginning of the line.
If your patient has a right hemianopia but they still read reasonably well, that’s a clue that they probably have some macular sparing, even if it doesn’t show on their field analyses.
No comments yet.