Can We Shrink Our Blind Spots?
The science press was atwitter with excitement about the blind spot this week.
Reports hailed good news out of the laboratory of Paul Miller, of the University of Queensland in Australia, showing that with exercise alone it is possible to shrink the naturally occurring blind spot in each eye. There "is quite an improvement," the study's author was quoted in livescience, "but people wouldn't notice, as we are typically unaware of our blind spots."
There's a certain irony, of course, in the idea that we all have a blind spot that doesn't actually cause us to experience, well, blindness. Why speak of a blind spot at all? And, if it's a blind spot that makes no difference to our lives, why is doing exercise to make it get smaller creating such a buzz?
The optic disc — the "blind spot" — has tended to be one of vision science's own blind spots, as philosopher Daniel Dennett was perhaps the first to notice. (My first serious work in philosophy, undertaken with neuroscientist Luiz Pessoa and philosopher Evan Thompson, pursued just this question. Our 1998 paper can be downloaded here.)
What's true is that there is a patch of retina that has no light-sensitive receptors on it. This is the result of basic anatomy: It's the place where the axons of retinal ganglion cells come together to form the optic nerve stretching back into the brain. So, light falling on the optic disc alone produces no neural event.
There are two reasons, one more fundamental than the other, why we don't notice this insensitivity in each of our eyes.
First, the eyes are in nearly constant motion, so what falls on the blind spot now, doesn't a quarter of a second later. Moreover, there are two eyes, so what falls on the blind spot of one eye isn't going to fall on the blind spot of the other eye.
But there is a deeper reason. Hold a bottle in your hands. Notice that you can feel its shape. And notice that you can do so even though you do not simultaneously touch every part of the bottle's surface. There are gaps between your fingers, for example. But you are not blind to the parts of the bottle between your fingers. They are there and you are, as it were, "in touch" with them.
And, so, with the eyes and the optic disk. We don't experience the absence of light corresponding to the optic disk any more than we experience the absence of light corresponding to the space behind our heads, or the absence of tactile stimulation from the parts of the bottle between our fingers.
So, the deeper reason why we don't experience any blindness with regard to the absence of stimulation corresponding to the optic disk is that the eye is not a camera and we don't see our retinal images. The gap that shows up on the retinal image is not a gap for us.
Miller and his colleagues showed that, with training, it is possible to enhance the sensitivity of neurons at the periphery of the optic disk to information projected to the optic disk. This is a striking example of neural plasticity. Perhaps they are right to suggest that it may have therapeutic implications for how we approach pathological blindness such as macular degeneration.
Perhaps there are also therapeutic lessons to be drawn from the fact that we aren't blind at the blind spot?
Alva Noë is a philosopher at the University of California, Berkeley, where he writes and teaches about perception, consciousness and art. You can keep up with more of what Alva is thinking on Facebook and on Twitter: @alvanoe
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