Blind spots great and small

You probably know that in each eye we have a blind spot. This is the where the optic nerve passes through the retina and is surprisingly large: it occupies perhaps 4 degrees of the horizontal visual field. The blind spots are offset from the centre of the eye by 10 degrees or so. The left one is to the left of centre and the right one to the right of centre.

So for a person of normal sight there is a small but significant gap in the visual field of each eye. You can demonstrate this for yourself with the example below. Close the right eye and fix the left eye on the cross. You may need to click on the image to enlarge it. Move your head nearer and further from the screen and when you get the distance correct the O will vanish!

Of course you can close the left eye and look at the O with
the right eye and this time the X will disappear.

So we are all partially blind. How can it be that we never notice this except under special conditions like the experiment above?

There are two schools of thought. One is that we simply ignore the missing area (the ‘passive’ theory). The other perhaps more intriguing one is that we fill in the missing information. We construct something plausible from what we see around where the blind spot is, using our understanding of the world (the ‘active’ theory).

In support of the active filling in theory consider this variation on the experiment above. As before close the right eye, gaze at the X and move your head closer and further from the screen until the O vanishes. You may need to click on the image to enlarge it. Keep gazing at the X, don’t let your eye wander.

The O and the X as before but now the O is against background of blue line shading sloping one way, the X a background of red shading in a different direction

Where the O was what do you see? Amazingly, not just the colour but the pattern are filled in, aren’t they? I reckon that’s 1 – 0 Active vs Passive.

The blind spot and the philosophical implications for perception has fascinated many, many writers, and there are lots of good articles out there. Two I found are MindHack, which includes a free ebook, and the very lucid article written by Richard Gregory and Patrick Cavanagh.

I can’t find (see? unsee?) my right blind spot now. The vision I lost from my stroke is a bigger blind spot that overlays it! So I could only try the diagrams above from the left eye. Remember the the spot is to the left of central vision, so it’s not in the area of vision I still have.

The fact of there being gap in the visual field because the optic nerve is above the retina seems surprising and I guess it’s an evolutionary accident.


About partialinsight

One evening I had a stroke. Half my sight vanished overnight. Adapting made me grasp how amazing the visual system and brain are. It also taught me to understand disability completely differently and I'm grateful for the lesson.
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One Response to Blind spots great and small

  1. Pingback: Filling in the Rainbow: More Adventures in the Blind Spot | partialinsight

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