Colourless peripheral vision? Yup.

Did you know you have no peripheral colour vision?
Look straight ahead. Without moving your eyes, take note of objects in your peripheral vision. You will see them in colour, right? But that’s an illusion.
You’re thinking: WTF?

Well, yeah, you can’t see colour peripherals because you have (virtually) no colour-sensing cells (“cones”) in the regions of your retina that receives peripheral light. That’s a fact.

So, how is it that you think you can see peripheral colour? Easy: you’re brain automatically adds colour based on colour information it gathers at those instants when you look more centrally to the side.
There’s a simple experiment you can perform with a friend to prove this:
  • Give a friend of yours three cards of the same size but of different solid colours.
  • Have him randomly select two cards, but have him hide that from you.
  • Have him put the two cards one on top of the other and show you one side. In other words, you’ll now know one of the colours – of the card facing you – but not the colour of the card in behind it.
  • You now sit still and stare straight ahead.
  • Have your friend hold the card by your ear (either one) so that you can just see the card peripherally, such that the colour you know is visible to you.
  • Tell your friend to slowly move the front card away, exposing the other one to your peripheral vision, at some time of his own choosing.
  • Your job is to notice and tell him when you see the colour of the card change – without moving your head or eyes.
  • You will not see the colour change at all, because your brain doesn’t know what the other card’s colour is. Even after your friend has moved the first card completely away, you will still see the same colour.
  • Turn your eyes toward the card and suddenly it will change colour as light reflecting from it finally reaches cones in your eye.
  • You will then feel a shiver run up your spine as you think of just easy it is to fool your eyes.
  • Now think of how important it is to accommodate these weird behaviours of human bodies when you design stuff – like control panels, warning lights, coloured signage, and so on.
Recently, quite by accident, I managed to recreate this experiment while on the way home from work.
I was on the northbound Yonge subway. The upcoming stations were College (which has brown walls) and then Wellesley (which has green walls). But I had been distracted by what I was reading, and as the train pulled into the “next” station, I didn’t know which station it was – College or Wellesley.
When I looked up, and straight ahead, my location was such that I could only see out the train’s window peripherally. I saw movement outside as we pulled into a station as the station walls zipped past. The walls were grey (no colour vision peripherally, so I was seeing in black and white).
Then I turned my head to the window, and suddenly the walls of the station turned green. It was a sudden change that coincided precisely with a saccade – – those sudden, jerk like movements of the eye that we perform automatically to gather visual information. One moment the walls were grey; the next they were green. One saccade oriented my eyes well enough that some cones detected the green colour of the walls and my brain did the rest, filling in everything in my vision that was “wall” with that colour, plus memories I had of Wellesley Station. And it was all automatic, without any conscious help at all from “me.”
Cool, eh?

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