Monday, 27 January 2014

Moving heads

BushesAt first there seemed nothing odd about it. I had often see part of a head showing just above a tall fence I know well. The fence is so high that, for most people, only the upper part of their head shows when they are walking alongside it. The sight of part of a head apparently skimming along a fence can appear a little odd at first but you get used to it.

Having only seen the head in peripheral vision I looked up, curious to see who it was. But there was no one there! There wasn't time for anyone to have traversed the whole visible section of the fence in the fraction of a second it took to look up. Bemused, I watched for a while, in case some one had ducked down out of sight for some obscure reason. After several minutes, it appeared to me that there was no one there. Except a ghost perhaps!

Exactly the same thing happened a little later. Once again I saw a head, in peripheral vision, behind the fence. As before, I looked up to see no one there. The chances of someone doing the same 'trick' twice, and so quickly that I missed it both times, were low. Especially as most people are not even tall enough to see me over the fence!

Then I noticed something I'd previously not seen. There were shadows from a nearby tall bush playing along the top few centimeters of the fence. There were about the same size as a head! Watching the shadows moving, as the bush was blown around in a strong breeze, I realised it was what I'd seen as 'moving heads'. So this was an example of a peripheral vision misperception.

There are some interesting points in this incident. For instance, the misperception relied on several factors coming together. The sun can only throw those particular shadows for a few times a year, at a specific time of day, due to the surrounding objects and angles involved. It also required the combination of a brisk wind and sunshine. And finally, it required expectation on my part.

Expectation, as well as coincidence, is an important factor in misperception. If someone else had been where I was for this incident, they probably wouldn't have noticed the 'moving heads' at all. Even if they had looked up, all they'd have seen is dancing shadows and thought nothing of it. My expectation of seeing heads had created the misperception for me.

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