Thursday, 29 November 2012

How far away is a reflected ghost?

ShadowA quick update on the 'door ghost' or 'doppelganger' story (catch up here). I saw the ghostly figure, reflected in frosted glass, once more. It definitely appeared to be standing BEHIND me in the reflection. However, I realised that I could see my hand, responsible for the misperception, in the same view in my peripheral vision! And it looked a lot closer, just as you would expect! And even though I could see both the 'original' and the 'reflection' in the same view, it still looked like there was someone standing behind me until I shifted position.

If you look at yourself in a mirror it is as if you see yourself through a window at the same distance on the other side. This has the effect of making your image in the mirror half your actual size, though, crucially, it doesn't appear that way. Any movement you make will appear half the amount you actually moved. So although you know you are, say, 1m from the mirror, your image shows you as apparently 2m away and half the size. There's an excellent discussion of this (with further explanatory links) here. Though an image looks twice as far away as it is, we also ' know' that it isn't really, introducing a contradiction which can produce strange perception effects. Research has shown (see previous link) that people generally misjudge the size and distance of objects in reflections. I believe that is a key point in understanding how the remarkably persistent 'door ghost' misperception works. Even seeing where my hand really is does not break the strong impression that is actually the feet of a person, or ghost, standing, rather menacingly, directly behind me!

I mentioned a few days ago that I thought that feeling mentally 'detached' may contribute to the effect. I still believe this is true but I think the basic mechanism is likely to be a fundamental misjudgment of size and distance. With a bland background offering no visual cues to size, my brain cannot correct its erroneous perception of my hand. It is only by shifting my hand, or my whole position, that additional visual information arrives that corrects the misperception. Is the mystery of the doppelganger finally solved? Maybe but it is certainly useful to have a reproducible misperception to observe regularly! And it looks as if there is another type of situation that can lead to misperceptions - reflections of objects against featureless backgrounds!

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