Tuesday, 19 January 2016

Insect vanishes then reappears

Apparent ghost insectI am short-sighted. However, even without glasses I can see all but the smallest objects. They might look fuzzy but I can see they are there. Until now, it seems!

Recently, while not wearing glasses, I noticed something small moving on a white wall. The small grey object moved away from me. I assumed it was an insect. Though insects are not common in winter, they are still around. But something struck me as odd about this insect. When I stepped forward to get a better look at it the thing vanished! So, a ghost insect then!

But this is where it gets even warder. As I stepped back the bisect reappeared in a new position. It then moved in exactly the same way as it had before. And as iI approached it again, it vanished again, only to reappear in the same position as before. Looking closely at the 'insect' it turned out to be a grey mark on a white wall. I have seen 'insects' in such circumstances before (see here) but they didn't move. So how can a grey mark on a white wall possibly move?

It may be something similar to autokinesis, which happens with a point source of light in an otherwise completely dark environment. The light appears to move when, in fact, it is stationary. I have come across autokinesis on dark ghost vigils. And I think a similar phenomenon may also happen in broad daylight when there is a small object seen against a featureless background, like a uniformly white wall. However, the daylight version appears much stranger. That's because the rest of the scene looks perfectly normal making the apparent movement look very real and decidedly inexplicable.

I tried a few experiments to see how the phenomenon behaved. Luckily it was completely reproducible. I started by approaching the grey mark to a point where I could clearly see what it really was. It did not move. Then, as I moved away, it started to drift. I saw it approach a corner of the room leaving no trace behind. As I approached what I saw as the position of the mark it vanished and reappeared above the point I was looking at. In other words, I must have moved my gaze downwards without being aware of it. I was able to do this sequence of observations repeatedly and the same thing happened each time. I felt surprise, on every occasion, to see the spot reappear above the point where I was looking. I then tried the same thing with my glasses ON. However, looking at the mark directly, it never moved. But when I put the mark in peripheral vision it appeared to be moving upwards. However, despite apparently continually moving it never seemed to get any higher, relative to other objects nearby.

Clearly this visual perception phenomenon, whether autokinesis or not, can cause some very strange observations. And given that I could see a less dramatic version with glasses on, it may not just affect short-sighted people. It is something to consider when you come across reports of small objects apparently moving across a featureless background.

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