Mental Illusion
I was travelling around the Copeland constituency and Borough today doing various things as part of the campaigns for the election of the next MP, Mayor, and councillors.
On the way home after my last stop I was driving through unlit sections of the A595 and was struck by two illusions. Neither was dangerous, both were interesting. I've titled this post with a reference to mental illusion because I'm convinced that each of these events was a cerebral phenomenon rather than an optical one.
The optical condition which set up both events is that it is a very dark night this evening. There was a moon visible ahead of me for part of the journey, but it was an almost perfect new moon, a very thin crescent which gave very little light. (Ironically much like the pictures others took of the eclipse yesterday which I did not see.) There were few if any stars visible, and the sections of the A595 I was driving on were in the countryside with no street lights, so it was extremely dark.
The first illusion was that on several occasions I fleetingly noticed the impression of a large black rectangular shadow, even darker than the sky, shaped like a bridge or the kind of gantry that motorway overhead signs are hung on. It was as if one was about to drive under such a gantry, but it was only an impression and vanished instantly.
The relevant part of the A595 has a very few long straight sections on which one could safely drive at or close to the national speed limit, but it also has some sharp bends which only a maniac would attempt to take at that kind of speed.
I don't need a satnav to find my way home along the A595, but because it was so dark, I had turned on my Satnav and was using the picture view to give myself a bit of extra warning of sharp bends ahead.
I realised after about the fourth time that I had the impression of a dark shape ahead, that it was almost exactly the same shape as the dark rim of the Satnav - a dark outline surrounding the bright image outlining the shape of the road ahead. The same shape - but much larger.
If the human eye had the ability which some cameras have, to magnify or shrink an image so as to "zoom in" on an area, there would have been a simple optical explanation for what I appeared to be seeing: persistence of vision. E.g. when returning one's gaze to the actual road ahead after a quick glance at it's image on the Satnav, there could be a transitory impression of the dark outline framing that image.
Now here is the interesting thing. Persistence of vision, which is what enables all films and video featuring movement to work, is normally a characteristic of the eye. However, the human eyeball does not have a "zoom" feature: our ability to change the focal length of the eye's lens is used purely for focussing.
But although the mark one eyeball does not have a "zoom feature," the brain does.
The most likely explanation of the illusion would appear to be that as I took occasional glances at the satnav to check for bends in the road ahead, the dark edge of the satnav was somehow retained in some recess of my short-term memory and occasionally briefly created a slight influence on the mental picture in my consciousness while I was looking at the road ahead.
The second "illusion" was an illustration of how much what we think we see is actually "filled in" by the brain.
I was approaching one of the villages in West Cumbria, on a stretch of the road where there was no illumination, and a vehicle came into view ahead of me, moving slightly more slowly. What I could actually see was just its' rear lights and outline: not much more than a shadow. The shape appeared significantly taller than a car, with a round top, and was evidently moving at about ten miles an hour below what would be the speed limit for a car, e.g. at the speed limit for a vehicle towing a trailer.
My mind interpreted this shape as a horse box being towed by a car which was hidden on the other side of it, the image generated in my head was that of a horse box. If I had turned off before reaching the lights of the village, and I had shortly afterwards been asked if I had seen any trailers on the A595 and what kind, I would have answered that I had seen a horse box.
Until the instant that the vehicle perhaps a hundred yards ahead of me reached an area illuminated by powerful street lights and came clearly into view. It was as if someone had flipped a switch and transformed the image in front of me on a projector screen: it wasn't a horse box at all, but a milk transporter.
It was the sort of moment when you experience what is normally hidden from you, namely how much of what we think we see is created by our brains filling in the details.
On the way home after my last stop I was driving through unlit sections of the A595 and was struck by two illusions. Neither was dangerous, both were interesting. I've titled this post with a reference to mental illusion because I'm convinced that each of these events was a cerebral phenomenon rather than an optical one.
The optical condition which set up both events is that it is a very dark night this evening. There was a moon visible ahead of me for part of the journey, but it was an almost perfect new moon, a very thin crescent which gave very little light. (Ironically much like the pictures others took of the eclipse yesterday which I did not see.) There were few if any stars visible, and the sections of the A595 I was driving on were in the countryside with no street lights, so it was extremely dark.
The first illusion was that on several occasions I fleetingly noticed the impression of a large black rectangular shadow, even darker than the sky, shaped like a bridge or the kind of gantry that motorway overhead signs are hung on. It was as if one was about to drive under such a gantry, but it was only an impression and vanished instantly.
The relevant part of the A595 has a very few long straight sections on which one could safely drive at or close to the national speed limit, but it also has some sharp bends which only a maniac would attempt to take at that kind of speed.
I don't need a satnav to find my way home along the A595, but because it was so dark, I had turned on my Satnav and was using the picture view to give myself a bit of extra warning of sharp bends ahead.
I realised after about the fourth time that I had the impression of a dark shape ahead, that it was almost exactly the same shape as the dark rim of the Satnav - a dark outline surrounding the bright image outlining the shape of the road ahead. The same shape - but much larger.
If the human eye had the ability which some cameras have, to magnify or shrink an image so as to "zoom in" on an area, there would have been a simple optical explanation for what I appeared to be seeing: persistence of vision. E.g. when returning one's gaze to the actual road ahead after a quick glance at it's image on the Satnav, there could be a transitory impression of the dark outline framing that image.
Now here is the interesting thing. Persistence of vision, which is what enables all films and video featuring movement to work, is normally a characteristic of the eye. However, the human eyeball does not have a "zoom" feature: our ability to change the focal length of the eye's lens is used purely for focussing.
But although the mark one eyeball does not have a "zoom feature," the brain does.
The most likely explanation of the illusion would appear to be that as I took occasional glances at the satnav to check for bends in the road ahead, the dark edge of the satnav was somehow retained in some recess of my short-term memory and occasionally briefly created a slight influence on the mental picture in my consciousness while I was looking at the road ahead.
The second "illusion" was an illustration of how much what we think we see is actually "filled in" by the brain.
I was approaching one of the villages in West Cumbria, on a stretch of the road where there was no illumination, and a vehicle came into view ahead of me, moving slightly more slowly. What I could actually see was just its' rear lights and outline: not much more than a shadow. The shape appeared significantly taller than a car, with a round top, and was evidently moving at about ten miles an hour below what would be the speed limit for a car, e.g. at the speed limit for a vehicle towing a trailer.
My mind interpreted this shape as a horse box being towed by a car which was hidden on the other side of it, the image generated in my head was that of a horse box. If I had turned off before reaching the lights of the village, and I had shortly afterwards been asked if I had seen any trailers on the A595 and what kind, I would have answered that I had seen a horse box.
Until the instant that the vehicle perhaps a hundred yards ahead of me reached an area illuminated by powerful street lights and came clearly into view. It was as if someone had flipped a switch and transformed the image in front of me on a projector screen: it wasn't a horse box at all, but a milk transporter.
It was the sort of moment when you experience what is normally hidden from you, namely how much of what we think we see is created by our brains filling in the details.
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