Here’s one that had me puzzled in the early hours.
Sometimes when I use my phone to check the time in the night, the image persists for a few seconds after I’ve blanked the screen. It was 3:15 this morning and I got to thinking about where that image is and what causes it to persist.
Is it stuck in the receptors of my retina? Or is it stuck in my brain somewhere? i.e. which side of the optic nerve is it on?
I’m talking here about a full colour image that sticks around for 4 or 5 seconds, but only when I’ve been asleep and it’s dark. I don’t mean the shadow you can get after staring at a light bulb or such. That, I imagine, is caused by receptors being overloaded and taking time to reset.
There are two types of after-images like this. One is where you continue to see the same image, which is what you are describing. This is basically caused by neurons being kinda weird. Neurons are a bit reluctant to start firing, and once they start firing they can be a bit reluctant to stop firing. So your eyes and your optic nerve continue to send the image even after it isn’t there. There’s some uncertainty about how much your brain is involved in this.
The other type is when you see a negative of the image, which is the shadow you are talking about seeing after staring at a light bulb. This is caused by the photoreceptors in your eyes basically being chemically depleted, and they send less of a signal back to your brain than the photoreceptors around them. The weakened signal causes a photographic negative of the image of sorts in your vision.
ecg, have you explored machine vision much? 25 years ago I was involved, as an editor, with a whole bunch of analog computation people (some at CalTech, others private) on computational analogues.
So,e extraordinary special-purchase chips were being designed, even then.