A couple of weeks ago, I had an experience that might be classified as synesthesia. First I’ll describe it, then I’ll explain how the details imply an explanation for synesthetes, if it was in fact an instance of synesthesia.
I was out for a run late in the evening. I’d gone three miles and had about half a mile left. At this point in my route there’s a pretty hefty hill to run up, which I took at a good clip. Afterward I was breathing heavy but not unusually so; the lack of oxygen getting to my brain probably has something to do with causing the experience. If I had any sense at all, I would have tried to isolate the sensory stimulus causing the image, but I didn’t think of it at the time.
In the center of my visual field what looked like an afterimage appeared, although it couldn’t have been an afterimage proper because there had been no visual stimulus to cause it. It just showed up. In shape the image was shaped like two guitar picks, one pointing up and one pointing down, with a little space in between them. They took up about 20 degrees of my visual field, vertically, and maybe 5 degrees horizontally. The color was incandescent yellow-white, exactly like the afterimage that you’ll see if you look too long at a light bulb. This is why I make the connection to afterimages. This vision lasted for about a minute and then faded. Notably, I did not experience this as emotionally meaningful in contrast with most synesthetes (except in terms of intellectual curiosity), but this may be due to my highly unusual emotionality.
Some context for the more theoretical part coming up next:
Negative afterimages are caused when the eye’s photoreceptors, primarily known as rods and cones, adapt to overstimulation and lose sensitivity. Newer evidence suggests there is cortical contribution as well. Normally, the overstimulating image is moved to a fresh area of the retina with small eye movements known as microsaccades. However, if the image is large or the eye remains too steady, these small movements are not enough to keep the image constantly moving to fresh parts of the retina. The photoreceptors that are constantly exposed to the same stimulus will eventually exhaust their supply of photopigment, resulting in a decrease in signal to the brain. This phenomenon can be seen when moving from a bright environment to a dim one, like walking indoors on a bright snowy day. These effects are accompanied by neural adaptations in the occipital lobe of the brain that function similar to color balance adjustments in photography. These adaptations attempt to keep vision consistent in dynamic lighting. Viewing a uniform background while these adaptations are still occurring will allow an individual to see the after image because localized areas of vision are still being processed by the brain using adaptations that are no longer needed.
This effect isn’t restricted to light and color, it’s also related to perceptual processing. If you watch a waterfall for too long and then look at a brick wall, the bricks will appear to move upwards because of the same sort of perceptual fatigue, with respect to processing motion.
Putting all of these pieces together, I figure this explains the mechanism for synesthesia. When a synesthete experiences synesthesia, it’s accompanied by a rather shocking loss of blood flow to large portions of the brain, to an extent that would typically suggest brain damage. It seems reasonable to me that loss of blood flow to a region related to a particular sort of perceptual processing would artificially cause this sort of afterimage fatigue. For instance, loss of blood flow to the visual cortices in the regions that process “red” and “dots” would cause a person to see green dots, like an afterimage.
If this is correct, it implies that something in the brain is miswired to turn off blood flows upon perceiving particular stimuli, or something similar to depress electrical activity. E.g. Upon processing the number ‘7’, a synesthete’s brain might turn off the blood supply to the neurons that correspond to “red” in that part of the visual field, causing it to appear green.
Aside from all of that, I want a term for the screen within our visual field where things like afterimages are projected. This is also where the homonculus appeared during my misadventure with marijuana. If anyone knows of a name for this screen, I’d love to hear it.