If you stare at a bright light for a few seconds and then look away, an after‑image (AI) will be seen. If you blink roughly once a second as the AI fades, it will eventually vanish suddenly.① It may reappear either gradually or suddenly but can now be made to appear and disappear at will. If the after‑image were as a result of retinal fatigue then it should not be possible to choose not to see it.
The next experiments were performed in a dark room. I positioned a computer monitor perpendicular to and about two metres away from one wall. I maximised a Notepad application window with a white background so that the monitor would provide a gradient of low‑level illumination along the wall. You will also need a wide spectrum, diffuse mains lamp. I used either a 15W compact fluorescent works or a 60W tungsten filament pearl bulb. Switch on the mains lamp and stare at it from a distance of about a metre for about fifteen seconds. Switch off the lamp and look at the laptop screen. You should see a cyan AI which will persist for about three-four minutes.
Regenerate the AI by looking at the lamp again. After switching off the lamp, close your eyes and cover them with cupped hands in order to cut out the light from the room. You should now see a yellow AI fading to a deep red at the edge. Wait until the centre of the AI has faded to the same deep red. This should take about as long as the first AI took to fade from view. Stand at 90° to the monitor, looking at the wall it is illuminating, uncover and open your eyes. You should now see an intense dark green AI. This AI will also last about three-four minutes before fading from view.
The blacking out of the eyes is crucial otherwise the AI simply fades. The brain seems to be running a long term averaging filter on the signal from the retina when that signal is above a certain threshold. In the absence of a signal, the brain seems to try to maintain the image by increasing the contrast. When the retinal image exceeds a certain threshold, the filtered image is subtracted from it. This is supported by results obtained by Manzotti.② Why should this mechanism have evolved? It may serve as a predator and prey detection mechanism. Animals like deer and meerkat hold their heads very still when startled. Kestrels hover with their heads motionless. This would cause the retinal image to be accumulated and any movement by predator or prey respectively will become immediately apparent in the difference image.
The second effect may be difficult to reproduce. It helps if you know what you are looking for. A less persistent AI is required so only stare at the lamp for about five seconds. Switch off the light and stand at 90° to the monitor, looking at the wall and close your eyes. Turn gradually towards the monitor, thus slowly increasing the illumination on your eyelids, until the AI switches from positive to negative. Watch until it fades from view. Now turn to face the darkest part of the room. The AI will reappear. Watch it until it fades from view then turn back towards the monitor until it reappears. I got bored after performing six such cycles with no loss in the intensity of the AI each time it was restored.
I make no attempt to explain what is happening or why this functionality exists. However, I suggest that it places the after‑image in the lateral geniculate nucleus which is known to contain four parvocellular layers which have a slow, sustained response to retinal signals and whose role is unclear.
In the first experiment, while waiting for the red to migrate to the centre of the AI, relax your gaze, sort of staring blankly rather than looking directly at the AI. Notice how the colours blend into each other. Now look directly at the AI. There are now bands of distinct colours. The boundaries between the colours appear pixelated. A variety of image enhancement processes are being applied here, e.g. contrast enhancement, sharpening and colour binning. These are the same processes that contribute to the unreal vividness of dreams. If woken up suddenly, I sometimes see this layer as a mist of pale blue and magenta spots overlaid onto my vision. An impressive psychedelic light show can seen if I cover my eyes after being woken up in a sunlit room. The effect only last about twenty seconds so you will miss it if you wake up gradually or do not cover your eyes promptly.
|①||Powell, Sumner & Bompas - The effect of eye movements and blinks on after‑image appearance and duration.|
Journal of Vision March 2015, Vol.15, 20.
|②||Manzotti - A Perception-Based Model of Complementary Afterimages|
SAGE Open January-March 2017: 1–10.
© Copyright 2021 Andrew Jarvis.