Introduction
This article explores how afterimages increase our detection of edge and line. This is a non technical article written for artists, designers, and others who wish to be empowered in their creative work.
Afterimage is one of two features of the visual system that act independently to enhance our edge detection and perception of form. The other is lateral inhibition. Lateral inhibition will be discussed in a later article.
If practical, view the images below on a laptop at brightest illumination in a dark room. Although afterimages begin immediately, appreciation of the images is enhanced if 20 to 30 seconds is allowed for them to develop more strongly. If you use readers, you may find that removing your readers when looking at the pictures is helpful, even if the pictures are a little blurred. You can also enhance the effects by covering one eye.
Photransduction for artists
When a photon of light strikes a retinal photoreceptor, it alters (bleaches) the rhodopsin molecules within the photoreceptor. This begins a cascade of events leading to the generation of a signal across the wall of the photoreceptor. This signal is the first step in a long and lightning fast chain of events that leads to conscious perception.
After light bleaches it, rhodopsin is unresponsive to further light until it is regenerated to its light-sensitive state. Because of this, an area of retina that is exposed to a bright stimulus becomes less responsive- "fatigued". Conversely, if an area of retina is exposed to darkness, sensitive rhodopsin molecules accumulate and the retina becomes hypersensitive.
Examples
The images below show how this affects what you see.
Begin by staring at this image for 20-30 seconds until it begins to shimmer, then divert your gaze downward to the “x” in the center of the gray box.
Now do the same with this one:
You probably appreciate that a bright image causes a dark afterimage, and a dark image causes a bright afterimage. The main goal up to this point is just to appreciate these basic afterimages.
Shimmering
Now let's turn our attention to shimmering.
Below, stare at the interface between the light and dark rectangles until the border shimmers strongly. Note the appearance of an evanescent, shimmering, super bright line to the right of the interface between the two squares, and an evanescent, shimmering, super black line just to the left of the interface. The longer you look, the more pronounced the effect becomes.
This shimmering is due to afterimaging. Here's how it happens:
The white box on the right bleaches rhodopsin and the photoreceptors become less sensitive (fatigued). This is what is occurring to the portion of your retina perceiving the bright box on the right. A dark afterimage occurs.
The black box on the left allows light-sensitive rhodopsin to accumulate in the portion of your retina that is perceiving it. The retina becomes hypersensitive. A bright afterimage occurs.
Putting this together: as you stare at the picture, a region of fatigued photoreceptors and a dark afterimage develops on the right, and a region of hypersensitive photoreceptors and a bright afterimage develops on the left.
So far, no shimmering.
If you were able to keep your eyes absolutely, perfectly still, not much would happen at the interface. But you can’t! Tiny horizontal involuntary flickering movements (saccades) are a designed feature of the visual system. If your gaze flicks to the right, suddenly you expose a line of hypersensitive photoreceptors/bright afterimage to bright light. You perceive a super bright line at the edge of the interface. The moment your gaze flicks to the left, you expose a line of fatigued photoreceptors/dark afterimage to darkness- and you perceive a super dark line.
But why the shimmering? You can't saccade to the left and right at the same time! You're not a reptile! So you cannot perceive the bright and dark line at the same time. Each must disappear when the other is perceived.
In summary, afterimages enhance the contrast of any interface between two surfaces of differing luminance by creating a shimmering dark line at the edge of the dark surface, and a shimmering bright line at the edge of the lighter surface.
Complementary Colors also Shimmer
Complementary colors will also shimmer, even if luminance is the same.
Conclusion
Afterimages are the inevitable result of the biochemical processes that capture light and convert it to neural signals. Saccades turn this biochemical imperative into a powerful tool for line and edge enhancement.
Why don’t we notice shimmering in our daily life? We would, if we were to stop and stare at an edge that was stationary with a large luminance difference, or an edge between two complementary colors. More subtle afterimages begin immediately, occur all the time, and have a profound effect on our appreciation of line and edge.
Afterimages enhance line and edge any time there is a discrepency in luminance or color. The greater the difference in luminance and the further that colors are away from one another on the color wheel, and the longer we stare, the more pronounced the effect.
Sometimes a piece of artwork invites the viewer to stare at a single location, and shimmering can result.
Mask IV 2019