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Hall of Illusions

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 Afterimage
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THINGS TO DO AND NOTICE

     Steadily fixate on the black lightbulb for thirty seconds or more. Try not to avert your gaze. Immediately turn your gaze to the white region on the right ajoining the bulb (or a blank white sheet of paper). You should see a glowing light bulb!

So What's Going On?

     The glowing white light bulb you see on the white screen after staring at the black light bulb figure is called an afterimage. When you focus on the black light bulb, light sensitive photoreceptors (whose job it is to convert light into electrical activity) in your retina respond to the incoming light.  Other neurons that receive input from these photoreceptors respond as well. As you continue to stare at the black light bulb your photoreceptors become desensitized (or fatigued).

      Your photopigment is "bleached" by this constant stimulation. The desensitization is strongest  for cells viewing the brightest part of the figure, but weaker for cells viewing the darkest part of the figure. Then, when the screen becomes white, the least depleted cells respond more strongly than their neighbors, producing the brightest part of the afterimage: the glowing light bulb. This is a negative afterimage, in which bright areas of the figure turn dark and vice versa. Positive afterimages also exist. 

     Most  afterimages last only a few seconds to a minute,  since in the absence of strong stimulation, most nerve cells quickly readjust.


     Desensitization of the retina can be important for survival.  A constant stimulus is usually ignored in favor of a changing one by the brain, because a changing stimulus is usually  more important.  But desensitization also leads to afterimages.


     Afterimages are constantly with us. When we view a bright flash of light, briefly look at the sun, or are blinded by the headlights of an approaching car at night, we see both positive and negative afterimages.


     To prevent permanent damage to your eyes, NEVER look at any bright light source, in particular the sun.  The British psychologist Kenneth Craik  burned a tiny hole in his right retina and permanently scarred his  eye at that spot,  when he stared directly into the  sun for two minutes. DON'T TRY THIS AT HOME!   For the first few  days following his  experiment--in which he wanted to find out whether such a lesion in the eye is visible--he saw a dim orange disk with closed eyes (positive afterimage) and a black afterimage with open eyes.  Fortunately, after a year or so,  Craik's vision at that location in his eye appeared to return to normal.  His brain cleverly filled-in information at this damaged piece of retina.

Second Experiment:
     Notice that if the afterimage is viewed on the screen or on a nearby sheet of white paper, it appears relatively small.  If it is viewed on a distant wall, however,  it appears much larger, even though the size and shape of the retinal image remains the same. The perceived size of the afterimage varies directly with the distance of the surface on which it is viewed. This relation is an instance of a more general perceptual relation known as Emmert's law:  The perceived size of a particular visual angle is directly proportional to its perceived distance.


     The illusion of afterimages appearing to vary in size despite a constant retinal image is precisely  what one would predict if perceived size is governed not only by visual angle but also by distance. The two seemingly different facts, that images of the same size lead to perceptions of different size (Emmert's law), and that images of different size lead to perceptions of the same size (size constancy), actually, illustrate the same principle: Distance is taken into account in computing  the  size on an perceived object from the size of the image falling onto the retina, which is another example illustrating that we don't  just see what our eye tells us.

Third Experiment:
     When you close your left eye and adapt to the figure with your right eye, only your right eye will see an afterimage when looking at a white region, not your left eye. The afterimage does not transfer between eyes. This  helps scientists determine where in the visual system this effect arises.  This type of aftereffect is caused by cells in the retina fatiguing, rather than by cells in the visual cortex, where information from the two eyes are combined. You may be surprised to find that there are afterimage effects that do transfer.

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