Consider two pieces of paper, one black and one white Let's say we were outside in bright sunlight (instead of being in this dingy lecture hall). What color do you imagine they would look like? Still black and white. This is actually a surprising fact!
The image of the black paper outdoors is actually more intense than the image of white paper indoors. Why does the black paper outdoors still look black even though it is physically more intense? We call this phenomenon brightness constancy. Perceived brightness is not equal to the actual physical intensity of the stimulus. Rather, perceived brightness depends on the surface reflectance, independent of the illumination conditions.
This is obviously a good thing that white paper looks white and black paper looks black regardless of the level of illumination. We don't sense the distal stimuli (the surface reflectance) directly because our eye is not rubbing up against the papers. Instead, light bounces off the paper and into our eyes. The visual stimulus that reaches the eyes depends both on the illumination level (indoor light vs. outdoor light) and the reflectance of the surfaces. But we are not actually interested in the visual stimulus itself. Rather, we are interested in the physical properties of the surfaces in the world (the reflectances of the black and white paper) that gave rise to the visual stimulus. So the visual system factors out the illumination and all we typically perceive is the relative reflectance (black vs. white) of the paper.
The light/dark adaptation mechanisms in the retina effectively re-normalize or divide by the average intensity in the image to compensate for (or factor out) the level of illumination. This helps the visual system achieve perceptual constancy: white looks white and black looks black regardless of the level of illumination.
Simulaneous brightness contrast: But, having a brightness percept that depends on the context (mean/average light level) also predicts some other (perhaps counter-intuitive) phenomena. Here's one example.
Compare the brightnesses of the two gray squares. The one on the right should appear brighter. In fact, this is a visual illusion; the two central gray squares are physically identical, one surrounded by white and the other surrounded by black. This illusion can be explained by what we know about the visual processing in the retina. Retinal responses depend on the local average image intensity. On the right, the background is black so the average intensity there is pretty small. Here we divide by a small number yielding a brighter percept. On the left, however, the background is white so the average intensity there is pretty large. Here we divide by a large number yielding a darker percept.
Here's a related visual illusion.
Testing the theory: At this time, it's worth pointing out how to do an experiment to test this theory. Let's say we do a detection/discrimination experiment. We have the subject adapt to a certain background/ambient level of illumination. Then, we measure the threshold for detecting a test light superimposed on that background. To do this properly, we use a forced choice so that the test light is presented on only half the trials. We measure hits and false alarms to get d' and determine the test light intensity that yields d'=1. Then, we repeat the whole experiment over again using a different level of background/ambient adapting light. What do you expect will be the outcome of this experiment? How will the detection threshold depend on the level of the adapting light? Why? Discuss...
How could you do an electrophysiological experiment to test this theory? Discuss...
Summary (so far): The visual system is designed to try to achieve a perceptual constancy. Black looks black and white looks white regardless of the level of illumination. The putative mechanism for brightness constancy is light adaptation - retinal responses depend on average intensity. Most of the time this works great. However, sometimes it screws up, e.g., yielding the simultaneous brightness contrast illusion.
Notice the illusory gray spots at the intersections, and the fact that these spots depend to some extent on where you fixate. Consider how ganglion cells respond to this stimulus.
The intersections of the white "streets" in the pattern are surrounded by more white. This results in more inhibition from the surround in on-center, off-surround receptive fields.
But why do the gray spots disappear when you look at them? Receptive fields sizes depend on retinal position. Those in central fovea (corresponding to where you fixate) are the smallest. They may be so small that they fit inside the intersection entirely, resulting in no difference in ganglion cell response between the intersections and the streets in central fovea. Does this explanation really work? Discuss...
Mach bands are a similar illusion with a similar kind of explanation: spatial filtering in ganglion cells. Once again, we see that the retina emphasizes edges. See text book or better, the online copy of the powerpoint media for details. Mach bands are typically not perceived near step edges (despite what your textbook says), but rather near gentle, ramp edges (as above).
Take this idea to the extreme: perhaps the brightness percept depends only on contrast at edges. The two squares at the bottom look like uniform gray, even though the actual intensities (plotted in the graph) are not. Removing the occluder (top) makes it look completely different. Now you see the edge and the squares appear to have different lightness. The contrast at the central edge is detected by retinal ganglion cells, and this determines the perceived lightness.
This figure shows different types of edges:
Brightness judgements can be influenced by high-level perceptual factors (e.g., 3D interpretation). Top: The two labeled squares are the same physical shade of gray and they appear the same. Bottom: The same two squares now look very different. This pattern is made of patches with the same gray shades , but has a different geometry (contours) leading to a different 3D interpretation and a strong brightness illusion. The brightness percept depends on the perceived reflectance of the surfaces. Because the light appears to come from above, the visual system infers that the upper square is receiving more incident light than lower square. Because the two gray patches have the same physical intensity, the upper one must be less reflective than lower one. The stronger brightness illusion at the bottom can not be explained by the light adaptation and spatial filtering mechanisms discussed above.
These are real photos illustrating the same effect. The same surfaces look different depending on their geometric arrangement.
Squares A and B are the same shade of gray, yet appear to have different brightness because B is seen as being in shadow.
The perception of transparency also affects perceived brightness.
Summary: Three processes in vision affect perceived brightness: