Above is an example of lightness constancy. Look at the checkerboard scene on the left: Which squares do you perceive to be the same? If you are like most people, your visual system creates the perception that square A and C are different, and A and B are identical, even though (and now look at the checkerboard scene on the right) they are not! Why did your brain do that? Why don’t you see what is actually on the retina?
Think about it, it happens a lot: Most times you see a coin, for example, you perceive it to be a circle. Yet think about the image that coin casts on the retina (which is the first stage of visual processing). That image is hardly ever circular and far more likely to elliptical. However, you perceive the coin to be round, just as a door is perceived to be a rectangle at all times, even though a rectangle shape is less likely on the retina than a parallelogram. In building our visual perception of the world of objects and scenes from those bottom-up signals impinging on our sensory receptors, the input is the retinal image, constantly changing as our head shifts, as we make eye movements, as we move relative to objects and scenes; however, our perceptions of color, shape, size, orientation, motion, and lightness remain constant; they do not change. It doesn’t appear as if everything is visually transmogrifying and morphing in front of our eyes, even though that is exactly what is happening on the retina. How does the brain achieve perceptual constancy, the ability to perceive the unchanging features of the world despite the ever-changing retinal image?
This week, please answer the following:
1) What is meant by perceptual constancy?
2) Choose either: size constancy, orientation constancy, shape constancy, lightness constancy, or color constancy (perceptual constancy is discussed in the lecture and the textbook. I discuss orientation constancy in the lecture. Demonstrations can be seen in Course Material and in my lecture). You might want to explore the famous “dress” controversy? Was it blue and gold or was it black and whatever!? What accounted for that? (Hint: Do some research on color constancy).
3) Describe this constancy in a paragraph. Again, you should do some research on how it is described in the field, and then armed with that information, present this information using your own words. Give evidence for it where possible.
4) What are the proposed ways that the visual brain achieves it? Again, what is the evidence?
5) What might happen to a person’s visual experiences of color, size, shape, orientation, motion, brightness, and other percepts if perceptual constancy did not occur? What would a person perceive? Can you find any evidence for this? (Note: This last part requires you do some research).
