Study Questions: Brightness Perception

1. Distinguish between "brightness" and "lightness". Define "reflectance". What do we mean when we say that lightness is an aspect of surface perception?

2. Define contrast. What is lightness constancy, and how does it relate to the coding of contrast? Since contrast is computed locally, how does that relate to an illusion of lightness?

3. What are "Mach bands"? How do you explain the perception of Mach bands by lateral inhibition?

4. People experience illusory dark spots in the center of the intersections of the white bars of the Hermann grid. Give a theory of the mechanism which generates the illusion based on the receptive field characteristics of retinal ganglion cells. Why do the dark spots move around as you move your eyes?

5. What is the difference between an illumination edge and a reflectance edge? What problems do shadows pose for lightness constancy? Explain how our perception of an object's lightness is affected by our perception of how the object is illuminated.