Color:  Light energy is characterized by a property known as wavelength.  Color, on the other hand, is a mental experience and is not a property of light.  The colors we experience depend on how neurons respond to light of different wavelengths.  There have been 2 major theories of color vision.  The first, called the trichromatic theory, maintains that there are 3 basic color sensations (such as red, green and violet).  Each of the sensations is produced by activity in a different visual nerve.  Any color we experience is actually a compound sensation, composed of a weighted combination of the 3 basic color sensations.  The other major theory of color vision is the opponent process theory.  It maintains that color sensations are organized into opposing pairs (red vs. green is one pair;  blue vs. yellow is the other).  The colors we see are combinations of either a red or a green sensation + either a blue or a yellow sensation.  The current evidence overwhelmingly favors the opponent-process theory.

Lightness and contrast:  It would seem reasonable that the greater the energy of light, the brighter it should appear.  Remarkably, the perception of brightness of a light and the lightness of a surface depends on far more than the light's intensity.  Human vision is characterized by very profound spatial interactions, in which the brightness of a light or the lightness of a surface depends very much on the relationship between the intensity of the light or surface and the intensity of the background, that is, on contrast.  We will discuss how the appearance of lights and surfaces depends on backgrounds and also consider a mysterious illusion known as "Mach Bands".  This discussion will take us into consideration of interactions among neighboring neurons in the retina and the brain, and we will explore the characteristics of what is known as a neuron's "receptive field".   We will also see that by finding the minimal contrast needed to detect a pattern it is possible to gain insight into how the visual system analyzes spatial patterns at its very earliest levels.

Perception of form:  Vision in everyday life depends critically on our ability to distinguish and recognize different shapes and forms.  We will begin by considering early approaches to perception of form developed in the late 19th and early 20th century.  One approach (Structuralism) is based on the idea that we learn to perceive form and the other (Gestalt) is based on the idea that form perception depends on innate processes that detect relationships among nearby elements.   The Gestalt psychologists were remarkably insightful.  We will see how many of their ideas have been verified in modern work, and we will see the way in which the visual system is constructed so that certain relationships among nearby contours are made to stand out from backgrounds.  Detection of such relationships may be instrumental in our ability to see form and shape.