Vision

The entire visual pathway—from the retina to the visual cortex—is paved with millions of neurons. From the time light enters the eye until the brain forms a visual image, vision relies upon the process of electrochemical communication between neurons. Each neuron has a cell body with branching fibers called dendrites and a single long, cylindrical fiber called an axon. When a neuron is stimulated it sends chemicals called neurotransmitters, which causes the release of electrical impulses along the axon. The point where information passes from one cell to the next is a gap called a synapse, and neurotransmitters affect the transmission of electrical impulses on to an adjacent cell. This synaptic transmission of impulses is repeated until the message reaches the appropriate location in the brain. In the retina, approximately 125 million rods and cones transmit information to approximately one million ganglion cells. This means that many rods and cones must converge onto one single cell. At the same time, however, information from each single rod and cone "diverges" on to more than one ganglion cell. This complicated phenomenon of convergence and divergence occurs along the entire optic pathway. The brain must transform all this stimulation into useful information and respond to it by sending messages back to the eye and other parts of the brain before we can see.

Our eyes adapt to an incredible range of light intensities—from the glare of sunlight on glistening snow to the glow of moonlight on rippling water. Although the pupil regulates to some degree the amount of light entering the eye, it is the rods and cones which allow our vision to adapt to such extremes. Rod vision begins in dim light at the level of darkness and responds for up to five orders of intensity. Cones function in bright light and are responsible for color vision and visual activity.

When light hits the surface of an object, it is either absorbed, reflected, or passes through—as it does through clear glass. The amount of pigment in an object helps us determine its color. The amount of light absorbed by an object is determined by the amount of pigment, or color, contained in that object. The more heavily pigmented the object, the darker it appears because it absorbs more light. A sparsely pigmented object, which absorbs very little light and reflects a lot back, appears lighter.