Light rays reflected by an object—for example, a pencil—enter the eye and pass through its lens. The lens projects an inverted image of the pencil onto the retina at the back of the eye. Signals produced by rod and cone cells in the retina then start on their way into the brain through the optic nerve and reach a major relay station, the LGN (lateral geniculate nucleus).

Signals about particular elements of the pencil then travel to selected areas of the primary visual cortex, or V1, which curves around a deep fissure at the back of the brain. From there, signals fan out to "higher" areas of cortex that process more global aspects of the pencil such as its shape, color, or motion.

Surprisingly, light rays must penetrate two layers of neurons in the retina before reaching the precious rods and cones at the back: a middle layer of bipolar cells, and a front layer of ganglion cells whose long axons (fibers that transmit electrical impulses to other neurons) form the optic nerve leading into the brain.

— Geoffrey Montgomery

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Illustration: Eade Creative Services, Inc./George Eade illustrator (visual system adapted from a drawing by Laszlo Kubinyi and Precision Graphics on page 9 of Images of Mind by Michael I. Posner and Marcus E. Raichle, Scientific American Library, ©1994; retinal cells adapted from a drawing by Carol Donner/Tom Cardamone Associated on page 37 of Eye, Brain and Vision by David H. Hubel, Scientific American Library, ©1998)