Editor's note: Brain Basics from Scientific American Mind is a series of short video primers on the brain and how we feel, think and act. Below is a synopsis of the second video in the series written by a guest on this blog, Roni Jacobson, a science journalist based in New York City.

By Roni Jacobson

Vision begins when light passes through the eye’s transparent covering, the cornea. The cornea bends the light and sends it through a small opening, the pupil, behind the surface of the eye to the lens. Unlike the cornea, which is a passive structure, the lens is controlled by muscles that change its shape to actively focus the image, an upside-down version of which projects onto the retina at the back of the eye. There, photoreceptor cells transform light energy into electrochemical signals that they transmit to cells in other layers of the retina. Photoreceptors are of two types: rods and cones. The rods enable us to see in dim light, but cannot pick up visual details or color. Cones, on the other hand, enable us to detect hue and fine structures such as the words on a screen or contours of a face.

Visual signals travel from the retina through the optic nerve, which connects the eyes to the central nervous system. The first stop is the thalamus in the center of the brain. The thalamus further processes the information and separates it into parallel paths based on color and detail or contrast and motion. Because the thalamus also receives inputs from higher regions of the brain, it may also change what we see based on its importance at the moment. From there, the data proceeds to the primary visual cortex at the back of the brain, where different cells similarly specialize in signaling, say, color, motion or edges. The brain processes these different pieces of information in parallel. At later stages of analysis, these pieces are combined into a perception of an object or scene.

Other Brain Basics videos:

Quick! What Is the Word for a Pair of Opposites?