Visual System

The human eye is nature's optical masterpiece. The eye can focus on a wide range of objects, bright and dull, large and small, far and near, all at dizzying speeds and with uncanny accuracy. We rely on our eyes to provide most of the information we perceive about the world, so much so that a significant portion of the brain is devoted entirely to visual processing. The anatomy of the visual system provides important clues about how the brain is structured in general, and about how we humans solve the complex problem of sight in particular.

Light enters the eye by first passing through the cornea, a clear layer of tissue that begins focusing the image. Behind the cornea a substance very much like thickened water, called aqueous humor, keeps the front of the eye firm and slightly curved. Light travels through this fluid to reach the iris (named for the Roman goddess of the rainbow). The iris is a beautifully colored and textured ring-shaped muscle that gives our eye its color. The hole in the center of the iris, called the pupil, dilates and constricts to control the amount of light entering the eye. By contracting or relaxing, the iris can change the size of the pupil to compensate for changing lighting conditions.

Once light passes through the pupil, it is further focused by a bit of clear, stiff, jelly-like tissue called the lens. The lens can squeeze tight into a ball or be stretched flat, allowing us to shift our focus between near and far objects. Behind the lens, another clear liquid, called the vitreous humor, fills most of the eyeball, and light passes through this liquid to finally come to a focus on the very back of the eye --- on a sheet of tissue called the retina.

The retina is responsible for converting light into neural signals that can be relayed to the brain. The retina consists of a team of five types of cells whose role it is to collect light, extract basic information about color, form, and motion, and pass the pre-processed image on to centers in the brain. These cell types are photoreceptors, bipolar cells, horizontal cells, amacrine cells, and ganglion cells. They are arranged within the retina in three layers, from the back to the front.

An image shining upon the retina traverses the three layers to reach the photoreceptor cells, which absorb the incoming light and transform it into electrochemical signals. Photoreceptors are divided into two subtypes, rods and cones, named for their shape. Rod cells are very sensitive to changes in contrast even at low light levels, but consequently are imprecise in detecting position (due to light scatter) and insensitive to color. Rods are generally located in the periphery of the retina and used for night vision. Cones are high-precision cells that are specialized to detect red, green, or blue light. They are generally located in the center of the retina in a region of high spatial acuity called the fovea.

Signals from the photoreceptors pass forward into the next layer of the retina containing horizontal, bipolar, and amacrine cells. These cells form small networks that are able to extract information about form and motion from an image. That information continues to the front of the retina where it is received by a layer of ganglion cells. The ganglion cells send out long, thin fibers that bundle together and plunge back down through the retina and out the back of the eye into the optic nerve, which carries them deep into the brain. The spot where the optic nerve exits the eye is devoid of cells, and forms a blind spot in each eye.

The optic nerves within each eye meet in the front part of the head at a point called the optic chiasm, which functions like a cloverleaf on a highway. All the fibers from the left half of each retina turn towards the right side of the brain, and the fibers from the right half of each retina turn towards the left side of the brain. The end result of this crossing is that the left half of the brain looks at the right visual world, and the right half of the brain looks at the left visual world.

A small group of fibers in the optic nerve splits off and travels down to brainstem nuclei, which are groups of cells governing reflex actions. Those fibers mediate automatic responses, such as adjusting the size of the pupil, blinking, and coordinating the movement of the eyes. The majority of fibers in the optic nerve take another path that leads to the very back of the brain, to a part of the occipital lobe called primary visual cortex, or V1.

On the way to V1, these fibers enter a nucleus in the center of the brain called the thalamus. The thalamus acts as a central depot for information coming into and going out of the cortex, and it has centers specialized for different types of information. The center that deals with vision is called the lateral geniculate nucleus (LGN), a layered structure with cells that respond to form, motion, and color. Fibers from the optic nerve enter the LGN, where streams of information about the visual image are further separated and then sent on to the primary visual cortex.

V1, also called striate cortex because of the distinctive stripe it bears, is responsible for creating the basis of a three-dimensional map of visual space, and extracting features about the form and orientation of objects. Once basic processing has occurred in V1, the visual signal enters the secondary visual cortex, V2, which surrounds V1. V2 is principally responsible for perceiving color and the relationships between form and color.

V2 and higher cortical areas are generally referred to as extrastriate areas, meaning outside of primary visual cortex. This nomenclature indicates how much more we understand about V1 than we do about any other area of visual cortex. Still, most of what we consider visual perception occurs in these extrastriate areas. They perform the two broad tasks of perceiving "what" forms are in the visual image and "where" objects are in space. The "what" tasks correspond to a stream of connections into the temporal lobe, which contains areas that recognize objects and faces. The "where" tasks are performed through a stream into the parietal lobe, which houses areas dedicated to perceiving movement and spatial relationships. Together, through simultaneous activity, these cortical centers allow us to very quickly see, understand, and respond to an enormous range of visual scenes.

Neural activity can be visualized using a number of techniques available to neuroscientists today. In the second part of this NeuroSeries, we will examine the function of neural activity in the visual system to develop a more thorough understanding of how our brains interpret images. We will also discuss exciting new studies that give us our first glimpses of how internally generated images are processed within the visual system of the brain.