Visual system

The visual system is the physiological foundation of visual perception. This is the ability to detect and process light to build a mental model of your surroundings. The system functions by detecting, transducing, and interpreting light within the visible range. It allows humans to construct images and understand the environment. This complex system is divided into two functional parts. The optical system includes the cornea and the lens. The neural system includes the retina and the visual cortex. Together, these parts allow for tasks like color vision and motion perception. They also enable stereopsis, which is the ability to see in three dimensions.

Seeing begins with the optical process of refraction. Light enters the eye and passes through the cornea. It then moves through the pupil, which is controlled by the iris. The lens provides further refraction to focus the light. The cornea and lens act as a compound lens together. This process projects an inverted, or upside-down, image onto the retina. The retina is a layer of tissue at the back of the eye. It contains many photoreceptor cells that use proteins called opsins. These opsins absorb photons, which are particles of light. This absorption triggers a signal transduction pathway. This pathway results in hyper-polarization of the photoreceptor cell.

There are two main types of photoreceptors in the human retina. These are called rods and cones. Each eye contains approximately 120 million rods. Rods are located mainly in the periphery of the retina. They are used to see in low light levels. Cones are found primarily in the center, or fovea. Each eye has about 6 to 7 million cones. Cones are responsible for day vision and color perception. There are three types of cones that absorb different wavelengths. These are often called short, middle, or long cones. They correspond to blue, green, and red light.

The retina also uses complex chemistry to respond to light. In the dark, a molecule called retinal has a bent shape. This shape is known as cis-retinal. When light hits the molecule, it changes to a straight form. This form is called trans-retinal. This change is called bleaching because the rhodopsin changes color. This chemical shift stops the release of glutamate. When glutamate stops, the bipolar cells can release neurotransmitters. These neurotransmitters then signal the ganglion cells. The ganglion cells conduct action potentials to the brain. About 1.2 million axons from these cells carry the information. This is a massive reduction from the 130 million photoreceptors.

Once the signal leaves the eye, it enters the optic pathway. The pulses travel through the optic nerve and the optic canal. They reach the optic chiasm, where nerve fibers decussate. This means some fibers cross from the left side to the right. Most fibers end in the lateral geniculate nucleus, or LGN. The LGN acts as a sorting and tagging station. It gauges the range of objects. It also adds a velocity tag to major objects. This tag helps the brain predict movement. The LGN then forwards these pulses to the primary visual cortex, known as V1.

The visual cortex is divided into many specialized areas. V1 performs edge-detection to understand spatial organization. It focuses on small color changes about 40 milliseconds after light hits. It also creates a saliency map to guide your attention. V2 handles illusory contours and helps determine depth. V3 helps the brain process global motion, such as speed. V4 is responsible for recognizing simple shapes. V5 and V6 work together to analyze complex motion. V5 analyzes your own self-motion. V6 analyzes how objects move relative to the background. Finally, the inferior temporal gyrus recognizes complex objects and faces.

The visual system also manages functions that do not involve making images. The pretectal area contains seven unique nuclei. These nuclei help with the accommodation reflex and REM sleep. The Edinger-Westphal nucleus helps adjust the lens and pupil size. The suprachiasmatic nucleus is part of the hypothalamus. It helps regulate your body clock by halting melatonin production at first light. These processes show that the visual system is about more than just sight. It is a vital part of how your whole body interacts with time and light.