The Human eye receives light as visual stimulation at the retina and it is transmitted through the optic nerve to the brain, where it is recognized as an image. Here, detailed explanations about the mechanism of seeing are provided.
How Does Human Eye Work?
1. Eyelid
The Eyelid acts as a lens cap and shutter of a camera. It protects the eye from dust and dryness, and regulates the amount of incoming light to prevent retinal damage.
2. Cornea and Crystalline Lens
The Cornea and crystalline lens act as a camera lens. The Cornea also functions as a filter to absorb ultraviolet rays. In a camera, focus is adjusted by movement of the camera lens. On the other hand, the eye has an auto-focus function to adjust focus by changing the thickness of the crystalline lens.
3. Iris
The Iris acts as a diaphragm of a camera. It controls the size of the “pupil” to regulate the amount of incoming light.
4. Vitreous
The Vitreous acts as a film compartment of a camera. It keeps a certain distance between the lens and the retina to project a properly-sized image onto the retina.
5. Retina
The Retina acts as a film or an image sensor of a digital camera. In a digital camera, incoming light is converted into electrical signals by the image sensor and the signals are processed by a computer to be recorded on a memory. In the visual system, the computer for processing the signals corresponds to the brain and the cable connecting the image sensor and the computer serve as the optic nerve.
How Is Human Vision Obtained?
Incoming light from the cornea passes through the vitreous to the outmost retinal layer called retinal pigment epithelium. Photoreceptor cells, which are located inside this layer, have a function to convert light into electrical signals.
The electrical signals converted by the photoreceptor cells are transmitted from cell to cell with the visual information. Then the signals are integrated at the “lateral geniculate nucleus” of the brain, after passing through the optic nerve.
The visual information converged on the lateral geniculate nucleus is further integrated and transmitted to the visual cortex and visual association area.
The information is ultimately integrated in the cerebrum and the sensations of color, motion, form, and distance are obtained in conjunction with other brain regions.
In this way, the visual information converted from light to the electrical signals is intricately processed, and then we can “see” and interpret the meaning of the information.
About Visual Disturbance
Visual impairments are caused by 1) refractive error such as myopia, hypermetropia, astigmatism and amblyopia, or 2) ocular diseases.
The major diseases causing blindness include glaucoma, diabetic retinopathy (DR), retinitis pigmentosa (RP) and age-related macular degeneration (AMD).
The cause for blindness depends on the disease. In glaucoma, the damage of the optic nerve may lead to blindness, and in DR, huge hemorrhage.
In RP and AMD, light can not be converted into electrical signals, which might lead to blindness.
This is because “photoreceptor cells”, converting light to an electrical signal, and “retinal pigment epithelium cells”, modulating the amount of “light-sensitive pigment” which is contained in photoreceptor cells and senses color and brightness, are damaged and fail to transmit the light stimulation to the brain.
R&D on Visual Prosthesis
