The visual system consists of a sensory organ (the eye) and components of the central nervous system (the retina, which contains photoreceptor cells, the optic nerve, the optic tract, and the visual cortex), and it provides organisms with the sense of sight (the ability to detect and process visible light) as well as the ability to form several non-image photo response functions. It detects and interprets information from the visible portion of the optical spectrum that is perceptible to that species in order to “construct a representation” of its environment. The visual system performs a variety of complex tasks, including light reception and formation of monocular neural representations, colour vision, the neural mechanisms underlying stereopsis and distance assessment to and between objects, object identification, motion perception, visual information analysis and integration, pattern recognition, and accurate motor coordination under visual guidance. Visual perception is the neuropsychological aspect of visual information processing; its dysfunction is referred to as visual impairment, and its complete absence is referred to as blindness. The pupillary light reflex (PLR) and circadian photoentrainment are examples of non-image generating visual functions that exist independently of visual experience.
Structure of Eye
Humans have two eyes, one on each side of the face. The eyes are located in bony holes in the skull called the orbits. Six extraocular muscles are responsible for eye movement control. The whitish sclera, a coloured iris, and the pupil make up the visible portion of the eye. On top of this lies a thin layer called the conjunctiva. The front portion of the eye is often referred to as the anterior region.
The eye is not perfectly round; rather, it is a fused two-piece structure made of an anterior (front) and posterior (rear) portion. The cornea, iris, and lens comprise the anterior section. The cornea is transparent and more curved than the posterior segment, which is composed of the vitreous, retina, choroid, and the outer white shell termed the sclera. The cornea is generally 11.5 millimetres (0.45 inch) in diameter and 0.5 millimetres (500 millimetres) thick in its centre. The remaining five-sixths of the chamber is occupied by the posterior chamber, which has a diameter of around 24 mm (0.94 in). The limbus connects the cornea and sclera. The iris is a pigmented circular structure that surrounds the pupil, which appears to be black. The iris’ dilator and sphincter muscles govern the size of the pupil, which affects the amount of light entering the eye.
The cornea, the pupil, and then the lens all receive light energy. The ciliary muscle controls how the lens shape changes for near focus (accommodation). Photons of light striking the retina’s light-sensitive cells (photoreceptor cones and rods) are converted to electrical signals and transferred to the brain via the optic nerve, where they are processed as sight and vision.
Mechanism
The information from the retina’s ganglion cells reaches the optic nerves, where it is converted to action potentials that move to a location called the optic chiasm (where the optic nerve fibres of both eyes cross in the midline and then form the optic tract). The path of visual information is slightly different here, as the ipsilateral temporal side of vision travels directly to the ipsilateral cortex, whereas the nasal side of vision travels to the contralateral occipital cortex. Thus, downstream of the optic chiasm, each optic tract receives information from both eyes, from the temporal ipsilateral and the nasal contralateral visual fields. This visual information is subsequently integrated into the thalamic’s lateral geniculate nuclei and projected to the visual cortex. Prior to reaching the thalamus, visual information may travel to other structures such as the pretectal nuclei and superior colliculus in the brainstem (to generate visual reflexes to focus on specific objects) or to the hypothalamic suprachiasmatic nuclei (to regulate circadian rhythms), among others.
When information reaches the thalamus, it must be organised in the same way that paperwork is organised at an office. Thus, the lateral geniculate nucleus comprises six layers of neural networks to facilitate the integration and organisation of information. Layers II, III, and V receive data from the contralateral temporal visual field, and layers I, IV, and VI receive data from the contralateral nasal visual fields. To add intrigue, the first and second levels are composed of magnocellular neurons, whereas the third, fourth, fifth, and sixth layers are composed of parvocellular neurons. Additionally, the retina contains magnocellular and parvocellular neurons, which are ganglion cell subtypes (the cells that receive information at the end of the retinal visual pathway). In the retina, ganglion cells of the “magnocellular” type receive information about black-and-white contrast and rapid changes in object placements, whereas neurons of the “parvocellular” type receive information about colour. Thus, the lateral geniculate nucleus contains two layers of neurons dedicated completely to the integration of information concerning black-and-white contrast and changes in the visual field, and four layers dedicated to colour mixing. All of these colour and contrast cues subsequently travel to the visual brain, where they are processed and interpreted.
Conclusion
The following physiological events occur:
- The refraction of light that enters the eye
- By accommodating the lens, the image is focused on the retina.
- Converging images
- The retina’s photochemical activity and its translation to nerve impulses
- To digest information in the brain and then to perceive
All of the eye’s components work in unison to enable us to see. At first, light enters the eye through the cornea, the clear front layer of the eye. Due to its dome-shaped structure, it bends light to assist the eye in focussing.
A portion of this light enters the eye via the pupil opening. The iris, the coloured portion of the eye, controls the amount of light that enters the pupil.
Light enters through the lens and is focused on the retina by the lens in conjunction with the cornea. When light strikes the retina, it is converted into electrical signals by specific cells called photoreceptors. The optic nerve transmits these impulses from the retina to the brain. The brain then converts the signals to the images we see.