Visual System | Functions | 7 Important Components

VISUAL SYSTEM

Contents

The visual system refers to the collective working of the sensory organ or the eye along with sections of the central nervous system (i.e. the retina containing photoreceptor cells, the optic tract, the optic nerve, and the visual cortex) and will contribute together to allow organisms the sense of vision, that is the capability of detection and processing visible light. These components are also responsible for enabling the generation of various non-image photo response functions and will permit the detection and interpretation of info from the optical spectrum perceptible to that specie for “forming a representation” of the surrounding. 

Image source: Rhcastilhos. And Jmarchn., Schematic diagram of the human eye enCC BY-SA 3.0

Functions of the Visual system

The visual system is responsible for carrying out a series of complex functions, such as:

  • The reception of light and the generation of monocular neural representations,
  • Color vision,
  • Analyzing distances to the object and between two targets,
  • Identifying specific objects of interest,
  • Perception of motion,
  • The assessment and collection of visual information,
  • Recognizing pattern.
  • Accurate motor coordination under proper visual guidance.

 
Visual perception refers to the neuropsychological portion of visual information processing. An abnormality or issue in the neurophysiological processing is known as visual impairment and a total absence of the same is known as blindness. The visual system also performs certain non-image-forming visual functions, (that is independent of visual perception) such as circadian photoentrainment and the pupillary light reflex or PLR.

Components of the Visual system

visual system
Schematic demonstration of human visual pathway. Image source: Miquel Perello NietoHuman visual pathwayCC BY-SA 4.0

EYE.

The eye is the main part of the visual system. Lightray falls on the cornea (gets refracted on the aqueous humor) and enters the eye via the pupil (controlled by the iris). After entering the eye the light rays suffer a series of refractions through the eye lens and vitreous humor. These series of refractions form an inverted image on the retinal surface.

RETINA.

The refracted light rays fall on the retina that contains a number of photoreceptor cells. The retina comprises two types of protein molecules contributing to conscious vision, namely rod opsins, and cone opsins. an opsin absorbs a refracted photon and directs the signal to the cell, hyper-polarizing the photoreceptor. Rod opsins are present near the boundary of the retina and help in visualizing at low light levels. Cone opsins are present near the center of the retina and help in visualizing color at normal light levels. We can found 3 categories of cone opsins for human eye namely short or blue, middle or green, and long or red respectively.

OPTIC NERVE.

The information signal processed in the retinal cells is transmitted to the brain cells by the optic nerve. Around 89% of the nerve fibers send the information signal to the lateral geniculate nucleus present in the thalamus. Here, parallel processing is undergone to perceive vision.

OPTIC CHIASM.

The optic fibers from the retina of both the eyes meet and cross at the optic chiasm, here, the simultaneous information signals from both the eyes are combined first and then separated based on the FOV (left FOV of both eyes and right field of views of both eyes). The right and left half of the corresponding FOV are sent to the left and right half of the brain, respectively for further analysis and central portion of the FOV is analyzed by both the parts of the brain. (FOV – field of views)

OPTIC TRACT.

The left optic tract carries the info signal of the right visualfield  and the right optic tract carries the information signal of the left visualfield (now present on the right side of the brain) and these optic tracts end in the LGN in the thalamus.

Simplified Image analysis of Mona Lisa . Image source: ClockLisa analysisCC BY-SA 3.0

LATERAL GENTICULATE NUCLEUS.

This is present in the thalamus of the brain and this is basically a system of sensory relay nucleus that relays the image information to the visual cortex.

VISUAL CORTEX.

Visual cortex is the main visual processing unit of the brain. It lies above the cerebellum at the rear part of the brain. Information about vision-related reflexes, color, and motion are processed in the visual cortex.

Visual cortex representation. Brodmann areas 17, 18 and 19. BA 17 is shown in red. BA 18 is orange. BA 19 is yellow. Image source: The original uploader was Washington irving at English Wikipedia., Brodmann areas 17 18 19CC BY-SA 3.0

What causes a change in the visual system?

Visual system of a person gets altered at times due to refractive or neural errors.

Refractive error of the eye:

Refractive errors such as myopia, hyperopia (when light rays falling on the eye does get focused behind the retina), ametropia, astigmatism (when the image formed on the retina is blurred due to uneven corneal structure), etc. can cause an alteration in the field of vision.

Neural errors:

Neural errors such as detached retina (when the retina gets separated from the layers present underneath), macular degeneration (when there is reduced or no vision near the central vision region), amblyopia (when the visual brain is poorly developed during childhood), traumatic brain injury damaging the optical nerves, stroke, etc. can cause an alteration in the field of vision.

Visual field defect demonstration. 1. Complete loss of vision in the right eye 2. Bitemporal hemianopia 3. Homonymous hemianopia 4. Quadrantanopia 5.& 6. Quadrantanopia with macular sparing. Image source: As eoHemianopsia enCC BY-SA 4.0

About Sanchari Chakraborty

I am an eager learner, currently invested in the field of Applied Optics and Photonics. I am also an active member of SPIE (International society for optics and photonics) and OSI(Optical Society of India). My articles are aimed towards bringing quality science research topics to light in a simple yet informative way. Science has been evolving since time immemorial. So, I try my bit to tap into the evolution and present it to the readers.

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