Visual system

?

Visual stimuli

  • The visual system is part of the central nervous system. It gives us the ability to process visual detail. It detects and interprets information from visible light to build a representation of the surrounding environment
  • The eye responds to visible light. Visible light refers to the narrow band of the spectrum of electromagnetic radiation. Visible light varies in wavelength measured by nanometers (nm). It varies from about 380 nm (violet) from the far left of the spectrum up to the far right - 760 nm (red)
  • Carries out a number of complex tasks including reception of light and formation of monocular representations, the buildup of a nuclear binocular perception from a pair of 2D projections, identifying and categorising visual objects, assessing distances to and between objects, and guiding body movements in relation to the objects seen
  • The psychological process of visual information is known as visual perception
  • Perceiving the colour of light: hue is determined by wavelength and corresponds roughly to colour names. Saturation is the intensity of the electromagnetic radiation (colour distance from grey). Brightness is the purity of the light (intense vs dull)
1 of 7

Location and anatomy of the eye

  • Light entering the eye is refracted as it passes through the cornea. It then passes through the pupil (controlled by the iris) and is further refracted by the lens. The cornea and lens act together as a compound to project an inverted image onto the retina
  • The lens' main job is to, along with the cornea, refract light to be focused on the retina. By changing shape, it functions to change the focal distance of the eye so it can focus on objects at various distances, allowing for a sharp real image of the object of interest to be formed on the retina
  • The cornea is the transparent front part of the eye that covers the iris and pupil. It refracts light, accounting for around two thirds of the eye's total optical power. Its focus is fixed, unlike that of the lens which can be adjusted to "tune" to focus
  • The pupil is the hole located in the centre of the iris of the eye that allows light to strike the retina. The iris regulates the amount of light by controlling the size of the pupil; bright light causes it to constrict (become smaller) whilst dark environments causes it to dilate (become bigger)
  • The retina is the third and inner coat of the eye which is a light-sensitive layer of tissue. The optics of the eye create an image of the visual world on the retina
2 of 7

The retina

  • The retina is the interior lining in the back of the eye. It is a layered structure with several layers of neurons interconnected by synapses. It contains millions of photoreceptor cells which are sensitive to light. Photoreceptors are responsible for transforming light into neural activity. They respond to light energy by firing electrical impulses in the nervous system. There are 2 types of photoreceptor cells found in the retina: rods and cones
  • Rods are sensitive to light of low intensity. On average, there is around 90 million rod cells in the human retina. They are more sensitive than cone cells and are almost entirely responsible for night vision. Rods are largely colour blind, which is one of the main reasons colours are much less apparent in darkness
  • Cones are responsible for colour vision, and function the best in relatively bright light. They are densely packed in the fovea centralis, which is the part of the retina which is responsible for good visual acuity (resolution). There are about 6-7 million cones in a human eye. There are 3 types of cones: S-cones, M-cones, and L-cones. This results in trichromatic vision, as each of the 3 cones respond to variation in colour in different ways
  • 4 main processing stages in the retina: photoreception, transmission to bipolar cells, transmission to ganglion cells, and transmission along the optic nerve to the brain
3 of 7

Inversion of images

  • When the light is refracted by the lens, and then projected onto the retina by the lens and cornea, it produces an inverted image, as the lens is curved and bends the light. The image is also projected as 2D
  • Light from the left side of the visual field strikes the right half of each retina, and vice versa
  • The job of the visual system and brain is to take the information that it receives from the world, e.g the 2D, upside down image projected by the lens and cornea, and correct it so that it is the right way up and 3D
4 of 7

Visual pathway from eye to brain

  • Information from the right side of the visual field on each retina cross at the optic chasm and travels to the left side of the brain. Information from the left side of the visual field travels to the right side of the brain
  • The information leaves the eye by way of the optic nerve, and there is a partial crossing of axons at the optic chasm. After the chasm, the axons are called the optic tract. The optic tract wraps around the midbrain to get to the lateral geniculate nucleus (LGN), where all the axons must synapse. From there, the LGN axons fan out through the deep white matter of the brain as the optic radiations, which will ultimately travel to the primary visual cortex, at the back of the brain
  • The LGN is a relay center in the thalamus for the visual pathway. It is the main central connection for the optic nerve to the occipital lobe. It is comprised of 6 layers. The top 4 layers are the parvocellular layers. They consist of small body neurons. They are responsive to colour and fine detail patterns. They react to slow or stationary objects. The bottom 2 layers of the LGN are the magnocellular layers. They are composed of a large body of neurons that are responsive to rods and movements. Underneath the 2 layers above are the koniocellular sub layers
5 of 7

Visual cortex

  • The visual cortex is a part of the cerebral cortex that plays an important role in processing visual information. It's located in the occipital lobe at the back of the skull
  • Visual information coming from the eye goes through the LGN, located in the thalamus, and then reaches the visual cortex
  • Both hemispheres of the brain contain a visual cortex. The visual cortex in the left hemisphere receives signals from the right visual field, whilst the visual cortex in the right hemisphere receives signals from the left visual field
  • The part of the visual cortex that receives the sensory inputs from the thalamus is the primary visual cortex, also known as Visual area one (V1), and the striate cortex. The extrastriate areas consist of visual areas two (V2), three (V3), four (V4), and five (V5).
  • The visual cortex contains 3 types of cells that respond to different types of stimuli. These cells are known as feature detectors. Simple cells detect straight lines and edges. Complex cells respond to orientation. Hypercomplex cells respond to length and width
6 of 7

Brain problems with vision

  • Cortical blindness is the total or partial loss of vision in a normal-appearing eye caused by damage to the brain's occipital cortex. It can be acquired or congenital, and may also be temporary in certain instances
  • Visual agnosia is an impairment in recognition of visually presented objects. It is not due to a deficit in vision, language, memory, or low intellect. While cortical blindness results from lesions to the primary visual cortex, visual agnosia is often due to damage to more anterior cortex such as the posterior occipital and/or temporal lobes in the brain
  • Prosopagnosia, also called face blindness, is a cognitive disorder of face perception in which the ability to recognise familiar faces, including one's own face, is impaired, while other aspects of visual processing and intellectual functioning remain intact. It can be caused by acute brain damage, or in the form of a congenital disorder
7 of 7

Comments

No comments have yet been made

Similar Psychology resources:

See all Psychology resources »See all Visual System resources »