The lateral geniculate nucleus (LGN) is a relay centre in the visual pathway from the eye to the brain. It receives signals from the retina via the axons of ganglion cells. The thalamus, a part of the brain located near the brainstem, houses the LGN.
- The thalamus which houses the lateral geniculate nucleus is a small structure within the brain, located just above the brain stem between the cerebral cortex and the midbrain and has extensive nerve connections to both.
- The LGN specializes in processing visual information from both eyes. It resolves relationships between different visual inputs, helping us understand the sequence of events and the location of objects in our field of view.
- Some of this processing involves signals from one eye, while others deal with information from both eyes to create a three-dimensional perception of the world. The LGN acts as a central connection for the optic nerve to the primary visual cortex in the occipital lobe. Both the left and right hemispheres of the brain have a lateral geniculate nucleus.
- There are three major cell types in the LGN, each connecting to different types of ganglion cells and playing specific roles in vision:
- P cells: Process information about colour and fine detail.
- M cells: Respond to motion.
- K cells: Involved in low-resolution processing.
A laser is a light source that can create a narrow and intense beam of electromagnetic radiation. Unlike a flashlight, which has a bulb that emits light in all directions, a laser beam focuses its light into a concentrated stream of photons. LASER stands for Light Amplification by Stimulated Emission of Radiation.
- Light waves are made up of tiny packets of energy called photons.
- Normal light emission happens when atoms or molecules release photons when they transition from higher energy states to lower ones. These emitted photons have random directions and energies, creating a diffuse light.
- The concept that makes lasers unique is stimulated emission. This occurs when an incoming photon interacts with an excited atom in the laser material. The photon’s energy triggers the excited atom in the material to emit a new photon with identical characteristics. The new photon has the same wavelength and so colour, phase and direction as the original.
- Laser material refers to the medium that is used to generate the laser light.
- This phenomenon creates a cascade effect. The newly emitted photon can itself stimulate another excited atom, leading to two identical photons travelling in the same direction. This process repeats, rapidly amplifying the initial light within the laser cavity.
- The cavity comprises two mirrors strategically positioned at the opposite ends of the laser material. One mirror is fully reflective, while the other partially reflects.
- As the amplified light bounces between the mirrors, it continues to stimulate more emissions, resulting in an intense beam of identical photons. The partially reflective mirror allows a portion of this intense light to escape as the laser beam, while the rest continues to contribute to the amplification within the cavity.
A light-emitting diode (LED) is a semiconductor device that emits light when an electric current flows through it. Electroluminescence is the process where this happens: voltage applied to the semiconductor makes electrons flow across a junction, releasing energy as light.
- Semiconductors, typically made from gallium nitride, are solid-state materials with unique properties that allow them to emit light at specific wavelengths, determining the perceived colour.
- LEDs typically emit one colour with a narrow range of wavelengths.
- Multicoloured LEDs combine three diodes emitting the RGB primary colours – red, green, and blue light.
- By adjusting the relative brightness of the primary colours, a vast array of colours can be created.
- Combining the three primary colours in equal proportions produces white light.
Light is electromagnetic radiation (radiant energy), which, detached from its source, is transported by electromagnetic waves (or their quanta, photons) and propagates through space. Even if humans had never evolved, stars would have emitted electromagnetic radiation since the first galaxies formed over 13 billion years ago.
- Simply stated, light is energy. Light is the way energy travels through space.
- Whilst the term light can be used to refer to the whole electromagnetic spectrum, visible light refers to the small range of wavelengths our eyes are tuned to.
- The term light can be used in three different ways:
- Light can be used to mean the whole of the electromagnetic spectrum from radio waves, through visible light to gamma rays. When this meaning is intended, the terms radiant energy or photon energy are placed in brackets after the term light in this resource.
- Light can be used to mean the range of wavelengths and frequencies that can be detected by the human eye. A better term is visible light which refers to the wavelengths that correspond with the colours between red and violet, the visible spectrum.
- Light can also be used to mean the range of wavelengths and frequencies between infra-red and ultra-violet. This usage is sometimes useful because the outer limits of the visible spectrum can differ under different lighting conditions and for different individuals.
- Remember that the precise experience of visible light is not the same for all individual humans and is not the same for all living things.
- Light travels through a vacuum at 299,792,458 metres per second but propagates more slowly through other media.
- When light interacts with matter it results in optical phenomena such as absorption, dispersion, diffraction, polarization, reflection, refraction, scattering and transmission.
Optic radiation
The optic radiations are tracts formed from the axons of neurons located in the lateral geniculate nucleus and leading to areas within the primary visual cortex. There is an optic radiation on each side of the brain. They carry visual information through lower and upper divisions to their corresponding cerebral hemisphere.
Lateral geniculate nucleus
The lateral geniculate nucleus is a relay centre on the visual pathway from the eyeball to the brain. It receives sensory input from the retina via the axons of ganglion cells.
The thalamus which houses the lateral geniculate nucleus is a small structure within the brain, located just above the brain stem between the cerebral cortex and the midbrain with extensive nerve connections to both.
The lateral geniculate nucleus is the central connection for the optic nerve to the occipital lobe of the brain, particularly the primary visual cortex.
Both the left and right hemispheres of the brain have a lateral geniculate nucleus.
There are three major cell types in the lateral geniculate nucleus which connect to three distinct types of ganglion cells:
- P ganglion cells send axons to the parvocellular layer of the lateral geniculate nucleus.
- M ganglion cells send axons to the magnocellular layer.
- K ganglion cells send axons to a koniocellular layer.
The lateral geniculate nucleus specialises in calculations based on the information it receives from both the eyes and from the brain. Calculations include resolving temporal and spatial correlations between different inputs. This means that things can be organised in terms of the sequence of events over time and the spatial relationship of things within the overall field of view.
Some of the correlations deal with signals received from one eye but not the other. Some deal with the left and right semi-fields of view captured by both eyes. As a result, they help to produce a three-dimensional representation of the field of view of an observer.
- The outputs of the lateral geniculate nucleus serve several functions. Some are directed towards the eyes, others are directed towards the brain.
- A signal is provided to control the vergence of the two eyes so they converge at the principal plane of interest in object-space at any particular moment.
- Computations within the lateral geniculate nucleus determine the position of every major element in object-space relative to the observer. The motion of the eyes enables a larger stereoscopic mapping of the visual field to be achieved.
- A tag is provided for each major element in the central field of view of object-space. The accumulated tags are attached to the features in the merged visual fields and are forwarded to the primary visual cortex.
- Another tag is provided for each major element in the visual field describing the velocity of the major elements based on changes in position over time. The velocity tags (particularly those associated with the peripheral field of view) are also used to determine the direction the organism is moving relative to object-space.
Optic nerve
The optic nerve is the cable–like grouping of nerve fibres formed from the axons of ganglion cells that transmit visual information towards the lateral geniculate nucleus.
The optic nerve contains around a million fibres and transports the continuous stream of data that arrives from rods, cones and interneurons (bipolar, amacrine cells). The optic nerve is a parallel communication cable that enables every fibre to represent distinct information about the presence of light in each region of the visual field.
Müller cells
Müller glia, or Müller cells, are a type of retinal cell that serve as support cells for neurons, as other types of glial cells do.
An important role of Müller cells is to funnel light to the rod and cone photoreceptors from the outer surface of the retina to where the photoreceptors are located.
Other functions include maintaining the structural and functional stability of retinal cells. They regulate the extracellular environment, remove debris, provide electrical insulation of the photoreceptors and other neurons, and mechanical support for the fabric of the retina.
- All glial cells (or simply glia), are non-neuronal cells in the central nervous system (brain and spinal cord) and the peripheral nervous system.
- Müller cells are the most common type of glial cell found in the retina. While their cell bodies are located in the inner nuclear layer of the retina, they span the entire retina.
As light crosses the boundary between two transparent media, the law of refraction (Snell’s law) states the relationship between the angle of incidence and angle of refraction of the light with reference to the refractive indices of both media as follows:
When electromagnetic radiation (light) of a specific frequency crosses the interface of any given pair of media, the ratio of the sines of the angles of incidence and the sines of the angles of refraction is a constant in every case.
- Snell’s law deals with the fact that for an incident ray approaching the boundary of two media, the sine of the angle of incidence multiplied by the index of refraction of the first medium is equal to the sine of the angle of refraction multiplied by the index of refraction of the second medium.
- Snell’s law deals with the fact that the sine of the angle of incidence to the sine of the angle of refraction is constant when a light ray passes across the boundary from one medium to another.
- Snell’s law can be used to calculate the angle of incidence or refraction associated with the use of lenses, prisms and other everyday materials.
- When using Snell’s law:
- The angles of incidence and refraction are measured between the direction of a ray of light and the normal – where the normal is an imaginary line drawn on a ray diagram perpendicular to, so at a right angle to (900), to the boundary between two media.
- The wavelength of the incident light is accounted for.
- The refractive indices used are selected for the pair of media concerned.
- The speed of light is expressed in metres per second (m/s).
A light source is a natural or man-made object that emits one or more wavelengths of light.
- The Sun is the most important light source in our lives and emits every wavelength of light in the visible spectrum.
- Celestial sources of light include other stars, comets and meteors.
- Other natural sources of light include lightning, volcanoes and forest fires.
- There are also bio-luminescent light sources including some species of fish and insects as well as types of bacteria and algae.
- Man-made light sources of the most simple type include natural tars and resins, wax candles, lamps that burn oil, fats or paraffin and gas lamps.
- Modern man-made light sources include tungsten light sources. These are a type of incandescent source which means they radiate light when electricity is used to heat a filament inside a glass bulb.
- Halogen bulbs are more efficient and long-lasting versions of incandescent tungsten lamps and produce a very uniform bright light throughout the bulb’s lifetime.
- Fluorescent lights are non-incandescent sources of light. They generally work by passing electricity through a glass tube of gas such as mercury, neon, argon or xenon instead of a filament. These lamps are very efficient at emitting visible light, produce less waste heat, and typically last much longer than incandescent lamps.
- An LED (Light Emitting Diode) is an electroluminescent light source. It produces light by passing an electrical charge across the junction of a semiconductor.
- Made-made lights can emit a single wavelength, bands of wavelengths or combinations of wavelengths.
- An LED light typically emits a single colour of light which is composed of a very narrow range of wavelengths.