The visual system of human beings is enables assimilation of visual perception from the environment. Visual perception refers to the ability to interpret light signals reaching the eyesight. Seeing is a complex activity and the visual system operates with very high precision to ensure constancy and to prevent the creation of blind spots in the line of vision. Visual perception starts when light travels into the eye through the pupil. The organs and cells involved in visual perception exhibit a high level of functional specialization. Perceptual constancy and an extremely high level of organization within the visual system facilitate effective recognition of objects. The cornea with the aid of the eye lens focuses an image onto the cells of the retina. These cells are specially adapted to detect photons of light. The retina serves as a transducer which enables the conversion of the light signal into more complex neural signals. Processing of visual signals also takes place in the retina. Neural impulses are transmitted into the lateral geniculate nucleus and then to the primary and the secondary cortex of the brain which is able to interpret and convey the necessary signal.
Structure of the Retina
The retina is a complex but highly organized structure used in visual perception. The retina in adult man has a diameter of about 22 mm and it is approximately 3mm thick. It comprises of three distinct layers of different neurons which appear to be interconnected through two different synapses. The normally functioning eye can perceive light from a wavelength of 700nm (Bruce et al., 1997).
The light impulses are focused by the cornea and the eye lens and they flow through the horizontal cells into the bipolar cells. The horizontal cells connect the different photoreceptor cells together while the bipolar cells connect the different layers of the retina. The amacrine cells allow the photo impulses to be picked up by the rest of the bipolar cells. The signals are then transferred to the ganglion cells.
The ganglion cells are more than twenty different types which occur in distinct regular lattice. The most common ganglion cells are the parasol ganglion cells and the midget ganglion cells. They are carried by the optic nerve which holds approximately 1.2 million optic nerve cells in an individual (Gauthier et al., 2009). They use action potentials and they have specialized axons which pick up the impulses. The retina also has many blood vessels which carry supply blood that carries food and nourishment.
The blind spot is a small area of the retina which is about 3mm. The blind spot lacks either of the photoreceptor cells (Bruce et al., 1997). The retina is defined by an area referred to as the ora serata. In between the ora serata is the yellow pigmented macula which has a special highly photo sensitive area referred to as the fovea at the center. The fovea is the thickest part of the retina and it contains the photoreceptive cones and rods. The central retina extends for about six mm along this area.
The retina has three different types of photoreceptor cells: the cones, photosensitive ganglion cells and the rods. These are specialized based on the different wavelengths of light they respond to and the different photosensitive receptors they contain. The rods occur towards the periphery of the eye in high numbers and range from 75 to 150 million in an adult individual. They are adapted to function in dim light. The rods contain rhodopsin as the photosensitive receptor. Rhodopsin contains opsin which is a protein, and retinal which is sensitive to light.
The cones occur in less numbers and they are about 7 million in an adult man. They are usually used for ordinary daytime vision. Cones are also responsible for color perception. They have a special type of retinal although some will also have different types of opsin. This special opsin is responsible for the ability to perceive color (Leeuwen et al., 2004).
The photosensitive ganglion neurons are found within the optic fiber. They are rare and are normally used for reflex responses which occur in extremely bright light. Synaptic contacts are made in the region between the rods and the cones. This region has two layers of neuropils which are the inner and the outer plexiforms (Leeuwen et al., 2004).
The lateral geniculate nucleus is arranged in six distinct layers. The topmost two layers are referred to as Magnocellular layers. They contain close to 100,000 neurons characterized by large cell bodies (Bruce et al., 1997). The bottom four layers are called the Parvocellular layers which comprises close to one million neurons characterized by small cell bodies. The Koniocellular layer separates the two layers and has very many tiny neurons.
Function of the Retina
The light stimulus enters the eye through the pupil. It is focused and inverted by the cornea and the eye lens onto the retina. The first cells to integrate the impulses in the retina are the receptors. Due to the fact that the layers of the retina are transparent, light is able to pass through with ease (Gauthier et al., 2009).
In the retina, the light signal passes through the aqueous humors and through the transparent layers of neural tissue. These layers are the ganglion cells and the bipolar cells. The light signals travel in a hierarchical pathway and reach the photoreceptors. Light impulses strike the photoreceptors on the surface of the retina provoking an efficient pathway that comprises of various chemical and electrical pathways. When light falls on a receptor on the retina, the light signal provokes a significantly proportional response to the bipolar cells through a neural synapse. The bipolar cells are then responsible for activating the retinal ganglion cells.
The receptors are able to communicate with other neighboring receptor cells via the amacrine and the horizontal cells. The horizontal cells and the amacrine cells lack special axons. Instead they respond by using graded potentials which determine whether they will release the neurotransmitters. These two types of cells are responsible for altering the synaptic signal which is from the receptors before it is transmitted to the ganglion cells at the optic nerve.
The fovea has been gauged to have a very high resolution limit of approximately ten thousand points. In the fovea, the rod and cone signals act respectively. The cones are responsible for color vision due to the opsin which is found in them. They are also activated by light stimuli during the day. This is referred to as photopic vision.
The ability of the cones to respond to light at differing wavelengths is referred to as the spectral sensitivity. Spectral sensitivity is divided into three groups based on the cones. These are the long, medium and short cones. The absence of any of the cone subtypes causes the different types of color blindness exhibited in human beings.
The rods are responsible for mediating lower vision. They also respond to dim light although they do not respond to pattern vision during daytime. This is referred to as scotopic vision. These signals are then transmitted into the brain via the neuronal system in the optic nerve. This image is the conveyed to the brain (Leeuwen et al., 2004).
The cone and rod signals combine and through several pathways the exposure to light hyperpolarizes the membrane. These pathways occur in the rods and the cones of the retina through patterned excitation. The outer cell segment of the cell membrane in the photo receptors contains a photopigment which depends on the type of the photoreceptor cell.
When the light signal activates the retinal, isomerization takes place. Isomerization causes the conversion of G proteins to transducins which act by triggering events that keep the cell is depolarized as the constant level of cyclic guanosine monophospate allows the sodium channels to stay open. The Na+ active transport triggers the retinal which is bound to the receptor protein to form a product referred to as the transretinal. This reaction causes the protein to degrade the cyclic guanosine monophospate and the Na+ channels are no longer bound causing the cell to become hyperpolarized (Gauthier et al., 2009).
When more sodium leaves the cell and potassium gets into the cell, this causes a flood of calcium ions into the cell. The calcium ions cause the pre synaptic vesicles to attach to the pre synaptic membrane thereby facilitating the releasing of the neurotransmitters which move across the synaptic cleft and bind to the post synaptic receptors.
The postsynaptic receptors are of two varying types based on their post synaptic reaction after they receive the neurotransmitters. Some of the postsynaptic receptors produce an excitatory potential after receiving the neurotransmitters. This results in the depolarization of the neuron. The other type of the post synaptic receptors depicts an inhibitory potential which results in the hyper polarization of the neuron.
Neural transmitters are released based on the amount of light available. In low levels of light, the neurotransmitters released are quantitatively more as compared to the levels produced in higher levels of light. This explains the shift in visual acuity in situations where an individual moves from bright to dim light (Gauthier et al., 2009).
The rods and the cones activate the bipolar cells which are in turn responsible for stimulating the ganglion cells. The retinal ganglion cells have two varying reactions which depend on the receptive field of the cell (Meister & Berry., 1999). The receptive field can either be the central or the annular areas. In case the light signals fall on the central area, the ganglion cells are activated. The cells ganglion cells are not activated when the light signals fall on the annular reception field.
A lot of preprocessing of the signals occurs within the retina due to the small ratio of retinal receptors as compared to the optic nerve fibers. The accuracy of visual perception normally relies on the processing of information within the fovea. More than ten percent of the axons of the optic neurons are specifically devoted to the fovea.
The electrical light signals move through the optic nerve into the optic chiasm. The optic chiasm contains the optic tract (Meister &Berry, 1999). This is where there is a differentiation between the left and right visual sides. The optic tract surrounds the cerebral peduncles of the brain. Most of the axons of the optic tract form synapses in the lateral geniculate nucleus. This allows access to the lateral geniculate nucleus which forms part of the hypothalamus.
The left side of the lateral geniculate nucleus receives input from the right side of the visual field. The right side of the lateral geniculate nucleus however receives input from the left visual side. Each of the two sides of the lateral geniculate nucleus receives visual input from each of the eyes.
The neurons found in the lateral geniculate nucleus have their axons forming synapse in the primary visual cortex which is comprised of several layers. This occurs through optic radiations. The lateral geniculate nucleus is also interconnected to the thalamic reticular nucleus, to the brainstem and to the superior colliculus (Bruce et al., 1997).
When the neural impulses reach the primary visual cortex, these axons from the lateral geniculate nucleus terminate in a layer of the cortex and the signal is transmitted within the different layers to reach the upper layer. When this happens, information from the right and the left eyes is mixed to form a uniform binocular vision. The lateral geniculate nucleus is therefore responsible for controlling the areas within the visual field which are focused on. The cortex has six different layers which respond to a particular section of the retina in each eye. Within the cortex, the ventral stream is responsible for analyzing the form and the color of objects while the dorsal stream is responsible for analyzing the position and the motion of an object. It is also responsible for determining spatial relationships between objects (Meister &Berry., 1999).
Spatial Encoding of the Image by the Retina
The retina is responsible for the spatial encoding of the image received to fit the limited capacity of the optic nerve. This occurs through compression of the received image. Spatial encoding allows information from the photoreceptor cells to get to the ganglion cells. The signals received are decorrelated by the bipolar and the ganglion cells in the center surround structures.
The center surround structures in the retina comprises of the on centers and the off centers. The off centers have a negatively weighted or inhibitory center and a positively weighted excitatory surround as opposed to the on centers which have a positively weighted or inhibitory center and a negatively weighted excitatory center.
These center structures depend on the connection affinities between the bipolar cells and the ganglion cells. The connection affinity between these two types of cells depends on the type and the number of the different ion channels which are imbedded into the synapses, located between the two branches of cells (Meister & Berry., 1999). The retina is therefore able to use these surround structures to enhance the distinguishing features and sharpen the edges of an object within the visual field.
After the image is spatially compressed by the different center surround structures, the resulting signal is sent through the optical nerve fibers through the optic chiasm into the lateral geniculate nucleus (Gauthier et al., 2009).The output from the lateral geniculate nucleus is then sent to the primary visual cortex and the secondary visual cortex.
Bruce, V., Green, P. & Georgeson, M. (1997). Visual perception. Sussex: Psychology Press.
Gauthier Jeffrey, Field Greg D., Sher Alexander, Greschner Martin, Shlens Jonathon, Litke Alan M. & Chichilnisky J.. (2009).Receptive Fields in Primate Retina Are Coordinated to Sample Visual Space More Uniformly. PLoS Biology, 7 (4): 55-69.
Leeuwen M T., Numan R. & Kamermans M. (2004). “Colour-constancy is coded in the retina” . Journal of Perception, 33(2) 5-21.
Meister M. & Berry J. (1999). “The neural code of the retina”. Neuron, 22(3): 433–50.