How visual images are captured and encoded by the retina and represented in the primary visual cortex

Paul B.

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How visual images are captured and encoded by the retina and represented in the primary visual cortex

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Abstract

Visual perception is the ability to interpret information from visual light reaching the eye, the resulting perception of this information is sight. Light is electromagnetic energy that is emitted in the form of waves, it is reflected off an object to enter the eye, and is refracted by the eye to be focused upon the retina at the back of the eye. The retina contains photo receptors that convert the light energy into neural activity. The retinal neuron axons form bundles, called the optic nerve, which passes through the orbit of the skull to enter the base of the brain near the pituitary. The retina as a whole is specialized to detect differences in the intensity of light falling on different parts of it, thus determining the aspects of shape and size of an object. Light is focused on the retina, to recreate the image that the light being collected has been reflected from, by the refractive action of the lens and cornea, such that light striking the curved surface of the cornea bends to converge on the back of the eye, and light entering the center of the eye passes straight to the retina, resulting in an inverted representation of the image the light is reflected from.

Introduction
The retina as a whole
1. How it works
2. The cells of the retina
3. Photoreceptors
4. The relative concentrations of rods and cones
5. The photo pigments
The bipolar cell
1. Types of bipolar cells
2. The bipolar cell organization
3. The link between bipolar and ganglion cells
4. Types of ganglion cell
The LGN
1. The visual receptive fields of the LGN cells
2. The major input into the LGN
The receptive fields in the retina
The neurones of the magnocellular pathway
Conclusion
Bibliography

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Extract

[...] The cells of the retina are organized in layers, and as such are said to have a laminar organization. Light must pass through the ganglion and predecessor cells before it reaches the photoreceptors, though due to the relative transparency of these cells there is minimal distortion of the image encoded by the light rays. This cellular organization is advantageous in that there is a pigmented epithelium below the photoreceptors that absorbs any light that passes entirely through the receptor, thus abolishing reflection of light within the eye that may cause image blurring. [...]

[...] For this to occur in each point in the retina there are a cluster of three receptor types, each type being maximally sensitive to either blue, red, or green, and the brain assigns colors to a received light wave based on a comparative readout of the stimulation of the three cone types, thus when all types of cone are equally activated we perceive the color white. Once transduction has taken place, there is considerable processing of the information within the retina before the information is passed on to the brain. [...]

[...] There are differences in the visual responses of the two types of ganglion cell; M cells have larger receptive fields, and conduct action potentials most rapidly in the optic nerve, and are also more sensitive to low contrast stimuli. P cells are sensitive to differences in wavelengths of light, i.e. color sensitive, and are called color opponent cells. They have similar ON and OFF centers as described earlier, except with different wavelengths of light as the stimulus. Two types of opponency are found; red versus green, and yellow versus blue, such that, for example, red light causing an ON response in the center will be antagonized by green light causing an OFF response in the surround. [...]

[...] Thus as, for example, the left visual field is viewed by both the nasal left retina and temporal right retina, and the information received from both is kept separate, in the right LGN the ipsolateral axons from the right eye synapse on cells in layers and and the contalateral axons from the left eye synapse on cells ion layers and 6. Layers 1 and 2 receive input from P-type ganglion cells and are called the magnocellular LGN layers. Layers 3 through 6 receive input from M-type ganglion cells, and are called the parvocellular LGN layers. [...]

[...] This map is often distorted as visual space is not sampled uniformly by cells of the retina, in that photoreceptors are in much higher density in the fovea, and there is often overlap of receptive fields meaning a discrete point of light will activate more and more cells further along in the pathway The major synaptic target of the LGN is the primary visual cortex (also called Brodmass's area 17, striate cortex, and located in the occipital lobe with the majority on the medial surface of the hemisphere. [...]

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