Explanation of the Visual Pathway.docx

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University of Toronto Scarborough
Catherine Nash

Explanation of the Visual Pathway • Light is reflected from objects and enters the eye (through the cornea, iris, lens, vitreous body) • The incoming light reacts with special types of neurons, at the back of the eye , called photoreceptors (photo transduction occurs here: convert light into neural signals that can stimulate biological processes); this layer of cells at the back of the eye which contains the photoreceptors is called the retina • The visual world can be divided into two halves; the right visual world and the left visual world • the light from the left visual world reflects onto the right retina of each eye, light from the right visual field reflect onto the left retina of each eye • there are 2 types of photoreceptor cells; rods and cones • rod cells: function in less intense light, concentrated at outer edge of the retina, approximately 125 million rods in the human eye, more sensitive than cone cells, responsible night vision • cone cells: responsible for colour vision, function best in bright light, densely packed in fovea (part of retina where retina reflected form point of focus in visual field lands) 6-7 million in human eye • the axons of the photoreceptors synapse with bipolar and horizontal cells (establish basis for brightness and colour contrast) • bipolar cells synapse with retinal ganglion cells and amacrine cells (enhance contrast effects that support form vision and establish the basis for movement detection) • axons of the retinal ganglion cells form a bundle (optic nerve) which exits eye • optic nerve from each eye cross over after leaving the eye, at a point called the optic chiasm • information from right visual field is represented in the left side of the brain and information from the left visual field is represented in the right side of the brain • after exiting the optic chiasm, majority of the fibres terminate the lateral geniculate nucleus (LGN) of the thalamus; this is called the geniculostriate pathway • another, less significant, perceptual pathway is called the collicular pathway (goes through superior colliculus, pulvinar and converges with the geniculate striate pathway in in extrastriate cortex) evolutionarily older pathway, involved with spatial orientation and eye movement control, not significant for the hollow-face illusion • M and P retinal ganglion cells synapse on to m and p neurons in the LGN (p cell = Parvocellular/midget cell, m cell = Magnocellular/ parasol cell) • the LGN has 6 principle layers of cells (inner 2 layers are called Magnocellular layers (layers 1 & 2), outer 4 are called Parvocellular layers(layers 3, 4, 5 & 6) • LGN is bilateral and exi
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