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Eye (Primary visual Cortex (V1) (V1-Spatial frequency (Some V1 cells…
Eye
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Retina
Interior lining of the back of the eye. There are three main layers: Photoreceptive layer, rods and cones (converts light into chemical signal); bipolar cell layer(conveys information from photoreceptors to ganglion cells); ganglion cell layer (receives information from bipolar cells)
Retinal Layers
Ganglian anf bipolar cells are transparent so light can reach the photoreceptors at the back. Horizontal amacrine cells integrate information like interneurones
Photoreceptors
Rods
120 million in retina. They are responsible for night vision and are sensitive to light insensitivity. Monochromatic information. They have poor acuity (sharpness of vision). Found mainly in peripheral retina.
Cones
6 million in retina. They are responsible for day vision and are sensitive to colour. Excellent acuity (sharpness). They are mainly found in fovea.
Rods and Cones
contain photopigment which changes light into nerve signals. Two component proteins are opsin and retinal. At rest photoreceptors depolarised and release glutamate which activates bipolar cells. Light causes hyperpolarisation, reducing glutamate release which activates bipolar cells. In photoreceptors and bipolar cells glutamate release is not regulates by Ap but membrane potential.
Ganglion Cells
Transmit information to the brain. Axons leave eye and form optic nerve, optic chiasm, optic tract. Axons terminate in lateral geniculate nucleus (thalamus) and superior colliculus
Receptor fields
Each retinal neurone 'covers' a small area in our field of vision. It changes the firing rate of a neurone. A photoreceptor receptive field is a tiny spot in your visual field that corresponds to the photoreceptors precise location on retina. Size of ganglion receptive field determines acuity - the smaller the better
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Periphery - large receptor fields as many receptors coverage on ganglion cells - signals are averaged.
Receptive fields are circular. There are two areas, one called the centre and the other is the surround. Centre and surround work in opposite ways, light that strikes the centre has opposite effects from light striking surround. Either 'on' or 'off' cells
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Perception
Scale
If we see an object thought to have great size but takes up little space on retina we perceive it as far away rather than miniature. Scale perception based on encoding of spatial frequency - we cannot perceive high frequency information from high distances
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Ponzo Illusion
Brain judges object size based on its background. Ponzo illusion is that we interpret conveying sides according to linear perception (as parallel lines receding into the distance). Involves visual association cortex which alters activity in V1 feedback
Damage
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Visual Agnosia
Prosopagnosia
Inability to recognise faces. Damage to fusiform face area - known to be involved with processing faces.Can do facial features but not faces as a whole
Visual Object Agnosia
fusiform area in tact, damage to visual association cortex
Damage to dorsal pathway
Akinetopsia (motion blindness)- inability to perceive movement. This is from damage to V1 and V5 in dorsal stream.
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Coding light and dark
Ganglion cells respond to light and dark using centre/surround receptive field. On areas increase firing it light hits it and off areas decrease firing if light hits it. They respond best to contrast-illuminating entire receptive field leads to cancelling out therefore no change in firing
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The optic Nerve
Ganglion call axons make up the optic nerve (each eye has one). Optic nerve coverage optic chiasma at base of brain: nasal half of axons cross to opposite hemisphere; lateral half of axons stay onto the same hemisphere. This allows both hemispheres to receive information from both eyes. Right visual field is processed in left hemisphere. Visual fields overlap which gives us depth perception
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Negative after image
Influenced by the ganglion cell firing rates and colour pairs. If ganglion cells are inhibited or excited they show rebound, firing slower for faster than normal. If you see green in a picture it inhibits some red-green ganglion cells for red on cells. When you are looking at a white square (reflecting natural-coloured light) these ganglion cells (e.g. red on cells) are no longer inhibited by green light ensuring that they fire faster than normal.
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