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Vision & PerceptionDr. Raj Patel OD - Vancouver Vision Clinic
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What we will cover today…
Eye Anatomy AccommodationRods & ConesConvergenceEdge Enhancement (Herman Grid
Illusion)Contralateral ProcessingCases
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LE 49-18
Cornea
Ciliary body
Suspensoryligament
Iris
Pupil
Aqueoushumor
Lens
Vitreous humor
Central artery andvein of the retina
Optic disk(blind spot)
Fovea (centerof visual field)
Opticnerve
RetinaChoroidSclera Photoreceptors:
Rods & Cones160K/mm in humans, 1 Billion/mm in hawk
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Retinal image (? Eye)
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Retinal Detachment
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Cataract
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Cataract surgery
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LASIK surgery
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Accomodation
The lens inside the eye changes shape to focus at different distances
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LE 49-19
Retina
Choroid
Suspensoryligaments
Front view of lensand ciliary muscle
Ciliarymuscle
Lens (rounder)
Lens (flatter)
Near vision (accommodation)
Distance vision
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What happens after 40!
Presbyopia
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How do we see ?
Lets go back to the retina …
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LE 49-22
Light Responses
Rhodopsin active
Na+ channels closed
Rod hyperpolarized
Bipolar cell eitherhyperpolarized ordepolarized, depending on glutamate receptors
No glutamatereleased
Dark Responses
Rhodopsin inactive
Na+ channels open
Rod depolarized
Bipolar cell eitherdepolarized orhyperpolarized, depending on glutamate receptors
Glutamatereleased
Rods:1. More sensitive to light – function better in dim light2. Become bleached in bright light3. Absorb all wavelengths of visible light, so they give monochrome vision4. Groups (up to 200) pass impulses to same sensory neuron of optic nerve, therefore less visual acuity5. More widely dispersed, giving a wider field of vision
Cones:1. Less sensitive to light2. Don’t bleach in bright light, function well in bright light3. 3 types of cone cells: sensitive to red, green & blue light4. Have their own individual neuron through which messages can be sent to brain, therefore better visual acuity5. Very concentrated near the fovea, giving one acute area of field of vision
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General Organization
Retina is only part of CNS visible 3 layers of neurons
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More Detail of Cell Types
Receptive field, convergence, resolution
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So how do we process all this?
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Edge Enhancement
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background
Provides greater contrast Many rods -> single ganglia = lower
acuity Cones have lower ratio with ganglia
= higher acuity Receptive fields. If smaller then
higher acuity Large fields in peripheral retina /
smallest in fovea
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The concept
Lateral Inhibition 2 other cells (horizontal and
amacrine) Below is 1 to 1 mapping (fovea)
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Drum roll …
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Herman Grid Illusion
Due to edge enhancement white areas of grid look whiter if they are next to black area
Optical illusion: grey areas appear at the intersections of the white lines which are not real. If all of grid is covered up apart from one white line, the grey areas disappear.
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Still Awake
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Contralateral Processing
Collect - integrate - perceive
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LE 49-24Leftvisualfield
Rightvisualfield
Lefteye
Righteye
Primaryvisual cortex
Lateralgeniculatenucleus
Optic nerve
Optic chiasm
3. Contralateral processing of Visual Stimuli*Both retinas receive stimuli from left & right fields of vision*Left & right optic nerves cross in optic chiasma*Right field of vision is processed in left side of brain (vice versa)*Neurons from optic nerve synapse with neurons in visual cortex of Cerebrum (occipital lobe)*Allows brain to Have perception of depth, distance & sizes
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An interesting case
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Where is the problem located?
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Thank You