Vision & PerceptionDr. Raj Patel OD - Vancouver Vision Clinic

What we will cover today…

Eye Anatomy AccommodationRods & ConesConvergenceEdge Enhancement (Herman Grid

Illusion)Contralateral ProcessingCases

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

Retinal image (? Eye)

Retinal Detachment

Cataract

Cataract surgery

LASIK surgery

Accomodation

The lens inside the eye changes shape to focus at different distances

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Retina

Choroid

Suspensoryligaments

Front view of lensand ciliary muscle

Ciliarymuscle

Lens (rounder)

Lens (flatter)

Near vision (accommodation)

Distance vision

What happens after 40!

Presbyopia

How do we see ?

Lets go back to the retina …

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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

General Organization

Retina is only part of CNS visible 3 layers of neurons

More Detail of Cell Types

Receptive field, convergence, resolution

So how do we process all this?

Edge Enhancement

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

The concept

Lateral Inhibition 2 other cells (horizontal and

amacrine) Below is 1 to 1 mapping (fovea)

Drum roll …

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.

Still Awake

Contralateral Processing

Collect - integrate - perceive

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

An interesting case

Where is the problem located?

Thank You