Dr. Raj Patel OD - Vancouver Vision Clinic Vision & Perception

What we will cover today… Eye Anatomy Accommodation Rods & Cones Convergence Edge Enhancement (Herman Grid Illusion) Contralateral Processing Cases

Photoreceptors: Rods & Cones 160K/mm in humans, 1 Billion/mm in hawk LE 49-18 Photoreceptors: Rods & Cones 160K/mm in humans, 1 Billion/mm in hawk Sclera Choroid Retina Ciliary body Suspensory ligament Fovea (center of visual field) Cornea Iris Optic nerve Pupil Aqueous humor Lens Central artery and vein of the retina Vitreous humor Optic disk (blind spot)

Retinal image (? Eye)

Retinal Detachment

Cataract

Cataract surgery

LASIK surgery

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

Near vision (accommodation) LE 49-19 Front view of lens and ciliary muscle Choroid Lens (rounder) Retina Ciliary muscle Suspensory ligaments Near vision (accommodation) Lens (flatter) Distance vision

What happens after 40! Presbyopia

How do we see ? Lets go back to the retina …

Rods: Cones: 1. More sensitive to light – function better in dim light 2. Become bleached in bright light 3. Absorb all wavelengths of visible light, so they give monochrome vision 4. Groups (up to 200) pass impulses to same sensory neuron of optic nerve, therefore less visual acuity 5. More widely dispersed, giving a wider field of vision LE 49-22 Dark Responses Light Responses Cones: 1. Less sensitive to light 2. Don’t bleach in bright light, function well in bright light 3. 3 types of cone cells: sensitive to red, green & blue light 4. Have their own individual neuron through which messages can be sent to brain, therefore better visual acuity 5. Very concentrated near the fovea, giving one acute area of field of vision Rhodopsin inactive Rhodopsin active Na+ channels open Na+ channels closed Rod depolarized Rod hyperpolarized Glutamate released No glutamate released Bipolar cell either depolarized or hyperpolarized, depending on glutamate receptors Bipolar cell either hyperpolarized or depolarized, depending on glutamate receptors

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

3. Contralateral processing of Visual Stimuli LE 49-24 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 Left visual field Right visual field Left eye Right eye Optic nerve Optic chiasm Lateral geniculate nucleus Primary visual cortex

An interesting case

Where is the problem located?

Thank You