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

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

3. 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)

4. Retinal image (? Eye)

5. Retinal Detachment

6. Cataract

7. Cataract surgery

8. LASIK surgery

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

10. 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

11. What happens after 40!
Presbyopia

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

13. 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

14. General Organization Retina is only part of CNS visible
3 layers of neurons

15. More Detail of Cell Types
Receptive field, convergence, resolution

16. So how do we process all this?

17. Edge Enhancement

18. 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

19. The concept Lateral Inhibition 2 other cells (horizontal and amacrine)
Below is 1 to 1 mapping (fovea)

20. Drum roll …

21. 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.

22. Still Awake

23. Contralateral Processing
Collect - integrate - perceive

24. 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

25. An interesting case

26. Where is the problem located?

27. Thank You