1. Neuroscience: Exploring the Brain, 3e
Chapter 9: The Eye

2. Introduction Significance of vision
Relationship between human eye & camera
Retina
Photoreceptors: Converts light energy into neural activity
Detects differences in intensity of light
Lateral geniculate nucleus (LGN)
First synaptic relay in the primary visual pathway
Visual information ascends to cortex interpreted and remembered

3. Properties of Light Light Electromagnetic radiation
Wavelength, frequency, amplitude

4. Properties of Light Light Energy is proportional to frequency
e.g., gamma radiation and cool colors - high energy
e.g., radio waves and hot colors - low energy

5. Properties of Light Optics Study of light rays and their interactions
Reflection
Bouncing of light rays off a surface
Absorption
Transfer of light energy to a particle or surface
Refraction
Bending of light rays from one medium to another

7. The Structure of the Eye
Gross Anatomy of the Eye
Pupil: Opening where light enters the eye
Sclera: White of the eye
Iris: Gives color to eyes
Cornea: Glassy transparent external surface of the eye
Optic nerve: Bundle of axons from the retina

8. The Structure of the Eye
Ophthalmoscopic Appearance of the Eye

9. The Structure of the Eye
Cross-Sectional Anatomy of the Eye

10. Image Formation by the Eye
Refraction of light by the cornea
Eye collects light, focuses on retina, forms images

11. Image Formation by the Eye
Accommodation by the Lens
Changing shape of lens allows extra focusing power

12. Image Formation by the Eye
The Pupillary Light Reflex
Connections between retina and brain stem neurons that control muscle around pupil
Continuously adjusting to different ambient light levels
Consensual
Pupil similar to the aperture of a camera

13. Image Formation by the Eye
The Visual Field
Amount of space viewed by the retina when the eye is fixated straight ahead

14. Image Formation by the Eye
Visual Acuity
Ability to distinguish two nearby points
Visual Angle: Distances across the retina described in degrees

15. Microscopic Anatomy of the Retina
Direct (vertical) pathway:
Ganglion cells 
Bipolar cells
Photoreceptors

16. Microscopic Anatomy of the Retina
Retinal processing also influenced lateral connections:
Horizontal cells
Receive input from photoreceptors and project to other photoreceptors and bipolar cells
Amacrine cells
Receive input from bipolar cells and project to ganglion cells, bipolar cells, and other amacrine cells

17. Microscopic Anatomy of the Retina
The Laminar Organization
Inside-out
Light passes through ganglion and bipolar cells before reaching photoreceptors

18. Microscopic Anatomy of the Retina
Photoreceptor Structure
Converts electromagnetic radiation to neural signals
Four main regions
Outer segment
Inner segment
Cell body
Synaptic terminal
Types of photoreceptors
Rods and cones

19. Microscopic Anatomy of the Retina
Regional Differences in Retinal Structure
Varies from fovea to retinal periphery
Peripheral retina
Higher ratio of rods to cones
Higher ratio of photoreceptors to ganglion cells
More sensitive to light

20. Microscopic Anatomy of the Retina
Regional Differences in Retinal Structure (Cont’d)
Cross-section of fovea: Pit in retina where outer layers are pushed aside
Maximizes visual acuity
Central fovea: All cones (no rods)
1:1 ratio with ganglion cells
Area of highest visual acuity

21. Phototransduction Phototransduction in Rods
Light energy interacts with photopigment to produce a change in membrane potential
Analogous to activity at G-protein coupled neurotransmitter receptor - but causes a decrease in second messenger

22. Phototransduction Phototransduction in Rods
Dark current: Rod outer segments are depolarized in the dark because of steady influx of Na+
Photoreceptors hyperpolarize in response to light

23. Phototransduction Phototransduction in Rods
Light activated biochemical cascade in a photoreceptor
The consequence of this biochemical cascade is signal amplification

24. Phototransduction Phototransduction in Cones
Similar to rod phototransduction
Different opsins
Red, green, blue
Color detection
Contributions of blue, green, and red cones to retinal signal
Spectral sensitivity
Young-Helmholtz trichromacy theory of color vision

25. All-cone daytime vision All-rod nighttime vision
Phototransduction
Dark and Light Adaptation
Dark adaptation—factors
Dilation of pupils
Regeneration of unbleached rhodopsin
Adjustment of functional circuitry
20–25 minutes
All-cone daytime vision
All-rod nighttime vision

26. Phototransduction Dark and Light Adaptation
Calcium’s Role in Light Adaptation
Calcium concentration changes in photoreceptors
Indirectly regulates levels of cGMP channels

27. Retinal Processing Transformations in the Outer Plexiform Layer
Photoreceptors release less neurotransmitter when stimulated by light
Influence horizontal cells and bipolar cells

28. Retinal Processing Receptive Field: “On” and “Off” Bipolar Cells
Receptive field: Stimulation in a small part of the visual field changes a cell’s membrane potential
Antagonistic center-surround receptive fields

29. Retinal Processing On-center Bipolar Cell
Light on (less glutamate); Light off -> more glutamate
‘Inverting’ synapse (inhib)

30. Retinal Output Ganglion Cell Receptive Fields
On-Center and Off-Center ganglion cells
Responsive to differences in illumination

31. Retinal Output Types of Ganglion Cells
Appearance, connectivity, and electrophysiological properties
M-type (Magno) and P-type (Parvo)ganglion cells in monkey and human retina

32. Retinal Output
Color-Opponent Ganglion Cells

33. Retinal Output Parallel Processing Simultaneous input from two eyes
Information from compared in cortex
Depth and the distance of object
Information about light and dark: ON-center and OFF-center ganglion cells
Different receptive fields and response properties of retinal ganglion cells: M- and P- cells, and nonM- nonP cells

34. Concluding Remarks
Light emitted by or reflected off objects in space  imaged onto the retina
Transduction
Light energy converted into membrane potentials
Phototransduction parallels olfactory transduction
Electrical-to-chemical-electrical signal
Mapping of visual space onto retina cells not uniform

35. End of Presentation

36. Retinal Processing Research in ganglion cell output by
Keffer Hartline, Stephen Kuffler, and Horace Barlow
Only ganglion cells produce action potentials
Research in how ganglion cell properties are generated by synaptic interactions in the retina
John Dowling and Frank Werblin
Other retinal neurons produce graded changes in membrane potential