1. Outline Of Today’s Discussion
The Human Eye: Phototransduction
Introduction To Retinal Ganglion Cells

2. Part 1
The Human Eye:
Phototransduction

3. The Eye & Phototransduction
Embryologically, the retina originates from the same tissue that spawns all neurons.
Like other brain cells, cells in the retina cannot be repaired.
Most importantly, the retina is the location of phototransduction.
Did you know that the photoreceptors “face the wrong way”?….

4. The Eye & Phototransduction
Light enters from here.
Anterior
Posterior
Photoreceptors face away.

5. The Eye & Phototransduction
So, this is the first instance of “something backwards” in our retina: Photoreceptors face away from the incoming light!
There are two categories of photoreceptor in the human eye: Rods, and cones.

6. The Eye & Phototransduction
Rods out number cones by approximately 100 million to 5 million, in each eye.
The most central part of the retina contains only cones. Cone density decreases with eccentricity, and in the far periphery, only rods are found.
Lumping rods and cones together, photoreceptor density (overall) decreases with eccentricity. So, vision is less sharp in the periphery.

7. The Eye & Phototransduction
For vertebrates, light increments lead to hyper-polarization, and light decrements lead to de-polarization: Invertabrates, vice-versa.
So, we have another instance of “something backwards” in our retina: More light, less transmitter release.

8. The Eye & Phototransduction
Opsin in rods (rhodopsin) responds best to 500 nm wavelengths.
The three different cone-opsins respond best to 440, 530, and 560 nm wavelengths.
But, at the point of photo-transduction, all wavelength information is lost. This is the principle of univariance.

9. The Eye & Phototransduction
Today’s “Top Three” List
The top three reasons why it is VULGAR to refer to human cones as ‘red’, ‘green’, and ‘blue’.
The number 3 reason is….

10. Part 2: The Eye & Phototransduction
3. Newton said so!

11. “The rays are NOT colored.”
“The Facts Of Light”
“The rays are NOT colored.”
Sir Isaac Newton

12. “The Facts Of Light”
“The red’s in the head.”
Billy Wooten

13. The Eye & Phototransduction
3. Newton said so!
2. The Peaks don’t match perception!

14. The Eye & Phototransduction
440 nm looks violet, not blue!
530 nm looks green…OK…but,
560 nm looks yellow, not red!

15. The Eye & Phototransduction
3. Because Newton said so!
2. The Peaks don’t match perception!
1. The Principle of Univariance!

16. The Eye & Phototransduction
All wavelength information is
lost at photo-transduciton.
Any cone absorb
any wavelength.

17. The Eye & Phototransduction
So, no vulgarity, please.

18. Learning Check
Be able to describe the three types of refractive errors, and the lenses used to correct them.
Be able to identify three reasons why it is inaccurate (“Vulgar”) to refer to our three cone types as red, green, and blue.
Be able to find your blind spot (each eye). That is, be able to “hide” your thumb in your blind-spot.

19. Retinal Ganglion Cells
Part 2
Introduction to
Retinal Ganglion Cells

20. Intro to Retinal Ganglion Cells
Here’s a
‘big picture’
of the eye.
Note, that the
retina is at the
posterior pole.

21. Intro to Retinal Ganglion Cells
The retina consists of three layers: Photoreceptors; Middle-layer (horizontal, bipolar, amacrine); Ganglion cells.
Let’s zoom in to see how these are arranged…

22. Intro to Retinal Ganglion Cells
Light enters from here.
Anterior
Posterior
Photoreceptors face away.

23. Intro to Retinal Ganglion Cells
The ganglion cells, especially, are involved in extracting meaningful visual information.
The most meaningful information for an organism would be the identification of object boundaries and edges.
Typically, objects in the environment differ from each other and/or their surroundings with respect to light intensity.
The ganglion cells specialize in detecting those luminance boundaries, and reporting them to later portions of the visual system.

24. Intro to Retinal Ganglion Cells
Location, Location, Location!
Photoreceptors face away from the incoming light. They are the most posterior of the three retinal layers.
Within the middle layer, the horizontal cells are closest to the photoreceptors. The bipolar cells are next. The amacrine cells are furthest middle-layer cells from the photoreceptors.
The ganglion cells are the “last stop” before exiting the retina.
The retina is no thicker than a postage stamp!

25. Intro to Retinal Ganglion Cells
All these cell types are in the retina.
Anterior
Posterior
The retina is no thicker than a stamp!

26. Intro to Retinal Ganglion Cells
A Memory Aid For Visual Anatomy: R.H. Bagols
Receptors
Horizontal Cells
Bipolar Cells
Amacrine Cells
Ganglion Cells
Optic Nerve
Lateral Ganiculate Nucleus
Striate Cortex (Primary Visual Cortex: “V1”; “Area 17)

27. Intro to Retinal Ganglion Cells
Ganglion cells don’t respond directly to light! They respond to the output of middle-layer cells.
There are approximately 1.25 million ganglion cells per eye.
So, there is a nearly 100-to-1 ‘data reduction’ within the retina. The summary specifies edges and boundaries.
By analogy, ganglion cells would summarize a 750-1,000 word essay into 7.5 to 10 words.

28. Intro to Retinal Ganglion Cells
Receptive Fields
Electrophysiological recordings are conducted to determine a ganglion cells receptive field.
The receptive field is the area of the photoreceptor mosaic to which a ganglion cell responds.
Equivalently, when the eye is immobilized, the receptive field corresponds to a region of space, since light travels in straight lines.
Receptive-field size is smallest in the fovea (the center of the macula). The smaller the receptive-field size, the sharper the vision.

29. Intro to Retinal Ganglion Cells
Center-Surround Antagonism
Ganglion cells exhibit center-surround antagonism: Stimuli that excite the center, suppress the surround, and vice versa.
Center-surround antagonism within a ganglion cell is a form of lateral inhibition.
Lateral inhibition - Antagonistic neural interaction between adjacent regions of a sensory surface, such as the retina.

30. Intro to Retinal Ganglion Cells
Center-Surround Antagonism
For ganglion cells, there are two types of center-surround antagonistic arrangements: “On-Center” and “Off-Center”.
“On-Center” and “Off-Center” cells are equally numerous, and their axons remain segregated through to the next stage in the visual pathway.

31. “On-Center” Ganglion Cell+ + + + - + + - + + + + - - - - - - -
Responds maximally to light increments in the center,
and light decrements in the surround.

32. “On-Center” Cells Love Stimuli like this.+ + + + - + + - + + + + - - - - - - -

33. “On-Center” Cells Hate Stimuli like this.+ + + + - + + - + + + + - - - - - - -

34. “Off-Center” Ganglion Cell+ + + + - + - - - - - - - - - + + +
Responds maximally to light decrements in the center,
and light increments in the surround.

35. “Off-Center” Cells Love Stimuli like this.+ + + + - - - + - - - + - - - - - - - + - + + +

36. “Off-Center” Cells Hate Stimuli like this.+ + + + - - - + - - - + - - - - - - - + - + + +

37. Ganglion cells have no orientation preference.

38. Intro to Retinal Ganglion Cells
The visual system
responds to
CONTRAST!!!

39. Intro to Retinal Ganglion Cells
(Lmax - Lmin)
_____________
(Lmax + Lmin)
Michelson Contrast =

40. High Contrast Stimuli

41. Low Contrast Stimuli

42. No-Contrast Stimuli
(No Difference Between Center and Surround)