Outline Of Today’s Discussion
1. The Human Eye: Phototransduction
2. Introduction To Retinal Ganglion Cells
Part 1
The Human Eye:
Phototransduction
The Eye & Phototransduction
1. Embryologically, the retina originates from the
same tissue that spawns all neurons.
2. Like other brain cells, cells in the retina cannot
be repaired.
3. Most importantly, the retina is the location of
phototransduction.
4. Did you know that the photoreceptors “face the
wrong way”?….
The Eye & Phototransduction
Light enters from here.
Anterior
Posterior
Photoreceptors face away.
The Eye & Phototransduction
1. So, this is the first instance of “something
backwards” in our retina: Photoreceptors face
away from the incoming light!
2. There are two categories of photoreceptor in the
human eye: Rods, and cones.
The Eye & Phototransduction
1.
Rods out number cones by approximately 100 million to
5 million, in each eye.
2.
The most central part of the retina contains only cones.
Cone density decreases with eccentricity, and in the far
periphery, only rods are found.
3.
Lumping rods and cones together, photoreceptor
density (overall) decreases with eccentricity. So, vision is
less sharp in the periphery.
The Eye & Phototransduction
•
For vertebrates, light increments lead to hyperpolarization, and light decrements lead to depolarization: Invertabrates, vice-versa.
•
So, we have another instance of “something backwards”
in our retina: More light, less transmitter release.
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.
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….
Part 2: The Eye & Phototransduction
3. Newton said so!
“The Facts Of Light”
“The rays are NOT colored.”
Sir Isaac Newton
“The Facts Of Light”
“The red’s in the head.”
Billy Wooten
The Eye & Phototransduction
3. Newton said so!
2. The Peaks don’t match perception!
The Eye & Phototransduction
440 nm looks violet, not blue!
530 nm looks green…OK…but,
560 nm looks yellow, not red!
The Eye & Phototransduction
3. Because Newton said so!
2. The Peaks don’t match perception!
1. The Principle of Univariance!
The Eye & Phototransduction
All wavelength information is
lost at photo-transduciton.
Any cone absorb
any wavelength.
The Eye & Phototransduction
So, no vulgarity, please.
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.
Part 2
Introduction to
Retinal Ganglion Cells
Intro to Retinal Ganglion Cells
Here’s a
‘big picture’
of the eye.
Note, that the
retina is at the
posterior pole.
Intro to Retinal Ganglion Cells
1. The retina consists of three layers:
Photoreceptors;
Middle-layer (horizontal, bipolar, amacrine);
Ganglion cells.
2. Let’s zoom in to see how these are arranged…
Intro to Retinal Ganglion Cells
Light enters from here.
Anterior
Posterior
Photoreceptors face away.
Intro to Retinal Ganglion Cells
1.
The ganglion cells, especially, are involved in extracting
meaningful visual information.
2.
The most meaningful information for an organism would
be the identification of object boundaries and edges.
3.
Typically, objects in the environment differ from each
other and/or their surroundings with respect to light
intensity.
4.
The ganglion cells specialize in detecting those luminance
boundaries, and reporting them to later portions of the
visual system.
Intro to Retinal Ganglion Cells
Location, Location, Location!
1.
Photoreceptors face away from the incoming light. They
are the most posterior of the three retinal layers.
2.
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.
3.
The ganglion cells are the “last stop” before exiting the
retina.
4.
The retina is no thicker than a postage stamp!
Intro to Retinal Ganglion Cells
All these cell types are in the retina.
Anterior
Posterior
The retina is no thicker than a stamp!
Intro to Retinal Ganglion Cells
A Memory Aid For Visual Anatomy: R.H. Bagols
1.
2.
3.
4.
5.
6.
7.
8.
Receptors
Horizontal Cells
Bipolar Cells
Amacrine Cells
Ganglion Cells
Optic Nerve
Lateral Ganiculate Nucleus
Striate Cortex (Primary Visual Cortex: “V1”; “Area 17)
Intro to Retinal Ganglion Cells
1.
Ganglion cells don’t respond directly to light! They
respond to the output of middle-layer cells.
2.
There are approximately 1.25 million ganglion cells per
eye.
3.
So, there is a nearly 100-to-1 ‘data reduction’ within the
retina. The summary specifies edges and boundaries.
4.
By analogy, ganglion cells would summarize a 750-1,000
word essay into 7.5 to 10 words.
Intro to Retinal Ganglion Cells
Receptive Fields
1.
Electrophysiological recordings are conducted to
determine a ganglion cells receptive field.
2.
The receptive field is the area of the photoreceptor
mosaic to which a ganglion cell responds.
3.
Equivalently, when the eye is immobilized, the receptive
field corresponds to a region of space, since light travels
in straight lines.
4.
Receptive-field size is smallest in the fovea (the center of
the macula). The smaller the receptive-field size, the
sharper the vision.
Intro to Retinal Ganglion Cells
Center-Surround Antagonism
1.
Ganglion cells exhibit center-surround antagonism:
Stimuli that excite the center, suppress the surround,
and vice versa.
2.
Center-surround antagonism within a ganglion cell is a
form of lateral inhibition.
3.
Lateral inhibition - Antagonistic neural interaction
between adjacent regions of a sensory surface, such as
the retina.
Intro to Retinal Ganglion Cells
Center-Surround Antagonism
•
For ganglion cells, there are two types of centersurround antagonistic arrangements: “OnCenter” 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.
“On-Center” Ganglion Cell
-
-
-
++ ++
- + +
++ + +
-
Responds maximally to light increments in the center,
and light decrements in the surround.
“On-Center” Cells Love Stimuli like this.
-
-
-
++ ++
- + +
++ + +
-
“On-Center” Cells Hate Stimuli like this.
-
-
-
++ ++
- + +
++ + +
-
“Off-Center” Ganglion Cell
+
---- - -
+
+
+
+
+
+
+
Responds maximally to light decrements in the center,
and light increments in the surround.
“Off-Center” Cells Love Stimuli like this.
+
+ +
+
-- +
-+
- -- - -- +
+
+
+
“Off-Center” Cells Hate Stimuli like this.
+
+ +
+
-- +
-+
- -- - -- +
+
+
+
Ganglion cells have no orientation preference.
Intro to Retinal Ganglion Cells
The visual system
responds to
CONTRAST!!!
Intro to Retinal Ganglion Cells
Michelson Contrast =
(Lmax - Lmin)
_____________
(Lmax + Lmin)
High Contrast Stimuli
Low Contrast Stimuli
No-Contrast Stimuli
(No Difference Between Center and Surround)