G9 Chapter 3 Neural Processing and Perception
Other cells in the retina. Not covered specifically in G9
We’ll first follow the neural activity upwards toward the brain – along the receptor -> bipolar ->
ganglion pathway. Recall the side view of the retina . . .
Cone
Rod
Horizontal
Bipolar
Amacrine
Ganglion
The Bipolar cells.
So called because it’s difficult to tell which is the “receiving” end and which is
the “sending” end. They transmit information from the receptors to the ganglion cells.
Why do they exist?
Perhaps to provide extra synapses between the receptors and ganglion cells.??
The first synapse –the receptor –> bipolar synapse heightens contrast. This is called lateral
inhibition. The lateral message is carried by the horizontal cells.
The second synapse –the bipolar –> ganglion synapse may allow a completely different type of
comparison, perhaps more global, perhaps wavelength comparisons.
This is pretty much the way things are in the brain – the comparisons, decisions, “thinking”, of
the brain is done in the synapses.
G9 Chapter 3a - 1
The Gossiping cells of the retina
Horizontal Cells
H
Receptors
H
Horizontal
Cells
H
B
B
B
G
G
G
Bipolar
Cells
Ganglion
Cells
These cells receive input from receptors and send inhibition to neighboring bipolar cells.
Since the inhibition is sent laterally, it’s called lateral inhibition.
One likely function of the horizontal cells is to heighten the contrast of borders between light
and dark. Example from Figure 3.9
Each vertical gray square seems slightly darker on the left.
This is the distribution of light intensity
This is the distribution of brightness, so our visual system is lying to us.
G9 Chapter 3a - 2
Goldstein’s description of lateral inhibition – G9 p 57
Output of each bipolar cell is the sum of the input and the total lateral inhibition.
Bipolar X: 100 – 10 – 10 = 80
Bipolar A: 100 – 10 – 10 = 80
Bipolar B: 100 – 10 – 2 = 88
Bipolar C:
20 – 10 – 2 = 8
Bipolar D: 20 - 2 - 2 = 16
Bipolar Y: 20 - 2 - 2 = 16
Lateral Inhibition may also explain simultaneous contrast G9 p 58
(To demonstrate, click on one of the squares and hit the Backspace key. Then press CTRL+Z to
undo.)
G9 also mentions the Hermann Grid. I will not test you over it.
G9 Chapter 3a - 3
The amacrine cells
Cone
Rod
Horizontal
Bipolar
Amacrine
Ganglion
These cells receive input from bipolar cells and send inhibition (probably) to neighboring
ganglion cells.
One conjecture is that the Amacrine cells are involved in inhibition associated with
wavelength. That is, they’re probably involved in perception of color.
Amacrine cells may act as switches between rod and cone systems driving the same ganglion
cell.
G9 Chapter 3a - 4
Ganglion cells
Cone
Rod
Horizontal
Bipolar
Amacrine
Ganglion
Only cells in retina that are clearly neurons. About 1 million per eye.
Axons form the optic nerve. So EVERYTHING WE SEE is carried to the rest of the brain over
the 1 million axons of the ganglion cells in that eye.
This is a 100:1 compression of information – 100+ million receptors down to 1 million RGCs.
There are over 20 specific types of retinal ganglion cells Y1 - p70
Two most well-studied
Parasol or M for Magni, larger in size with large receptive fields.
The parasol ganglion cells apparently carry information about movement, location, and
about gross features of the visual environment.
Midget or P for Parvi, smaller with smaller receptive fields.
The midget ganglion cells apparently carry information about wavelength and about fine
details of the visual environment.
G9 Chapter 3a - 5
The blind spot
The axons of the ganglion cells cross the surface of the retina and gather together at one place.
There, they go toward the back of the head, out of the eye. That spot, obviously, has no
receptors. It is called the blind spot.
Five of the
approximately 1
million ganglion
cells
Blind
spot
Receptive Fields – the collection of receptors connected to a given neuron. G9 p 60
Could play VL 2.9 here – it lasts about 10 minutes.
Each retinal ganglion cell is connected to a field of receptors 100s or 1000s of them.
Those receptors make up the ganglion cell’s receptive field.
Receptors
Receptive Field
G
Neuron : G for ganglion cell.
G9 Chapter 3a - 6
Center-Surround Characteristics of Receptive Fields G9 p 60
Most receptive fields are characterized as center surround.
This means that there is a center, circular area, with a donut-like surround.
Stimulation of the center yields one type of response (either an increase in activity or a
decrease). Stimulation of the surround yields the opposite type of response.
This type of receptive field is called an antagonistic center surround field.
An Off-center
An On-center
On-surround RGC
+
Off-surround RGC
+
Effects of various types of stimuli on a On-center/Off-surround receptive field . . .
Spot of light in center – positive response
response
+
Yippee!
-
Spot of light in surround – diminished
Spot of darkness in center – diminished response
response
Bummer.
Spot of darkness in surround – positive
+
Bummer.
-
+
-
-
Yippee!
Diffuse light – no response
Diffuse darkness – no response for same reason
Because center and surround cancel each other
+
-
+
I see nothing.
I see dark
-
+
-
I see light
+
I see nothing.
I see light
Edge of light – a response almost anywhere it is, in any orientation.
+
-
I see light.
+
-
This suggests that an antagonistic circular surround receptive field is an ideal shape for
detecting light-dark borders of any orientation – a wonderful building block for perception.
G9 Chapter 3a - 7
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