The Retina – the Neural System of the Eye Begins on G9 p 26
Retina: A layer of cells including receptors and neurons on the back of the interior wall of the eye.
Figure 2.3 from G9
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Major types of neural cells making up the retina include:
Pigment Epithelium
Cone
Rod
Horizontal
Bipolar
Amacrine
Ganglion
Note that light passes through ALL of the cells in the retina before it strikes the receptors.
Why do the receptors face away from the light?
Nutrition.
It’s more efficient to supply the nutrient-carrying blood to the receptors from the back of the eye.
Blood supply to the retina is not shown in this figure.
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The receptors G9 p 26
Two types – Rods and Cones. (Hmm – why do we have two types? Why not just one?)
Chemistry of the receptors
Receptors are filled with a complex chemical called the visual pigment
Each pigment molecule is made up of two parts – a long protein called opsin and a much smaller
component called retinal. In the dark, they are joined together.
When light strikes it, the combined molecule changes shape. (First response of our bodies to light.)
Prolonged exposure to light.
Prolonged exposure to light causes the two parts of the molecule to become separated.
When its molecules are separated, the visual pigment is not sensitive to light.
Prolonged exposure to darkness.
The molecule components re-join and become sensitive to light again.
At any given time.
At any given time, a proportion of the visual pigment molecules are joined and the rest are separated.
The overall sensitivity of the receptors depends on the proportion that are joined.
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Distribution of Rods and Cones G9 p. 28 (Aren’t you curious: Why two types?)
90°
90°
Cones
0°
This figure, taken from the web, is not quite to
scale, but it shows clearly that the center of the
retina is filled with only cones, while the
periphery is filled with rods and also a few
cones.
Blind spot
Macular Degeneration –Destruction of the cones in the center of the retina, leading to loss of central
vision.
Retinitus pigmentosa – Destruction of the rods in the periphery of the retina, leading to loss of peripheral
vision.
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Key differences in light handling ability of rods and cones.
1) Rod visual pigment is 1000 times more sensitive to light then that of cones.
2) There are three types of cones visual pigment. More later.
Dealing with overall differences in light levels G9 p 27
In daylight.
Rods. Nearly all visual pigment molecules are separated. Rods are max’d out.
Rods are not used for vision in daylight.
Cones. A working proportion of visual pigment molecules are joined and ready to respond to light.
Cones are typically used for vision in daylight.
Initially in darkness.
Rods. Rod visual pigment begins to recombine.
Cones. A substantial proportion of cone molecules
were already combined.
So initially, vision is based on cones.
After 2nd minute in darkness.
Rods. Rod visual pigment continues to
recombine.
Cones. All of the cone visual pigment is
available, but sensitivity plateaus cause cone
visual pigment is not terribly sensitive to low
light.
After about 7 minutes in darkness.
Rods. Rod visual pigment continues to recombine, making the rods more sensitive than the cones.
Cones. Cones are “dead to vision” because the light level is too low to active the cone visual
pigment.
After about 30 minutes in darkness
Rods. Rod visual pigment at maximum sensitivity.
Cones. Cones are still “dead to vision”.
So all vision is based on rods.
Why did pirates wear eye patches when on their ships?
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Differences between receptors in sensitivity to wavelength – Spectral Sensitivity G9 p 33-34.
Spectral Sensitivity Curves of the four types of receptors.
R=
Rods
Most sensitive to 500 nm
S=
Short wavelength cones
Most sensitive to 419 nm
M = Medium wavelength cones Most sensitive to 531 nm
L=
Long wavelength cones.
Most sensitive to 558 nm
419
500
531
558
Note that no receptor responds
to light that we perceive as red.
This reminds us that our
perception is the result of a
combination of receptor
activity, not the result of one
receptor alone.
There are far fewer S cones than there are M and L cones. This explains why it’s so difficult to read fine
blue print.
Cone vision’s overall curve shown in Figure 2.19
is primarily determined by the M and L cones.
The S receptors are actually sensitive to ultraviolet
radiation, but the lens absorbs UV radiation, so
none of it strikes the receptors.
Note that we do not have receptors whose
maximum sensitivity is greater than 650 nm, yet we
clearly have hue experiences associated with
wavelengths up to 700 nm.
Purkinje Shift
When it’s close to dark, rods begin to take over vision and cones are not activated by the faint light.
The result is that blue-green objects, like trees, become brighter relative to yellow or red objects.
This shift is called the Purkinje shift.
In the daylight, the red flower will usually seem brighter.
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At night, the blue flower will seem brighter. You won’t even be able to see the red flower because night
vision uses only cones when cannot respond at all to the long wavelength light that we perceive as red.
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Neural Processing
After the receptors, all responses of the visual system are responses of neurons.
The Nervous System –
Central Nervous System – Brain and spinal cord - About 1 trillion cells yes, TRILLION.
Peripheral Nervous System – Nerve cells in the muscles, skins, etc outside the spinal cord
The Brain – About 85 billion (yes, BILLION) neurons – about 8% of the central nervous system.
Cerebral Cortex –
About 15 – 30 billion neurons – less than 3% of the CNS
A 2 mm thick sheet of neurons surrounding other brain structures
Cerebellum -
About 60 billion neurons
A structure on the back of the brain primarily involved in motor control.
Note that many more neurons are involved in motor control than are involved in thinking.
Cerebral Cortex
15-30 billion neurons
Cerebellum
60 billion neurons
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Neurons – the individual cells of the cerebral cortex and the cerebellum G9 p 33
This is Goldstein’s picture of a neuron.
It’s as good as any others I’ve seen.
Three main parts –
Dendrites – Input area
Cell body – Processing area
Axon – Output area
Neuron Behavior
G9 p 36
Neurons do two things
1) Nothing
2) Engage in action potentials each of which lasts about .001 second.
So if you record the “behavior” of a neuron, you’ll either record a constant negative voltage – the neuron
resting – or you’ll record a positive spike of voltage – the neuron engaging in an action potential.
Action Potential
G9 p 38
A sudden (.001 second or so) change in the electrical balance between the inside and the outside of a
neuron. Described well on page 38.
Records of neuron behavior G9 p 37
Pictures of neuron behavior often look like a series of vertical spikes. Each spike is an action potential.
Soft Stimulus
Medium Stimulus
Strong Stimulus
Every neuron responds to changes in strength of stimulation by changing its rate of response.
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Synapses G9 p 39
Input to a neuron – getting a neuron to emit an action potential
Around the dendrites of a typical neuron is a mix of chemicals around the dendrites.
Some of the chemicals may be excitatory
neurotransmitter substances.
Others may be inhibitory neurotransmitter
substances.
The response rate of a neuron depends on the
proportion of excitatory to inhibitory
transmitter substances.
If the ratio of excitatory to inhibitory substance is “typical” for a neuron, it response randomly, at what is
called the base rate of activity. That rate may be 10s of action potentials per second.
If the ratio of E to I is high, the neuron will respond at a rate higher than the base rate.
If the ratio of E to I is low, the neuron will respond at a rate below the base rate.
Above base rate
At base rate
Below base rate
“Typical”
Amount of E relative to I transmitter substance
Output of a neuron G9 p 39
When each action potential of a neuron reaches the end of the axon, it causes the release of some of the
neuron’s own store of neurotransmitter substance.
That substance was stored in small neural containers called vesicles.
Neuron communication – neuron upchucking
So neurons communicate by throwing neurotransmitter
substance at each other.
If a neurons dumps a lot of excitatory neurotransmitter
substance, its activity will cause following neurons to
increase their rate of activity.
If a neurons dumps a lot of inhibitory neurotransmitter
substance, its activity will cause following neurons to
decrease their rate of activity.
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Synapses – gaps between neurons - the places where neurons communicate
The places were neurotransmitter substances get “dumped” and then have the potential to activate other
neurons are called synapses.
The word, synapse, means, roughly, neural gap. It is also used as a verb – meaning to connect with,
neurally.
Almost all neurons synapse with 100s of other neurons.
This means that the neurotransmitter dump of a neuron can influence the firing rates of 100s of other
neurons.
Convergence – summing of neuron output
If two neurons both release excitatory neurotransmitter substances, the result on following neurons will be
double the excitation of either one acting alone.
If two neurons both release inhibitory neurotransmitter substances, the result on the following neurons will
be double the inhibition of either one acting alone.
A
A
B
B
C
C
Convergence of Excitation
Convergence of Inhibition
Implications of Summative Convergence
If neither the output of A nor of B is enough to affect C’s responses, perhaps the output of both together will
be.
Thus, neural convergence allows “weak” inputs to summate and ultimately achieve usable effects on the
neurons which receive their outputs.
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Excitatory / Inhibitory convergence – Neural comparisons (Not in G9)
Suppose that A releases excitatory neurotransmitter substance and B releases inhibitory neural transmitter
substance.
Building
block
circuit
Implications –
If A and B both release neurotransmitter substance, C will be unaffected because the two will cancel each
other out. C will continue to respond at its base rate.
But if A releases excitatory but B releases nothing, C will respond above its base rate. (Assuming A’s
excitatory release is sufficient to drive C.)
Or if A releases nothing but B releases inhibitory neurotransmitter substance, C will be inhibited,
responding below its base rate.
So this circuit is a comparison circuit.
C responds based on a comparison of the activity of A and the activity of B.
A greater than B:
A equal to B:
A less than B:
C responds above its base rate.
C responds at its base rate.
C responds below its base rate.
This simple circuit is the building block for all of neural processing. We’ll see it again and again as we
study perceptual phenomena – in color perception, in motion perception, in sound localization, to name a
few.
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Rod and Cone Systems
The whole collection of rod receptors and their connections to other cells form the rod system.
Similarly, the whole collection of cone receptors and their connections to other cells form the cone system.
Convergence differences between rod and cone systems
Rods are individually more sensitive than cones – about 1000 times more.
But, in addition, the rods are wired so that they are far more convergent on following cells than are cones.
This makes the rod system much more sensitive than the cone system.
Rod system wiring
Cone system wiring
In the above, a faint stimulus to the rods will have 5 times as strong an effect on the red following neuron.
But a faint stimulus to the cones will have an equally faint effect on each of the 5 following neurons in the
cone system.
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Studying perception in infants – preferential looking experiments G9 46
In the study of infants, pairs of stimuli are presented.
If the infant looks equally long at each, then the conclusion is that the infant cannot tell the difference
between them.
If the infant looks more at one than the other, then the conclusion is that the infant can tell the difference
between them.
Often, the stimuli are designed to take advantage of infant spontaneous looking preferences – preferences
for looking at objects with interior contours rather than a homogeneous field, for example.
So an infant with the appropriate ability to see detail will look more at the rightmost
stimulus in the pair to the left.
As the interior contours get closer and closer together, the similarity between the two stimuli becomes
greater.
Ultimately, the infant looks equally often at both circles. This is an indication that the infant cannot tell the
difference between them.
The result, from a study of visual acuity in infants, is shown in Figure 2.39
Visual acuity plateaus to a level comparable to adults
at just after 1 year.
Measured using
evoked potentials
Interestingly, the change appears to be due to
decreases in the sizes of the cones as the infant ages.
From Figure 2.40 . . .
Measured using
preferential looking
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Questions to consider 1. Why two types of receptor? Why not just one type?
One outfielder cannot cover the whole outfield, so baseball has left fielders, center fielders, and right
fielders.
My guess is that the range of intensities to which we’re sensitive is too large to be covered by just one
receptor type. Two receptor types – one for night and one for day provides better coverage of the range of
lights in which we have to operate.
Range of intensities in which we exist is about 1 million to 1. That is, the brightest sunlight in which we
are able to use our eyes is about 1 million times more intense than the faintest starlight in which we can still
see shadows.
But the range of responses of receptors and the bipolar – ganglion cells to which they connect is only about
800 to 1. This means that significant changes in intensity would be represented by very small changes in
response rate of the cells involved. This would likely result in many intensity changes going unnoticed.
So the two receptors “divide” up the intensity range, with the rods taking the faint lights and the cones
taking the bright lights.
2. Why 3 types of cone? Why not just one type?
The short answer is that having three types of cones makes it MUCH easier for us to identify different
wavelengths than if we had just one. If we had only one receptor type, the world would be only shades of
gray.
The fact is that the more cone types you have, the better able you are to discriminate wavelengths – the
richer your color perception is.
More on this in the color perception chapter.
3. Why don’t we have more than 3 types of cones? Wouldn’t our color vision be even better than it is
now?
The answer is yes. However it turns out that each type of cone requires many thousands of extra
neurons to process and integrate the information from that type. But the brain has other things to do than to
provide good color vision. What about music? Form? Touch? Arithmetic? Literature? Facebook?
So the choice is to devote extra neurons to better color vision or devote them to processing auditory
information, or form, or touch, or something else. The bottom line is that our brain is a compromise. More
on this in the chapter on color
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