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How do our eyes respond to light?

Why do our eyes have two different sets of
receptors – rods and cones?
Figure 3.1 The electromagnetic spectrum, showing the
wide range of energy in the environment and the small
range within this spectrum, called visible light, that we
can see.

Electromagnetic spectrum
◦ Energy is described by wavelength.
◦ Spectrum ranges from short wavelength gamma rays
to long wavelength radio waves.
◦ Visible spectrum for humans ranges from 400 to 700
nanometers.
◦ Most perceived light is reflected light of surfaces.
Figure 3.2 An image of the cup is focused on the retina,
which lines the back of the eye. The close-up of the
retina on the right shows the receptors and other
neurons that make up the retina.

The cornea, which is fixed, accounts for
about 80% of focusing.

The lens, which adjusts shape for object
distance, accounts for the other 20%.
◦ Accommodation results when ciliary muscles are
tightened which causes the lens to thicken.
 Light rays pass through the lens more
sharply and focus near objects on retina.
Figure 6.2 Cross section of the vertebrate eye
Note how an object in the visual field produces an inverted image on the retina.
The Eye and Its Connections to the Brain
Pupil-opening in the center of the eye that
allows light to pass through
Lens-focuses the light on the retina
Retina-back surface of the eye that contains the
photoreceptors
The Fovea-point of central focus on the retina
blind spot-the point where the optic nerve
leaves the eye
(a) Object far –lens relaxed
(b) Object near- no accommodation,
object out of focus
(c) Object near – lens
accommodates, object in focus
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The near point occurs when the lens can no longer
adjust for close objects.
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Presbyopia - “elder eye”
◦ Distance of near point increases
◦ Due to hardening of lens and weakening of ciliary muscles
◦ Corrective lenses are needed for close activities, such as
reading

Myopia or nearsightedness - vision good for
near obects.

Inability to see distant objects clearly
◦ Image is focused in front of retina
◦ Caused by
 Refractive myopia - cornea or lens bends
too much light
 Axial myopia - eyeball is too long
(a) Distant object out of
focus
(b) No problem with near
object (Nearsighted)
(c) Distant object brought
into focus with lens
Focusing of light by the myopic (nearsighted) eye.

Solutions for myopia
◦ Move stimulus closer until light is focused on the
retina
 Distance when light becomes focused is
called the far point.
◦ Corrective lenses can also be used.
◦ LASIK surgery can also be successful.

Hyperopia or farsightedness - inability to see
nearby objects clearly
◦ Focus point is behind the retina.
◦ Usually caused by an eyeball that is too short
◦ Constant accommodation for nearby objects can lead
to eyestrain and headaches.
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
Transduction
First take a look at receptors we find in the
retina.
KW 8-5
KW 8-6
(a) Rod receptor showing discs in the outer segment.
(b) Close-up of one disc showing one visual pigment
molecule in the membrane.

Rod Receptor have outer segments, which
contain:
◦ Visual pigment molecules, which have two
components:

 Opsin - a large protein
 Retinal - a light sensitive molecule
Visual transduction occurs when the retinal
absorbs one photon.
◦ Retinal changes it shape, called isomerization.
Figure 3.6 (a) Rod receptor showing discs in the outer segment. (b) Close-up of one disc showing one
visual pigment molecule in the membrane. (c) Close-up showing how the protein opsin in one visual
pigment molecule crosses the disc membrane seven times. The light-sensitive retinal molecule is attached
to the opsin at the place indicated.
Figure 3.7 Model of a visual pigment molecule. The horizontal part of the
model shows a tiny portion of the huge opsin molecule near where the retinal
is attached. The smaller molecule on top of the opsin is the light-sensitive
retinal. The model on the left shows the retinal molecule’s shape before it
absorbs light. The model on the right shows the retinal molecule’s shape after
it absorbs light. This change in shape is one of the steps that results in the
generation of an electrical response in the receptor
.


Prior physical evidence showed that it takes
one photon to isomerize a pigment molecule.
Purpose of Hecht experiment - to determine
how many pigment molecules need to be
isomerized for a person to see
Figure 3.9 The observer in Hecht et al.’s (1942) experiment could see
a spot of light containing 100 photons. Of these, 50 photons reached
the retina, and 7 photons were absorbed by visual pigment molecules.

Results showed:
◦ a person can see a light if seven rod receptors are
activated simultaneously.
◦ a rod receptor can be activated by the
isomerization of just one visual pigment molecule.

Differences between rods and cones
◦ Shape
 Rods - large and cylindrical
 Cones - small and tapered
◦ Distribution on retina
 Fovea consists solely of cones.
 Peripheral retina has both rods and cones.
◦ More rods than cones in periphery.
◦ Number - about 120 million rods and 5 million cones
Figure 6.2 Cross section of the vertebrate eye
Note how an object in the visual field produces an inverted image on the retina.
Figure 3.12 The distribution of rods and cones in the
retina. The eye on the left indicates locations in degrees
relative to the fovea. These locations are repeated
along the bottom of the chart on the right. The vertical
brown bar near 20 degrees indicates the place on the
retina where there are no receptors because this is
where the ganglion cells leave the eye to form the optic
nerve.
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Blind spot - place where optic nerve leaves the eye
◦ We don’t see it because:
 one eye covers the blind spot of the other.
 it is located at edge of the visual field.
 the brain “fills in” the spot.
Figure 3.14 There are no receptors at the place where the optic nerve
leaves the eye. This enables the receptor’s ganglion cell fibers to flow
into the optic nerve. The absence of receptors in this area creates the
blind spot.

Macular degeneration
◦
◦
◦
◦
Fovea and small surrounding area are destroyed
Creates a “blind spot” on retina
Most common in older individuals
“Wet” causes vision loss due to abnormal blood
vessel growth
◦ “Dry” results from atrophy of the retinal pigment,
which causes vision loss through loss of
photoreceptors
Cones: short,
medium, long
rods
What about cones and color vision? Separate lecture on
color vision.
Method used in all experiments:
◦ Observer is light adapted
◦ Light is turned off
◦ Once the observer is dark adapted, she adjusts the
intensity of a test light until she can just see it.
Figure 3.18 Viewing conditions for a dark adaptation
experiment. The image of the fixation point falls on the
fovea, and the image of the test light falls in the
peripheral retina.

Experiment for rods and cones:
◦ Observer looks at fixation point but pays attention
to a test light to the side.
◦ Results show a dark adaptation curve:
 Sensitivity increases in two stages.
 Stage one takes place for three to four minutes.
 Then sensitivity levels off for seven to ten minutes
- the rod-cone break.
 Stage two shows increased sensitivity for another
20 to 30 minutes.
Figure 3.19 Three dark adaptation curves. The red line is the twostage dark adaptation curve, with an initial cone branch and a later
rod branch. The green line is the cone adaptation curve. The black
curve is the rod adaptation curve.

Process needed for transduction:
◦
◦
◦
◦
◦
◦
Retinal molecule changes shape
Opsin molecule separates
The retina shows pigment bleaching.
Retinal and opsin must recombine to respond to light.
Cone pigment regenerates in six minutes.
Rod pigment takes over 30 minutes to regenerate.


Rods and cones send signals vertically through
◦ bipolar cells.
◦ ganglion cells.
◦ ganglion axons.
Signals are sent horizontally
◦ between receptors by horizontal cells.
◦ between bipolar and between ganglion cells by
amacrine cells.


126 million rods and cones converge to 1
million ganglion cells.
Higher convergence of rods than cones
◦ Average of 120 rods to one ganglion cell
◦ Average of six cones to one ganglion cell
◦ Cones in fovea have one to one relation to ganglion
cells

Rods are more sensitive to light than cones.
◦ Rods take less light to respond
◦ Rods have greater convergence which results in
summation of the inputs of many rods into ganglion
cells increasing the likelihood of response.
◦ Trade-off is that rods cannot distinguish detail
Figure 3.26 The wiring of the rods (left) and the cones (right). The dot
and arrow above each receptor represents a “spot” of light that
stimulates the receptor. The numbers represent the number of
response units generated by the rods and the cones in response to a
spot of intensity of 2.0.

All-cone foveal vision results in high visual
acuity
◦ One-to-one wiring leads to ability to
discriminate details
◦ Trade-off is that cones need more light to
respond than rods
Rods do
not pick
up on
location.
Cones pick
up on
location.
Figure 3.28 Neural circuits for the rods (left) and the cones (right).
The receptors are being stimulated by two spots of light.
Rods
Cones