EXAM 1 SCORES
Quote of the day
"It's not what you look at that matters, it's what you see.”
- Henry David Thoreau (1817-1862)
Mean = 83.7
Median = 84.7
Minimum = 61.5
Maximum = 99.5
Standard Deviation = 8.9
Assignments
Read chapters 12 and 13 in Course Pack
Start working on Study Guide 3 as soon as you
receive it.
The Visual System
Exam 2 is on Monday, November 9. It will cover
the somatosensory system and as much of the
visual system as we have completed.
The visual system processes patterns of light energy
Some properties of light
• Light is a form of electromagnetic radiation emitted by the
oscillation of electrically charged material. The visible spectrum
is only a small portion of the electromagnetic spectrum.
• Light travels very fast (about
300,000 km/s)
• Light travels in straight lines
• Light can be reflected,
absorbed, or refracted.
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Eyes
• Eyes across the animal kingdom take many different forms and are
placed in different configurations.
Eye movements effectively increase the size of the visual
field.
Placement of eyes leads to more or less overlap of the two
visual fields.
• Animals with eyes on the sides of the head have a wide angle visual
field, but little binocular vision.
The eye, like a camera, needs a mechanical optical system for
optimizing transmission of light to the receptors.
• The eyeball is moved by the extraocular muscles.
• Brightness
• Focus
The main parts of the eye
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Cornea
Sclera
Iris
Pupil
Lens
Retina
Optic Nerve
Light passes through several different structures before it
reaches the receptor cells
• The cornea is the clear
covering over the front of the
eye.
• The pupil is the opening
through which light passes.
• The iris regulates the amount
of light passing through the
pupil.
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The lens focuses light on the back of the eyeball. This
process is called accommodation.
• Because different objects are at different distances from the
eye, there must be a mechanism to adjust the light!s focus.
Light entering the eye projects an inverted image onto the retina
• Up to this point the eye works very much like a camera.
The muscles
attached to the
lens contract or
relax, changing the
curvature of the
lens.
Q: The image in our eye is upside down, so why don!t we
perceive the world as upside-down?
The retina is the “screen” upon which light is projected
• The retina contains three main layers of cells.
A: We do, but our brain adjusts so that our visual image of the world
corresponds to the spatial coordinates that we experience
through other senses.
Light passes through all the layers of the retina before
reaching the receptor cells.
The retina contains two types of photoreceptors, rods and cones
• Rods and cones are specialized for processing different types of
information.
• Once the
photoreceptors are
activated, information is
transmitted back
through the bipolar
cells and ganglion cells
to reach the brain via
axons of the optic
nerve.
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Cones are mainly located in the fovea and rods in the remainder
of the retina.
Differences between rods and cones
• Structure
• The fovea contains
almost exclusively
cones.
• Type of light-sensitive chemical (photopigment)
• The periphery contains
mostly rods.
• Numbers (120 million rods vs 6 million cones)
• The site at which the
optic nerve exits the
eye contains no photoreceptors and is called
the “blind spot”.
• Distribution across retina
Transduction in the visual system
• When light strikes the retina,
it interacts with lightsensitive molecules in the
rods and cones.
• These molecules are called
photopigments.
• The photopigments are
contained in the rod and
cone outer segments.
Light causes retinal to change its shape and split away from
the opsin molecule.
•
When retinal leaves its binding site on opsin, the opsin changes shape,
triggering several subsequent steps that happen inside the cell.
•
These steps ultimately affect ion channels in the cell membrane.
•
The change in the retinal portion of the photopigment is the only step in
vision that depends on light.
A photopigment
molecule consists of two
components
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Opsin: A large protein
•
Retinal: A small molecule,
derived from Vitamin A
Light ultimately causes the closing of a sodium channel that is
normally open
•
The whole process occurs in less than 1 millisecond (1/1000 s)
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Light causes photoreceptors to hyperpolarize
•
This might seem counterintuitive (backwards)… but…
In the dark, sodium channels are open, so the rod or cone is depolarized.
In the dark, neurotransmitter (glutamate) is constantly being released.
•
Because we are hardly ever in complete
dark or extremely bright light, it is useful
to modulate the photoreceptor!s
membrane potential up and down about
some intermediate point.
Painting by Giorgio de Chirico
• Light causes the sodium channels to close, so the photoreceptor is
hyperpolarized. Glutamate release decreases or stops.
Key Concept: Modulation of neural activity around a
set point.
Many sensory neurons are “spontaneously” active. This activity can
be modulated upward or downward, depending on how stimulus
conditions change.
Relatively long-term changes in sensitivity of the eye
and retina
• After-images
• Light adaptation
The “set point” can also be modulated upward or downward
according to average conditions at a given time.
Dark adaptation
• Dark adaptation
Rods and cones dark-adapt at different rates
• Cones adapt quickly, but remain relatively insensitive.
• In response to a decrease in average brightness, the pupil
dilates
• Rods adapt slowly (about 20 minutes) but become very sensitive.
• Photopigments that were “bleached” by light regenerate in the
dark, making the receptors more sensitive.
• Rods and cones dark-adapt at different rates.
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“On”-center and “off”-center cells
Transmission of information in the retina
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Rods and cones contact
bipolar cells.
Bipolar cells contact
ganglion cells.
Some bipolar cells are
excited by glutamate and
are depolarized by dark
and hyperpolarized by light.
Some bipolar cells are
inhibited by glutamate and
are depolarized by light and
hyperpolarized by dark.
“On”-center and “off”-center cells
•
Ganglion cells are also on-center or off-center, depending on the nature
of the bipolar cells from which they receive input.
Depending on the kinds of receptors present on the cell membrane, bipolar cells
may be excited by light (on-center) or excited by dark (off-center)
Information is first represented as graded potentials, then
action potentials
• Rods and cones produce
graded potentials (receptor
potentials)
• Bipolar cells also produce
graded potentials
• Ganglion cells produce
action potentials which are
sent to the brain via their
axons in the optic nerve
Information processing in the retina
• The retina does not just collect and transduce information about
patterns of light. It also performs important information
processing tasks.
• The rod pathway amplifies the signal under low-light conditions.
• The cone pathway provides information about color and fine
structure of visual images
• Lateral inhibition sharpens contrast and contours.
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