Vision Flashcards
• How far one sees is dependent on
how far light travels before it strikes one’s eyes.
Arab philosophy a thousand years ago reasoned that if you
saw by sending out sight rays, they couldn’t get to the stars
that fast. Then he demonstrated that light rays bounce off an
object in all directions, but you see only those rays that reflect off the object and strike your retina
• Sensation:
registration by the sensory organ (eyes) of a physical stimuli from the environment (different form perception)
• Sensory info is influenced by past experience
• Perception
subjective interpretation of sensations by the brain
• Perception of vision is not in the eyes; it’s in the brain
• Law of specific nerve energies
states that activity by a particular nerve always conveys the same type of information to the brain – so brain codes info in terms of which neurons are active.
• The brain processes info depending on which neurons are active (And strength of activity) at any given point.
• Example: impulses in one neuron indicate light; impulses in another neuron indicate sound – the brain somehow interprets action potentials coming from specific neurons as e.g. sound.
= Each of our senses has specialized receptors that are sensitive to a particular kind of energy
Transduction of light (how does light enter the eye)
- Enters the eye through an opening in the center of the iris called the pupil
- Light is focused by the lens (adjustable) and the cornea (not adjustable) onto the rear surface of the eye known as the retina, which is lined with visual receptors (cones = process color and high accuracy, rods process less precise, more periphery, info)
- The left side of the world strikes the right side of the retina and vice versa
- From above strikes, the bottom half of the retina and vice versa
- The inversion of the image poses no problem for the nervous system. Remember, the visual system does not duplicate the image. It codes it by various kinds of neuronal activity.
Cornea = clear, outer covering.
Light comes in –> bent by cornea, goes in, bent again by lens we get an inverted and flipped image. (with respect to left-right and up-down)
Common Refractive Errors
- Myopia (nearsightedness) light falls short of the retina
- Hyperopia (farsightedness) light falls beyond the retina
In normal eyes lens figure light directly onto retina.
Myopia = image formed BEFORE retina = unclear image. (e.g. from elongated eyeball, or having too much of a curve on cornea) – can’t focus on far objects
Hyperopia = can’t focus on near points of object – eyeball may be too short, or lens is too flat (often case in older adults, lens looses elasticity)
Myopia
(nearsightedness) light falls short of the retina
Myopia = image formed BEFORE retina = unclear image. (e.g. from elongated eyeball, or having too much of a curve on cornea) – can’t focus on far objects
Hyperopia
(farsightedness) light falls beyond the retina
Hyperopia = can’t focus on near points of object – eyeball may be too short, or lens is too flat (often case in older adults, lens looses elasticity)
Route within retina (how is info processed in retina)
Receptors (back of eye) bipolar cells (closer to center of eye) (amacrine cells, get info from bipolar, send to other bipolar, refine input to ganglion cells so e.g. responding to specific shapes, directions, etc) ganglion cell ganglion axons forms optic nerve (optic nerve leaves at the “blind spot” = not receptors = blind spot (but everything in blind spot in one eye is visible to the other eye)) travel back to brain
(all cells between light and receptors are transparent, so light can passe through them.
Photoreceptors (the pink neurons)
• Located at the back of the eye
• Respond to light
• Sends signals to other cells closer to the eye
• Photoreceptors converts light to electrical signals.
Bipolar cell:
• receives input from photoreceptors
Horizontal cell:
• links photoreceptors and bipolar cells
The horizontal cells make inhibitory contact onto
bipolar cells, which in turn make synapses onto amacrine cells
and ganglion cells. All these cells are within the eyeball.
Amacrine cell:
• links bipolar cells and ganglion cells
Retinal ganglion cell:
• Their axons gives rise to the optic nerve
Optic nerve
Optic Nerve
• Axons of ganglion cells exit through the back of the eye and travel to the brain
• The point at which it leaves is called the blind spot
• it contains no receptors
Blind spot
where optic nerve leaves eye
no receptors = blind
but everything in blind spot in one eye is visible to the other eye = not ACTUALLY blind in practice.
Fovea
- Central portion of the retina and allows for acute and detailed vision
- Packed tightly with receptors known as cones
- (nearly free of any ganglion axons and blood vessels) = nearly unimpeded vision (due to tight packing of receptors)
- Each receptor attaches to a single bipolar cell and a single ganglion cell known as a midget ganglion cell – each cone has direct route to brain (midget ganglion cells provide 70 percent of input to brain – your vision is dominated by the fovea)
- Each cone in the fovea has a direct line to the brain which allows the registering of the exact location of input
The Periphery of the Retina
- Greater number of receptors called rods (about 20 to 1 to cones – higher for species active at night)
- Detailed vision is less in peripheral vision
- Toward the periphery of the retina, more and more receptors converge onto bipolar and ganglion cells. –> result, the brain cannot detect the exact location or shape of a peripheral light source.
- In the periphery, your ability to detect detail is limited by interference from other nearby objects
- Allows for the greater perception of much fainter light in peripheral vision
Photoreceptors
A photoreceptor cell is a specialized type of neuroepithelial cell found in the retina that is capable of visual phototransduction. The great biological importance of photoreceptors is that they convert light (visible electromagnetic radiation) into signals that can stimulate biological processes. hey use the photopigment rhodopsin or a related molecule.
Rods and cones exist
Both rods and cones contain photopigments, chemicals
that release energy when struck by light. Photopigments
consist of 11-cis-retinal (a derivative of vitamin A) bound to
proteins called opsins, which modify the photopigments’ sensitivity
to different wavelengths of light. Light converts 11-cisretinal
to all-trans-retinal, thus releasing energy that activates
second messengers within the cell.
(The light is absorbed in this
process. It does not continue to bounce around the eye.)
Rods: • More numerous than cones • Abundant in periphery • Sensitive to low levels of light (dim light) – not for bright light because bright light bleaches them. • Used mainly for night vision • One type of pigment only
Cones:
• Highly responsive to bright light
• Specialized for color and high visual acuity
• In the fovea only (OBS! Google says there are cones outside fovea (mostly “blue” cones)
• From book: abundant in and near fovea
• Three types of pigment
Rods
A photoreceptor
Rods: • More numerous than cones • Abundant in periphery • Sensitive to low levels of light (dim light) – not for bright light because bright light bleaches them. • Used mainly for night vision • One type of pigment only
Cones
Cones:
• Highly responsive to bright light
• Specialized for color and high visual acuity
• In the fovea only (OBS! Google says there are cones outside fovea (mostly “blue” cones)
• From book: abundant in and near fovea
• Three types of pigment
Color Vision
- Visible light is a portion of the electromagnetic spectrum
- The perception of color depends on the wavelength of light
- Humans perceive wavelengths between 400-700 nanometers (nm)
- Infrared goggles allow us to see longer waves.
Color Vision Theories
- The specificity of color perception depends on specific receptors within the eye
- Two major interpretations of color vision
- Trichromatic theory/Young-Helmholtz theory
- Opponent-process theory
Trichromatic theory/Young-Helmholtz theory
- Young realized color vision needs explanation that is biological, beyond physics of light. (later revised by Helmholtz)
- Color perception occurs through the relative rates of response by three kinds of cones
- Short-wavelength (blue)
- Medium-wavelength (green)
- Long-wavelength (red)
- The ratio of activity across the three types of cones determines the color
- More intense light increases the activity of all three cones without much change in their ratio of responses. = light appears brighter, but with same color.
- The nervous system determines the color of the light by comparing the responses of different types of cones.
- We have fewer blue cones, but approx same amount of red and green cones
Although the short-wavelength (blue) cones are about evenly distributed across the retina, the other two kinds are distributed haphazardly, with big differences among individuals
In the retina’s periphery, cones are so scarce that you have no useful color vision
Opponent-Process Theory
we perceive in terms of opposites.
That is, the brain has a mechanism that perceives
color on a continuum from red to green, another from yellow to blue, and another from white to black. After you
stare at one color in one location long enough, you fatigue that
response and swing to the opposite.
Part of the explanation for this process pertains to the
connections within the retina. For example, imagine a bipolar
cell that receives excitation from a short-wavelength cone and
inhibition from long- and medium-wavelength cones. It increases
its activity in response to short-wavelength (blue) light
and decreases it in response to yellowish light. After prolonged
exposure to blue light, the fatigued cell decreases its response.
Because a low level of response by that cell usually means yellow,
you perceive yellow.
• Proposes we perceive color at level of visual cortex, and not at level of bipolar cells in the eye.
Limitations of Color Vision Theories
- Both the opponent-process and trichromatic theory have limitations
- Ex. Color constancy, the ability to recognize color despite changes in lighting
- Retinex theory: cortex compares information from various parts of the retina to determine the brightness and color for each area – the classic picture with dark/light grey, that are really the same tone
- = visual perception requires reasoning and inference, not just retinal stimulation.
• Retinex theory:
cortex compares information from various parts of the retina to determine the brightness and color for each area - can explain Color constancy, the ability to recognize color despite changes in lighting
Color Vision Deficiency/ color blindness
impairment in perceiving color differences (complete color blindness, only perceiving black and white, is rare) – many animals have 4 types of color cones, so in that sense, all humans are color deficient.
• Gene responsible is contained on the X chromosome = because men only have one X chromosome = more men than women have deficiency.
• Caused by either the lack of a type of cone or a cone that has abnormal properties
• Most common form is difficulty distinguishing between red and green
• Results from the long- and medium-wavelength cones having the same photopigment
• In monkeys, it has been shown that by “adding” the lacking type of cone, monkey can adapt and learn to see the lacking color = brain is adaptive!
• Some women have 4 types of color cones – can finer distinguish between certain colors
Mammalian visual system
Receptors (back of eye) –> bipolar cells (closer to center of eye) –> (amacrine cells, get info from bipolar, send to other bipolar, refine input to ganglion cells so e.g. responding to specific shapes, directions, etc) –> ganglion cell –> ganglion axons forms optic nerve (optic nerve leaves at the “blind spot” = not receptors = blind spot (but everything in blind spot in one eye is visible to the other eye))–> travel back to brain.
–> Optic chiasm
• Junction of the optic nerves from each eye
• Axons from the nasal (inside) half of each retina cross over to the opposite (contralateral) side of the brain.
• Axons from the temporal (outer) half of each retina remain on the same (ipsilateral) side of the brain.
• Information from the left visual field goes to the right side of the brain; information from the right visual field goes to the left side of the brain.
–> most ganglion axons go to LGN + small number to superior colliculus And other areas (e.g. hypothalamus (part controlling waking-sleeping schedule)
–> from LGN, to other parts of thalamus + visual cortex