Chapter 3 Flashcards
What are the 4 basic steps to vision?
1) distal stimulus
- ex; the tree
2) Light is reflected and transformed to create an image of the tree on the retina
3) Receptor processes: receptors transform light into electricity
4) Neural processing: signals travel in a network of neurons
What is vision based on?
Visible light, which is a small part of the electromagnetic spectrum.
What is the electromagnetic spectrum?
Continuum of electromagnetic energy that is produced by electric charges and is radiated as waves.
What is a wavelength?
The distance between the peaks of the electromagnetic waves.
What wavelengths are part of the visible light?
Between 400 to 700 nanometers (mm)
What wavelengths are associated with blue, green, yellow orange and red?
Blue —> short wavelengths
Green —> middle wavelengths
Red (yellow, orange) —> long wavelengths
What does light pass through before hitting the retina at the back of the eye?
1) pupil
2) focused by the cornea
3) focused by lens
4) hits retina
What is the retina?
The network of neurons that covers the back of the eye and that contains the receptors for vision
What are the receptors for vision called?
Photoreceptors
What are the two types of photoreceptors?
Rods and cones
What are the outer-segments to the rods and the cones?
They are the part of the receptors that contain light-sensitive chemicals called visual pigments that react to light and trigger electrical signals.
How do the neural signals transduced from light exit the eye?
Signals from the receptors flow through the network of neurons that make up the retina and emerge from the back of the eye in the optic nerve.
Which part of the retina contains only cones?
Fovea
What part of the retina contains both rods and cones, but rods in an important amount?
Peripheral retina
What is the name of the condition which is most common in older people that destroys the cone-rich fovea and a small area that surrounds it?
Macular degeneration
What are the consequences of macular degeneration for vision?
This condition creates a blind region in central vision, so when a person looks directly at something, they lose sight of it.
Name the condition that is a degeneration of the retina that is passed from one generation to the next and that attacks the peripheral rod receptors?
Retinitis pigmentosa
What are the consequences of the condition called retinitis pigmentosa on vision?
It results in poor vision in the peripheral visual field.
Eventually, in severe cases, the foveal cone receptors are also attacked, resulting in complete blindness.
What is the blind spot of the eye?
An area in the retina where there are no photoreceptors, which is where the nerve fibres that make up the optic nerve leave the eye.
Why aren’t we actually aware of the blind spot?
1) the blind spot is located off to the side of our visual field, where objects are not in sharp focus.
2) a mechanism in the brain “fills in” the place where the image disappear.
What is the cornea?
The transparent covering of the front of the eye.
It accounts for 80% of the eye’s focusing power.
It is fixed in place, so it cannot adjust its focus.
What is the lens of the eye?
It supplies the remaining 20% of the eye’s focusing power.
It can change its shape to adjust the eye’s focus for objects located at different distances.
How is the change in shape of the lens in the eye achieved?
By the action of ciliary muscles, which increase the focusing power of the lens by increasing its curvature.
What is the process controlled by the adjustable lens?
Accommodation
What is accommodation?
The change in the lens’s shape that occurs when the ciliary muscles at the front of the eye tighten and increase the curvature of the lens so that it gets thicker
What are called the errors that can affect the ability of the cornea and/or lens to focus the visual input onto the retina?
Refractive errors
What is the refractive error that often occurs in normal aging called presbyopia?
As people get older, their ability to accommodate decreases due to the hardening of the lens and weakening of the ciliary muscles.
How can presbyopia be dealt with?
By wearing reading glasses that bring near objects into focus by replacing the focusing power that can no longer be provided by the lens.
What is myopia?
Inability to see distant objects clearly
What causes myopia?
It occurs when the optical system brings parallel rays of light into focus at a point in front of the retina, so that the image that reaches the retina is blurred.
What are the two causes of myopia?
1) refractive myopia
- in which the cornea and/or the lens bends the light too much
2) axial myopia
- in which the eyeball is too long
What is hyperopia?
People can see distant objects clearly but have trouble seeing nearby objects
What are the causes of hyperopia?
The focus point for parallel rays of light is located behind the retina, usually because the eyeball is too short
What is transduction?
The transformation of one form of energy into another form of energy.
Where does visual transduction occur?
In photoreceptors
What are the two parts to visual pigments?
A long protein called opsin
A much smaller light-sensitive component called retinal
What happens to the retinal when light hits it?
The visual pigment molecule absorbs the light, which causes the retinal within to change its shape, from being bent, to being straight.
That is called isomerization
What is isomerization?
The process of the retinal changing shape from bent to straight when it is hit by light
It activates thousands of charged molecules to create electrical signals in receptors
How do the visual pigments influence the aspects of visual perception?
1) how we adjust to darkness
2) how well we see light in different parts of the spectrum
The process of increasing sensitivity in the dark.
Dark adaptation
How is dark adaptation measured?
By determining a dark adaptation curve
What is the dark adaptation curve?
The function relating sensitivity to light to time in the dark, beginning when the lights are extinguished
What is light-adapted sensitivity?
The sensitivity measured in the light
Because it is measured while the eyed are adapted to the light