Lecture 19: Vision Flashcards
What are the 4 features of vision?
- colour
- shapes
- depth
- movement
What are the features of vision referred to as?
perceptions
Where are the perceptions received?
in the brain
What form is information sent into the brain?
action potentials
What is the stimulus for vision?
light
Where is the sensation of vision located?
in the brain
Visible light is only a small band of the electromagnetic spectrum. What are the wavelengths of visible light?
400-750 nm
What are the two features that embody our visual system?
wavelength and amplitude
Has red got a longer or shorter wavelength than violet?
longer
What does the eye do?
it detects _______ and processes _______ information to create visual __________ and guides _______
it detects light and processes visual information to create visual perceptions and guides behaviour
The eye consists of what two basic components?
- optical component
- neural component
What is the function of the optical component of the eye?
to collect and focus light onto the plane of the retina
What is the function of the neural component of the eye?
to convert ________ into patterned changes of ___________ _________ that the brain can decode to create __________ ____________
to convert energy into patterned changes of membrane potential that the brain can decode to create visual perceptions
What is the white bit of the eye called?
the sclera
At the front of the eye, the sclera is modified to form what structure?
the cornea
How is the sclera modified to form the cornea?
the sclera is white because the cells are white and fibrous but the cornea is translucent
Where is the cornea located?
at the front of the eye, covering the iris
Is the cornea vascular?
no, it is avascular
What two things side in front of the cornea?
the conjunctiva and tears
Why is the cornea avascular?
to allow light through unhinged by pigmented cells
Because the cornea is avascular, how does it get nutrients and get rid of CO2?
through diffusion
Where are the extraocular muscles and what are they for?
they are off to the side of the eye and they are needed for moving the eye around in the skull - this is necessary for balance
Where is the aqueous humor?
behind the cornea and in-front of the lens
What is another name for aqueous humor?
the anterior chamber
What is the role of the aqueous humor?
it is fluid filled for inflating the eye and maintaining intraocular pressure
What is the normal intraocular pressure in the aqueous humor and what is the name of a condition when this pressure increases?
15mmol Hg
glaucoma
Describe the lens of the eye
it is a fibrous capsule with crystallised lens cells held in place by zonular fibres
Is the lens avascular? Why or why not?
yes it is
this is to help get light in unhinged
Where is the virteous humor?
behind the lens
What is the virteous humor made of?
protein (collagen)
Where is the retina? What is it part of?
at the back of the eye, coming around the sides- it is part of the brain because it is an extension of the thalamus
Where are the choroid pigmented epithelium and where are they located?
they are located behind the retina and they are densely pigmented cells with black pigment to absorb light
What is the name of a disorder when the choroid pigmented epithelium are not pigmented and what effect does this have?
this is albinism and this means that the light is not absorbed and so instead it reflects all around the back of the eye so there are visual problems
Describe the retina ganglion cells
these line the back of the eye and leave the eye at the optic disc and form the optic nerve going to the brain
What is the optic disc?
the blind spot of the eye
What is the fovea?
this is a depression in the retina responsible for the strongest vision
Is the retina vascularised?
yes
Define refraction
this is the bending of light as it passes from one substance to another
What is refraction related to?
the difference between the refractive indices of the two media, and to the curvature of the refractive surface
Which part of the eye is most responsible for the bending of light?
the cornea
What is refractive power and how is it measured?
this is the ability of a substance to bend light and it is measured in diopters which is the reciprocal of focal length in metres
A lens that converges light to a focal point 1m beyond the lens has a power of how many diopters?
1
A 2 diopter lens would convert rays what distance beyond the lens?
0.5m
A 10 diopter lens would convert the rays what distance beyond the lens?
0.1m
What is the refractive power of the eye?
60 diopters
What is the near response?
when you shift your gaze from distant to near objects
During the near response, what three processes occur?
- accomodation
- constriction of the pupil
- convergence of the eyes
Define accomodation
the contraction/relaxation of ciliary muscles to alter the lens shape and change the refractive power
Describe what the ciliary muscles and zonular fibres are doing when looking at distant objects
the ciliary muscles are relaxed due to low parasympathetic activity, so the zonular fibres are taut and the lens is flattened
Describe what the ciliary muscles and zonular fibres are doing when shifting from looking far away to looking close up
the parasympathetic activation of ciliary muscles increases so these contract, there is tension removed from the zonular fibres so the lens becomes more spherical due to natural elasticity
When gazing at close up objects, how does the refractive power change compared to looking at far away objects?
when changing gaze from looking far away to close up, the refractive power increases from 60 to 75 diopters
Describe myopia
This is a dysfunction in accomodation where people are near sighted because their eyeball is too long. This means that people can see closeup but not far away
What causes myopia?
the eye is elongated which means that light is focused in front of the retina so far objects are left out of focus
How can myopia be corrected?
with a concave lens
Describe hypermetropia
This is far-sightedness which means that you can see far away but not up close.
What causes hypermetropia?
The eyeball is too short which means that light focuses behind the retina and so close up objects are out of focus
How can hypermetropia be corrected?
with convex lenses to bring the focal point forward
Another optical defect is astigmatism. What is this?
This is what the curvature of the cornea and/or the lens is spherical which causes there to be a different amount of refraction in different places. This means there is different focal points at the back of the retina.
How can astigmatism be corrected?
with a cylindrical lens
Another optical defect is presbyopia. What is this?
This is an age-related loss of accomodation caused by the lens losing elasticity so the near point gets further and further away
What is perfect vision called?
emmetropia
How can presbyopia be corrected?
with convex lenses to restore near vision
Define near point
the point closest to you eyes at which you can see clearly
What is a cataract?
This is when the lens becomes opaque because the cells in the lens die. This causes light to scatter not converged so there is a poor quality image
How can cataracts be fixed?
through surgery to get the lens removed and a plastic one in to replace it
however, you will still need glasses and you will have myopia because you cannot accomodate post surgery
As well as accomodation, another process that occurs during the near response is constriction of the pupil. What happens when there is constriction of the pupil?
the improved depth of focus and fewer optical aberrations by excluding edges of the lens (ie. the peripheries are reduced)
As well as accomodation, another process that occurs during the near response is convergence of the eyes. What happens during convergence of the eyes?
Objects remain in _______ on corresponding parts of the _________
objects remain in register on corresponding parts of the retinae
