Topic 2: The Eye and the Retina Flashcards
A band of energy within the electromagnetic spectrum; The stimulus for vision
Light
The ability to see an object depends on…
Visible light being reflected from that object into the eye
Light is described/measured by…
Wavelength (nm)
The light within the spectrum that we can see
Visible spectrum (for humans, 400nm to 700nm)
What are the 7 types of light on the electromagnetic spectrum, in order from shortest waves to longest waves?
Gamma rays
X-rays
Ultraviolet
Visible light
Infrared
Microwaves
Radio waves
What colours do we perceive short and long wavelengths to be?
Short = blues
Long = reds
The eyes contain… and transform light into…
The sensory receptors for vision, perceptual information
Opening of the eye through which light from the environment enters
Pupil
The “whites” of the eyes
Sclera
How does the sclera protect the eye?
Tough membrane
What happens to the sclera when it’s irritated?
Red eyes
Clear, transparent, outermost surface of the sclera that allows light to pass
Cornea
What process does the cornea begin?
Focusing light on to the back of the eye
What happens when the cornea is damaged?
Blurry vision
Controls the diameter and size of the pupil as well as the amount of light reaching the back
Iris
Where is the iris located?
Just behind the cornea
What reflex is associated with the iris?
Pupillary reflex
What part of the eye is associated with eye colour?
Iris
The iris — in bright light and — in dim light
Relaxes, contracts
Adjustable focusing element of the eye
Lens
What characteristic of the lens allows it to focus objects at different distances?
Flexible
What is the most useful part of the eye for seeing in focus?
Lens
Light-sensitive surface at the back of the eye that contains the sensory receptors for vision
Retina
Sensory receptors for vision
Photoreceptors
What are the two types of photoreceptors?
Rods, cones
Compare and contrast rods and cones
Rods: Cylinder-shaped, vision at dim light levels (night vision)
Cones: Cone-shape, vision in bright light (colour vision and fine detail)
Night vision (dim light)
Scotopic vision
Colour vision and fine detail (bright light)
Photopic vision
Outer segments of rods and cones contain…
Light-sensitive visual pigment molecules
Describe the functioning of the light-sensitive visual pigment molecules in rods and cones
Pigments react to light resulting in electrical signals that flow through neurons and the optic nerves
Bundles of millions of nerve fibers that carry signals from the retina into the brain
Optic nerves
Retinal area “away from the center”
Peripheral retina
Describe the distribution of rods and cones in the peripheral retina and fovea
Peripheral retina: Contains both photoreceptor types, but many more rods
Fovea: Only contains cones
Region at the center of the retina specialized for high visual acuity
Fovea
Describe the “sharpness” of vision in the different areas of the retina
Vision is “sharpest” in the center of the visual field (fovea) compared to the periphery
Area of the eye with an absence of photo receptors
Optic disc
Area at the back of the eye where axons forming the optic nerve leave the eye, and blood vessels enter/leave
Blind spot (produced by the optic disc)
Why don’t we notice the blind spot?
Visual system corrects for the blind spot problem on its own:
- The location is different in each eye in relation to the fovea.
- Visual fields overlap, so using both eyes allows you to see without blind spots
Which parts of the eye form the two-part system that focuses light on the retina?
Cornea and lens
Describe the distribution of work to focus light across the lens and cornea
80% of the work is done by the cornea (Issue: not flexible)
20% of the work is done by the lens
(Flexible, changes shape)
Changing the shape of the lens is accomplished by…
Ciliary muscles
Describe the process of focusing light on the retina
As the object moves closer to the eye, the light rays become angled, and the focus point gets pushed back.
The focus point is no longer directly on the retina, so the image appears blurry
Lens adjusts so that the focus point lands on the retina
Change in the lens’s shape that occurs when the ciliary muscles increase the lens curvature
Visual accommodation
What does visual accommodation do with respect to the focus point?
Changing the curvature moves the focus point to the retina so things no longer look blurry
Issues with the cornea or lens’s ability to focus incoming light onto the retina
Refractive errors
The ability to accommodate decreases, lens hardens and is less flexible; Often occurs in normal aging
Presbyopia
Inability to see distant objects clearly
Myopia
Explain why distant objects look blurry to someone with myopia
Light focused on a point in front of the retina
True or False: Vision occurs in the retina
False: It occurs in the brain
How many types of photopigments do we have?
3 types of cone photopigments (chromatic) and 1 type of rod photopigment
What are the two parts of photopigments?
Opsin: A long protein
Retinal: A smaller-light sensitive component
Describe why retinal photopigments are more crucial than opsin
Retinal is isomerized by light
When retinal and opsin are combined, the resulting molecule can absorb visible light - Also means when separated it cannot absorb light
Describe the process of transforming light energy into electrical energy
- Incoming light hits the retina
- Photopigment molecules absorb the light
- Retinal is isomerized
- Chemical chain reaction generates an electrical signal that is passed to adjacent neurons and ultimately to the optic nerve and brain
Visual adaptation that occurs in the dark, during which sensitivity to light increases over time
Dark adaptation
During dark adaptation, the increase in sensitivity to light over time is associated with…
Regeneration of the rod and cone visual pigments
Describe the two phases in which sensitivity increases during dark adaptation
Rapidly in the first 3 to 4 minutes then levels off (cones adapt)
At about 10 minutes, it begins increasing again and continues to do so for ~ 20-30 mins (rods adapt)
The sensitivity of the eyes after they have completely adapted to the dark
Dark-adapted sensitivity
True or False: The cones are much more sensitive to light at the end of dark adaptation
False: More sensitive at the beginning
The point at which vision in the dark shifts from cone vision to rod vision.
Rod-cone break
The rod-cone break and the time needed to become adapted can be explained by…
Differences in the rates of visual pigment restoration
The separation of opsin and retinal causing the molecule to become lighter in colour
Visual pigment bleaching
Does visual pigment regeneration happen more quickly in cones or rods?
Cones
Describe the visual pigment regeneration process
Isomerization causes retinal to separate from opsin, causing the molecule to become lighter in colour (no longer work)
Retinal re-attaches to opsin and returns to its original bent shape
Relate visual pigment regeneration to dark adaptation
The visual pigment regeneration process is continuous, but isomerization doesn’t occur in the dark
This means the concentration of unbleached, regenerated pigment can build back up in the dark
This is responsible for the increasing sensitivity during dark adaptation.
Our sensitivity to light at any point depends on…
The concentration of the unbleached visual pigment that is present.
Sensitivity of photoreceptors to different parts of the visible spectrum
Spectral sensitivity
Rods are more sensitive to which type of wavelength of light?
Short wavelengths (blues and greens)
Relate spectral sensitivity to dark adaptation
As vision shifts from cones to rods during dark adaptation, our vision becomes more sensitive to short-wavelength
A tendency for sensitivity of the eye to shift towards the blue- green end of the colour spectrum at low levels of illumination
Purkinje shift
Describe the pathway of signals travelling through the retina
Signals from the photoreceptors travel to bipolar cells and then to ganglion cells.
These then become the optic nerves and carry the information to our brain
What happens when several photoreceptors synapse onto a single ganglion cell?
Neural convergence
Why does a lot of neural convergence happen in the retina?
Disproportionate amount of photoreceptors compared to ganglion cells (126:1)
How do differences in rod and cone convergence lead to differences in perception?
Rods result in more sensitivity to light because they converge a lot (better vision in low light).
Cones result in better visual acuity because they converge a little (seeing fine details)
Why are rods more sensitive to light than cones?
It takes less light to generate a response from a rod than from a cone because more convergence allows for each cell to contribute less energy to the excitation of a ganglion cell, whereas less convergence for cones requires more energy per cell to reach the threshold for excitation
Why does less convergence for cones result in better visual acuity?
Stimulating cones leads to the excitation of more ganglion cells because they converge less (less cones per ganglion cell), providing the visual system with more information
The area of the retina that must be stimulated by light to cause the ganglion cell to respond.
Receptive field
In humans, ganglion cells have — receptive fields.
Centre-surround
Can occur when light hits the retinal ganglion cells (RGCs) in a pattern that isn’t ideal for their centre-surround arrangement
Centre-surround antagonism
How does centre-surround antagonism relate to edge detection?
Helps with edge detection by making edges look more distinct, which means we are better able to separate shapes from one another.
