Lecture 7 The first steps in vision: From light to neural signals Flashcards
Steps 1-3 of how the eye works
- Light passes through the cornea, some of this light enters the eye through the pupil
- Then through the aqueous humor (nourishment)
- Next the lens to focus the image on the retina
Steps 4-7 of how the eye works
- Vitreous humour,
- Retina, light is converted into electrical signals
- Fovea,
- Optic disc, information transmitted onwards to the brain
Key info about the fovea
maximum resolution focus, responsible for high acuity vision it is densely saturated with cone receptors to help us see fine detail.
Emmetropia
- Normal vision
Light rays hit at the back of the eye with no blur
Myopia
Short sighted
if the eyeball is too long or the cornea is too curved
raised light converge before they hit the retina so the image appears blurry
how glasses correct myopia
diverges light rays entering the eye so they focus a little further back. landing directly on the retina instead of infront of it.
Hyperopia
long sighted
can’t see objects close as light converges too far back of the eye
Accomodation
the eyes ability to adjust focus to see objects clearly at different distances
Key facts about accomodation
Happens in a quarter of a second
15 dioptre accommodation range at birth
How eyes accommodate to see near objects
lens becomes thicker, this allows the light rays to refract more strongly
How eyes accommodate to see objects far away
lens is pulled thin, allowing the light rays to refract slightly
How the ability to accomodate is affected by age
Presbyopia - ability declines with age
1D drop every five years until 30
Less than 2.5D range by age 40-50 yrs
Astigmatism definition
Blur due to irregular curvature of cornea
Image not focusing on the eye due to lens or the shape of the cornea
Retina: vertical connections
photoreceptors
bipolar cells
ganglion cells
Retina: Horizontal connections
Horizontal cells
Amacrine cells
Role of photoceptor’s
converts light into electrical signals that the brain interprets as sight
Key facts about cones
See in colours and fine details in light
densely packed in the fovea
Key facts about rods
responsible for vision in low light
no rods in the fovea
responsible for our peripheral vision
Three cone types
Short, medium, long
No S cones at the fovea
Difference between photoreceptor peaks at different wavelengths
Rod sensitivity peaks at a lower wavelength than cone sensitivity (20 deg)
What is the blind spot
Where the optic nerve connects to the retina, lacks photoreceptors
Rods maximum density
between 0-10 mm on either side of the fovea
Fundus definition
interior lining of the eye, including retina, vascular tree, macula, fovea, optic disc
Bipolar cells: Midget
Fovea, cones, low convergence (1:1), good resolution
