3. Early Visual Processing Flashcards
Issac Newton theory of light
light acts like a particle
James Clerk Maxwell theory of light
light has wavelike properties (produces diffraction patterns)
(Visible) Light
- light is electromagnetic radiation (like gamma rays, radio, radar, etc.)
- visible from ~380 to ~760 nm (billionths of a metre)
- the eye transduces light energy → neural impulses
Ḥasan Ibn al-Haytham
- called the “father of optics” and a “pioneer of modern optics”
- wrote Book of Optics (1011-1021):
- vision produced by light reflecting from surfaces into the eye
- visual perception occurs in the brain-not just the eye
- perception is subjective and affected by individual experience
- laid the foundation for the scientific method
How light passes through the eye
- light first strikes the cornea: concentrates light rays
- passes through aqueous humour
- passes through pupil (hole in centre of the iris)
- pupil dilates (gets larger) in the dark to let in more light
- contracts in bright light to protect the eye
- passes through crystalline lens
- passes through vitreous humour to retina
C, A, P, L, V, R
What is the pupil and what does it do?
A hole in the center of the iris, the iris being a muscle that controls the size of the pupil
- pupil dilates (gets larger) in the dark to let in more light
- contracts in bright light to protect the eye
- sunglasses should have UV protection to guard against retinal and corneal damage
e.g., iggak (caribou antler goggles) worn by the Inuit protect against snow blindness (sunburned corneas)
The Crystalline Lens and accommodation
accomodation: ciliary muscles change shape of the lens, altering its focal length, which keeps image focused on retina
elasticity reduces with age, making near point (minimum distance at which you can focus) move farther away: presbyopia
the retina
- receptors (rods and cones) point to the back of the eye
- synapse with bipolar cells (have two long extensions)
- which connect to ganglion cells (2 types): P-cells and M-cells
they also have horizonal cells and amacrine cells
horizontal cells
make lateral connections among receptors and bipolar cells
amacrine cells
laterally connect among bipolar and ganglion cells
duplex retina theory
(Schultze, 1866):
- observed that retinas of nocturnal animals (e.g., owls) only contained rods
- diurnal animals (e.g., pigeons) only contained cones
- animals active during day and night had both rods and cones
duplicity theory
(von Kries, 1896):
- related rods and cones to scotopic (dark) and photopic (light) vision
Rods
we have 120-125 million of them
only located in the periphery
high sensitivity
scotopic(dark vision)
black and white vision
Cones
we have 5-6 million of them
located mostly in the fovea but the amount of them decreases as you get further and further away from it
low sensitivity
photoptic(light vision)
colour/day vison
fovea centralis
used for directed looking
- densest concentration of receptors in the eye
- only has cones (peripheral retina contains rods & cones)
explain the dark adaptation curve
different pigments in rods and cones
when going from light to dark environments, the rods need time to adjust so for the first 7ish minutes you are using cones. Once the rods have recovered they regain sensitivity and we switch over to them
- Boll (1876) found photosensitive pigment in rods: bleached in the light and regenerated in the dark
- rhodopsin comprised of retinal and opsin
- when hit by light, retinal changes shape (isomerization), causing a chain of events that culminates in a neural signal
rod monochromats
due to a genetic defect, have only rods on their retinas
isomerization
when hit by light, retinal changes shape causing a chain of events that culminates in a neural signal
rhodopsin splits into retinal and opsin
what is the absolute threshold for light?
one photon of light is the minimum to change the shape of a pigment molecule
Hecht, Shlaer, & Pirenne (1942): measured absolute threshold
pigment regeneration
Rushton (1961): measured using retinal densitometry
- shone thin, dim beam of light onto the retina
- some bounces off the back of the eye and is reflected out
- receptor pigment absorbs light–until it bleaches out, causing more light to be reflected out
- measured amount of reflected light over time: indicates time for pigment to regenerate
- result: cones take 6 min., rods take 30 min.
- pigment is re-formed, with the help of (beta carotene →) vitamin A + enzymes
Snellen Chart
measures foveal acuity only, not an absolute measurement
this is the typical eye exam chart we know
- normal is 20/20 vision (what you can see at 20 feet vs. distance for normal person to see)
- 20/200 (or worse) is legally blind, at 20 feet away they have the visibility of someone 200 feet away
diopters
used by optometrists to measure the reciprocal of focal length (m) of corrective lens
- negative = concave lens (for nearsightedness)
- positive = convex lens (for farsightedness)
visual angle
measurement of size of retinal image in degrees
tan (α) = size ÷ distance = 2.4 cm ÷ 70 cm = 0.034, therefore α ≈ 2° (a quarter at arms length)
- note: 1° = 60’ (minutes of arc), and 1’ = 60” (seconds of arc)
- with 20/20 vision, details of 1’ can be resolved (size of a quarter at the distance of a football field)
Visual acuity
refers to the clarity or sharpness of vision, and it is a measure of the eye’s ability to distinguish fine details. It specifically relates to how well someone can resolve two points or objects as separate from one another. Higher visual acuity means better detail perception.
Visual acuity depends on several factors:
-The sharpness of the focus on the retina.
-The health and function of the retina, particularly the
cone cells in the fovea, which are responsible for
detailed central vision.
-The brain’s ability to process visual information.