Lecture 3 Flashcards
Light definition
Electromagnetic radiation with a specific frequency that falls within the observable spectrum.
- Wavelength usually expressed in nanometers.
Cornea
Light first passes through the cornea.
The cornea protects the structures inside the eye and refracts and bends the light rays.
- This happens because the cornea tissue is denser than air.
Pupil
The pupil is an opening in the iris. It appears to be black because light that enters the eye is absorbed by the retina.
- The size of the pupil determines how much light enters the eye.
Lens
After the light passes through the cornea, it is refracted again by the lens to focus light on the retina.
- Achieved by ciliary muscles that bend or flatten the lens.
- 20% of refraction happens in the lens, 80% in the cornea.
Retina
- The inside lining of the eye.
- Cones and rods: Around 7 million receptors are cones, the other 123 million are rods.
- Light passes through ganglion cells and bipolar cells before reaching the cones/rods.
The retina comprises 130 million light-sensitive receptors that are responsible for signal transduction.
Cones
Mostly packed in the ‘pit’ (fovea) in the central part of the retina (macula).
- Responsible for perception of colour and fine details (foveal vision if central and sharp).
Rods
Mostly centred around the fovea.
- Sensitive to low intensity light.
- Colourblind.
- Responsible for perception of low movement, peripheral vision, night vision.
Blind spot
Where the optic nerve leaves the eye.
- Discovered by anatomist Mariotte.
- Cephalopods don’t have a blind spot as retinal axons pass over the back of the retina. Our brain fills the blind spot with immediate surroundings.
Hyperopia (hypermetropia)
Eye is too short, focal place (where light is focused) lies behind the retina.
- The person is farsighted and cannot focus on close objects.
- This is corrected with convex lens+
Myopia
Eye is too long, focal plane lies before the retina.
- The person is nearsighted and sees distant objects as blurred.
- This is corrected with concave lens-
Presbyopia
Type of farsightedness due to hardening/diminished elasticity of the lens as we get older (<40 years),
- Corrected with reading glasses - convex lens.
Astigmatism
Imperfections in the spherical curvature of the cornea of the lens results in multiple focal points (blurry vision at all distances).
- Can be corrected with a cylinder.
Cataract
Clouding of the lens (age, diabetes).
Macular degeneration
Retinal degeneration of the fovea, the focus is black and distorted.
Glaucoma + what causes it
Worsening/loss of peripheral vision
- Failure of nerve cells due to increased eye pressure.
Receptive fields of ganglion cells (also how light passes through the eye)
Compressed information from cones/rods are sent via bipolar cells to ganglion cells, that relays the information to the optic nerve.
- One ganglion cell receives input from multiple cones/rods.
- Donut like - On centre v. Off centre cells.
On-centre cells
They are neurons with receptive fields that are primarily activated by a stimulus presented in the central region of their receptive field (donut hole)
- Can offer an explanation for some visual illusions like Hermann grid.
Hermann grid + receptive field
The white bands are equally intense throughout, but gray circles appear at their intersections.
- Receptive field 1 receives more surround inhibition than 2.
Meaning more on light surround = more inhibition = firing at slower rate than 2 –> 1 is perceived less intense than 2.
Mach bands + receptive field
The luminance is the same within one band, but perceived luminance within the band varies. The contrast is exaggerated on the edges.
- Receptive field 2 receives more surrounding inhibition than 1. Meaning more light on surround = more inhibition = firing at slower rate than 1 –> 2 perceived as less intense than 1.
Visual pathway explained
The ganglion cells feed forward information to V1. The nasal part of the optic nerve crosses at the optic chiasm(a), the temporal part continues to the ipsilater (same) side. This results in the projection of the left visual field in the right hemisphere.
Lateral geniculate nucleus in the visual pathway
The vast majority of the nerve fibres in the optic tract project to the LGN in the dorsal part of the thalamus.
- LGN is the main relay station in the pathway to the primary visual cortex.
Optic nerve is damaged/severed
All vision is lost in that eye.
Optic chaism(a) is damaged/severed
Outer part of the visual field is lost in both eyes.
Visual pathway from LGN to V1 is damaged/severed
Visual field is lost in both eyes.
