SWeek 7 ELM 14: Sensory Systems I - Vision

Light can be described as both an electromagnetic wave and a stream of photons.
The intensity of light corresponds to the amplitude of the electromagnetic wave (larger amplitude means brighter light).
The color of light is determined by its wavelength.
Humans can only see a narrow range of the electromagnetic spectrum.
Different people might have slightly different color perception, which can be due to conditions like color blindness.
Wavelength is calculated as $wavelength = speed/frequency$ .

Key components of the eye include:
- Cornea: Provides the greatest refractive power.
- Lens: Accommodates by changing shape to adjust refractive power.
- Iris
- Zonule fibers
- Aqueous humor
- Ciliary muscle
- Vitreous humor
- Sclera
- Retina: Part of the brain responsible for detecting light.
- Fovea: Area of the retina with maximum visual acuity.
- Optic nerve: Transmits visual information to the brain.
The image formed on the retina is inverted.
The cornea has a refractive power of approximately 42 diopters (D), while the lens contributes about 12 D.

Common vision problems include:
- Myopia (Short-sightedness): Corrected with concave lenses.
- Hyperopia (Far-sightedness): Corrected with convex lenses.
- Astigmatism: Caused by irregularities in the cornea or lens.

Light passes through several layers of retinal cells before reaching the photoreceptors (rods and cones).
The order of cells that light passes through is:
- Rods and Cones
- Bipolar cells
- Retinal ganglion cells
- Amacrine cells
- Horizontal cells
- Pigment epithelium
Retinal neurons are mostly glutamatergic excitatory neurons, along with inhibitory neurons.
The retina is considered "inside-out" because light must pass through several cell layers before reaching the photoreceptors.

Located at the back of the retina.
Essential for recycling retinaldehyde, which is crucial for the function of rods and cones.
Helps rods and cones manage oxidative stress.
The pigmented layer makes the pupil appear black.
Tapetum lucidum: layer just behind the RPE found in some animals that causes eyeshine.

Cones: Primarily located in the fovea, providing maximum visual acuity; function best in bright light.
Rods: Responsible for night vision and peripheral vision; 1000 times more sensitive to light than cones.
Color discrimination is poorer in the periphery and at night due to the properties of rods.

Opsin: A G protein-coupled receptor (GPCR) with 7 transmembrane domains.
- There are different types of opsins found in the three types of cones, rods, and melanopsin-containing retinal ganglion cells (5 opsins in total).
Retinal: A derivative of Vitamin A, which is the same for every opsin.
- Retinal absorbs light and changes conformation, leading to bleaching.
Photopigment: Formed by the combination of opsin(s) and retinal.

Photoreceptors are unique because they are depolarized in the dark due to open sodium channels.
In the light, sodium channels close, causing the membrane to hyperpolarize.
Thus, the presence of a stimulus (light) leads to hyperpolarization.
Photoreceptors detect darkness due to this mechanism.

When light is absorbed by retinal, it causes a conformational change in the rhodopsin molecule.
This leads to the activation of transducin.
Activated transducin activates phosphodiesterase.
Phosphodiesterase reduces the concentration of cGMP.
The reduction in cGMP leads to the closure of sodium channels and hyperpolarization.

Humans have 3 types of cones (trichromats).
Dogs, reptiles, mice, cats, and horses have 2 types of cones (dichromats).
Dolphins, whales, and seals have 1 type of cone (monochromats).
Mantis shrimps have 12-16 cone types.
Differential activation of cone cells is necessary for the brain to create the perception of color.

The genes for red and green opsins are located on the X chromosome, while the gene for blue opsin is on Chromosome 7.
Approximately 6% of men have some form of color vision anomaly.
2% of men lack a gene for either red or green opsins.
1% of women have color vision anomalies.
Less than 0.001% of people lack all color vision.
Some women are tetrachromats due to having two red alleles.
Ishihara test: Used to test for color blindness.

Horizontal cells
- Light intensity adaptation
- Spatial processing
- Color processing (opponency)
Amacrine cells
- Directional motion detection
- Light adaptation modulation
- Circadian rhythm modulation
- Sensitivity of night vision
Bipolar cells
- OFF bipolar cells
- ON bipolar cells
- Center-surround receptive fields (e.g., of an ON bipolar cell)

Retinal ganglion cells (RGC)
- Further process color, motion, and shapes.
- Serve as the only output cells of the retina, firing action potentials.
Intrinsically photosensitive retinal ganglion cells (ipRGC)
- Can detect light via melanopsin.
- Melanopsin does not contribute to image formation but affects:
  - Circadian rhythms
  - Pupil size
  - Body temperature

Visual information from the left visual field is processed by the right side of the brain, and vice versa.
The pathway involves:
- Eye
- Optic nerve
- Optic chiasma
- Lateral geniculate nucleus (LGN)
- Primary visual cortex (V1, striate cortex) located in the occipital lobe
Orientation selectivity is present in V1.

Eyes and vision are vital for survival for many animals, leading to diverse visual strategies.
The vertebrate retina has a laminar structure, with photoreceptors located behind other cells.
Retinal cells perform early processing of visual information before it is sent to the brain.
Vertebrate vision depends on rhodopsin, transducin, and hyperpolarization (Na+ dark current).
Conscious visual information is processed in the lateral geniculate nucleus (LGN) and cortical area V1. Also, orientation selectivity in V1.