Vision and Light

Illusions highlight the difference between sensation and perception.
Some illusions create afterimages due to quirks in human vision.
Example: Staring at a turquoise, black, and yellow flag, then seeing a red, white, and blue afterimage on a white screen.

Vision is very intricate; nearly 70% of the body's sensory receptors are in the eyes.
Almost half the cerebral cortex is involved in processing visual information.
Vision is a dominant sense but can be tricked.

Light is electromagnetic radiation that travels in waves.
The frequency of a light wave determines its hue (color).
High frequency (short waves) are perceived as blueish colors.
Low frequency (long waves) are perceived as reddish colors.
The amplitude of a light wave relates to its brightness (intensity).
Lower amplitude results in dull, muted colors.
Higher amplitude results in bright colors.
Visible light is a small portion of the electromagnetic spectrum, which includes gamma rays, X-rays, and radio waves.
Eyes' photoreceptors convert light energy into nerve impulses, similar to how mechanoreceptors in ears and chemoreceptors in the tongue function.

Eyebrows and eyelashes protect the eyes (e.g., keep sweat away, trigger blinking).
Eyelids and lacrimal apparatus further protect the eyeballs.

Shape: Irregularly spherical, about 2.5 cm in diameter in adults.
Mostly hollow, filled with fluids to maintain shape.
Only the anterior sixth of the eyeball is visible; the rest is protected by fat and tethered by extrinsic eye muscles within the bony orbit of the skull.
Global luxation: Rare condition where the eyeball pops out, usually due to trauma.
The eyeball wall has three layers: fibrous, vascular, and inner.

Outermost layer made of connective tissue.
Sclera: The white part of the eye.
Cornea: Transparent anterior part that allows light to enter. Damage to the cornea can be very painful due to many pain receptors.

Middle layer containing the posterior choroid, which supplies blood to the eye layers.
Ciliary body: Ring of muscle tissue surrounding the lens.
Iris: The colored part of the eye composed of smooth muscle. It is shaped like a flattened donut.
Sphincter muscles in the iris contract and expand to control the size of the pupil.
Pupil: The opening in the iris that allows light to enter the eye. It constricts in bright light to protect the eye.

Includes the retina, which contains photoreceptors.
Lens: Convex, transparent disc that focuses light onto the retina.
Retina: Contains millions of photoreceptors that convert light into electrical signals.

Photoreceptors: Rods and cones.
Retina has two layers:
- Outer pigmented layer: Absorbs light to prevent scattering.
- Inner neural layer: Contains neurons including photoreceptors, bipolar neurons, and ganglion neurons.
Bipolar neurons: Synapse with photoreceptors on one end and ganglion neurons on the other, forming a bridge.
Ganglion neurons: Their axons form the optic nerve.

Light hits the retina, activating photoreceptors.
Signals pass to bipolar cells, then to ganglion cells, generating action potentials.
Axons of ganglion cells form the optic nerve (cranial nerve II), transmitting impulses to the thalamus and then to the visual cortex in the brain.

Cones:
- Located near the center of the retina.
- Detect fine detail and color.
- Three types: red, green, and blue sensitive.
- Require bright conditions to be activated.
Rods:
- More numerous and light-sensitive.
- Do not detect color; register grayscale (black and white).
- Located around the edges of the retina, responsible for peripheral vision.

Rods: Many rods (up to 100) connect to a single ganglion cell, making it difficult to discern detailed images. They provide information about shape and light/dark contrast.
Cones: Each cone connects to its own ganglion cell, allowing for detailed color vision in bright conditions.

Afterimages: Photoreceptors continue firing action potentials even after the stimulus is removed (e.g., bright lights or colors).
Cone Fatigue: Staring at a colored image for too long can cause cones to become desensitized.
Process:
- Staring at the turquoise stripes fatigues blue and green cones.
- Looking at a white screen (containing all colors) results in only red cones responding.
- The afterimage appears red.
- Rods create a black and white negative afterimage, leading to dark areas appearing light and vice versa.

Vision is complex but fallible.
Understanding visual mistakes helps understand the system.
Studied eye structure (fibrous, vascular, inner layers) and retinal neurons (photoreceptors, bipolar cells, ganglion neurons).
Distinguished between rods and cones and explained the flag illusion mechanics.