VISION--HOW WE SEE THINGS

VISION--HOW WE SEE THINGS

Vision is a complex, two-part process that begins when your eyes detect light and ends when your brain constructs the final, meaningful image. It involves a precise pathway from the outside world to the visual cortex.

👁️ Step 1: In the Eye (Capturing and Focusing Light)

1. Light Entry and Bending: Light rays reflected from an object first pass through the transparent, dome-shaped cornea, which performs the majority of the light-bending (refraction) needed for focusing.

2. Light Regulation: The light then travels through the pupil, an opening whose size is controlled by the iris (the colored part of the eye). The iris constricts the pupil in bright light and dilates it in low light, much like a camera aperture.

3. Final Focusing: The light passes through the lens, a clear structure that changes shape (a process called accommodation) to fine-tune the focus, ensuring the image lands sharply on the retina. Due to this process, the image projected onto the retina is upside down and reversed.

4. Signal Conversion (Retina): The focused light hits the retina, the light-sensitive tissue at the back of the eye. This layer contains specialized photoreceptors:

   • Rods: Handle vision in dim light (low resolution, black and white).

   • Cones: Handle color vision and fine detail (require bright light).

   • These cells convert the light energy (photons) into electrical signals (neural impulses).

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🧠 Step 2: To the Brain (Transmission and Interpretation)

1. Optic Nerve Transmission: The electrical signals are collected by the optic nerve (Cranial Nerve II), which carries the information from the back of the eye toward the brain.

2. The Crossover (Optic Chiasm): The optic nerves from both eyes meet at the optic chiasm. Here, the nerve fibers from the nasal (inner) half of each retina cross over to the opposite side of the brain. This ensures that the right visual field (everything you see to your right) is processed by the left hemisphere, and the left visual field is processed by the right hemisphere.

3. Relaying Information: Most signals make a stop at the Lateral Geniculate Nucleus (LGN), a major relay center in the thalamus.

4. Cortical Processing: The signals are finally sent to the Primary Visual Cortex (V1), which is located in the Occipital Lobe at the very back of the brain. This is where sight becomes vision. The brain performs several crucial processing tasks:

   • It flips the inverted image from the retina, so you perceive the world right-side up.

   • It combines the two slightly different signals from your eyes to create a single, continuous image with depth perception (3D vision).

   • The information is then routed into specialized pathways for further processing: the "What" pathway (for recognizing objects, faces, and color) and the "Where" pathway (for determining motion and spatial location).

Diagram of Brain

Information from our eyes goes to areas at the very back of the brain. We've all seen cartoons where the rabbit gets hit on the head and the rabbit sees stars. This can actually happen in human beings (trust me, not a good thing to do at home!). If you take a hard enough blow to the back of the head, this brain area bangs against back of your skull. This stimulates it and you can see stars and flashing lights. Remember those two hemispheres? Each hemisphere processes half the visual information. Visual information that we see on the left gets processed by the right hemisphere. Information on the right gets processed by the left hemisphere. Remember, wires that bring in information to the brain are "crossed"--visual information from the left goes to the right brain.

Diagram of Brain; VIDEO