1. Energy: Light

Before any perception can happen, there must be a physical stimulus. For vision, this energy is light.

What it is

Light is a narrow band of the electromagnetic radiation spectrum that our eyes can detect (roughly 400–700 nm). It can be conceptualized as both a wave and a stream of particles called photons.

How we get it

We rarely see "pure" light from a source. Instead, we see light that has interacted with objects in the world.

Key Interactions

• Reflected: The light bounces off a surface (this is how we see most objects).
• Absorbed: The light is taken in by the surface and not transmitted (e.g., a black shirt absorbs most light).
• Refracted: The light is bent as it passes through a new medium, like when it enters your eye's cornea.

The "information" your brain will ultimately process is contained in the wavelengths and intensities of the light that is reflected from objects. Before hitting the neural tissue, light must travel through the eye's optical structures. It enters via the cornea (which performs the initial refraction/bending), passes through the pupil, and is further focused by the lens to project an inverted image onto the retina at the back of the eye.

2. Transduction: From Light to Neural Signal

Transduction is the crucial process of converting that light energy into an electrical and chemical signal the brain can understand. This entire process happens in the retina.

Where

The process begins in the photoreceptors (rods and cones), the back-most layer of the retina.

• Rods: Specialized for scotopic (dim light) vision. They are highly sensitive but do not process color.
• Cones: Specialized for photopic (bright light) vision. They are responsible for high visual acuity (detail) and color vision.

The Chemical Process

A photon of light is absorbed by a molecule called a photopigment located in the outer segment of a rod or cone. This photopigment is made of two parts: opsin and retinal.

When light is absorbed, the retinal changes shape and separates from the opsin called "bleaching." This separation triggers a chemical cascade that hyperpolarizes the photoreceptor (makes it more negative).

This hyperpolarization causes the photoreceptor to decrease its release of the neurotransmitter glutamate. (In the dark, photoreceptors are depolarized and steadily release glutamate).

3. Pathway: to the Brain

The new neural signal now travels from the retina to the primary visual cortex, getting organized and processed at each stop.

A. In the Retina (Processing)

The signal travels through two main pathways:

Vertical Pathway: This is the main route.

• Bipolar Cells:
  • Midget Bipolar Cells: In the fovea, they get input from just one cone. This 1:1 connection is why the fovea has high acuity (detail).
  • Diffuse Bipolar Cells: In the periphery, they input from many photoreceptors. This is why peripheral vision is highly sensitive to dim light but has low acuity.
• Ganglion Cells: These cells receive the signal from the bipolar cells. Their long axons bundle together to form the optic nerve, which is the cable that leaves the eye.
  • P ganglion cells: Receive input from midget cells. Involved in processing color and fine detail (Parvocellular pathway).
  • M ganglion cells: Receive input from diffuse cells. Specialized for processing motion.

Lateral Pathway (Horizontal): Horizontal Cells connect photoreceptors to each other. They are responsible for lateral inhibition, which sharpens contrast and is the reason ganglion cells have center-surround receptive fields.

B. Optic Chiasm

The optic nerves from both eyes meet at the optic chiasm. Here, information from the visual fields is sorted: Left Visual Field goes to Right Hemisphere, Right Visual Field goes to Left Hemisphere.

C. LGN

The optic tract synapses in the Lateral Geniculate Nucleus (LGN), a part of the thalamus. The LGN has 6 layers that keep information separate (retinotopic map, M-cell/P-cell separation, left/right eye separation).

D. The Cortex (V1 and Beyond)

Primary Visual Cortex (V1): Signals travel from LGN to V1 in the occipital lobe. V1 creates a retinotopic map with cortical magnification. It uses simple cells to detect orientation and is organized into hyper columns.

Extrastriate Cortex ("What" and "Where"):

• Ventral Stream ("What"): Travels to temporal lobe. Responsible for object recognition. Cells in IT cortex respond to complex objects like faces.
• Dorsal Stream ("Where"): Travels to parietal lobe. Responsible for location, motion, and action.

4. Interpretation: Making Sense of the Signals

A. Bottom-Up Processing

Based on the sensory data coming in. Example: Gestalt Principles automatically group simple features from V1 (Proximity, Good Continuation, Similarity).

B. Top-Down Processing

Based on context, memories, and expectations to interpret. Example: Bayesian Approach. When you see an ambiguous "B" / "13" figure, context determines the interpretation. Prior knowledge dictates the final interpretation.