Sensation and Perception of Colour Vision

2. The Neural Chain

Once the photoreceptors are activated, the signal travels through a specific sequence of cells:

1. Photoreceptors (Rods and Cones): Detect the light and wavelengths.
2. Bipolar Cells: Receive the signal from the photoreceptors.
3. Ganglion Cells: Process the information further before sending it toward the brain.
4. Optic Nerve: Carries the processed visual information from the eye to the brain.

Exam Tip: Remember the sequence of the visual pathway: Light $\rightarrow$ Rods/Cones $\rightarrow$ Bipolar Cells $\rightarrow$ Ganglion Cells $\rightarrow$ Optic Nerve.

Theories of Colour Vision

Psychologists use two primary theories to explain how we perceive color, and interestingly, both theories involve the number "three" but at different stages of the process.

Trichromatic Theory (The First Stage)

The trichromatic theory suggests that color perception begins at the level of the cones. There are exactly three types of cone cells, each containing a different photopigment sensitive to specific wavelengths:

• Short wavelengths (Blue)
• Medium wavelengths (Green)
• Long wavelengths (Red)

By combining the signals from these three types of cones, the brain can produce our perception of a multicolored world.

Opponent-Process Theory (The Second Stage)

While the cones detect the initial wavelengths, the opponent-process theory explains how that information is processed in the ganglion cell layer. This theory suggests that color information is separated into three sets of opposing channels:

• Red vs. Green
• Blue vs. Yellow
• Black vs. White

This explains why we perceive certain colors as opposites on a color wheel.

Colour Adaptation and Afterimages

The opponent-process theory provides a clear explanation for the phenomenon of afterimages.

When you stare at a specific color for an extended period, your cones undergo colour adaptation—their sensitivity to that specific color decreases. When you look away, the "opponent" color temporarily wins because the ability of the adapted cones to inhibit the opposing color is reduced.

Worked Example: The Afterimage Effect If you stare intensely at a bright red shape and then look at a white wall:

1. Your "red-driving" cones adapt (Stage 1).
2. They signal less red and hold back the green channel less effectively.
3. You see a green afterimage (Stage 2).

Summary of the Two-Stage Model

To visualize how these theories work together, consider the following flowchart of color vision:

1. Stage 1 (Trichromatic): Three cone types detect wavelengths (Red, Green, Blue).
2. Stage 2 (Opponent-Process): Information is processed via three opposing pairs (Red-Green, Blue-Yellow, Black-White) in the ganglion cells.