4.11 Understanding Lateral Inhibition in Vision

Lateral inhibition is a process that enhances contrast in visual images through the interaction between neurons in the visual system, particularly involving horizontal cells and photoreceptors (rods and cones).
Horizontal cells act as inhibitory interneurons and are responsible for connecting and influencing adjacent photoreceptors.
These cells primarily use GABA (gamma-aminobutyric acid) as an inhibitory neurotransmitter, meaning they inhibit the activity of neighboring neurons when activated.

Horizontal cells are crucial in the lateral inhibition process.
They mediate the flow of signals between photoreceptors, creating a network that allows for the spread of inhibitory signals.
Activation of a photoreceptor leads to inhibition of surrounding photoreceptors:
- For example, if photoreceptor C is activated, it sends inhibition to photoreceptors B and D.

The Mach Band effect is a visual illusion that illustrates lateral inhibition's role in creating perceived contrast at the edges of borders between areas of different light intensity.
In the illustration involving bands of gray, as one approaches a darker band, the left edge of the border appears lighter, while the right edge appears darker, leading to an illusion of enhanced contrast even though the bands themselves are uniform in color.

Uniform Bands of Gray: The bands are solid colors with no actual change in brightness at the borders, but perceived changes occur due to lateral inhibition.
Contribution of Photoreceptors and Light Intensity:
- A lighter band corresponds to stronger light intensity and thus a lighter shade of gray.
- A darker band has lower light intensity.
Photoreceptors and Inhibition Dynamics:
- Each edge of the bands is processed by photoreceptors that send signals about light intensity to the brain.
- For example:
  - Left Edge of a Border:
    - Photoreceptor receiving high light intensity (active) experiences more intense signal and transmits this to the brain. It receives some inhibition from its neighbor (which is firing intensely) but also less from the opposite side (where light intensity is lower), leading to a perceived lightening effect.
  - Right Edge of a Border:
    - Here, with lower firing of connected photoreceptors, the right photoreceptor experiences greater inhibition (due to strong firing from the left photoreceptor), resulting in a darker perception of that edge.

The perceived brightness at the edges is not only about the intensity of the signal sent by photoreceptors but also significantly influenced by the level of inhibition.
The overall perception of an area (light or dark) depends on both:
- The strength of the signal (rate of action potentials) sent by photoreceptors.
- The extent of inhibition received from neighboring photoreceptors.

A graph may depict light intensity levels along a gradient, showing high intensity on one side diminishing to low intensity on the other side, aiding in understanding:
- Photoreceptors in high-intensity areas fire more rapidly.
- Increased firing leads to more inhibition sent to neighbors through horizontal cells.

Activated Photoreceptors: Send action potentials based on light exposure, thus influencing the visual signal.
Inhibition Flow: Activation leads to a decrement of signal strength through inhibition, mediated by horizontal cells.
The balance between activation and inhibition at the borders of light and dark bands creates the illusion of enhanced contrast, crucial for visual perception in distinguishing boundaries and forms in our environment.