This is the third in a series of posts on visual processing.
Local sensitivity adjustment was introduced in the last post, and is explored more below.
The Checker Square Illusion demonstrates local sensitivity adjustment.
Square B is clearly brighter than square A. However, square A and square B have identical objective luminance. B is brighter because it exists within a shadow region. Brightness is subjective. Brightness is what you experience.
In the painting below, I am impressed by the detail I can perceive in the figure kneeling on the ground on the lower left side. His white shirt sleeve is quite bright. Much of his detail is easy to perceive.
Visitors in the Tyrol
Franz con Defregger 1835-1921
Frye Art Museum, Seattle
This is not the slow process of global dark adaptation. It is immediate and localized. Local sensitivity adjustment allows me to simultaneously perceive detail within regions that vary greatly in their luminance.
Our visual processing system is essentially capable of adjusting sensitivity locally. This is like a camera being able to adjust ISO according to local conditions in a scene. This allows us to perceive detail in shadow regions. This remarkable ability is deeply wired into the visual system and and not unique to humans.