Biology uses relative measures
Visual illusion 1: Both gray rings have the same hue
Visual system measure brightness relative to the background
Visual illusion 2: Both women are the same size in both photos
Our brain compares objects using perspective measures
Key point: Neurobiology doesn’t use absolute measures or values
Relative values, adaptation, and habituation appear at all levels of neuronal processing
From Kandel, Schwartz, Jessel, Principles of Neural Science, 3rd edn., 1991, pgs. 411 & 444