Overview

How bright is that patch of light? What is white? What is black? What is a middle gray? These are simple-sounding questions, but the answers are complex and lead us to consider many of the fundamental mechanisms of perception. The fact that we have light-sensing receptors in our eyes might seem like a good starting point. But in fact, the receptor signals tell us very little. The nerves that transmit information from the eyes to the brain transmit nothing about the amount of light falling on the retina. Instead, they signal the relative amount of light: how a particular patch differs from a neighboring patch, or how a particular patch of light has changed in the past instant. Neurons in the early stages of the visual system do not behave like light meters; they behave like change meters.

The signaling of differences is not special to lightness and brightness. This is a general property of many early sensory systems, and we will come across it again and again throughout this book. The implications of this are fundamental to the way we perceive information. The fact that differences, and not absolute values, are transmitted to the brain accounts for contrast illusions that can cause substantial errors in the way data is "read" from a visualization. The signaling of differences also means that the perception of lightness is nonlinear, and this has implications for the gray-scale coding of information.