Sizing Things Up; 105 Mind-Bending Illusions; Special Editions; by Vilayanur S. Ramachandran and Diane Rogers-Ramachandran; 4 Page(s)

The great German physicist Hermann von Helmholtz not only discovered the first law of thermodynamics (the conservation of energy) but also invented the ophthalmoscope and was first to measure nerve impulse velocity. He is, in addition, widely regarded as the founding father of the science of human visual perception--and is, to both of us, an inspiration.

We have often emphasized in our writings that even the simplest act of perception involves active interpretation, or "intelligent" guesswork, by the brain about events in the world; it involves more than merely reading out the sensory inputs sent from receptors. In fact, perception often seems to mimic aspects of inductive thought processes. To emphasize perception's thoughtlike nature, Helmholtz used the phrase "unconscious inference." Sensory input (for example, an image on the retina at the back of the eye) is interpreted based on its context and on the observer's experience with, and knowledge of, the world. Helmholtz used the word "unconscious" because, unlike for many aspects of thinking, no conscious cogitation is typically required for perception. By and large it is on autopilot.