The A/S ratio is the proportion of the brain not directly connected with either receptor inputs or motor outputs, where A is the association cortex and S is the sensory cortex. The A/S ratio can be measured for any organism with a brain, and the higher the A/S ratio, the more complex the brain is.
It was proposed by Donald Hebb in 1949.
Book: Lerner, Dr. Richard M.. Concepts and Theories of Human Development. 2002. L. Erlbaum. Mahwah, N.J.. 978-0-585-38736-9. 49569701.