Editor’s note: Brain Basics from Scientific American Mind is a series of short video primers on the brain and how we feel, think and act. Below is a synopsis of the tenth video in the series written by a guest on this blog, Roni Jacobson, a science journalist based in New York City.

By Roni Jacobson

Russian physiologist Ivan Pavlov stumbled upon the work that would later make him famous while studying the digestive system. He noticed that the dogs in his lab would start to salivate—a natural biological response that helps break down food—when the research assistant who normally fed them entered the room rather than when they started eating. From this observation, Pavlov developed the principles of classical conditioning, in which an organism associates a neutral stimulus—footsteps or a bell, for example—with a stimulus such as food that ignites a biological response—say, salivation. After a while, the neutral stimulus starts to provoke the biological response on its own. That is, the dogs salivate in response to footsteps or a bell.

Psychologist John Watson expanded upon Pavlov’s research in a series of controversial experiments involving a young child known as Little Albert. By sounding an unpleasant noise every time the boy played with a white rat, Watson made Little Albert afraid not only of the rat but also of other white, fluffy things including other small animals and a white furry coat. Watson went on to establish behaviorism, an attempt to explain people through observable behavior without considering feelings, memories or other internal mental states. While behaviorists uncovered many important psychological principles, their approach was insufficient to capture the full complexity of the human condition. One additional piece of the puzzle is evolution: over millions of years, organisms become biologically predisposed to react to certain stimuli and to learn some associations over others.

Other Brain Basics videos:

Children Reason Differently from Adults

Multitask at Your Own Risk

Is Your Sense of Humor in Your Genes? Geneticists Crack the Code

Acts of Kindness Explained

Remember When…How Your Brain Builds A Memory

Terrified or Hopping Mad? What’s Going on Inside You

A Transformation of Light: How We See

Quick! What Is The Word for a Pair of Opposites?

The Hidden Power of Others Over You