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experiences in some way or other, and then integrating in some way the new and old together. This integration requires constant and dynamic changes to the brain structures that represent what we know and how we use it. 1 One way this could happen is that when we perceive something new, we actually re-enact in our mind’s eye the previous related perceptions. For example, if we encounter an apple – having encountered many previously -- perhaps we actually imagine (seeing) one or more previous apples or episodes involving apples, or imagine (performing) one or more instances of biting an apple or throwing one, and/or imagine tasting and smelling one. We might also imagine hearing the word “apple”, producing the sounds of the word, seeing the written form of the word, or even hearing, seeing, or producing synonyms or related words in our first (e.g., “Granny Smith”) or second (e.g., “pomme”) languages. We propose that understanding an apple is tantamount to executing this entire set of processes, and thus, that the circuits for understanding are very complicated and take up a large portion of the brain.

Of course, not all of what we understand is directly available to the senses; we can clearly understand beliefs and emotions as well as physical objects and overt actions. Brain researchers generally take the view that previous experience guides understanding of abstract concepts in much the same way that it guides the understanding of the more concrete entities. For example, we can understand the emotional states of other people by imagining being in those states ourselves. When I see someone feeling happy or sad, I can evoke examples from my own previous experience of feeling happy or sad (perhaps even for the same reasons), and by feeling the emotion, I can understand it. The closer my previous experience is to the perceived one, the better the “understanding”. This principle holds whether one is trying to understand objects or people. An important distinction between people and objects when trying to understand their actions is that people, but not objects, have intentions.

The Social Brain

The critical question for neurobiologists is how does the brain understand. In particular, we want to know whether the same brain circuits that are used when we experience things personally are also used when we try to understand another person having the same experiences, whether these experiences are concrete, like grasping a cup or hitting a baseball, or more abstract like feeling sad or fearful or in pain. We also want to know whether understanding the simple concrete perceptions and these highly complex emotional states are mediated similarly in the brain. Further, we are interested in the overlap between conscious and unconscious understanding and shared or personal understanding. One way to address these questions is to examine the brain structures responsible for specific types of personal sensations, actions, and cognitive processes, and to see if these same ones play a role when individuals attempt to understand these functions in others. Using modern techniques of physiology, experiments of this type have been conducted in both monkeys and humans, with some surprising results.

It is now possible to measure brain activity of humans while having a wide range of different kinds of experiences. Such human neuroimaging

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