Philosophers and psychologists have long debated the nature of emotions such as happiness. Are they states of supernatural souls, cognitive judgments about goal satisfaction, or perceptions of physiological changes? Advances in neuroscience suggest how brains generate emotions through a combination of cognitive appraisal and bodily perception.
Suppose that something really good happens to you today: you win the lottery, your child gets admitted to Harvard, or someone you've been interested in asks you out. Naturally, you feel happy, but what does this happiness amount to? On the traditional dualist view of a person, you consist of both a body and a soul, and it is the soul that experiences mental states such as happiness. This view has the appealing implication that you can even feel happiness after your body is gone, if your soul continues to exist in a pleasant location such as heaven. Unfortunately, there is no good evidence for the existence of the soul and immortality, so the dualist view of emotions and the mind in general can be dismissed as wishful thinking or motivated inference.
There are currently two main scientific ways of explaining the nature of emotions. According to the cognitive appraisal theory, emotions are judgments about the extent that the current situation meets your goals. Happiness is the evaluation that your goals are being satisfied, as when winning the lottery solves your financial problems and being asked out holds the promise of satisfying your romantic needs. Similarly, sadness is the evaluation that your goals are not being satisfied, and anger is the judgment aimed at whatever is blocking the accomplishment of your goals.
Alternatively, William James and others have argued that emotions are perceptions of changes in your body such as heart rate, breathing rate, perspiration, and hormone levels. On this view, happiness is a kind of physiological perception, not a judgment, and other emotions such as sadness and anger are mental reactions to different kinds of physiological stages. The problem with this account is that bodily states do not seem to be nearly as finely tuned as the many different kinds of emotional states. Yet there is undoubtedly some connection between emotions and physiological changes.
Understanding how the brain works shows that these theories of emotion - cognitive appraisal and physiological perception - can be combined into a unified account of emotions. The brain is a parallel processor, doing many things at once. Visual and other kinds of perception are the result of both inputs from the senses and top-down interpretations based on past knowledge. Similarly, the brain can perform emotions by interactively combining both high-level judgments about goal satisfactions and low-level perceptions of bodily changes. The judgments are performed by the prefrontal cortex which interacts with the amygdala and insula that process information about physiological states. Hence happiness can be a brain process that simultaneously makes appraisals and perceives the body. For details about how this might work, see the EMOCON model of emotional consciousness.