TA mechanistic approach to mental disorder should be based on psychobiological mechanisms rather than symptoms. We suggest that learning processes and associated brain plasticity are core mechanism that can be studied in animals and humans. In addition to the hedonic value, the learning phase, i.e. habituation, acquisition, extinction, extinction memory, the role of stimulus properties, for example cue versus context and event timing need to be considered, as well as the processing of prediction erros.
The study of extinction of conditioned fear has made great progress with the use of Pavlovian fear conditioning in rodents. In recent years, however, there has been a shift toward more realistic behavioral scenarios, in which an animal encounters danger while pursuing rewards. Approach-avoidance conflict tasks can reveal different behavioral strategies employing different prefrontal-amygdalo-striatal circuits. Extinction of approach-avoidance conflict is relevant to understanding obsessive compulsive disorder.
The first cells living billions of years ago likely had to detect and respond to danger in order to survive. Life is about not being dead, and behavior is a major way that organisms hold death off. Although behavior does not require a nervous system, complex organisms have brain circuits for detecting and responding to danger, the deep roots of which go back to the first cells. But these circuits do not make fear. Fear is a human invention; a construct we use to account for what happens in our minds when we become aware that we are in harm’s way. This requires a brain that can personally know that it existed in the past, that it is the entity that might be harmed in the present, and that it will cease to exist in the future.