The amygdala is an almond-shaped structure deep within the temporallobe and was first identified by Burdach in the early 19thcentury. Burdach originally described a group of cells thatare now known as the basolateral complex. Subsequently, a largenumber of structures that surround the basolateral complexhave been identified in many species and constitute what isnow known as the amygdaloid complex. This structure has attractedcontinued interest because of its central role in emotionalprocessing. The word emotion is a difficult concept that describessubjective experiences and feelings such as pain, fear, desire,and hope as well as aspects of the behavior of individualsboth public and private. In the past, emotions had traditionallybeen viewed as exclusively human and distinct from other aspectsof brain function such as cognition and sensory perception.This separation between cognition and emotion persisted despitethere being little doubt that emotions can have a major impacton various aspects of mental function. In the biological studyof emotions, perhaps the single most influential contributionwas that of Charles Darwin. In his seminal Expression of Emotionsin Man and Animals published in 1872, Darwin (40) suggestedthat there are some fundamental aspects of emotion that findsimilar expression in the behavior of both man and animals.This was the first indication that it may be possible to makeinferences about human emotion by examining animal behavior.Around the same time William James (87) and C. G. Lange (111)independently suggested that emotions are the cognitive responsesthat accompany our physiological responses to external stimuli.This idea came to be called the James-Lange theory of emotion.Thus, in James' words, when we see a bear "we don't run becausewe are afraid but are afraid because we run" (88). Togetherwith Darwin's proposal, these ideas suggested that it is possibleto study emotions by examining the physiological responsesto stimuli.
The first neurophysiological theories of emotion emerged fromthe work of Cannon and Bard in the 1920s. Cannon and Bard werecritical of the James-Lange theory and instead suggested thatthe hypothalamus and its projections to the cortex and brainstem were the central element that both evaluated and initiatedemotional responses (25). Subsequently, Papez (197), reviewingthe anatomical and clinical data in 1937, added more medialtemporal structures to the circuitry involved in emotionalexpression. These ideas were further expanded by Paul McLean(172), who named these forebrain circuits the "visceral brain"and introduced the concept of the limbic system. While McLeanincluded the amygdala in the limbic system, the involvementof the amygdala in emotional processing arose from the now classic studies of Klüver and Bucy (101, 102) who examinedthe behavioral effects of medial temporal lobe lesions in monkeys.These animals showed a range of effects including marked changesin emotional behavior that were described by them as "psychicblindness" and has come to be known as the Klüver Bucysyndrome. However, these lesions were quite large and includedthe amygdala, hippocampus, and surrounding cortical areas. Subsequently, Weiskrantz (288) showed that more restrictedamygdala lesions could replicate the results of Klüverand Bucy, cementing the fundamental role of the amygdala inemotional processing. These studies made it clear that theamygdala is an essential component of the circuitry that assignsemotional significance and produces appropriate behavioralresponses to salient external stimuli (69, 96, 116, 231).
While initial studies on the role of the amygdaloid complexused avoidance conditioning and instrumental learning (245),the study of emotions reached a new level of analysis withthe development of the study of fear conditioning. Fear conditioningis a simple Pavlovian conditioning task in which a neutralstimulus, such as a tone or a light, is paired with an aversivestimulus, typically a footshock. Following a relatively smallnumber of such pairings, the neutral stimulus subsequentlyelicits a behavioral state similar to that evoked by the aversivestimulus alone. The fear response consists of freezing (a cessationof movement), sweating, and changes in heart rate and bloodpressure. In humans, there are also cognitive effects such asfeelings of dread and despair associated with these autonomiceffects. This learned behavior is rapidly acquired and longlasting. The simple nature of this learning task, and the readilymeasured physiological changes that accompany it, have madethe study of fear conditioning a very attractive model forthe study of learning and memory consolidation. Furthermore,because of the physiological similarities between animal andhuman fear, fear conditioning is seen as relevant to the genesisof anxiety disorders in humans (41, 43, 237), thus providingan additional incentive to study fear. These advances haveled to a rapid increase in the number of studies examiningthe role of the amygdala in fear, learning, and memory in general.Although some controversy persists in the precise role of thestructures involved (114, 198), these studies have provideddefinitive evidence implicating the amygdala and its afferentand efferent projections in fear processing in mammals (1,43, 96, 115).
In this review, we discuss the anatomy and physiology of theamygdala and the mechanisms proposed to underlie its involvementin fear conditioning.