The amygdala.

Abstract

The basolateral amygdala channels sensory inputs to other parts of the brain. This becomes obvious in ‘conditioning’ experiments, when neutral stimuli, such as lights or tones, are paired with emotionally significant stimuli, such as shocks or food. For example, a light paired with food can serve as a positive reinforcer by changing neural transmission in the basolateral amygdala which sends signals to the striatum, leading to approach behavior. By contrast, a light paired with shock could cause the basolateral amygdala to send signals to the central nucleus of the amygdala, which would result in the somatic, autonomic and endocrine signs of fear, as well as increased attention to the stimulus. These ‘conditioned’ effects often depend on activation of N-methyl-D-aspartate receptors within the basolateral amygdala. More long-lasting fear-like effects, often not dependent on conditioning, may involve outputs to the lateral bed nucleus of the stria terminalis and may be more akin to anxiety than fear. Outputs to the striatum may also be involved in avoidance of stimuli paired with aversive events. Outputs to the hippocampus may influence the development of conscious memories of good or bad times. Finally, reciprocal connections between the cortex and the basolateral amygdala may be involved in the representations of these positive or negative events in memory. The amygdala (that’s, a’mig’da’la) is a structure in the brain that looks like an almond and makes you feel scared. The amygdala is involved in fear, anxiety and aggression. Monkeys without an amygdala are not appropriately afraid of humans or of the boss monkey of their troop. Rats without an amygdala might play with their native predators, such as cats or snakes. A person with a dysfunctional amygdala finds everyone trustworthy, and can’t sense when someone else is afraid. It’s also now clear that the basolateral amygdala is involved in more positive behaviors, such as in approaching things that appeal. The anatomist Burdach coined the term (Latin for ‘almond’) in 1819, to describe an almond-shaped cell mass located deep in the human temporal cortex. The structure originally described as the amygdala is composed of several distinct groups of cells (the lateral, basal and accessory basal nuclei), now collectively termed the ‘basolateral amygdala’. Several structures surrounding the basolateral amygdala, including the central, medial and cortical nuclei, were traditionally included in the ‘amygdaloid complex’. Unfortunately, this complex has gradually come to be called the amygdala, even though it doesn’t make anatomical sense to group the different structures together. When we are afraid, or think of things that make us scared, more blood flows to the amygdala. This even occurs when we have no conscious awareness that we are looking at something scary, like a face, for example. The amygdala filters sensory information and acts as a sort of ‘interpretation’ channel. The basolateral amygdala receives sensory information from the thalamus and cortex and then forwards a signal to the appropriate target areas (see Figure). Thus, it is useful to think of the basolateral amygdala as the amygdala proper, and to think of its several target areas as parts of a broader network that serves more specialized functions. Because the basolateral amygdala is critical for emotion, a better understanding of the chemicals within these brain circuits should lead to improved pharmacological treatments for emotional dysfunction in psychiatric disorders. Adolphs R, Tranel D, Damasio H, Damasio AR: Fear and the human amygdala. J Neurosci 1995, 15:5879-5891. Aggleton JP: The Amygdala: Neurochemical Aspects of Emotion, Memory and Mental Dysfunction. Edited by Aggleton JP. New York: John Wiley-Liss and Sons; 1992. Cahill L, McGaugh JL: Mechanisms of emotional arousal and lasting declarative memory. Trends Neurosci 1998, 21:294-299. Holland PC, Gallagher M: Amygdala circuitry in attentional and representational processes. Trends Cognitive Sci 1999, 3:65-73. LeDoux JE: The Emotional Brain: the Mysterious Underpinnings of Emotional Life. New York: Simon & Schuster; 1996. McGinty JF: The Extended Amygdala: Are the Central Nucleus of the Amygdala and the Bed Nucleus of the Stria Terminalis Differentially Involved in Fear Versus Anxiety? New York: Annals of the New York Academy of Sciences; 1999. Whalen PJ, Rauch SL, Etcoff NL, McInerney SC, Lee MB, Jenike MA: Masked presentations of emotional facial expressions modulate amygdala activity without explicit knowledge. J Neurosci 1998, 18:411-418.