FIGURE 2.

Model of amygdala circuitry during fear conditioning and extinction. The model contains both known and hypothetical neural connections. Solid and dashed lines indicate strengthened and weakened connections, respectively, during the learning processes. (A) During fear conditioning, CEm output activity is enhanced by two intra-amygdala pathways. First, there are enhanced excitatory signals that originate from LA. In LA, there is disinhibition of local inhibitory circuits due to the effects of several biogenic amines. This disinhibition leads to increased neural activity in BA, ITCd, and CEl. Fear neurons in BA may receive direct excitatory inputs from LA. Augmented activity of fear neurons, mediated through reciprocal connections with PL regions, is necessary to activate CEm output neurons. Second, strong inhibition of CEloff neurons and signaling from ITCv to CEm output neurons are reduced by the LA–CElon–CEloff–CEm and LA–ITCd–ITCv–CEm pathways, respectively. (B) During fear extinction, enhanced neural activity in LA is reduced by increased suppression from local inhibitory circuits and depotentiation of glutamatergic synapses. Decreased connectivity between extinction-resistant neurons in LA and fear neurons in BA could also contribute to fear extinction. Furthermore, IL cortex sends glutamatergic projections to ITCv and possibly extinction neurons of BA. These IL projections may participate in suppression of BA fear neurons via local inhibitory circuits.