The main measures of fear that we have used are reflexive responses, such as freezing behavior or autonomic changes, that are rapidly and automatically elicited upon encountering a danger signal. It is now clear that the same brain system underlies fear learning in rats and humans. The detailed mechanisms of fear, which have been uncovered through animal studies, can thus be applied to understanding fear processing in the human brain. With the neural system underlying fear learning now understood in considerable detail, we are beginning to explore the cellular mechanisms in these circuits that underlie fear learning. This is being pursued through studies of multichannel single unit recordings in awake behaving animals, intracellular recordings in anesthetized animals, and whole cell recordings in brain slices.
Conceptual issues being explored include the following.
Functional inactivation of the lateral and basal nuclei of the amygdala by muscimol infusion prevents fear conditioning to an explicit CS and to contextual stimuli. NMDA and AMPA receptors are post-synaptic to auditory thalamic afferents. Fear conditioning enhances different temporal components of tone-evoked spike trains in auditory cortex and lateral amygdala. Fear conditioning induces associative long-term potentiation in the amygdala. A mixed-trial fMRI study.