Brainstem correlates of defensive states in humans

Background: Brainstem auditory evoked potentials (BAEP) reflect the activation of brainstem nuclei in the first milliseconds after presentation of an auditory stimulus. These electrophysiological correlates of neural processing are highly automatic and not influenced by cognitive factor or task demands; however, data from patients with anxiety disorders suggest deviations in the BAEP. It has been hypothesized that these differences reflect heightened activation of structures involved in defensive states, such as the amygdala and locus coeruleus, projecting to the inferior colliculus, one of the brainstem generators of wave V of the BAEP. The present study investigated ibis possibility by testing BAEP during experimentally induced anxiety in healthy volunteers. Methods: In this study, BAEP were recorded from healthy normal volunteers under threat of shock, compared with safe conditions. Results: The first experiment (n = 12) showed that shock anticipation increased the amplitude of wave V A replication experiment (n = 13) confirmed this finding. Conclusions: Although BAEP are highly robust with respect to attentional manipulations, they are affected by transient activation of the fear system due to threat of shock. This finding indicates that some of the electrophysiological brainstem abnormalities observed in anxiety disorders can be replicated in healthy control subjects by inducing a transient state of anxiety.