Temporal characteristics of enhancement of startle by stimulation of the amygdala.

A previous study demonstrated that electrical stimulation of the amygdala facilitated the acoustic startle reflex. The purpose of the present study was to determine the temporal relationship between activation of the amygdala and its enhancement of the acoustic startle reflex. A single 0.1 msec pulse was delivered to the central nucleus of the amygdala at various times before or after the onset of 20 msec noise burst or a 0.1 msec click. Startle was enhanced when the stimulation occurred within 5 msec of the startle stimulus onset. Electromyographic recordings from the neck muscles demonstrated that the short 6 msec latency startle response was facilitated even when amygdala stimulation was presented 1.25 msec after the onset of the startle stimulus. This indicates that the time for the effects of amygdala stimulation to reach the brain stem startle circuit is less than 5 msec, suggesting a very direct pathway from the amygdala to the startle circuit. The similarity of this effect to fear-potentiated startle is also discussed.