AMYGDALA EFFERENTS MEDIATING ELECTRICALLY-EVOKED STARTLE-LIKE RESPONSES AND FEAR POTENTIATION OF ACOUSTIC STARTLE

One-pulse stimulation of the ventral amygdalofugal pathway (VAF), vent rolateral midbrain, or medial medulla evoked startle-like responses. R efractory periods tested by 2-pulse stimulation ranged from 0.2 to 0.5 ms in both midbrain and medulla sites and from 0.4 to 0.8 ms in VAF s ites. Symmetric collision effects between midbrain and medulla sites s uggest that fast-conducting axons between the sites mediate the respon se. Asymmetric collision effects between VAF and midbrain suggest that strong synapses between these sites mediate the response, with a tran smission time of 1 ms. No collision was observed between contralateral sites. Bilateral lesions of midbrain sites blocked VAF responses and fear-conditioned startle, but failed to block acoustic startle, and pa rtially blocked medulla responses. A new neural model of fear-potentia ted startle is proposed.