The human startle reflex and pons activation: a regional cerebral blood flow study

Using positron emission tomographic measurements of regional cerebral blood flow, we report activation of a medial pons area in humans during acoustic startle stimulation. Eight healthy volunteers were scanned during rest and when presented startle-eliciting stimuli. We performed a theory-driven directed search for activity in the nucleus reticularis pontis caudalis, situated in the pons. Because habituation of cerebellar activity during acoustic startle repetition has been reported [Timmann, D., Musso, C., Kolb, F.P., Rijntjes, M., Juptner, M., Muller, S.P., Diener, H.C. & Weiller, C. (1998) J. Neurol. Neurosurg. Psychiatry 65, 771-773], we also predicted habituation in the cerebellum and in the pons as a function of startle repetition. Measurements of eye electromyography validated the presence of a startle response and its habituation. Analysis of regional cerebral blood flow revealed higher neural activity during startle stimulation than at rest in a medial pons area consistent with the location of the pontine reticular nucleus. As a consequence of startle repetition, regional cerebral blood flow increased in the medial cerebellum, and habituated in the ventral cerebellum and in a ventral pons area separate from the pontine reticular nucleus. In the ventral pons, but not in the pontine reticular nucleus, regional cerebral blood flow and the startle reflex were positively correlated. In the cerebellum both positive and negative correlations with the startle reflex were observed. Thus we conclude that the neurofunctional correlates of the startle circuit and its habituation in humans are similar to that previously described in animals.