Prepulse inhibition (PPI) is a neurological phenomenon in which a weaker prestimulus (prepulse) inhibits the reaction of an organism to a subsequent strong startling stimulus (pulse). The stimuli are usually acoustic, but tactile stimuli (e.g. via air puffs onto the skin) and light stimuli are also used. When prepulse inhibition is high, the corresponding one-time startle response is reduced. Prepulse inhibition (PPI) is a neurological phenomenon in which a weaker prestimulus (prepulse) inhibits the reaction of an organism to a subsequent strong startling stimulus (pulse). The stimuli are usually acoustic, but tactile stimuli (e.g. via air puffs onto the skin) and light stimuli are also used. When prepulse inhibition is high, the corresponding one-time startle response is reduced.