The monotonic dependency of prepulse inhibition of the acoustic startle reflex on the intensity of the startle-eliciting stimulus

Prepulse inhibition (PPI) of the acoustic startle reflex is a translational behavioural paradigm for the assessment of sensorimotor gating deficit which has been demonstrated in a number of neuropsychiatric conditions. PPI refers to the reduction of the reflexive startle response to a 'pulse' stimulus when its presentation is shortly preceded by a weak 'prepulse' stimulus. We have recently examined the expression of PPI as a function of the startle-eliciting 'pulse' stimulus intensity in mice and in humans. One major discrepancy that emerged was the finding that healthy human subjects, unlike normal mice, did not show a clear monotonic reduction of PPI magnitude (as indexed by % reduction in startle reactivity) with increasingly intense pulse stimulus. This lack of correspondence between species may potentially weaken the translational power of the PPI paradigm. Here, we re-examined this issue in 31 healthy subjects across three levels of pulse stimulus intensity (95, 105 and 115 dB). A clear linear reduction of PPI as a function of pulse intensity was revealed when subjects failing to respond to the lowest pulse stimulus were excluded. Inclusion of such non-responders, on the other hand, resulted in a trend towards an inverted U-shape function as reported previously. The present study thus clarifies an apparent divergence between mouse and man, and provides important qualification to the "First Law of Reflex Modification" proposed by Hoffman and Ison which suggests that the absolute reduction in startle reactivity resulting from a prepulse stimulus preceding the startle-eliciting pulse stimulus is fixed by the prepulse intensity regardless of the pulse stimulus intensity.