Prepulse modulation of the startle reaction and the blink reflex in normalhuman subjects

Blink reflexes are usually considered the most representative and consisten t response of the auditory startle reaction (ASR), and they are often the o nly response evaluated in human psychophysiological studies. However, audit ory stimuli also induce an auditory blink reflex (ABR), the physiological c haracteristics and brainstem circuitry of which may be different from those of the ASR. This study aimed to investigate whether there were differences between the orbicularis oculi (OOc) responses elicited with the ABR (OOcAB R) and those elicited with the ASR (OOcASR) regarding their behavior to pre pulse modulation. For comparison, we also examined the OOc responses to sup raorbital nerve stimulation (OOcEBR). Electromyographic responses were simu ltaneously recorded from the OOc, masseter (MAS) and sternocleidomastoid (S CM) muscles. ABRs were considered when auditory stimuli induced responses l imited to the OOc, and ASRs were considered when responses were induced in all muscles recorded from. Prepulse stimuli were either a weak electrical s timulation at the third finger (somatosensory prepulse) or a weak acoustic tone (auditory prepulse) that preceded the response-eliciting stimuli by in tervals ranging from 0 to 200 ms. Prepulse effects differed according to pr epulse modality, but the OOcABR and the OOcASR were always modulated in the same way. In both responses, somatosensory prepulses induced facilitation from 20 to 50 ms, followed by inhibition beyond 75 ms, and auditory prepuls es induced no facilitation but a significant inhibition beyond 30 ms. In th e OOcEBR, both somatosensory and acoustic prepulses induced facilitation of R1 and inhibition of R2 beyond 30 ms. Our results suggest that the OOcABR and the OOcASR exhibit the same physiological behavior regarding prepulse m odulation. It is hypothesized that prepulse facilitation is due to direct i mpingement of subthreshold excitatory inputs onto the facial motoneurons wh ile prepulse inhibition results from the engagement of a presynaptic inhibi tory circuit in the brainstem.