Prepulse inhibition (PPI) of the startle reflex is defined
as the attenuation of the startle response to a startling
stimulus (pulse), when such a stimulus is briefly preceded
by a stimulus of subthreshold intensity (prepulse). PPI
is thought to be neither learned nor due to conscious response
inhibition, as it occurs at stimulus onset asynchronies
(SOAs) too short to enable the activation of a volitional
response. The present study explored the latter of these
assertions by investigating (a) the degree to which human
subjects are able to detect prepulses at SOAs of 30, 60
and 120 ms, and (b) whether such detection is related to
inhibition. Startle eyeblink reflex and detection were
measured in 39 participants subjected to an acoustic startle
paradigm. Results revealed a significant trend in prepulse
detection according to SOA, with highest detection rates
at the 120-ms SOA (75%). However, trials on which detection
occurred did not differ from trials without detection on
measures of startle inhibition. This suggests that PPI
is independent of awareness of the prepulse.