Abstract
Prepared responses can be involuntarily triggered at short latency following the presentation of a loud (114-124 dB) startling acoustic stimulus (SAS), a phenomenon known as the “StartReact” effect. However, the origin of startle stimulus may impact its effectiveness in producing a startle. The present experiment used matched intensity stimuli originating from different directions to determine if differences exist in the probability of eliciting a startle reflex, as well as its amplitude. Participants performed a simple reaction time (RT) task requiring a wrist extension movement following a visual go-signal where a SAS (115 dB) was occasionally presented 200ms prior to the go-signal through headphones, or from loudspeakers located in front and behind the participant. Results indicated that there was a significantly lower probability of eliciting startle when the SAS was presented through headphones, but no difference in the probability, expression, or RT facilitation of a startle reflex when the SAS was presented via a loudspeaker in front or behind participants. Because participants were unable to determine the origin of the SAS from when delivered through the speakers, a follow-up experiment was performed where participants were aware of the origin of the SAS. Results showed that the probability of eliciting a startle reflex, as well as its amplitude, was moderately higher when the SAS was presented from the front versus the back. These results suggest that standard acoustic intensity measurements are potentially insufficient for gauging the true magnitude of an intense stimulus reaching the auditory apparatus.
