Recent experiments involving the use of a startling acoustic stimulus during a simple reaction time (RT) task have shown that premotor RT (PMT) can be significantly reduced when participants are startled (Valls-Sole et al., 1999;​ Carlsen et al., in press). This effect is not produced by an early startle reflex adding on to a later voluntary response, as EMG profiles remain largely unmodified. Further, the lack of an effect of the startle on final position and aiming accuracy suggests that the response produced is indeed the prepared response. These findings suggest that a startle stimulus may act to trigger a prepared movement earlier in comparison to voluntary initiation (Carlsen et al., 2000).

It has been shown that individuals habituate to a startling stimulus at different rates depending on the required activity level of the participant in the task (Brown et al., 1991a;​ Siegmund et al., 2001). The aim of the first study of this thesis was to determine the rate at which participants habituate to a startle during the completion of a RT task. Participants performed a targeted wrist extension in a Simple RT task. An auditory startle stimulus (124 dB) replaced the imperative stimulus in some of the trials. For the duration of the experiment, startle response electromyographic (EMG) activity continued to be produced, but not on every startle trial, indicting habituation was not complete after 20 startle trials. PMT in the startle (ST) condition was significantly shorter than control PMT however, within the ST condition, when a measurable EMG burst in the SCM was present, PMT was significantly shorter than when no SCM burst was present.

It has been suggested that a startling stimulus activates structures in the lower CNS that are common to both the startle and voluntary response pathways, acting to trigger a preprogrammed movement (Valls-Sole et al. 1999). In a Choice RT paradigm, however, it is thought that cortical processing must occur before a response can be prepared, since the appropriate response is not known in advance (Schluter et al., 1998 Schluter et al., 2001). The second experiment of the thesis addressed the issue of response preparation through the use of a Choice RT paradigm. Participants performed targeted wrist flexion / extension movements involving 1, 2, or 4 Stimulus-response (S-R) alternatives. Results showed that while in the Simple RT situation PMT was significantly shorter when participants were startled, that no difference in PMT was observed when participants were startled in the Choice RT situations. Furthermore, more errors occurred when participants were startled during the Choice RT conditions suggesting that the startle may actually interfere with ongoing cortical processes. These results support the hypothesis that a startle acts to trigger a prepared response, rather than only increasing systemic activation.