Startle activation is additive with voluntary cortical activation irrespective of stimulus modality


When a startling acoustic stimulus (SAS: >120dB) is presented during a simple reaction time (RT) task it has been shown to trigger the prepared movement through an involuntary initiation pathway. To investigate the time course of initiation related activity, a previous study (Maslovat et al. 2014) presented a SAS at various times following the go-signal (i.e., during the RT interval), with RT results suggesting that the activations related to both the voluntary and startle-related initiation processes were additive. In the current study the predictions of an additive neural activation model were tested by replicating the methods of Maslovat et al., but changing the modality of the go-signal. As voluntary RT latencies are delayed for visual stimuli compared to acoustic stimuli it was hypothesised that the time course of additive activation would be similarly delayed. Participants performed 150 RT trials requiring a targeted 20° wrist extension in response to a visual go-signal. In 20% of trials, a SAS was randomly presented 0, 25, 50, 75, 100, or 125 ms following the go-signal. As predicted, RT to the visual go-signal (186 ms) was increased compared to the previous study (127 ms; Maslovat et al.). Furthermore, results showed that RTs for the 25 to 125 ms SAS delays (110, 132, 146, 158, 172 ms, respectively) were very similar to RT values predicted by an additive initiation model (112, 130, 145, 161, 175 ms). Together the results support an additive neural model of startle and voluntary initiation-related activation, even when the go-signal is of a different modality than the SAS. In addition, these results indicate that RT differences due to stimulus modality are attributable to processes occurring prior to the increase in initiation-related activation.

Acknowledgments: Research supported by NSERC