Startle decreases reaction time to active inhibition

In reaction time (RT) tasks where fast ballistic movements are required, the requisite action is generally preplanned to enable the quickest responses. When a loud acoustic stimulus (e. g., >120 dB) that elicits a startle response is presented during the preplanning phase, the movement is triggered involuntarily and at a sufficiently short enough latency to discount normal cortical initiation processes. It has been suggested that the startle triggers the action by providing sufficient additional activation to surpass the initiation threshold. It is unclear, however, whether similar RT shortening due to startle would occur in the absence of an excitatory motor output. Thus, in the current study, participants performed a flexion force offset (i.e., inhibition) task within a simple RT paradigm. A startling acoustic stimulus (SAS) was presented in place of the usual "go" signal on several trials. Results from startle trials showed that the inhibitory command could be elicited substantially earlier by an SAS (latency of similar to 78 ms) compared to control trials (120 ms). This suggests active inhibition is preprogrammed and can be triggered early by startle in similar way to traditional "excitatory" tasks. Additionally, early startle-related EMG activity superimposed with the triggered offset suggests that the nature of the inhibitory command used in the current experiment involves the active suppression of ongoing motor output.