Response triggering by an acoustic stimulus increases with stimulus intensity and is best predicted by startle reflex activation

Abstract

In a simple reaction time task, the presentation of a startling acoustic stimulus has been shown to trigger the prepared response at short latency, known as the StartReact effect. However, it is unclear under what conditions it can be assumed that the loud stimulus results in response triggering. The purpose of the present study was to examine how auditory stimulus intensity and preparation level affect the probability of involuntary response triggering and the incidence of activation in the startle reflex indicator of sternocleidomastoid (SCM). In two reaction time experiments, participants were presented with an irrelevant auditory stimulus of varying intensities at various time points prior to the visual go-signal. Responses were independently categorized as responding to either the auditory or visual stimulus and those with or without SCM activation (i.e., SCM+/−). Both the incidence of response triggering and proportion of SCM+ trials increased with stimulus intensity and presentation closer to the go-signal. Data also showed that participants reacted to the auditory stimulus at a much higher rate on trials where the auditory stimulus elicited SCM activity versus those that did not, and a logistic regression analysis confirmed that SCM activation is a reliable predictor of response triggering for all conditions.

Altered frontoparietal activity in acoustic startle priming tasks during reticulospinal tract facilitation: An fNIRS study

“…Since the measurement channel for the M1 had low specificity in this study, further confirmation is still needed. Furthermore, as suggested by other researchers ( Carlsen et al, 2003 ; Carlsen and Maslovat, 2019 ; Maslovat et al, 2021 ), using SCM + as a marker of successful induction of the SE should be considered in future studies.…”

“…This phenomenon is called the StartleReact effect (SE) and is recognized as the result of the rapid transmission of motion commands mainly via the RST ( Carlsen and Maslovat, 2019 ). Despite not always occurring simultaneously ( Leow et al, 2018 ), early activation of the sternocleidomastoid muscle (SCM) due to the startle reflex is strongly correlated with early initiation of movement in this paradigm ( Maslovat et al, 2021 ). The SCM activation latency in the time window of 30 to 130 ms provides a convenient marker of the SE ( Carlsen et al, 2011 ; van Lith et al, 2018 ).…”

“…The SCM activation latency in the time window of 30 to 130 ms provides a convenient marker of the SE ( Carlsen et al, 2011 ; van Lith et al, 2018 ). Therefore, analyses that simultaneously incorporate SCM response time and limb muscle activity enable better dissection of RST function in motor control ( Maslovat et al, 2021 ). With this approach, researchers have revealed increased motor unit discharge ( Skarabot et al, 2022 ), additional muscle contraction ( Fernandez-Del-Olmo et al, 2014 ), a greater range of motion during motor initiation ( McInnes et al, 2020 ), and even better motor ( Rahimi and Honeycutt, 2020 ) or speech output ( Swann et al, 2022 ) in stroke survivors.…”

Startle-triggered responses indicate reticulospinal drive is larger for voluntary shoulder versus finger movements

“…Indeed, when a loud acoustic stimulus also elicits a startle reflex (i.e. SCM +), which is an indication of activation in the reticular formation, RT reduction and the incidence of response triggering are increased relative to when no startle reflex is elicited (SCM −) 24 – 26 . While this does not mean that SCM − trials are devoid of reticulospinal activation, these previous results clearly show that the presence of SCM activation leads to a more robust StartReact effect that is unbiased by stimulus intensity.…”

Response preparation of a secondary reaction time task is influenced by movement phase within a continuous visuomotor tracking task

“…Furthermore, only 20% (4/20) of participants exhibited StartReact responses in over 40% of SAS trials. While it has been reported that, on occasion, some SAS trials are triggered at a short latency without an observed startle response (SCM−), this misclassification rarely happens with a SAS of 120 dB and/or in prepared conditions (e.g., simple RT tasks) (Maslovat et al, 2021). These results suggest that in general, participants were unable to consistently prepare the probe RT task to a sufficiently high level required for a StartReact effect to be elicited.…”