A broadband acoustic stimulus is more likely than a pure tone to elicit a startle reflex and prepared movements

Abstract

A loud acoustic stimulus that elicits a startle reflex has long been used to study the neurophysiology of cortical and subcortical neural circuits. More recent investigations have shown that startle can act as an early trigger for prepared actions, suggesting a brainstem role in the preparation and initiation of actions. However, in order to attribute any startle-triggered voluntary responses to activation in subcortical structures it is necessary to measure a startle-related activity in these structures. The current study investigated the most effective stimulus for eliciting a detectible startle reflex. While more intense stimuli are more likely to elicit a startle reflex, the current study examined whether broadband noise is more likely than a pure tone to produce a startle at various intensities above 100 dB. Participants performed a button release reaction time task in response to either a 1 kHz tone or a broadband noise pulse with intensities ranging from 82 to 124 dB. Reaction time and EMG from the wrist extensors and the sternocleidomastoid (SCM) were measured. Results showed that startle-related SCM EMG was elicited more frequently by broadband noise compared to pure tones. The higher proportion of startle reflexes observed in SCM was associated with a higher incidence of the voluntary task being triggered early. A higher incidence of startle following broadband noise is attributed to the activation of a larger proportion of the basilar membrane; thus, a lower intensity broadband noise stimulus may be used to elicit startle reflex at a similar rate as a higher intensity pure tone.

Keywords: Intensity; reaction time; reticular formation; startle; stimulus frequency.

Figure 1

Probability of observing a startle response in sternocleidomastoid (SCM). Mean (SE) probability is shown across stimulus intensities for 1 kHz tone (gray) and broadband noise (black). *indicates significant difference between stimulus types at each intensity (P < 0.05).

Figure 2

Premotor reaction time (RT) across intensities and stimulus types. Mean (SE) premotor RT is shown for 1 kHz tone (gray) and broadband noise (black), and whether or not a SCM startle response was (SCM+: solid lines) or was not (SCM−: dashed lines) observed, as a function of stimulus type and intensity. Note that each mean is made up of a different number of trials (see Results section for details), thus for intensities above control (82 dB) the size of the markers at each intensity represents the proportion of trials that makes up that mean. *indicates a significant (P < 0.05) difference between factors.