Startle produces early response latencies that are distinct from stimulus intensity effects

Phasic electrodermal responses associated with whole-body instability: presence and influence of expectation

While much is understood about somatic contributions to postural control, there is less consideration for the potential involvement of the autonomic nervous system (ANS) as integral for maintenance of stability. The purpose of this study was to examine autonomic responses, as measured through electrodermal recordings, evoked in response to whole-body perturbations to standing balance. We hypothesized that phasic electrodermal responses (EDRs) would be consistently observed in response to evoked perturbations and that response amplitude would depend on the capacity to predict perturbation timing. Temporally unpredictable and self-initiated (predictable) backward perturbations evoked in healthy participants (n=15) elicited compensatory feet-in-place reactions with tibialis anterior activation 125.1+/-60.2 ms following perturbation onset. EDRs were consistently observed starting 1883.6+/-329.1 ms after perturbation and reaching their peak at 4016.6+/-896.9 ms. Amplitude was significantly larger in the unpredictable task (1.1+/-0.84 micromho) compared to the predictable task (0.45+/-0.55 micromho, P<0.001). Amplitude was largest in the first block of five trials (P<0.0001), then remained constant for subsequent trials in each condition. Post-hoc analysis indicated that trials with an unplanned compensatory step (3.5%) were 137.0+/-176.6% larger than feet-in-place reactions (P=0.02). Elevated EDRs during initial trials and unanticipated reactions suggest that these measures could be used to assess the perceived 'novelty' of applied perturbations, having implications for interpreting characteristics of the evoked somatic reactions. The persistence of perturbation-evoked EDRs even after thirty trials may also highlight an important role for phasic ANS responses in compensatory postural control.

Fast visuomotor processing made faster by sound

Reaction time to a visual event can be dramatically reduced if the visual stimulus is accompanied by a startling sound. The mechanism may involve a motor programme being stored and triggered early by the sound. However, in a choice reaction task the required response is not known in advance, and so cannot be stored. In this case startling sound does not usually speed up the reaction and may even be detrimental to performance. Here we show that the reaction time of a special type of visually evoked movement can be substantially reduced by startling sound, even though the movement requires choice. The task involved stepping onto an illuminated target that sometimes moved mid-step left or right, requiring a foot trajectory adjustment. These adjustments occur at much shorter latency than conventional visuomotor reaction tasks and are thought to involve subcortical brain areas. The presence of the sound, which carried no information, shortened the already fast mean response time of 134 ms by approximately 20 ms. We attribute this to auditory-visual interaction since sound alone had no effect. Although we observed startle responses, the quickening effect was not contingent upon their presence. Given minimum motor and sensory conduction time, we estimate that the loud sound reduced the central visuomotor processing time by at least 30%.

Choice reaction times for human head rotations are shortened by startling acoustic stimuli, irrespective of stimulus direction

Auditory startle reflexes can accelerate simple voluntary reaction times (StartReact effect). To investigate the role of startle reflexes on more complex motor behaviour we formulated two questions: (1) can auditory startle reflexes shorten choice reaction times?; (2) is the StartReact effect differentially modulated when startling auditory stimuli are delivered ipsilaterally or contralaterally to an imperative 'go' signal? We instructed 16 healthy subjects to rotate their head as rapidly as possible to the left or to right in response to a guiding visual imperative stimulus (IS), in both a simple and choice reaction protocol. Startling acoustic stimuli (113 dB) were delivered simultaneously with the IS (from either the same or opposite side) to induce the StartReact effect. We recorded kinematics of head rotations and electromyographic responses. The StartReact effect was present during choice reaction tasks (56 ms onset reduction; P < 0.001). The presentation side of the startling stimulus (left/right) did not influence the effect in choice reaction tasks. We observed a directional effect in simple reaction tasks, but this probably occurred due to a flooring effect of reaction times. Onsets of EMG responses in neck muscles were not influenced by the direction of the acoustic startling stimulus. Startling acoustic stimuli decrease reaction times not only in simple but also in choice reaction time tasks, suggesting that startle reflexes can accelerate adequate human motor responses. The absence of a clear directional sensitivity of reaction times to startling acoustic stimuli suggests that the acceleration is not highly specific, but seems to provide a global preparatory effect upon which further tailored action can be undertaken more quickly.

Unilateral reaction time task is delayed during contralateral movements

Performing unlearned unimanual tasks when simultaneously carrying out another task with the contralateral hand is known to be difficult. The dual task interference theory predicts that reaction time will be delayed if the investigated task is performed in the course of ongoing contralateral movements. Ballistic movements can be performed at maximal speed in simple reaction time (SRT) experiments when subjects have adequately prepared the motor system needed for movement execution. When fully prepared, activation of subcortical motor pathways by a startling auditory stimulus (SAS) triggers the whole reaction. In this study, we have examined dual task interference with reaction time in eight healthy volunteers. They were presented with a visual imperative signal to perform unilateral SRT either in a baseline condition (control trials) or while carrying out contralateral rhythmic oscillatory movements (test trials). A SAS was introduced in 25% of the trials in both conditions. SRT was significantly delayed in the interference test trial when compared to control trials either with or without SAS (P<0.001). Control and test trials with SAS were significantly faster than those without SAS in both conditions (P<0.001). However, there were no significant differences in the percentage SRT shortening induced by SAS or in the percentage SRT delay observed in the test trials. Our results suggest that performing rhythmic oscillatory movements with one limb slows SRT in the contralateral limb and that this effect is likely related to motor preparation changes. The effect described here can be of interest for physiological studies of interlimb coordination and the mechanisms underlying the dual task interference phenomenon.