Preparedness for landing after a self-initiated fall.

A startling auditory stimulus (SAS) causes a faster execution of voluntary actions when applied together with the imperative signal in reaction time tasks (the StartReact effect). However, speeding up reaction time may not be the best strategy in all tasks. After a self-initiated fall, the program for landing has to be time-locked to foot contact to avoid damage, and therefore advanced execution of the program would not be convenient. We examined the effects of SAS on the landing motor program in 8 healthy subjects that were requested to let themselves fall from platforms either 50 or 80 cm high at the perception of a visual imperative signal and land on specific targets. In trials at rand…

Speeding up gait initiation and gait-pattern with a startling stimulus.

Human gait involves a repetitive leg motor pattern that emerges after gait initiation. While the automatic maintenance of the gait-pattern may be under the control of subcortical motor centres, gait initiation requires the voluntary launching of a different motor program. In this study, we sought to examine how the two motor programmes respond to an experimental manipulation of the timing of gait initiation. Subjects were instructed to start walking as soon as possible at the perception of an imperative signal (IS) that, in some interspersed trials was accompanied by a startling auditory stimulus (SAS). This method is known to shorten the latency for execution of the motor task under prepar…

Matter of mind

The effects of a startle on awareness of action

The execution of a ballistic movement within a reaction time task paradigm is significantly speeded up when an unexpected startling auditory stimulus (SAS) is delivered together with the imperative signal. Using Libet's clock, we investigated whether acceleration involves also the subjective appraisal of the time of task execution. In trials containing the SAS, reaction time shortened to 68.7% of control values. However, subjective judgment of task execution remained a similar time with respect to the imperative signal as in control trials. The dissociation between task execution and its subjective perception indicates the existence of separate circuits for action execution and action aware…

Chapter 9 The effects of a prepulse on the StartReact phenomenon

Publisher Summary This chapter presents a study to investigate whether the inhibitory effects of the prepulse are different in the condition of motor preparation with respect to that of unexpectancy and whether the inhibition of the startle reaction by a prepulse is associated with the inhibition of the StartReact phenomenon. The chapter demonstrates that the procedure could help in ascertaining whether reflex and volitional components are actually combined in the StartReact phenomenon. Eight healthy volunteers, six men and two women, aged between 25 and 52 years were investigated and the startle reaction was recorded with surface electrodes over the right orbicularis oculi muscle and, in t…

Excitability of subcortical motor circuits in Go/noGo and forced choice reaction time tasks

The size of the response to a startling auditory stimulus (SAS) may reflect the excitability of the reticulospinal tract. In this study, we examined whether there was any excitability change in the reticulospinal tract during preparation for execution of two types of choice reaction time task: a forced choice reaction time task (fCRT) and a Go/no-Go task (GnG). In 13 healthy volunteers we used three types of trials: control trials in which subjects were requested to perform ballistic wrist movements during fCRT or GnG tasks; test trials in which a SAS was presented with the visual cue, and baseline trials in which SAS was presented alone. Latency and area of the responses to SAS were measur…

Bipedal distribution of human vestibular‐evoked postural responses during asymmetrical standing

Galvanic vestibular stimulation (GVS) evokes responses in muscles of both legs when bilateral stimuli are applied during normal stance. We have used this technique to assess whether asymmetrical standing alters the distribution of responses in the two legs. Subjects stood either asymmetrically with 75% of their body weight on one leg or symmetrically with each leg taking 50% of their body weight. The net response in each leg was taken from changes in ground reaction force measured from separate force plates under each foot. The net force profile consisted of a small initial force change that peaked at approximately 200 ms followed by an oppositely directed larger component that peaked at ap…