The StartReact effect on self-initiated movements

Abstract

Preparation of the motor system for movement execution involves an increase in excitability of motor pathways. In a reaction time task paradigm, a startling auditory stimulus (SAS) delivered together with the imperative signal (IS) shortens reaction time significantly. In self-generated tasks we considered that an appropriately timed SAS would have similar effects. Eight subjects performed a ballistic wrist extension in two blocks: reaction, in which they responded to a visual IS, and action, in which they moved when they wished within a predetermined time window. In 20-25% of the trials, a SAS was applied. We recorded electromyographic activity of wrist extension and wrist movement kinematic variables. No effects of SAS were observed in action trials when movement was performed before or long after SAS application. However, a cluster of action trials was observed within 200 ms after SAS. These trials showed larger EMG bursts, shorter movement time, shorter time to peak velocity, and higher peak velocity than other action trials (P < 0.001 for all), with no difference from Reaction trials containing SAS. The results show that SAS influences the execution of self-generated human actions as it does with preprogrammed reaction time tasks during the assumed building up of preparatory activity before execution of the willed motor action.

Figure 1

Representative trials from a single subject. RT: trial obtained in response to an imperative signal (IS). RS: trial obtained in response to an imperative signal, delivered together with a startling auditory stimulus (SAS). AT: trial obtained in a self-initiated action performed within the required time window. AS: trial obtained in a self-initiated action performed within the required time window short time after a SAS. OOc: EMG activity recorded from the orbicularis oculi muscle. EMG_F: EMG from wrist flexor muscles. EMG_E: EMG from wrist extensor muscles. Wrist extension is shown as an upwards change in the angle and velocity traces.

Figure 2

Histogram of the temporal distribution of EMG latency for AT trials in bins of 50 ms along the epoch of interest.

Figure 3

Upper part: histogram of the temporal distribution of all EMG latencies for AS trials in bins of 50 ms normalized with respect to the time of startling auditory stimulus (SAS) application, at time 0. Lower part: representative trials from a single subject. ASb: trial before startle. ASc: trial close after SAS. ASl: trial late after startle. OOc: EMG activity recorded from the orbicularis oculi muscle. EMG_F: EMG from wrist flexors. EMG_E: EMG from wrist extensors. Wrist extension is shown as an upwards change in the angle and velocity traces.

Figure 4

Distribution of all EMG latencies for ASc trials along the whole epoch in which the SAS was applied. Temporal relationship between time of SAS application across the time window and latency of Action close to SAS (ASc) trials from SAS.