The effects of an auditory startle on obstacle avoidance during walking

Abstract

Movement execution is speeded up when a startle auditory stimulus is applied with an imperative signal in a simple reaction time task experiment, a phenomenon described as StartReact. The effect has been recently observed in a step adjustment task requiring fast selection of specific movements in a choice reaction time task. Therefore, we hypothesized that inducing a StartReact effect may be beneficial in obstacle avoidance under time pressure, when subjects have to perform fast gait adjustments. Twelve healthy young adults walked on a treadmill and obstacles were released in specific moments of the step cycle. On average the EMG onset latency in the biceps femoris shortened by 20% while amplitude increased by 50%, in trials in which an auditory startle accompanied obstacle avoidance. The presentation of a startle increased the probability of using a long step strategy, enlarged stride length modifications and resulted in higher success rates, to avoid the obstacle. We also examined the effects of the startle in a condition in which the obstacle was not present in comparison to a condition in which the obstacle was visibly present but it did not fall. In the latter condition, the obstacle avoidance reaction occurred with a similar latency but smaller amplitude as in trials in which the obstacle was actually released. Our results suggest that the motor programmes used for obstacle avoidance are probably stored at subcortical structures. The release of these motor programmes by a startling auditory stimulus may combine intersensory facilitation and the StartReact effect.

Figure 1. Schematic diagram of the experimental setup

The electromagnet is attached to a bridge over the front of the treadmill. The obstacle falls onto the treadmill in front of the subject's left foot after the electromagnet has been switched off by a trigger from the computer. The three obstacle release phases were late stance (LSt), early swing (ESw) and mid-swing (MSw).

Figure 2. Examples of EMG responses for obstacle avoidance

EMG activity of biceps femoris, rectus femoris, tibialis anterior and gastrocnemius medialis in response to an obstacle release at mid-swing. Representative trials from one subject. A, no startle trial. B, startle trial. The vertical dotted line indicates the obstacle release moment. The vertical continuous line shows when the startle was given. The shaded area represents mean and ±.2 s.d.s of EMG activity of the control stride. Superimposed (continuous line) is the trace of the representative trial. The obstacle was released at 72.8% of the step cycle in A and at 71.1% of the step cycle in B, which accounts for the slight delay of the control stride in B with respect to A (difference of 20 ms).

Figure 3. EMG effects of startling auditory stimulus on obstacle avoidance

Mean values and standard deviation of onset latencies (A) and amplitudes (B) of EMG activity in biceps femoris, rectus femoris, tibialis anterior and gastrocnemius medialis muscles in response to an obstacle for no startle and startle trials. Obstacle release phases were late stance (LSt), early swing (ESw) and mid-swing (MSw). *P < 0.05 between startle and no startle conditions.

Figure 4. Stride modification effects of startling auditory stimulus on obstacle avoidance

Mean percentages and standard deviation of stride shortening or lengthening for no startle and startle trials. Obstacle release phases were late stance (LSt), early swing (ESw) and mid-swing (MSw). *P < 0.05. Note: the number of trials in each condition was different (see Table 1) since there were few late stance trials in which subjects performed a short step strategy (or early swing and mid-swing trials in which subjects performed a long step strategy). This may partly explain why significance was only obtained for early swing and mid-swing in stride shortening.

Figure 5. Responses to startling auditory stimulus in obstacle conditions

A, averaged EMG data of all subjects for biceps femoris in three different startle conditions (no obstacle, stationary obstacle and obstacle avoidance). Obstacle avoidance condition without startle (grey trace) is added for comparison. Open circles in the traces indicate mean onset latency of each condition. The vertical dotted line indicates the obstacle release moment. The vertical continuous line shows when the startle was given. The time window used to compare the amplitudes of the various conditions is indicated by a box. B, mean percentages and standard deviation of stride shortening or lengthening for startle conditions.