Precues enable multiple response preprogramming: Evidence from startle

When one or more response dimensions in a choice reaction time (RT) task are provided beforehand (or precued), RT decreases, indicating that the precued part of a response was prepared in advance. In this study, a startling acoustic stimulus was used to investigate the amount of preprogramming that occurs when part of the response is precued because a startling stimulus has been shown to directly trigger preprogrammed responses. Participants performed wrist flexion/extension movements in a precued one to four choice RT paradigm. A control auditory stimulus (82 dB) or a startling acoustic stimulus (124 dB) accompanied the visual "go" signal on each trial. Although RT latencies were significantly reduced by the startle, many more errors were observed in the startled conditions. Importantly, the errors seen at short RT latencies largely reflected multiple movements to the cued response alternatives, suggesting that multiple responses were preprogrammed.     

Interaction between startle and voluntary reactions in humans

The startle reaction (SR) is usually understood as an involuntary reaction to an unexpected sensory input. However, there is evidence that the mechanisms involved in the SR contribute also to the execution of actions that we consider voluntary. We need to exert a fine control of the motor output, which usually involves inhibition of the reflex reaction but it may also imply letting the SR manifest, shaped in accordance with our willed actions. The latter occurs mainly with relatively simple ballistic movements but it has also been shown to occur in complex movements. In this review, we describe arguments published in the literature in favour of the contribution of subcortical motor tracts involved in the generation of the SR to the execution of voluntary movements. Some of these studies have brought insight on the mechanisms accounting for motor preparation and execution of voluntary movements.     

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

Recent experiments pairing a startling stimulus with a simple reaction time (RT) task have shown that when participants are startled, a prepared movement was initiated earlier in comparison to voluntary initiation. It has been argued that the startle acts to trigger the response involuntarily. However, an alternative explanation is that the decrease in RT may be due to stimulus intensity effects, not involuntary triggering. Thus the aim of the current investigation was to determine if RT simply declined in a linear fashion with increasing stimulus intensity, or if there was a point at which RT dramatically decreased. In the present experiment participants completed 50 active wrist extension trials to a target in response to an auditory stimulus of varying stimulus intensity (83–123 dB). The presented data show that RTs associated with a startle response are separate from stimulus intensity facilitated responses. Furthermore, this startle facilitation is more highly associated with sternocleidomastoid electromyographic (EMG) activity, rather than the EMG from the widely used startle response indicator muscle orbicularis oculi.     

Reaction time facilitation by acoustic task-irrelevant stimuli is not related to startle

Previous research has been interpreted to suggest that the startle reflex mediates the RT facilitation observed if intense, accessory acoustic stimuli are presented coinciding with the onset of a visual imperative stimulus in a forewarned simple RT task. The present research replicated this finding as well as the facilitation of startle observed during the imperative stimulus. It failed, however, to find any relationship between the size of the blink startle reflex elicited by the accessory acoustic stimuli, which differed in intensity and rise time, and RT or RT facilitation observed on trials with accessory acoustic stimuli. This finding suggests that the RT facilitation is not mediated by the startle reflex elicited by the accessory acoustic stimuli.     

Temporal stability of the emotion-modulated startle response

In the present study, we examined the stability of one measure of emotion, the emotion-modulated acoustic startle response, in an undergraduate sample. Using the acoustic startle paradigm on two different occasions, we measured stability of affective modulation of the startle response during and following the presentation of pictures selected to be of positive, negative, or neutral emotional valence. The two assessments were separated by 4 weeks. Two groups of subjects were compared: one group that viewed the same pictures at each assessment and a second group that viewed different pictures at the second assessment. We found that viewing different pictures at two assessments separated by 4 weeks yielded moderate stability of the emotion modulation of startle magnitude, whereas subjects who viewed the same pictures at both assessments showed poor stability. Furthermore, this difference was due to the stability of responses to high versus low arousal pictures, not to differences in valence.     

Motor planning: effect of directional uncertainty with continuous spatial cues

We have investigated the effect of directional uncertainty on the planning of reaching movements. For this purpose, we have used sections of annuli as spatial cues to indicate the directional range within which the target would be presented. The results showed that the reaction time of the reaching response increased with cue range and with the angle between the center of the cue and the target. In addition, the initial direction of movement was biased toward the center of the cue. These results conformed to the predictions of the capacity-sharing model. This model assumes that the processing resources used for motor planning are limited and distributed as a function of the range of directions indicated by the cue, and that when the target appears, these resources are reallocated to represent the response to be executed.     

A startle speeds up the execution of externally guided saccades

The control of eye movements depends in part on subcortical motor centres. Gaze is often directed towards salient visual stimuli of our environment with no conscious voluntary commands. To further understand to what extent preprogrammed eye movements can be triggered subcortically, we carried out a study in normal volunteers to examine the effects of a startling auditory stimulus (SAS) on externally guided saccades. A peripheral visual cue was presented in the horizontal plane at a site distant 15° from the fixation point, and subjects were instructed to make a saccade to it. SAS was presented together with the peripheral visual cue in 20% of trials. To force rapid visual fixation at the end of the saccade, targets were loaded with a second cue, a small arrow pointing towards the right or the left (or a neutral sign), not distinguishable with peripheral vision. Subjects were requested to perform a flexion/extension wrist movement, according to the direction of the arrow (or not to move if the second cue was the neutral sign). SAS presented together with the visual target caused a significant shortening of the latency of saccadic movements. The wrist movements performed as a response to the second cue had similar reaction times regardless of whether the trial contained a SAS or not. Our results show that voluntary saccades to peripheral targets are speeded up by activation of the startle circuit, and that this effect does not cause a significant disturbance in the execution of simple in-target cues. These results suggest that subcortical structures play a main role in preparation of externally guided saccades.     

A startling absence of emotion effects: Active attention to the startle probe as a motor task cue appears to eliminate modulation of the startle reflex by valence and arousal

Research has shown that during emotional imagery, valence and arousal each modulate the startle reflex. Here, two imagery-startle experiments required participants to attend to the startle probe as a simple reaction time cue. In Experiment 1, four emotional conditions differing in valence and arousal were examined. Experiment 2, to accentuate potential valence effects, included two negative high arousal, a positive high arousal and a negative low arousal condition. Imagery effectively manipulated emotional valence and arousal, as indicated by heart rate and subjective ratings. Compared to baseline, imagery facilitated startle responses. However, valence and arousal failed to significantly affect startle magnitude in both experiments and startle latency in Experiment 1. Results suggest that emotional startle modulation is eclipsed when the probe is significant for task completion and/or cues a motor response. Findings suggest that an active, rather than defensive, response set may interfere with affective startle modulation, warranting further investigation.     

The effect of warning stimulus modality on blink startle modification in reaction time tasks

The present study investigated the effects of lead stimulus modality on modification of the acoustic startle reflex during three reaction time tasks. In Experiment 1, participants (N = 48) were required to press a button at the offset of one stimulus (task relevant) and to ignore presentations of a second (task irrelevant). Two tones that differed in pitch or two lights served as signal stimuli. Blink startle was elicited during some of the stimuli and during interstimulus intervals. Skin conductance responses were larger during task-relevant stimuli in both groups. Larger blink facilitation during task-relevant stimuli was found only in the group presented with auditory stimuli, whereas larger blink latency shortening during task-relevant stimuli was found in both groups. Experiment 2 (N = 32) used only a task-relevant stimulus. Blink magnitude facilitation was significant only in the group presented with tones, whereas blink latency shortening was significant in both groups. Experiment 3 (N = 80) used a go/nogo task that required participants to press a button if one element of a compound stimulus ended before the second, but not if the asynchrony was reversed. The offset asynchrony was varied between groups as a manipulation of task difficulty. Startle magnitude facilitation was larger during acoustic than during visual stimuli and larger in the easy condition. The present data indicate that startle facilitation in a reaction time task is affected by stimulus modality and by task demands. The effects of the task demands seem to be independent of lead stimulus modality.     

Reaction time latencies of eye and hand movements in single- and dual-task conditions

The goal of this study was to investigate whether ocular and hand motor systems operate independently or whether they share processes. Using dualtask methodology, reaction time (RT) latencies of saccadic eye and hand motor responses were measured. In experiment 1, the hand and eye motor systems produced rapid, aimed pointing movements to a visual target, which could occur either to the left or right of a central fixation point. Results showed that RT latencies of the eye response were slower in the dual-task condition than in the single-task condition, whereas the RT latencies of the hand response were virtually the same in both conditions. This interference effect indicated that the ocular and manual motor systems are not operating independently when initiating saccadic eye and goal-directed hand movements. Experiment 2 employed the same experimental paradigm as experiment 1, except for one important modification. Instead of a goal-directed hand movement to the target stimulus, subjects had to make a button-press response with either the index or middle finger of the right hand dependent upon whether the stimulus occurred to the right or left of the control fixation point. The aim of experiment 2 was to investigate the issue whether the observed interference effect in experiment 1 was specific or non-specific (e.g. overhead costs due to coordinating any two responses). The finding that saccadic eye movements and button-press responses in the dual-task condition could be initiated without delay relative to the single-task conditions, supports the specific interference interpretation.     

Lesions of the inferior olive do not affect long- or short-term habituation of the acoustic startle response in rats

Short- and long-term habituation of the acoustic startle response were assessed in a group of inferior olive-lesioned rats. Neither short- and long-term habituation, nor the performance of the reflex, were affected by the lesion. Since the cerebellar vermis is essential for long-term habituation of this reflex, we suggest that climbing fibres are not involved in this form of learning, which would therefore be mediated by the other cerebellar input, presumably the mossy fibres.     

The facilitation of motor actions by acoustic and electric stimulation

The presentation of a loud acoustic stimulus during the preparation of motor actions can both speed movement initiation and increase response vigor. Several recent studies have explored this phenomenon as a means to investigate the mechanisms and neural correlates of movement preparation. Here, we sought to determine the generality of this effect across sensory modalities, and in particular whether unexpected somatosensory stimulation can facilitate movements in a manner similar to loud sounds. We show that electric and acoustic stimuli can be similarly effective in inducing the early release of motor actions, in both reaction time and anticipatory timing tasks. Consistent with recent response activation models of motor preparation, we also demonstrate that increasing the intensity of electric stimuli induces both progressive decreases in reaction time and increases in response vigor. Additionally, we show that the early release of motor actions can be induced by electric stimuli targeting predominantly either muscle afferents or skin afferents. Finally, we show that simultaneous acoustic and electric stimulation leads to earlier releases of anticipatory actions than either unimodal stimulus. These findings may lead to new avenues for experimental and clinical exploitation of the effects of accessory sensory information on movement preparation and initiation.     

Effects of a startle stimulus on response speed and inhibition in a go/no‐go task

Two studies examined the interaction of an acoustic startle stimulus and visual go/no‐go task stimuli on startle reactivity and task performance. In the first study, an acoustic stimulus (50 ms, 100 dB noise) was presented alone or with a green (go) or red (no‐go) circle; in the second study, a prepulse (50 ms, 75 dB noise) was presented alone or 120 ms before the startle stimulus or circle. The startle stimulus speeded responses to the go stimuli and increased the covert false alarm rate in the no‐go condition (measured by EMG activity in the hand), although very few overt errors were made in the no‐go condition. Startle response magnitude was increased by a circle but decreased by a prepulse. The speeding of go responses caused by a startle stimulus was attenuated by the occurrence of a startle response, suggesting that an intense accessory stimulus can facilitate responding to an imperative stimulus, and that the startle response to that intense stimulus can interfere with that facilitation.     

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 Libets 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 awareness.     

Startle blink facilitation during the go signal of a reaction time task is not affected by movement preparation or attention to the go signal

Startle reflex eliciting stimuli presented at the onset of the go signal in a simple forewarned reaction time (RT) task (at a SOA of 0 ms) elicit larger blink reflexes than do stimuli presented later (e.g., at a SOA of 150 ms) or during inter trial intervals. The present study investigated whether this facilitation is affected by attention to the go signal or motor preparation. Participants performed a forewarned reaction time task that crossed the requirements for a speeded response (Hold versus Move) and for a discrimination task performed with the go signal (Report versus No report). Relative to control reflexes, blinks elicited at a SOA of 0 ms were facilitated and blinks elicited at a SOA of 150 ms were inhibited. RTs were slower on trials that required attention to the go signal and in both attention conditions and at both SOAs shortened in the presence of a blink-eliciting stimulus. However, neither attention to the go signal nor motor preparation affected blink facilitation at the 0 ms SOA. This finding suggests that the blink reflex facilitation observed at a SOA of 0 ms with the onset of a go signal reflects on the summation of sub- and supra-threshold activations of the startle pathway.     

Discrete and continuous planning of hand movements and isometric force trajectories

 We have previously demonstrated that, in preparing themselves to aim voluntary impulses of isometric elbow force to unpredictable targets, subjects selected default values for amplitude and direction according the range of targets that they expected. Once a specific target appeared, subjects specified amplitude and direction through parallel processes. Amplitude was specified continuously from an average or central default; direction was specified stochastically from one of the target directions. Using the same timed response paradigm, we now report three experiments to examine how the time available for processing target information influences trajectory characteristics in two-degree-of-freedom forces and multijoint movements. We first sought to determine whether the specification of force direction could also take the form of a discrete stochastic process in pulses of wrist muscle force, where direction can vary continuously. With four equiprobable targets (two force amplitudes in each of two directions separated by 22° or 90°), amplitude was specified from a central default value for both narrow and wide target separations as a continuous variable. Direction, however, remained specified as a discrete variable for wide target separations. For narrow target separations, the directional distribution of default responses suggested the presence of both discrete and central values. We next examined point-to-point movements in a multijoint planar hand movement task with targets at two distances and two directions but at five directional separations (from 30° to 150° separation). We found that extent was again specified continuously from a central default. Direction was specified discretely from alternative default directions when target separation was wide and continuously from a central default when separation was narrow. The specification of both extent and direction evolved over a 200-ms time period beginning about 100 ms after target presentation. As in elbow force pulses, extent was specified progressively in both correct and wrong direction responses through a progressive improvement in the scaling of acceleration and velocity peaks to the target. On the other hand, movement time and hand path straightness did not change significantly in the course of specification. Thus, the specification of movement time and linearity, global features of the trajectories, are given priority over the specific values of extent and direction. In a third experiment, we varied the distances between unidirectional target pairs and found that movement extent is specified discretely, like direction, when the disparity in distances is large. The implications of these findings for contextual effects on trajectory planning are discussed. The independence of extent and direction specification and the prior setting of response duration and straightness provide critical support for the hypothesis that point-to-point movements are planned vectorially. Received: 6 August 1996 / Accepted: 18 December 1996     

Effects of stimulus and task factors on achilles tendon reflexes evoked early during a preparatory period

The effects of intensity, duration and modality of a warning signal on tendon (T) reflexes evoked during the initial phase of a preparatory period of 4 sec were investigated. Reflexes were evoked simultaneously in both legs, from 0 to 350 msec after warning signal onset in steps of 50 msec. The required response was a plantar flexion of the right foot. A facilitation of reflexes was seen within 150 msec after warning signal onset, showing a somewhat longer latency for visual as compared to auditory signals. An effect of intensity was found in the auditory modality only, where the louder of two warning signals yielded a clear peak at 100 msec while the softer stimulus caused no significant departure of Achilles tendon reflexes from baseline. The time course of facilitation in the auditory modality was influenced by warning signal duration as well, although this effect was only marginally significant. There were no effects of physical warning signal parameters on reaction time. A comparison with an experiment in which non-signal stimuli were presented alone, pointed to aspects of the preparatory process which were manifest at the spinal level as early as 200 msec after warning signal onset.     

Colour vision can contribute to fast corrections of arm movements

Can chromatic information be used for the fast on-line control of action? In order to find out we asked subjects to tap a red square as quickly as possible. In half of the trials the red square’s position changed as soon as the subject’s hand started to move. We examined how quickly subjects could adjust their movements to this change. In half of the trials there was also a green square of the same luminance as the red one. If there was a green square, and the red square’s position changed, the change consisted of the two squares exchanging places, so that all that really changed was the squares’ colours. In such cases subjects could not have adjusted their movements without having analysed the colour. Nevertheless, subjects could respond adequately within as little as 120 ms. This was even so if the squares’ luminances changed considerably at the moment that the subject’s hand started to move. Thus, chromatic information can be used for the fast on-line control of action     

Context influences on the preparation and execution of reaching movements

The ability of rapidly adapting our motor behaviour in order to face the unpredictable changes in the surrounding environment is fundamental for survival. To achieve such a high level of efficiency our motor system has to assess continuously the context in which it acts, gathering all available information that can be relevant for planning goal-oriented movements. One still-debated aspect of movement organization is the nature and timing of motor planning. While motor plans are often taken to be concerned with the setting of kinematic parameters as a function of perceptual and motor factors, it has been suggested that higher level, cognitive factors may also affect planning. To explore this issue further, we asked 18 right-handed human participants to perform speeded hand-reaching movement toward a visual target in two different experimental settings, a reaction time (RT) paradigm (go-only task) and a countermanding paradigm. In both tasks participants executed the same movements, but in the countermanding task no-stop trials were randomly intermixed with stop trials. In stop trials participants were required to withhold the ongoing movement whenever a stop signal was shown. It is known that the presence of stop trials induces a consistent increase of the RTs of no-stop trials with respect to the RTs of go-only trials. However, nothing is known about a similar effect for movement times (MTs). We found that RTs and MTs exhibit opposing tendencies, so that a decrease in the RT correspond to an increase in the MT and vice versa. This tendency was present in all our participants and significant in 90% of them. Furthermore we found a moderate, but again very consistent, anticorrelation between RTs and MTs on a trial-by-trial base. These findings are consistent with strategic changes in movement programmes for the very same movements under different cognitive contexts, requiring different degrees of feedback-driven control during movement.     

Modulation of the blink reflex and P3 component of the startle response during an interoceptive challenge

The blink reflex component of the startle response is potentiated during processing of exteroceptive unpleasant stimuli. In contrast, blink magnitudes are often inhibited during interoceptive challenges. We measured respiration, blink magnitudes, and the P3 component to the acoustic startle probes in 34 participants while breathing against a mild resistance (mask‐with‐tubing) compared to breathing with no mask. Breathing through a mask with tubing resulted in increased inspiratory resistance as indicated by increased flow rate and tidal volume, a compensatory breathing pattern. Blink magnitudes to probes presented during the mask‐with‐tubing condition were inhibited compared to no‐mask. Likewise, the probe P3 component was smaller during breathing through a mild resistance. These data suggest that startle inhibition during interoceptive challenges might be due to a shift in attention towards the mildly unpleasant interoceptive stimuli.