Where is movement initiated

Up to now, most of the research on the so called StartReact effect has dealt with reaction time tasks, where the temporal aspects of the reaction can be experimentally controlled. Some authors, though, have investigated how the preparation to perform an internally generated action modified the execution of the same or another action in a reaction time task [ 28 , 35 ]. They showed that, when subjects had to react during the building up of an internally generated preparation to act, the reaction time responses were delayed in comparison to the same reaction done without an ongoing internal preparation.

The delay was different according to whether the responses were congruent or not; that is, the delay of the reaction time response was less prominent when the responses required were the same for both action and reaction compared to when the response required for the reaction time task was different than the one being built internally. The authors concluded that subjects could not take advantage of the ongoing preparation of internally generated actions to perform externally driven reactions and, therefore, they suggested that the two circuits are clearly separated.

It is to note, though, that the main source of delay in reaction time in these conditions might have been in the processing of the afferent signal rather than in the execution since the same delay was observed in simple and complex reaction time tasks. Using a similar paradigm Hughes et al. The authors concluded that the stimulus-driven system could take advantage of the high degree of preparation of the voluntary system, and thus both systems might share common central preparatory mechanisms.

In an attempt to reconcile these two opposite views, we should consider what may account for the differences in processing and similarities in execution.

Likely, the processing time for an afferent input to reach the motor execution areas may take longer when our sensorimotor areas are engaged in an internally generated buildup of excitability since this may block externally generated sensory inputs that would potentially interfere with such process [ 37 ], thus likely accounting for the reported delay attributed to the afferent circuit.

However, when subjects are highly prepared, the amount of excitability builtup in the execution system may change the situation. Beyond a certain point in preparation, everything in the motor system may be ready to trigger the prepared motor programme.

In our opinion, this high degree of readiness in the motor system is what actually leads to the StartReact phenomenon and may also explain the faster execution of the congruent response found by Hughes et al. Our findings in the present study are in agreement with the considerations made in the previous paragraph. Therefore we believe that there was no interference between both systems.

Probably subjects did not shift from an internally driven command to an externally driven command but just anticipated the execution of the prepared movement when they reached an adequate level of excitability enhancement in their motor pathway. The observations reported by Kumru et al.

It has also been demonstrated that just the desire to act leads to a state of high excitability [ 39 , 40 ]. Accordingly, preparation for a motor act requires building up the programme with an increase in excitability in all structures that participate in its execution along the motor pathway. Not only response latency but also other characteristics of the resulting movement were similar for ASc and RS trials. Therefore, our results suggest that the mechanisms engaged in movement preparation for volitional execution share some similarities with those engaged in responding to an external cue.

Notwithstanding, cortical preparation is different for both tasks: long preparation in internally generated commands and just a premotor potential in movement execution in a reaction time paradigm. Motor preparatory activity has been described in brain areas up to 2 seconds before voluntary actions [ 8 , 41 ], and, in case of choice actions, prefrontal cortex activity occurs several seconds before action [ 42 ]. The Bereitschaftpotential, a slowly rising negative action potential recorded from the scalp, appears in movements made with free will [ 8 , 43 , 44 ] and may indicate some long lasting preparation that begins well in advance of the generation of descending motor commands.

The difference may be due to raised expectation of the cue after forewarning in reaction time tasks. Although there are limitations in free will studies because of the implicit difficulties in introducing temporal experimental constraints, we believe that a motor act can be considered an act of free will when subjects have a certain freedom [ 45 ] such as decision on the time to act, as they had in our experiment.

Our subjects were free to choose the time point of action, and, given the even distribution of the AT trials within the 2-second period allowed, we can assume that they did so. However, even if actions were performed at will, our subjects responded to the SAS as an external stimulus, suggesting that the effect of SAS on action occurred at a similar site as that on reaction.

A comparative study between reaction, and action was done by Cunnington et al. These authors explored subjects performing finger tapping as a reaction to a visual stimulus or at their own will.

For both conditions similar activations at medial motor and selected cortical areas were observed. However, activation within the basal ganglia was found only for self-generated movements. Their subjects showed a similar level of supplementary motor area SMA and cingulate cortex activation for both movement conditions, but at pre-SMA the activation timing was earlier for self-initiated movements 1. These findings indicate that pre-SMA and basal ganglia play a relevant role in differentiating internally generated from externally triggered actions [ 45 ].

In our study, the fact that responses in reaction time tasks were similar in general shape to those shown by actions performed close after SAS delivery, supports the idea that the influence of SAS in both, action and reaction, has to occur at a level where the two types of tasks coincide, out of cortical and basal ganglia processing.

We consider that a potential site along the motor pathway for the SAS to release the prepared motor programme is located at subcortical motor centers with enough hierarchy to activate a sufficient number of motoneurons for full motor programme execution.

Previous research has suggested that this formation is likely the brainstem reticular formation [ 21 , 22 , 26 , 33 ]. However, in an ingenious recent work, Alibiglou and MacKinnon [ 46 ] have advocated that the effects of a startle on movement execution require a cortical transit. Our results indicate that, if this is the case, such cortical structure should have reached a similar level of preparation in both conditions at a certain point to be ready to release the motor programme after SAS delivery.

Our study has some limitations. We did not stress accuracy in the kinematics of movement performance but just requested a quick initial reaction. We considered that latency would be a more reliable measure than movement kinematics in terms of comparison with previous studies concerning movement preparedness. Therefore, we do not know if some differences would exist in the characteristics of the movement between internally generated actions and externally triggered reactions if we had stressed accuracy in movement performance.

We cannot completely exclude a carry-over effect between blocks, although they were presented with a time separation of 30 minutes which might have been enough to prevent significant effects. The fact that no changes were observed in the size of the OOc response as an index of the effect of the startling stimulus in trials of interest supports this.

Finally, our reaction time task was actually an anticipation timing task, as subjects were familiarized with the time frame in which the response had to be performed. However, it has been shown that this type of reaction time task responds to the presentation of a SAS in a similar way as simple reaction time tasks, except for some modulation according to the level of information given [ 23 , 24 ].

Our results suggest that, whether subjects prepare their motor system to respond to an external cue or to perform a willed movement, they do so by enhancing the excitability of structures of the motor pathway at subcortical level, where they are accessible to a SAS. The release of the motor programme by SAS does not require the combination of two stimuli, and, therefore, it does not seem to involve reinforcement of afferent input but rather a timely occurrence in the preparation phase of movement execution.

A startling auditory stimulus anticipates the execution of self-generated human actions in the same way as it shortens the latency of task execution in simple reaction time trials. The anticipation may depend on the coincidence between SAS and the assumed buildup of preparatory activity just before execution of the willed motor action.

The programme for willed execution of fast actions may entail the preparation of subcortical motor structures that can be accessed by a startling stimulus to trigger the response. Castellote et al. This is an open access article distributed under the Creative Commons Attribution License , which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Article of the Year Award: Outstanding research contributions of , as selected by our Chief Editors.

Read the winning articles. Journal overview. Special Issues. Castellote , 1,2 M. Van den Berg, 3 and J. Academic Editor: Jean Blouin. Received 30 Apr Revised 17 Jul Accepted 01 Aug Published 11 Sep Abstract Preparation of the motor system for movement execution involves an increase in excitability of motor pathways.

Introduction Movement execution requires previous preparation of the motor system at various levels. Methods 2. Subjects Eight healthy subjects 5 females and 3 males, aged 28—52 took part in the study. Recording and Stimulation Subjects were sitting comfortably in a chair.

Procedure Subjects were facing a computer screen positioned at approximately 1 meter distance at eye level. Results Subjects performed the movement resulting in an overall mean wrist extension angular displacement of degrees without significant differences among conditions , ,.

RS: values for the experimental trials in response to the IS with simultaneous startle. Table 1. EMG and kinematic values for trials in which subjects performed a fast reaction with a wrist extension or a fast wrist extension at will. 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.

Wrist extension is shown as an upwards change in the angle and velocity traces. Figure 2. Figure 3. Lower part: representative trials from a single subject.

ASb: trial before startle. ASc: trial close after SAS. ASl: trial late after startle. Table 2. Figure 4. Supplementary Materials. References M. Floeter and J. View at: Google Scholar A. Pascual-Leone, J. Wassermann, J. Brasil-Neto, L. Harry might have altered patterns of walking due to damage in the leg area of the motor cortex of the right side of his brain. To help Harry regain efficient walking ability, the physiotherapist helps him perform sequences or patterns of walking by practising activation and control of specific muscle groups in his left leg.

At first, Harry will need lots of concentration to use the correct muscles as his brain is laying down new neural pathways to compensate for the damaged areas. But as this practice is repeated and the new pathways are established and strengthened, correct movement becomes easier without much concentration.

This same principle of neuroplasticity also applies for learning in the healthy brain. So if you are a ballet dancer or a gymnast, a swimmer or a soccer player, a watch-maker or micro-surgeon, your brain connections in your motor system will be different depending on the practice and skill you have with fine movement of different parts of your body.

This article was co-written with Zita Arends, who is a physiotherapist in stroke rehabilitation and aged care. Read other articles in our Brain Control series, here. Festival of Social Science — Aberdeen, Aberdeenshire. Brain Res ; : — Selective coding of motor sequence in the supplementary motor area of the monkey cerebral cortex. Exp Brain Res ; 82 : — Neuronal activity in the primate premotor, supplementary, and precentral motor cortex during visually guided and internally determined sequential movements.

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Sign In. Advanced Search. Search Menu. Article Navigation. Close mobile search navigation Article Navigation. Volume Article Contents Abstract. Self-initiated versus externally triggered movements: II.

The effect of movement predictability on regional cerebral blood flow. Harri Jenkins , I. Harri Jenkins. Oxford Academic. Google Scholar. Marjan Jahanshahi. Markus Jueptner. Richard E. David J. Revision received:. Cite Cite I. Select Format Select format. Permissions Icon Permissions. Abstract Event-related potential studies in man suggest a role for the supplementary motor area SMA in movement preparation, particularly when movements are internally generated.

Table 1 Significant rCBF increases during self-initiated movement compared with rest. Area activated. Extent of area activated relative to AC—PC line. Talairach co-ordinates of peak activation. Z score of peak activation. Foci of significant change in rCBF for comparison indicated in the table title. The mean percentage rCBF increase compared with rest is given for each area, measured at the pixel of maximal significance.

Open in new tab. Table 2 Significant rCBF increases during externally triggered movement compared with rest. See footnote to Table 1 for explanation of data and unlisted abbreviations. Table 3 Significant rCBF increases during self-initiated movement compared with externally triggered movement. Table 4 Significant rCBF increases during externally triggered movement compared with self-initiated movement. Open in new tab Download slide. Brain Res. J Neurophysiol.

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