The Characteristics And Mechanisms Of Anticipatory Posture Adjustments Under Acoustic Startle Priming In Normal Subjects And Stroke Patients

Objective: To observe the effect of Acoustic Startle Reaction(ASR)on kinematic and muscle response characteristics of Anticipatory Postural Adjustment(APA)in normal people and stroke subjects,and to explore the cortical activation features of ASR-induced motor initiation as well as potential central regulatory mechanisms.Methods: This study was divided into four parts.Firstly,the trunk APA response,motor reaction time and motion smoothness from the center of gravity of the trunk to the distal end of the upper limb in normal and stroke subjects in the reaching tasks were analyzed from the perspective of kinesiology via the portable inertial measurement unit system.Meanwhile,the effects of the ASR on the activations of the trunk,upper and lower limb muscles of normal people and stroke subjects were observed based on the reaching movement paradigm in the sitting position via wireless surface electromyography system.A further exploration of the onset latency and amplitude of anticipatory muscle activation(AMA)in the trunk and arm under fine finger movements was also performed.Finally,the underlying neuromodulatory mechanisms behind the ASR were further explored by functional near-infrared technology.Results: All subjects showed a significant shortening of motor response time to startle stimuli.The kinematic analysis results additionally suggested that there were differences in the A/Compensatory PAs smoothness between different moving sides,and the CPA smoothness Jerk value of every part was significantly higher than that of the APA smoothness,and it showed a more obvious trend towards the distal end.No significant effects of the ASR effect on APA delay of center-of-gravity and all smoothness metrics were found.Bilateral AMAs dysfunction is commonly seen in all stroke patients,manifested as lower incidence of AMAs,altered AMAs latency,and decreased amplitude of bilateral muscle AMAs,etc.,and ASR can improve these AMA performances in stroke patients.The time interval from muscle activation to peak contraction during ASR was significantly reduced,and the AMA amplitude of the lower trapezius muscle on the contralateral side and the latissimus dorsi muscle on the same side were significantly improved compared with the normal state.The results of near-infrared exploration indicated that the right dorsolateral frontal cortex and the frontoparietal network in which it belongs may be the high-level regulatory center of ASR and reticulospinal tract facilitation.Reduced activity in the ipsilateral primary motor cortex suggested increased inhibition of the nonmotor side during ASR-induced motor initiation.Conclusions: The ASR effect may involve facilitation of loops in the frontoparietal network,ascending reticular activating system,and cortico-reticulospinal tract,which can induce explicit motor onset and early onset of anticipatory muscle activation,and improve the activation amplitude of some trunk muscles,but did not appear to adversely affect movement.The features of ASR-related neural network represented by the facilitation of the reticulospinal tract has great application potential in the field of rehabilitation after cerebral injury.These findings obtained from this study will contribute to the construction of a series of quantitative evaluation indicators and the design of intervention methods for subsequent use of reticulospinal tract regulation to promote stroke recovery.Part Ⅰ: Effect of Acoustic Startle Reaction on Anticipatory Postural Adjustments in Healthy and Stroke Subjects: A Kinematic AnalysisObjective: To explore the characteristics of movement initiation and postural adjustment on trunk and upper limb joints brought about by the ASR effect from the perspective of kinesiology.Methods: 14 male stroke patients and 14 age-and sex-matched healthy subjects were recruited.The subjects were asked to use their left and right hands to complete the tasks of reaching,reaching to grasp a cup or ball according to two different initiation signals(80 d B and 114 d B stimuli).By using the inertial measurement unit system,the movement trajectory and velocity signals of the subject’s trunk and bilateral upper limbs were collected for comparison and analysis.The movement reaction time,the APA delay of the center of gravity,and the smoothness of motion from trunk to arm under the time window of A/CPAs among different movement sides(from healthy people,non-paralyzed side and paralyzed side)and difference trials(with startle stimuli or not).Results: A significant difference between different sound stimuli states(F1,886 = 71.00,P < 0.001)was found in motor reaction time,and the motor reaction time of startle was significantly shorter than that of normal stimuli.Significant main effects on the APA smoothness of the shoulder and elbow between different sides(P < 0.05)were also observed,and there were significant differences between healthy subjects and the non-paralyzed side and between healthy and paralyzed sides(P < 0.05).The changes in the CPA smoothness of the shoulder and elbow joints on different movement sides were basically similar to the APA smoothness,but the CPA Jerk values(smoothness)of the corresponding parts were significantly increased compared with the APA Jerks.And this difference got bigger closer to the hands.In addition,no significant effects of ASR on other indicators except reaction time were found in this study(P > 0.05).Conclusions: The ASR effect can clearly induce the advance of movement,but the effect on the APA latency of trunk was not obvious.This study found that the use of APA/CPA smoothness can reflect the kinematic differences between stroke and healthy subjects in movement initiation,and the CPA smoothness had a large increment in Jerk compared with the APA smoothness,which may represent the enhancement of sensorimotor feedback adjustments.Part Ⅱ: Effects of Acoustic Startle Reaction on Anticipatory Muscle Activation in Normal and Stroke Subjects: A Surface Electromyography StudyObjective: To demonstrate the task-specificities of AMAs among different forwardreaching tasks and explore the ASR effect on AMAs in occurrence proportions,AMAs onset latency or amplitude within these tasks in both healthy and stroke population.Methods: Ten healthy and ten stroke subjects were recruited.Participants were asked to complete the three forward-reaching tasks(reaching,reaching to grasp a ball or cup)on the left and right hand respectively with two different starting signals(warning-Go,80 d B and warning-startle,114 d B).The surface electromyography of Anterior deltoid,Flexor Carpi Radialis and Extensor Carpi Radialis(ECR)on the moving side was recorded together with signals from bilateral Sternocleidomastoid muscles(SCMs),Lower Trapezius,Latissimus Dorsi,and Tibialis Anterior.Proportions of valid trials,the incidence of ASR,AMAs incidence of each muscle and their onset latency and amplitude were involved in analyses.Difference of these variables across different movement sides(healthy,nonparetic and paretic),normal or startle conditions and the three tasks were explored.the ECR AMAs onset was selected to further explore the ASR effect on the incidence of AMAs.Results: Comparisons between movement sides revealed a widespread AMAs dysfunction in subacute stroke survivors which was manifested as lower AMAs onset incidence,changed onset latency and smaller amplitude of AMAs in bilateral muscles.However,a significant main effect of different tasks was only observed in AMAs onset latency of muscle ECR(F = 3.56,P = 0.03,η2p = 0.011),but the significance disappeared in the subsequent analysis of the stroke subjects only(P > 0.05).Moreover,the following post hoc comparison indicated significant earlier AMAs onsets of ECR in task Cup when comparing with Reach(P < 0.01).For different stimuli conditions,a significance was only revealed on shortened premotor reaction time under startle for all participants(F = 60.68,P < 0.001,η2p = 0.056).Furthermore,stroke survivors had a significantly lower incidence of ASR than healthy subjects under startle(P < 0.01).But all performed higher incidence of ECR AMAs onset(P < 0.05)than with normal signal.In addition,the incidence of ECR AMAs of both nonparetic and paretic sides could be increased significantly via startle(P ≤ 0.02).Conclusions: Healthy people have task-specific AMAs of muscle ECR when they perform forward-reaching tasks with different hand manipulations.However,this task-specific adjustment is lost in subacute stroke survivors.ASR can improve the incidence of AMAs for all subjects in the forward-reaching tasks involving precision manipulations,but not change AMAs onset latency and amplitude.Part Ⅲ: Effects of Acoustic Startle Reaction on Anticipatory Muscle Activations in Fine Motor Movements of the Upper Extremity: A Surface Electromyography StudyObjective: To verify the task specificity of AMAs in fine motor activities of upper extremity and explore the effect of the reticulospinal tract facilitation via acoustic startle on AMAs.Methods: A total of 13 healthy male right-handed subjects were recruited in this study.Participants were asked to complete the 4 reaching tasks(reaching,grasp a cup,use thumb or little finger to pinch a card)on the right hand,respectively,following two different starting signals(warning-Go,80 d B and warning-startle,114 d B).Electromyographic signals were recorded from the subjects’ bilateral sternocleidomastoid,lower trapezius(LT),latissimus dorsi(LD),lumbar erector spinae and right anterior deltoid,right ECR and right FCR.The premotor reaction time,muscle contraction time to peak,activation latency,A/CPA amplitude of the tested muscles were compared across different tasks and starting signals.Results: With the activation of reticulospinal tract,the premotor reaction time was significantly shortened in all task(P < 0.01),and the contraction time to peak of all muscles was significantly reduced(P < 0.01).Meanwhile,the APA amplitudes of contralateral LT and ipsilateral LD were significantly higher than those in normal(P < 0.05),but the timing of muscle activation onset and the A/CPA amplitudes of ECR/FCR were not affected(P > 0.05).The AMA activation latency of ECR in the little finger grip task was significantly shorter than that in reaching(P = 0.02).Conclusions: Forearm-specific AMA modulation persists in fine manipulation and inexperienced tasks of the fingers.Activation of the reticulospinal tract leads to earlier initiation of expected movements and accelerates muscle activation,which can increase APA amplitudes in some trunk muscles,but has no significant effect on AMA onset latency and A/CPA amplitudes in forearm muscles.Part Ⅳ: Cortical Activation Features of Acoustic Startle Reaction Priming: A Functional Near-infrared StudyObjective: To explore the potential role of RST facilitation in the Acoustic Startle Priming(ASP)paradigm and observe the cortical changes induced by ASP reaching tasks.Methods: Twenty healthy participants were included in this study.The reaching tasks were performed with their left and right hands.Participants were instructed to get ready after the warning cue and complete the reaching task as soon as they heard the Go cue.Half of the testing trials were set as control trials with an 80-d B Go cue.The other half of the trials had the Go cue replaced with 114-d B white noise to evoke the ASR effect,inducing reticulospinal tract facilitation.The response of the bilateral SCMs and the anterior deltoid was recorded via surface electromyography.Startle trials were labeled as exhibiting a positive or negative ASR effect,according to whether the SCM was activated early(30 to 130 ms after the Go cue)or late,respectively.Functional near-infrared spectroscopy was used to synchronously record the oxyhemoglobin and deoxyhemoglobin fluctuations in bilateral motor-related cortical regions.The β values representing cortical responses were estimated via the statistical parametric mapping technique and included in the final analyses.Results: Separate analyses of data from movements of the left or right side revealed significant activation of the right dorsolateral prefrontal cortex during RST facilitation.Moreover,left frontopolar cortex activation was greater in positive startle trials than in control or negative startle trials during left-side movements.Furthermore,decreased activity of the ipsilateral primary motor cortex in positive startle trials during ASP reaching tasks was observed.Conclusions: The right dorsolateral prefrontal cortex and the frontoparietal network to which it belongs may be the regulatory center for the ASR effect and RST facilitation.In addition,the ascending reticular activating system may participate this process.The decreased activity of the ipsilateral primary motor cortex suggests enhanced inhibition of the nonmoving side during the ASP reaching task.These findings provide further insight into the ASR effect and RST facilitation.