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The long-latency "reflexive" response (LLR) following an upper limb mechanical perturbation is generated by neural circuitry shared with voluntary control. This feedback response supports many task-dependent behaviors and permits the expression of goal-directed corrections at latencies shorter than voluntary reaction time. An extensive body of literature has demonstrated that the LLR shows flexibility akin to voluntary control, but it has not yet been tested whether instruction-dependent LLR changes can also occur in the absence of an overt voluntary response. The present study used kinesthetic motor imagery (experiment 1) and instructed participants to execute movement with the unperturbed contralateral limb (experiment 2) to explore the relationship between the overt production of a voluntary response and LLR facilitation. Activity in stretched right wrist flexors were compared with standard "do not-intervene" and "compensate" conditions. Our findings revealed that on ~40% of imagery and ~50% of contralateral trials, a response occurred during the voluntary epoch in the stretched right wrist flexors. On these "leaked" trials, the early portion of the LLR (R2) was facilitated and displayed a similar increase to compensate trials. The latter half of the LLR (R3) showed further modulation, mirroring the patterns of voluntary epoch activity. By contrast, the LLR on "non-leaked" imagery and contralateral trials did not modulate. We suggest that even though a hastened voluntary response cannot account for all instruction-dependent LLR modulation, the overt execution of a response during the voluntary epoch in the same muscle(s) as the LLR is a prerequisite for instruction-dependent facilitation of this feedback response.NEW & NOTEWORTHY Using motor imagery and contralateral responses, we provide novel evidence that facilitation of the long-latency reflex (LLR) requires the execution of a response during the voluntary epoch. A high proportion of overt response "leaks" were found where the mentally simulated or mirrored response appeared in stretched muscle. The first half of the LLR was categorically sensitive to the appearance of leaks, whereas the latter half displayed characteristics closely resembling activity in the ensuing voluntary period.

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It is well known that increasing the complexity of the required response results in a corresponding increase in simple reaction time (RT). This "response complexity effect" has typically been attributed to increased time required to prepare some aspect of the response; however, most studies examining the response complexity effect have used an unpredictable foreperiod, which does not allow for optimal preparation to occur. Thus, it is conceivable that response complexity effects are influenced by an inability to predict the occurrence of the go-signal. In order to examine this possibility, participants (N = 36) were randomly assigned to one of four groups that differed in predictability of the go signal: 1) 2500-3500 ms random foreperiod; 2) 3000 ms constant foreperiod; 3) 1000 ms constant foreperiod; 4) 3000 ms constant foreperiod with a 1000 ms countdown timer. Participants performed one of three different key-press responses in a simple RT paradigm: 1) single key-press; 2) three key-presses with an equal/isochronous time interval between presses; 3) three key-presses with an unequal/non-isochronous time interval between presses. Results confirmed that while the countdown timer group had an overall reduced RT, response complexity effects were present and of similar magnitude for all groups in all testing blocks. This confirms that predictability of the go signal does not affect the response complexity effect.

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Perturbations delivered to the upper limbs elicit reflexive responses in stretched muscle at short- (M1: 25-50 ms) and long- (M2: 50-100 ms) latencies. When presented in a simple reaction time (RT) task, the perturbation can also elicit a preprogrammed voluntary response at a latency (< 100 ms) that overlaps the M2 response. This early appearance of the voluntary response following a proprioceptive stimulus causing muscle stretch is called a triggered reaction. Recent work has demonstrated that a perturbation also elicits activity in sternocleidomastoid (SCM) over a time-course consistent with the startle response and it was, therefore, proposed that the StartReact effect underlies triggered reactions (Ravichandran et al., Exp Brain Res 230:59-69, 2013). The present work investigated whether perturbation-evoked SCM activity results from startle or postural control and whether triggered reactions can also occur in the absence of startle. In Experiment 1, participants "compensated" against a wrist extension perturbation. A prepulse inhibition (PPI) stimulus (known to attenuate startle) was randomly presented before the perturbation. Rather than attenuating SCM activity, the responses in SCM were advanced by the PPI stimulus. In Experiment 2, participants "assisted" a wrist extension perturbation. The perturbation did not reliably elicit startle but despite this, two-thirds of trials had RTs of less than 100 ms and the earliest responses began at ~ 70 ms. These findings suggest that SCM activity following a perturbation is the result of postural control and is not related to startle. Moreover, an overt startle response is not a prerequisite for the elicitation of a triggered reaction.

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When we move, our ability to detect tactile events on the moving limb is reduced (e.g., movement-related tactile suppression). This process prevents unimportant sensory information from bombarding our central nervous system. This study investigated whether movement-related suppression can be modulated according to task relevance, while introducing a novel motor-driven complex upper limb movement. In three experiments, participants performed volitional self-driven and passive motor-driven reaching and grasping movements. Over the course of the movement, weak electrical stimulation was presented at task-relevant (i.e., index finger) and irrelevant sites (i.e., forearm) on the moving limb. In Experiment 1, participants displayed reduced detectability during movement (90% resting detection). This was true for all locations on the moving limb irrespective of task-relevance and during both self and motor-driven movements. In Experiments 2 and 3 a range of stimulus amplitudes were presented to one task-relevant location during both self and motor-driven movements (Experiment 2A), to a task-relevant and irrelevant site (Experiment 2B) and during a targeted and pantomime/no target reach (Experiment 3). This allowed us to estimate perceptual thresholds and assess the magnitude of movement-related suppression. During both self and motor-driven movements participants exhibited movement-related suppression. Suppression was greater at the irrelevant site (forearm) than at the relevant site (index finger) of the limb. Further, the magnitude of suppression varied with task relevance such that pantomime movements elicited more suppression than targeted movements. Collectively, these experiments suggest that although tactile suppression may be a general consequence of movement, suppression can be modulated in a relevance-dependent manner.

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The current study examined the processes involved in the preparation of sequencing and timing initiation for multi-component responses. In two experiments, participants performed a reaction time (RT) task involving a three key-press sequence with either a simple (isochronous) or complex (non-isochronous) timing structure. Conditions involved a precue that provided information about all features of the movement (simple RT), no features of the movement (choice RT), sequencing only, or timing structure only. When sequencing was precued, RT decreased significantly as compared to choice RT, indicative of advance preparation of sequencing. When timing was precued, RT decreased significantly compared to choice RT when the timing structure was simple, suggesting that some aspect of timing preparation can occur prior to the go stimulus. However, even when the timing structure was known in advance, RT was still affected by timing complexity, confirming that some aspect of timing preparation cannot occur until after the onset of the stimulus and thus occurs during the RT interval. To explain these findings, we propose a two-component model of preparation for the timing initiation structure in which timing selection occurs in advance but timing implementation must occur following the go signal. These results support and extend previous findings regarding the independence of the processes associated with response sequencing and timing initiation.

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Movement preparation of bimanual asymmetric movements takes more time than bimanual symmetric movements in choice reaction-time conditions. This bimanual asymmetric cost may be caused by increased processing demands on any stage of movement preparation. The authors tested the contributions of each stage of movement preparation to the asymmetric cost by using the additive factors method. This involved altering the stimulus contrast, response compatibility, and response complexity. These manipulations changed the processing demands on stimulus identification, response selection, and response programming, respectively. Any manipulation with a larger reaction time cost than control suggests that stage contributes to the bimanual asymmetric cost. The bimanual asymmetric cost was larger for incompatible stimuli, which supports that response selection contributes to the bimanual asymmetric cost.

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The purpose of the current study was to examine the processes involved in the preparation of timing during response initiation and execution through the use of a startling acoustic stimulus (SAS). In Experiment 1, participants performed a delayed response task in which a two key-press movement was to be initiated 200 ms after an imperative signal (IS) with either a short (200 ms) or long (500 ms) interval between key-presses. On selected trials, a SAS was presented to probe the preparation processes associated with the initiation delay and execution of the inter-key interval. The SAS resulted in a significant decrease in the initiation time, which was attributed to a speeding of pacemaker pulses used to time the delay interval, caused by an increased activation due to the SAS. Conversely, the SAS delayed the short inter-key interval, which was attributed to temporary interference with cortical processing. In Experiment 2, participants performed a 500-ms delayed response task involving two key-presses 200 ms apart. In this condition, the SAS resulted in significantly decreased initiation time and a delayed inter-key interval (p = .053). Collectively, these results support a different timeline for the preparation of the delay interval, which is thought to be prepared in advance of the IS, and the inter-key interval, which is thought to be prepared following the IS. This conclusion provides novel information with regard to timing preparation that is consistent with models in which response preparation, initiation, and execution are considered separate and dissociable processes.

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Perturbations applied to the upper limbs elicit short (M1: 25-50 ms) and long-latency (M2: 50-100 ms) responses in the stretched muscle. M1 is produced by a spinal reflex loop, and M2 receives contribution from multiple spinal and supra-spinal pathways. While M1 is relatively immutable to voluntary intention, the remarkable feature of M2 is that its size can change based on intention or goal of the participant (e.g., increasing when resisting the perturbation and decreasing when asked to let-go or relax following the perturbation). While many studies have examined modulation of M2 between passive and various active conditions, through the use of constant foreperiods (interval between warning signal and a perturbation), it has also been shown that the magnitude of the M2 response in a passive condition can change based on factors such as habituation and anticipation of perturbation delivery. To prevent anticipation of a perturbation, most studies have used variable foreperiods; however, the range of possible foreperiod duration differs between experiments. The present study examined the influence of different variable foreperiods on modulation of the M2 response. Fifteen participants performed active and passive responses to a perturbation that stretched wrist flexors. Each block of trials had either a short (2.5-3.5 seconds; high predictability) or long (2.5-10.5 seconds; low predictability) variable foreperiod. As expected, no differences were found between any conditions for M1, while M2 was larger in the active rather than passive conditions. Interestingly, within the two passive conditions, the long variable foreperiods resulted in greater activity at the end of the M2 response than the trials with short foreperiods. These results suggest that perturbation predictability, even when using a variable foreperiod, can influence circuitry contributing to the long-latency stretch response.

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Unexpected presentation of a startling auditory stimulus (SAS>120 decibels) in a reaction time (RT) paradigm results in the startle reflex and an early release (<100ms) of the preplanned motor response (StartReact effect). Mechanical perturbations applied to the upper limbs elicit short- (M1) and long-latency (M2) stretch reflexes and have also been shown to initiate intended motor responses early (<100ms). Ravichandran et al. (2013) recently proposed that unexpected delivery of a perturbation could also elicit a startle response and therefore the StartReact effect may be responsible for the early trigger of a preplanned response. To investigate this further, we examined startle incidence, RT, and stretch reflex modulation for both expected and unexpected perturbations. In Experiment 1, participants performed active (ACT) and passive (DNI) conditions to an expected large perturbation (similar to previous studies examining M2). The startle response was not observed; however, the perturbation still elicited the voluntary response at short latency (<100ms) and goal-dependent modulation of the M2 response was observed. In Experiment 2, participants performed ACT and DNI conditions to a weak auditory stimulus or a small wrist perturbation. On unexpected trials we probed startle circuitry with a large perturbation or SAS. The SAS consistently elicited a startle response in both ACT and DNI conditions, but startle-like activity was only observed on 17.4% of ACT perturbation probe trials. Our findings suggest that while unexpected upper limb perturbations can be startling, startle triggering of the preplanned voluntary response is not the primary mechanism responsible for goal-dependent modulation of the M2 response.

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The current studies examined the processes involved in response sequencing and timing initiation for complex, multiple-component movements. Participants performed a 3 key-press sequence in simple and choice reaction time (RT) paradigms (Experiment 1), or a study time paradigm that allowed the participants to control the foreperiod delay, which is thought to reflect advance preparation duration (Experiment 2). Sequencing complexity was manipulated by using either the same hand and effector for all key presses (low complexity) or different hands/effectors across key presses (high complexity) while timing initiation complexity was manipulated by using either an isochronous (low complexity) or nonisochronous (high complexity) timing pattern. Increasing sequencing complexity had little effect on simple RT but increased participant-controlled foreperiod delay (i.e., study time). Conversely, increasing timing initiation complexity had no effect on foreperiod delay but increased simple RT. These results provide compelling evidence that in a simple RT paradigm, sequencing preparation is performed during the foreperiod while preparation of timing initiation is delayed until the RT interval. Furthermore, choice RT increased with sequencing complexity and was relatively unaffected by timing initiation complexity, indicative of sequencing preparation occurring during the choice RT interval and preparation of timing initiation occurring online. Collectively, the data indicate a dissociation and independence of the preparation of timing initiation and sequencing for complex movements. (PsycINFO Database Record

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Stretching a muscle of the upper limb elicits short (M1) and long-latency (M2) reflexes. When the participant is instructed to actively compensate for a perturbation, M1 is usually unaffected and M2 increases in size and is followed by the voluntary response. It remains unclear if the observed increase in M2 is due to instruction-dependent gain modulation of the contributing reflex mechanism(s) or results from voluntary response superposition. The difficulty in delineating between these alternatives is due to the overlap between the voluntary response and the end of M2. The present study manipulated response accuracy and complexity to delay onset of the voluntary response and observed the corresponding influence on electromyographic activity during the M2 period. In all active conditions, M2 was larger compared with a passive condition where participants did not respond to the perturbation; moreover, these changes in M2 began early in the appearance of the response (∼ 50 ms), too early to be accounted for by voluntary overlap. Voluntary response latency influenced the latter portion of M2, with the largest activity seen when accuracy of limb position was not specified. However, when participants aimed for targets of different sizes or performed movements of various complexities, reaction time differences did not influence M2 period activity, suggesting voluntary activity was sufficiently delayed. Collectively, our results show that while a perturbation applied to the upper limbs can trigger a voluntary response at short latency (<100 ms), instruction-dependent reflex gain modulation remains an important contributor to EMG changes during the M2 period.

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Movement preparation of bimanual asymmetric movements is longer than bimanual symmetric movements in choice reaction time conditions, even when movements are cued directly by illuminating the targets (Blinch et al. in Exp Brain Res 232(3):947-955, 2014). This bimanual asymmetric cost may be caused by increased processing demands on response programming, but this requires further investigation. The present experiment tested the demands on response programming for bimanual movements by temporally separating the preparation of each arm. This was achieved by precuing the target of one arm before the imperative stimulus. We asked: What was prepared in advance when one arm was precued? The answer to this question would suggest which process causes the bimanual asymmetric cost. Advance movement preparation was examined by comparing reaction times with and without a precue for the left target and by occasionally replacing the imperative stimulus with a loud, startling tone (120 dB). A startle tone releases whatever movement is prepared in advance with a much shorter reaction time than control trials (Carlsen et al. in Clin Neurophysiol 123(1):21-33, 2012). Participants made bimanual symmetric and asymmetric reaching movements in simple and 2-choice reaction time conditions and a condition with a precue for the left target. We found a bimanual asymmetric cost in 2-choice conditions, and the asymmetric cost was significantly smaller when the left target was precued. These results, and the results from startle trials, suggest (1) that the precued movement was not fully programmed but partially programmed before the imperative stimulus and (2) that the asymmetric cost was caused by increased processing demands on response programming. Overall, the results support the notion that bimanual movements are not the sum of two unimanual movements; instead, the two arms of a bimanual movement are unified into a functional unit. When one target is precued, this critical unification likely occurs during response programming.

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When a startling acoustic stimulus (SAS) is presented in a simple reaction time (RT) task, response latency is significantly shortened. The present study used a SAS in a psychological refractory period (PRP) paradigm to determine if a shortened RT1 latency would be propagated to RT2. Participants performed a simple RT task with an auditory stimulus (S1) requiring a vocal response (R1), followed by a visual stimulus (S2) requiring a key-lift response (R2). The two stimuli were separated by a variable stimulus onset asynchrony (SOA), and a typical PRP effect was found. When S1 was replaced with a 124dB SAS, R1 onset was decreased by 40-50ms; however, rather than the predicted propagation of a shortened RT, significantly longer responses were found for RT2 on startle trials at short SOAs. Furthermore, the 100ms SOA condition exhibited reduced peak EMG for R2 on startle trials, as compared to non-startle trials. These results are attributed to the startling stimulus temporarily interfering with cognitive processing, delaying and altering the execution of the second response. In addition to this "startle refractory period," results also indicated that RT1 latencies were significantly lengthened for trials that immediately followed a startle trial, providing evidence for longer-term effects of the startling stimulus.

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A recent study by Marinovic et al. (J. Neurophysiol., 2013, 109: 996-1008) used a loud acoustic stimulus to probe motor preparation in a simple reaction time (RT) task. Based on decreasing RT latency and increases in motor output measures as the probe stimulus approached the "go" stimulus, the authors concluded that response-related activation increased abruptly 65 ms prior to the imperative stimulus, a result in contrast to previous literature. However, this study did not measure reflexive startle activity in the sternocleidomastoid (SCM) muscle, which has been used to delineate between response triggering by a loud acoustic stimuli and effects of stimulus intensity and/or intersensory facilitation. Due to this methodological limitation, it was unclear if the data accurately represented movement-related activation changes. In order to provide a measure as to whether response triggering occurred on each trial, the current experiment replicated the study by Marinovic et al., with the collection of muscle activation in the SCM. While the replication analyses involving all trials confirmed similar results to those reported by Marinovic et al., when data were limited to those in which startle-related SCM activation occurred, the results indicated that movement-related activation is constant in the 65 ms prior to action initiation. The difference between analyses suggests that when SCM activation is not considered, results may be confounded by trials in which the probe stimulus does not trigger the prepared response. Furthermore, these results provide additional confirmation that reflexive startle activation in the SCM is a robust indicator of response triggering by a loud acoustic stimulus.

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Symmetric, target-directed, bimanual movements take less time to prepare than asymmetric movements (Diedrichsen et al. in Cerebral Cortex 16(12):1729-1738, 2006; Heuer and Klein in Psychol Res 70(4):229-244, 2006b). The preparation savings for symmetric movements may be related to the specification of symmetric amplitudes, target locations, or both. The goals of this study were to determine which symmetric movement parameters facilitate the preparation of bimanual movements and to compare the size of the facilitation for different parameters. Thirty participants performed bimanual reaching movements that varied in terms of the symmetry/asymmetry of starting locations, movement amplitudes, and target locations. Reaction time savings were examined by comparing movements that had one symmetric parameter (and two asymmetric parameters) to movements with all asymmetric parameters. We observed significant savings (~10 ms) for movements with symmetric amplitudes and movements with symmetric target locations. Reaction time costs were examined by comparing movements that had two asymmetric parameters (and one symmetric parameter) to movements with all symmetric parameters. We observed significant reaction time costs (~13 ms) for all movements with asymmetric amplitudes. These results suggest that movement preparation is facilitated when amplitudes or target locations are symmetric and that movement preparation suffers interference when amplitudes are asymmetric. The relative importance of the three parameters to movement preparation, from most to least important, is movement amplitudes, target locations, and then starting locations. Interference with asymmetric amplitudes or target locations may be caused by cross-talk between concurrent processes of parameter specification during response programming.

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The preparation of multiple element movements has been examined for decades, with no clear explanation offered for the disparate results observed. Results from 2 experiments are presented and, in conjunction with previous results, a theoretical interpretation is offered regarding the preparatory processes that occur before, during and after the reaction time (RT) interval for multiple element movements during both simple and choice RT paradigms. In Experiment 1, number of elements and timing complexity were manipulated in a simple RT key-press task, using a startling acoustic stimulus to probe advance preparation. Both startle and nonstartle RT increased with number of movement elements and for a movement with increased timing complexity, providing evidence that the control of response timing occurs during the RT interval. In Experiment 2, the production of key-press movements of varying number of elements was compared in a simple versus choice RT paradigm. Results indicated that simple RT was affected by the number of elements, yet choice RT was not. Additionally, choice RT trials showed significantly longer interresponse intervals compared with those observed in simple RT trials, providing evidence for online processing in choice RT. The results of both studies, together with previous findings, suggest that planning of the timing of the onsets of the elements is prepared during simple RT, whereas planning of other aspects of the sequence of elements seems to occur in the foreperiod prior to the "go" signal. Conversely, in the choice RT paradigm, timing seems to be controlled online. This explanation may bring closure on difficulties encountered in over 50 years of research examining response preparation for complex movements.

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The goal of this study was to determine the process or processes most likely to be involved in reaction-time costs for spatially cued bimanual reaching. We used reaction time to measure the cost of bimanual symmetric movements compared to unimanual movements (a bimanual symmetric cost) and the cost for bimanual asymmetric movements compared to symmetric movements (a bimanual asymmetric cost). The results showed that reaction times were comparable for all types of movements in simple reaction time; that is, there was neither a bimanual symmetric cost nor an asymmetric cost. Therefore, unimanual, bimanual symmetric, and bimanual asymmetric movements have comparable complexity during response initiation. In choice conditions, there was no bimanual symmetric cost but there was a bimanual asymmetric cost, indicating that the preparation of asymmetric movements is more complex than symmetric movements. This asymmetric cost is likely the result of interference during response programming.

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The present study was designed to investigate the mechanism associated with dual-task interference in a psychological refractory period (PRP) paradigm. We used a simple reaction time paradigm consisting of a vocal response (R1) and key-lift task (R2) with a stimulus onset asynchrony (SOA) between 100ms and 1500ms. On selected trials we implemented a startling acoustic stimulus concurrent with the second stimulus to determine if we could involuntarily trigger the second response. Our results indicated that the PRP delay in the second response was present for both control and startle trials at short SOAs, suggesting the second response was not prepared in advance. These results support a response preparation bottleneck and can be explained via a neural activation model of preparation. In addition, we found that the reflexive startle activation was reduced in the dual-task condition for all SOAs, a result we attribute to prepulse inhibition associated with dual-task processing.

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Muscles involved in rapid, targeted movements about a single joint often display a triphasic [agonist (AG1)-antagonist (ANT)-agonist (AG2)] electromyographic (EMG) pattern. Early work using movement perturbations suggested that for short movements, the entire EMG pattern was prepared and initiated in advance (Wadman WJ, Dernier van der Gon JJ, Geuze RH, Mol CR. J Hum Mov Stud 5: 3-17, 1979), whereas more recent transcranial magnetic stimulation evidence indicates that the ANT may be programmed separately (MacKinnon CD, Rothwell JC. J Physiol 528: 633-645, 2000) with execution of the bursts occurring serially (Irlbacher K, Voss M, Meyer BU, Rothwell JC. J Physiol 574: 917-928, 2006). The purpose of the current study was to investigate the generation of triphasic EMG bursts for movements of different amplitudes. In experiment 1, participants performed rapid elbow extension movements to 20° and 60° targets, and on some trials, a startling acoustic stimulus (SAS), which is thought to trigger prepared motor commands at short latency, was delivered at the onset of AG1. For short movements, this perturbation elicited ANT and AG2 early, suggesting the agonist and antagonist bursts may have been programmed independently. In contrast, the same manipulation did not disrupt EMG timing parameters for the long movements, raising the possibility that ANT and AG2 were not fully programmed in advance of movement onset. In experiment 2, an SAS was delivered later in the movement, which produced early onset of both ANT and AG2. We propose that the triphasic pattern is executed serially but believe the trigger signal for initiating the ANT burst occurs not in relation to the AG1 burst, but rather in close temporal proximity to the expected onset of ANT.

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