Bedforms and sedimentary structures related to supercritical flows in glacigenic settings

Upper-flow-regime bedforms, including upper-stage-plane beds, antidunes, chutes-and-pools and cyclic steps, are ubiquitous in glacigenic depositional environments characterized by abundant meltwater discharge and sediment supply. In this study, the depositional record of Froude near-critical and supercritical flows in glacigenic settings is reviewed, and similarities and differences between different depositional environments are discussed. Upper-flow-regime bedforms may occur in subglacial, subaerial and subaqueous environments, recording deposition by free-surface flows and submerged density flows. Although individual bedform types are generally not indicative of any specific depositional environment, some observed trends are similar to those documented in non-glacigenic settings. Important parameters for bedform evolution that differ between depositional environments include flow confinement, bed slope, aggradation rate and grain size. Cyclic-step deposits are more common in confined settings, like channels or incised valleys, or steep slopes of coarse-grained deltas. Antidune deposits prevail in unconfined settings and on more gentle slopes, like glacifluvial fans, sand-rich delta slopes or subaqueous (ice-contact) fans. At low aggradation rates, only the basal portions of bedforms are preserved, such as scour fills related to the hydraulic-jump zone of cyclic steps or antidune-wave breaking, which are common in glacifluvial systems and during glacial lake-outburst floods and (related) lake-level falls. Higher aggradation rates result in increased preservation potential, possibly leading to the preservation of complete bedforms. Such conditions are met in sediment-laden jökulhlaups and subaqueous proglacial environments characterized by expanding density flows. Coarser-grained sediment leads to steeper bedform profiles and highly scoured facies architectures, while finer-grained deposits display less steep bedform architectures. Such differences are in part related to stronger flows, faster settling of coarse clasts, and more rapid breaking of antidune waves or hydraulic-jump formation over hydraulically rough beds. © 2020 The Authors. Sedimentology published by John Wiley & Sons Ltd on behalf of International Association of Sedimentologist

Error rate estimation by mixture decomposition

AbstractWe show how error rate estimation may be viewed as a problem of mixture decomposition. We apply the idea to the average conditional error rate approach to estimation and explore the effectiveness of one particular decomposition method by simulation

TMS coil orientation and muscle activation influence lower limb intracortical excitability

Introduction: Previous research with transcranial magnetic stimulation (TMS) indicates that coil orientation (TMS current direction) and muscle activation state (rest or active) modify corticospinal and intracortical excitability of upper limb muscles. However, the extent to which these factors influence corticospinal and intracortical excitability of lower limb muscles is unknown. This study aimed to examine how variations in coil orientation and muscle activation affect corticospinal and intracortical excitability of tibialis anterior (TA), a lower leg muscle. Methods: In 21 young (21.6 ± 3.3 years, 11 female) adults, TMS was administered to the motor cortical representation of TA in posterior-anterior (PA) and mediolateral (ML) orientations at rest and during muscle activation. Single-pulse TMS measures of motor evoked potential amplitude, in addition to resting and active motor thresholds, were used to index corticospinal excitability, whereas paired-pulse TMS measures of short-interval intracortical inhibition (SICI) and facilitation (SICF), and long-interval intracortical inhibition (LICI), were used to assess excitability of intracortical circuits. Results: For single-pulse TMS, motor thresholds and test TMS intensity were lower for ML stimulation (all P < 0.05). In a resting muscle, ML TMS produced greater SICI (P < 0.001) and less SICF (both P < 0.05) when compared with PA TMS. In contrast, ML TMS in an active muscle resulted in reduced SICI but increased SICF (both P ≤ 0.001) when compared with PA TMS. CONCLUSION: TMS coil orientation and muscle activation influence measurements of intracortical excitability recorded in the tibialis anterior, and are therefore important considerations in TMS studies of lower limb muscles.Brodie J. Hand, George M. Opie, Simranjit K. Sidhu, John G. Semmle

Motor cortex plasticity is greater in endurance-trained cyclists following acute exercise

First published September 8, 2022Previous research using transcranial magnetic stimulation (TMS) has shown that plasticity within primary motor cortex (M1) is greater in people who undertake regular exercise, and a single session of aerobic exercise can increase M1 plasticity in untrained participants. This study aimed to examine the effect of an acute bout of exercise on M1 plasticity in endurance-trained (cyclists) and sedentary individuals. 14 endurance-trained cyclists (mean ± SD; 23 ± 3.8 years) and 14 sedentary individuals (22 ± 1.8 years) performed two experimental sessions. One session included an acute bout of high-intensity interval training (HIIT) exercise involving stationary cycling, while another session involved no-exercise (control). Following exercise (or control), I-wave periodicity repetitive TMS (iTMS) was used (1.5 ms interval, 180 pairs) to induce plasticity within M1. Motor evoked potentials (MEP) induced by single and paired-pulse TMS over M1 were recorded from a hand muscle at baseline, after HIIT (or control) exercise, and after iTMS. Corticospinal and intracortical excitability was not influenced by HIIT exercise in either group (all P &gt; 0.05). There was an increase in MEP amplitude after iTMS, and this was greater after HIIT exercise (compared with control) for all subjects (P &lt; 0.001). However, the magnitude of this response was larger in endurance cyclists compared with the sedentary group (P &lt; 0.05). These findings indicate that M1 plasticity induced by iTMS was greater in endurance-trained cyclists following HIIT. Prior history of exercise training is, therefore, an important consideration for understanding factors that contribute to exercise-induced plasticity.Brodie J. Hand, George M. Opie, Simranjit K. Sidhu and John G. Semmle

Modulation of I-Wave Generating Pathways With Repetitive Paired-Pulse Transcranial Magnetic Stimulation: A Transcranial Magnetic Stimulation - Electroencephalography Study

OnlinePublObjectives: Repetitive paired-pulse transcranial magnetic stimulation (iTMS) at indirect (I) wave intervals increases motorevoked potentials (MEPs) produced by transcranial magnetic stimulation (TMS) to primary motor cortex (M1). However, the effects of iTMS at early and late intervals on the plasticity of specific I-wave circuits remain unclear. This study therefore aimed to assess how the timing of iTMS influences intracortical excitability within early and late I-wave circuits. To investigate the cortical effects of iTMS more directly, changes due to the intervention were also assessed using combined TMSelectroencephalography (EEG). Material and Methods: Eighteen young adults (aged 24.6 ± 4.2 years) participated in four sessions in which iTMS targeting early (1.5-millisecond interval; iTMS1.5) or late (4.0-millisecond interval; iTMS4.0) I-waves was applied over M1. Neuroplasticity was assessed using both posterior-to-anterior (PA) and anterior-to-posterior (AP) stimulus directions to record MEPs and TMS-evoked EEG potentials (TEPs) before and after iTMS. Short-interval intracortical facilitation (SICF) at interstimulus intervals of 1.5 and 4.0 milliseconds was also used to index I-wave activity. Results: MEP amplitude was increased after iTMS (p < 0.01), and this was greater for PA responses (p < 0.01) but not different between iTMS intervals (p = 0.9). Irrespective of iTMS interval and coil current, SICF was facilitated after the intervention (p < 0.01). Although the N45 produced by AP stimulation was decreased by iTMS1.5 (p = 0.04), no other changes in TEP amplitude were observed. Conclusions: The timing of iTMS failed to influence which I-wave circuits were potentiated by the intervention. In contrast, decreases in the N45 suggest that the neuroplastic effects of iTMS may include disinhibition of intracortical inhibitory processes.Ryoki Sasaki, Brodie J. Hand, John G. Semmler, George M. Opi

Motor cortex plasticity and visuomotor skill learning in upper and lower limbs of endurance-trained cyclists

Published online: 7 October 2021Purpose: Studies with transcranial magnetic stimulation (TMS) show that both acute and long-term exercise can influence TMS-induced plasticity within primary motor cortex (M1). However, it remains unclear how regular exercise influences skill training-induced M1 plasticity and motor skill acquisition. This study aimed to investigate whether skill training-induced plasticity and motor skill learning is modified in endurance-trained cyclists. Methods: In 16 endurance-trained cyclists (24.4 yrs; 4 female) and 17 sedentary individuals (23.9 yrs; 4 female), TMS was applied in 2 separate sessions: one targeting a hand muscle not directly involved in habitual exercise and one targeting a leg muscle that was regularly trained. Single- and paired-pulse TMS was used to assess M1 and intracortical excitability in both groups before and after learning a sequential visuomotor isometric task performed with the upper (pinch task) and lower (ankle dorsiflexion) limb. Results: Endurance-trained cyclists displayed greater movement times (slower movement) compared with the sedentary group for both upper and lower limbs (all P  0.05). Furthermore, endurance-trained cyclists demonstrated a greater increase in M1 excitability and reduced modulation of intracortical facilitation in resting muscles of upper and lower limbs after visuomotor skill learning (all P < 0.005). Conclusion: Under the present experimental conditions, these results indicate that a history of regular cycling exercise heightens skill training-induced M1 plasticity in upper and lower limb muscles, but it does not facilitate visuomotor skill acquisition.Brodie J. Hand, George M. Opie, Simranjit K. Sidhu, John G. Semmle

Age-related changes in motor cortex plasticity assessed with non-invasive brain stimulation: an update and new perspectives

Published online: 16 July 2021It is commonly accepted that the brains capacity to change, known as plasticity, declines into old age. Recent studies have used a variety of non-invasive brain stimulation (NIBS) techniques to examine this age-related decline in plasticity in the primary motor cortex (M1), but the effects seem inconsistent and difficult to unravel. The purpose of this review is to provide an update on studies that have used different NIBS techniques to assess M1 plasticity with advancing age and offer some new perspective on NIBS strategies to boost plasticity in the ageing brain. We find that early studies show clear differences in M1 plasticity between young and older adults, but many recent studies with motor training show no decline in use-dependent M1 plasticity with age. For NIBS-induced plasticity in M1, some protocols show more convincing differences with advancing age than others. Therefore, our view from the NIBS literature is that it should not be automatically assumed that M1 plasticity declines with age. Instead, the effects of age are likely to depend on how M1 plasticity is measured, and the characteristics of the elderly population tested. We also suggest that NIBS performed concurrently with motor training is likely to be most effective at producing improvements in M1 plasticity and motor skill learning in older adults. Proposed NIBS techniques for future studies include combining multiple NIBS protocols in a co-stimulation approach, or NIBS strategies to modulate intracortical inhibitory mechanisms, in an effort to more effectively boost M1 plasticity and improve motor skill learning in older adults.John G. Semmler, Brodie J. Hand, Ryoki Sasaki, Ashley Merkin and George M. Opi