Facilitated event-related power modulations during transcranial alternating current stimulation (tACS) revealed by concurrent tACS-MEG

Transcranial alternating current stimulation (tACS) is increasingly used in cognitive neuroscience to study the causal role of brain oscillations for cognition. However, online effects of tACS largely remain a “black box” because of an intense electromagnetic artifact encountered during stimulation. The current study is the first to employ a spatial filtering approach to recover, and systematically study, event-related oscillatory dynamics during tACS, which could potentially be altered in various directions. TACS facilitated pre-existing patterns of oscillatory dynamics during the employed mental rotation task, but did not counteract or overwrite them. In addition, control analyses and a measure to quantify tACS artifact suppression are provided that can enrich future studies investigating tACS online effects

Sustained Aftereffect of alpha-tACS Lasts Up to 70 min after Stimulation

Transcranial alternating current stimulation (tACS) has been repeatedly demonstrated to increase power of endogenous brain oscillations in the range of the stimulated frequency after stimulation. In the alpha band this aftereffect has been shown to persist for at least 30 min. However, in most experiments the aftereffect exceeded the duration of the measurement. Thus, it remains unclear how the effect develops beyond these 30 min and when it decays. The current study aimed to extend existing findings by monitoring the physiological aftereffect of tACS in the alpha range for an extended period of 90 min post-stimulation. To this end participants received either 20 min of tACS or sham stimulation with intensities below their individual sensation threshold at the individual alpha frequency (IAF). Electroencephalogram (EEG) was acquired during 3 min before and 90 min after stimulation. Subjects performed a visual vigilance task during the whole measurement. While the enhanced power in the individual alpha band did not return back to pre-stimulation baseline in the stimulation group, the difference between stimulation and sham diminishes after 70 min due to a natural alpha increase of the sham group

Vegetative and edaphic responses in a Northern mixed conifer forest three decades after harvest and fire: Implications for managing regeneration and carbon and nitrogen pools

Research Highlights: This experiment compares a range of combinations of harvest, prescribed fire, and wildfire. Leveraging a 30-year-old forest management-driven experiment, we explored the recovery of woody species composition, regeneration of the charismatic forest tree species Larix occidentalis Nutt., and vegetation and soil carbon (C) and nitrogen (N) pools. Background and Objectives: Initiated in 1967, this experiment intended to explore combinations of habitat type phases and prescribed fire severity toward supporting regeneration of L. occidentalis. At onset of the experiment, a wildfire affected a portion of the 60 research plots, allowing for additional study. Our objective was to better understand silvicultural practices to support L. occidentalis regeneration and to better understand the subsequent impacts of silvicultural practices on C and N pools within the vegetation and soil. Materials and Methods: We categorized disturbance severity based on loss of forest floor depth; 11 categories were defined, including controls for the two habitat type phases involved. We collected abundance, biomass, and C and N concentrations for the herbaceous layer, shrubs, and trees using nested quadrats and 6 to 10 experimental units per disturbance category plot. Moreover, we systematically sampled woody residue from transects, and forest floor, soil wood, and mineral soil with a systematic grid of 16 soil cores per disturbance category plot. Results: We found that (1) disturbance severity affected shrub species richness, diversity, and evenness within habitat type phases; (2) L. occidentalis regenerates when fire is part of the disturbance; (3) N-fixing shrub species were more diverse in the hotter, drier plots; (4) recovery levels of C and N pools within the soil had surpassed or were closer to pre-disturbance levels than pools within the vegetation. Conclusions: We confirm that L. occidentalis regeneration and a diverse suite of understory shrub species can be supported by harvest and prescribed fire, particularly in southern and western aspects. We also conclude that these methods can regenerate L. occidentalis in cooler, moister sites, which may be important as this species’ climate niche shifts with climate change

Iron cycling and stable Fe isotope fractionation in Antarctic shelf sediments, King George Island

Iron (Fe) fluxes from reducing sediments and subglacial environments are potential sources of bioavailable Fe into the Southern Ocean. Stable Fe isotopes (δ56Fe ) are considered a proxy for Fe sources and reaction pathways, but respective data are scarce and Fe cycling in complex natural environments is not understood sufficiently to constrain respective δ56Fe “endmembers” for different types of sediments, environmental conditions, and biogeochemical processes. We present δ56Fe data from pore waters and sequentially extracted sedimentary Fe phases of two contrasting sites in Potter Cove (King George Island, Antarctic Peninsula), a bay that is affected by fast glacier retreat. Sediments close to the glacier front contain more easily reducible Fe oxides and pyrite and show a broader ferruginous zone, compared to sediments close to the icefree coast, where surficial oxic meltwater streams discharge into the bay. Pyrite in sediments close to the glacier front predominantly derives from eroded bedrock. For the high amount of easily reducible Fe oxides proximal to the glacier we suggest mainly subglacial sources, where Fe liberation from comminuted material beneath the glacier is coupled to biogeochemical weathering processes (likely pyrite oxidation or dissimilatory iron reduction, DIR). Our strongest argument for a subglacial source of the highly reactive Fe pool in sediments close to the glacier front is its predominantly negative δ56Fe signature that remains constant over the whole ferruginous zone. This implies in situ DIR does not significantly alter the stable Fe isotope composition of the accumulated Fe oxides. The nonetheless overall light δ56Fe signature of easily reducible Fe oxides suggests pre-depositional microbial cycling as it occurs in potentially anoxic subglacial environments. The strongest 56Fe-depletion in pore water and most reactive Fe oxides was observed in sediments influenced by oxic meltwater discharge. The respective site showed a condensed redox zonation and a pore water δ56Fe profile typical for in-situ Fe cycling. We demonstrate that the potential of pore water δ56Fe as a proxy for benthic Fe fluxes is not straight-forward due to its large variability in marine shelf sediments at small spatial scales (- 2.4‰ at the site proximal to oxic meltwater discharge vs. -0.9‰ at the site proximal to the marine glacier terminus, both at 2 cm sediment depth). The controlling factors are multifold and include the amount and reactivity of reducible Fe oxides and organic matter, the isotopic composition of the primary and secondary ferric substrates, sedimentation rates, and physical reworking (bioturbation, ice scraping). The application of δ56Fe geochemistry may prove valuable in investigating biogeochemical weathering and Fe cycling in subglacial environments. This requires, however (similarly to the use of δ56Fe for the quantification of benthic fluxes), that the spatial and temporal variability of the isotopic endmember is known and accounted for. Since geochemical data from subglacial environments are very limited, further studies are needed in order to sufficiently assess Fe cycling and fractionation at glacier beds and the composition of discharges from those areas

Characterizing low-frequency artifacts during transcranial temporal interference stimulation (tTIS)

Transcranial alternating current stimulation (tACS) is a well-established brain stimulation technique to modulate human brain oscillations. However, due to the strong electro-magnetic artifacts induced by the stimulation current, the simultaneous measurement of tACS effects during neurophysiological recordings in humans is challenging. Recently, transcranial temporal interference stimulation (tTIS) has been introduced to stimulate neurons at depth non-invasively. During tTIS, two high-frequency sine waves are applied, that interfere inside the brain, resulting in amplitude modulated waveforms at the target frequency. Given appropriate hardware, we show that neurophysiological data during tTIS may be acquired without stimulation artifacts at low-frequencies. However, data must be inspected carefully for possible low-frequency artifacts. Our results may help to design experimental setups to record brain activity during tTIS, which may foster our understanding of its underlying mechanisms.</p

Ongoing modifications to root system architecture of Pinus ponderosa growing on a sloped site revealed by tree-ring analysis

Abstract Our knowledge of the root system architecture of trees is still incomplete, especially concerning how biomass partitioning is regulated to achieve an optimal, but often unequal, distribution of resources. In addition, our comprehension of root system architecture development as a result of the adaptation process is limited because most studies lack a temporal approach. To add to our understanding, we excavated 32-year-old Pinus ponderosa trees from a steep, forested site in northern Idaho USA. The root systems were discretized by a low magnetic field digitizer and along with AMAPmod software we examined their root traits (i.e. order category, topology, growth direction length, and volume) in four quadrants: downslope, upslope, windward, and leeward. On one tree, we analyzed tree rings to compare the ages of lateral roots relative to their parental root, and to assess the occurrence of compression wood. We found that, from their onset, first-order lateral roots have similar patterns of ring eccentricity suggesting an innate ability to respond to different mechanical forces; more root system was allocated downslope and to the windward quadrant. In addition, we noted that shallow roots, which all presented compression wood, appear to be the most important component of anchorage. Finally, we observed that lateral roots can change growth direction in response to mechanical forces, as well as produce new lateral roots at any development stage and wherever along their axis. These findings suggest that trees adjust their root spatial deployment in response to environmental conditions, these roots form compression wood to dissipate mechanical forces, and new lateral roots can arise anywhere and at any time on the existing system in apparent response to mechanical forces

Similar EEG Activity Patterns During Experimentally-Induced Auditory Illusions and Veridical Perceptions

Hallucinations and illusions are two instances of perceptual experiences illustrating how perception might diverge from external sensory stimulations and be generated or altered based on internal brain states. The occurrence of these phenomena is not constrained to patient populations. Similar experiences can be elicited in healthy subjects by means of suitable experimental procedures. Studying the neural mechanisms underlying these experiences not only has the potential to expand our understanding of the brain's perceptual machinery but also of how it might get impaired. In the current study, we employed an auditory signal detection task to induce auditory illusions by presenting speech snippets at near detection threshold intensity embedded in noise. We investigated the neural correlates of auditory false perceptions by examining the EEG activity preceding the responses in speech absent (false alarm, FA) trials and comparing them to speech present (hit) trials. The results of the comparison of event-related potentials (ERPs) in the activation period vs. baseline revealed the presence of an early negativity (EN) and a late positivity (LP) similar in both hits and FAs, which were absent in misses, correct rejections (CR) and control button presses (BPs). We postulate that the EN and the LP might represent the auditory awareness negativity (AAN) and centro-parietal positivity (CPP) or P300, respectively. The event-related spectral perturbations (ERSPs) exhibited a common power enhancement in low frequencies