Arrow Research search

Author name cluster

Agnes Flöel

Possible papers associated with this exact author name in Arrow. This page groups case-insensitive exact name matches and is not a full identity disambiguation profile.

23 papers
1 author row

Possible papers

23

YNIMG Journal 2026 Journal Article

Harmonizing the stimulation dose of focal transcranial direct current stimulation across target sites

  • Axel Thielscher
  • Dayana Hayek
  • Oula Puonti
  • Ulrike Grittner
  • Felix Blankenburg
  • Rico Fischer
  • Gesa Hartwigsen
  • Shu-Chen Li

Focal transcranial direct current stimulation (tDCS) using center-surround electrode montages enables region-specific cortical targeting, and holds promise for both cognitive neuroscience and clinical interventions. However, systematic examinations of dose-response relationships and their regional differences are lacking, hampering informed selections of suited stimulation parameters. In this preparatory methodological study, we present a modeling-based framework to support harmonized empirical dose-response studies of focal tDCS across different target areas. It covers three steps: Determining the approximate electric field strength that had led to behavioral and physiological effects in related prior tDCS studies. In our case, this led to a field strength of 0.2 V/m on average across magnetic resonance images (MRIs) from 43 participants and eight target areas related to different cognitive and motor functions. Second, optimizing the radii of center-surround montages for each target area to - on average across participants - achieve the intended field strength while maximizing focality. An additional test of cross-sample generalization in an independent sample confirms that the intended target field strength is achieved on average for new participants. Third, the pre-determined montage radii and a method for the individualized positioning of the center-surround electrode montages are provided for prospective planning in empirical dose-response studies. By harmonizing the electric field strength between different target regions at the group level, but preserving inter-individual variability, our framework will enable systematic analyses to relate the field strength to behavioral and neuroimaging outcomes, and to assess differences of these relations across regions. The described computational tools are open-source, allowing other researchers to tailor our framework to their specific research questions; and are currently used in a multi-center study involving approximately 1000 datasets.

Details DOI

YNICL Journal 2026 Journal Article

Simulating tDCS-induced electric fields in stroke patients: Realistic-lesion head models are needed

  • Ikko Kimura
  • Marcus Meinzer
  • Daria Antonenko
  • Robert Darkow
  • Agnes Flöel
  • Axel Thielscher

INTRODUCTION: Transcranial direct current stimulation (tDCS) is tested as tool for post-stroke rehabilitation in aphasia, and individualized simulations of tDCS-induced electric fields (E-fields) can guide its application. However, the accuracy of simulations is challenged by complex and variable tissue properties of stroke lesions. Here, we assessed the impact of stroke lesions on tDCS-induced E-fields realistically in terms of lesion size, shape, and conductivity. METHODS: Structural and diffusion MRI datasets of stroke patients with aphasia (n = 13, six females, age = 38-70 years) and age-matched healthy controls (n = 13, eight females, age = 24-76 years) from a previous study were analyzed. Simulated E-fields were first compared between healthy head models with and without artificial lesions homogenously filled with cerebrospinal fluid. Then, the effects of lesion heterogeneity were tested by comparing E-fields for models of stroke patients with homogenous versus inhomogeneous (realistic) lesion conductivity informed by diffusion-to-conductivity mapping. RESULTS: Adding artificial lesions to healthy head models altered the E-field strengths (|E|) near the target region-of-interest (ROI) by up to 47%. Diffusion-to-conductivity mapping revealed substantial variability in lesion conductivities within and across patients. Modifying homogenous to realistic lesion models showed mostly small to moderate |E| differences within the ROI depending on montage type, lesion size, and lesion-to-target distance. CONCLUSION: Stroke lesions affect tDCS-induced E-fields with substantial variability across montages and individuals. These findings support the use of head models that include realistic representations of the shape, size and conductivity of the lesions to improve the accuracy of individualized tDCS simulations and guide personalized stimulation protocols in stroke rehabilitation.

Details DOI

YNICL Journal 2023 Journal Article

7T amygdala and hippocampus subfields in volumetry-based associations with memory: A 3-year follow-up study of early Alzheimer’s disease

  • Laura Göschel
  • Lea Kurz
  • Andrea Dell'Orco
  • Theresa Köbe
  • Peter Körtvélyessy
  • Ariane Fillmer
  • Semiha Aydin
  • Layla Tabea Riemann

INTRODUCTION: The hippocampus is the most prominent single region of interest (ROI) for the diagnosis and prediction of Alzheimer's disease (AD). However, its suitability in the earliest stages of cognitive decline, i.e., subjective cognitive decline (SCD), remains uncertain which warrants the pursuit of alternative or complementary regions. The amygdala might be a promising candidate, given its implication in memory as well as other psychiatric disorders, e.g. depression and anxiety, which are prevalent in SCD. In this 7 tesla (T) magnetic resonance imaging (MRI) study, we aimed to compare the contribution of volumetric measurements of the hippocampus, the amygdala, and their respective subfields, for early diagnosis and prediction in an AD-related study population. METHODS: Participants from a longitudinal study were grouped into SCD (n = 29), mild cognitive impairment (MCI, n = 23), AD (n = 22) and healthy control (HC, n = 31). All participants underwent 7T MRI at baseline and extensive neuropsychological testing at up to three visits (baseline n = 105, 1-year n = 78, 3-year n = 39). Analysis of covariance (ANCOVA) was used to assess group differences of baseline volumes of the amygdala and the hippocampus and their subfields. Linear mixed models were used to estimate the effects of baseline volumes on yearly changes of a z-scaled memory score. All models were adjusted to age, sex and education. RESULTS: Compared to the HC group, individuals with SCD showed smaller amygdala ROI volumes (range across subfields -11% to -1%), but not hippocampus ROI volumes (-2% to 1%) except for the hippocampus-amygdala-transition-area (-7%). However, cross-sectional associations between baseline memory and volumes were smaller for amygdala ROIs (std. ß [95% CI] ranging between 0.16 [0.08; 0.25] and 0.46 [0.31; 0.60]) than hippocampus ROIs (between 0.32 [0.19; 0.44] and 0.53 [0.40; 0.67]). Further, the association of baseline volumes with yearly memory change in the HC and SCD groups was similarly weak for amygdala ROIs and hippocampus ROIs. In the MCI group, volumes of amygdala ROIs were associated with a relevant yearly memory decline [95% CI] ranging between -0.12 [-0.24; 0.00] and -0.26 [-0.42; -0.09] for individuals with 20% smaller volumes than the HC group. However, effects were stronger for hippocampus ROIs with a corresponding yearly memory decline ranging between -0.21 [-0.35; -0.07] and -0.31 [-0.50; -0.13]. CONCLUSION: Volumes of amygdala ROIs, as determined by 7T MRI, might contribute to objectively and non-invasively identify patients with SCD, and thus aid early diagnosis and treatment of individuals at risk to develop dementia due to AD, however associations with other psychiatric disorders should be evaluated in further studies. The amygdala's value in the prediction of longitudinal memory changes in the SCD group remains questionable. Primarily in patients with MCI, memory decline over 3 years appears to be more strongly associated with volumes of hippocampus ROIs than amygdala ROIs.

Details DOI

YNIMG Journal 2022 Journal Article

Brain simulation as a cloud service: The Virtual Brain on EBRAINS

  • Michael Schirner
  • Lia Domide
  • Dionysios Perdikis
  • Paul Triebkorn
  • Leon Stefanovski
  • Roopa Pai
  • Paula Prodan
  • Bogdan Valean

The Virtual Brain (TVB) is now available as open-source services on the cloud research platform EBRAINS (ebrains.eu). It offers software for constructing, simulating and analysing brain network models including the TVB simulator; magnetic resonance imaging (MRI) processing pipelines to extract structural and functional brain networks; combined simulation of large-scale brain networks with small-scale spiking networks; automatic conversion of user-specified model equations into fast simulation code; simulation-ready brain models of patients and healthy volunteers; Bayesian parameter optimization in epilepsy patient models; data and software for mouse brain simulation; and extensive educational material. TVB cloud services facilitate reproducible online collaboration and discovery of data assets, models, and software embedded in scalable and secure workflows, a precondition for research on large cohort data sets, better generalizability, and clinical translation.

Details DOI

YNIMG Journal 2021 Journal Article

Inter-individual and age-dependent variability in simulated electric fields induced by conventional transcranial electrical stimulation

  • Daria Antonenko
  • Ulrike Grittner
  • Guilherme Saturnino
  • Till Nierhaus
  • Axel Thielscher
  • Agnes Flöel

Variations in head and brain anatomy determine the strength and distribution of electrical fields in humans and may account for inconsistent behavioral and neurophysiological results in transcranial electrical stimulation (tES) studies. However, it is insufficiently understood which anatomical features contribute to the variability of the modelled electric fields, and if their impact varies across age groups. In the present study, we tested the associations of global head anatomy, indexed by extra- and intra-cranial volumes, with electric field measures, comparing young and older adults. We modelled six "conventional" electrode montages typically used in tES studies using SimNIBS software in 40 individuals (20 young, 20 older adults; 20-35, 64-79 years). We extracted individual electric field strengths and focality values for each montage to identify tissue volumes that account for variability of the induced electric fields in both groups. Linear mixed models explained most of the inter-individual variability of the overall induced field strength in the brain, but not of field focality. Higher absolute head volume and relative volume of skin, skull and cerebrospinal fluid (CSF) were associated with lower overall electric field strengths. Additionally, we found interactions of age group with head volume and CSF, indicating that this relationship was mitigated in the older group. Our results demonstrate the importance to adjust brain stimulation not only according to brain atrophy, but also to additional parameters of head anatomy. Future studies need to elucidate the mechanisms underlying individual variability of tES effects in young and older adults, and verify the usefulness of the proposed models in terms of neurophysiology and behavior in empirical studies.

Details DOI

YNIMG Journal 2020 Journal Article

Beneficial effects of cerebellar tDCS on motor learning are associated with altered putamen-cerebellar connectivity: A simultaneous tDCS-fMRI study

  • Matthias Liebrand
  • Anke Karabanov
  • Daria Antonenko
  • Agnes Flöel
  • Hartwig R. Siebner
  • Joseph Classen
  • Ulrike M. Krämer
  • Elinor Tzvi

Non-invasive transcranial stimulation of cerebellum and primary motor cortex (M1) has been shown to enhance motor learning. However, the mechanisms by which stimulation improves learning remain largely unknown. Here, we sought to shed light on the neural correlates of transcranial direct current stimulation (tDCS) during motor learning by simultaneously recording functional magnetic resonance imaging (fMRI). We found that right cerebellar tDCS, but not left M1 tDCS, led to enhanced sequence learning in the serial reaction time task. Performance was also improved following cerebellar tDCS compared to sham in a sequence production task, reflecting superior training effects persisting into the post-training period. These behavioral effects were accompanied by increased learning-specific activity in right M1, left cerebellum lobule VI, left inferior frontal gyrus and right inferior parietal lobule during cerebellar tDCS compared to sham. Despite the lack of group-level changes comparing left M1 tDCS to sham, activity increase in right M1, supplementary motor area, and bilateral middle frontal cortex, under M1 tDCS, was associated with better sequence performance. This suggests that lack of group effects in M1 tDCS relate to inter-individual variability in learning-related activation patterns. We further investigated how tDCS modulates effective connectivity in the cortico-striato-cerebellar learning network. Using dynamic causal modelling, we found altered connectivity patterns during both M1 and cerebellar tDCS when compared to sham. Specifically, during cerebellar tDCS, negative modulation of a connection from putamen to cerebellum was decreased for sequence learning only, effectively leading to decreased inhibition of the cerebellum. These results show specific effects of cerebellar tDCS on functional activity and connectivity in the motor learning network and may facilitate the optimization of motor rehabilitation involving cerebellar non-invasive stimulation.

Details DOI