Biology subjects

Laakso, I.

Publications and source records attributed to Laakso, I..

6 recordsLinked to original sources

bioRxiv2026

Evaluation of Neuronal Activation Thresholds for Low-Frequency Electromagnetic Exposure Using Morphologically Realistic Neuron Models

International guidelines for low-frequency electromagnetic field exposure (LF EMF) are primarily intended to prevent substantiated adverse effects. In the frameworks, limits on internal electric fields are linked to external exposure levels through computational dosimetry. However, the relationship between internal electric fields and these adverse effects remains incompletely understood. In particular, current approaches often overlook the morphological complexity and diversity of cortical neurons, which may limit the realism of neuronal activation estimates used to support these assessments. This study evaluates LF EMF-induced neural activation using 25 morphologically realistic neuron models spanning all cortical layers, embedded within 11 detailed human head models. The internal electric fields were simulated for uniform magnetic field exposures (100 Hz-100 kHz) along the three anatomical directions, and excitation thresholds were computed using a multi-scale framework combining voxel-based dosimetry with biophysical neuron simulations. A real-world exposure scenario involving a child near an acousto-magnetic article-surveillance deactivator was also analyzed. Thresholds varied across cell type, morphology, cortical location, subject anatomy, frequency, and exposure direction, with L2/3 pyramidal, L4 basket, and L5 thick-tufted pyramidal cells showing the lowest thresholds. Despite this variability, all simulated thresholds were conservative with respect to the basic restrictions and dosimetric reference limits set by IEEE ICES and ICNIRP. The smallest margin occurred at 100 kHz, where the threshold remained a factor of 2.8 above the corresponding limit. These findings indicate that current LF EMF exposure limits remain conservative when evaluated using highly detailed, morphology-based CNS activation models.

neuroscience↗

bioRxiv2026

Distinct cortical excitability and connectivity profiles within the human SMA complex

Introduction and aimUnderstanding brain functions increasingly relies on a network-based perspective, emphasizing interactions across distributed regions. High-order cognitive functions, like language and executive processes, engage such networks rather than isolated cortical areas, yet mapping their dynamics in humans remains challenging. The supplementary motor area (SMA) complex serves as a crucial hub, functioning as an interface between cognitive and sensorimotor processing. Navigated transcranial magnetic stimulation combined with electroencephalography (TMS-EEG) enables causal, time-resolved assessment of cortical excitability and connectivity with high temporal and spatial precision. Extending prior proof-of-concept work, this study aims to provide normative neurophysiological signatures of the SMA complex (pre-SMA and SMA). MethodsTwenty-one healthy subjects underwent a TMS-EEG session where six stimulation targets over the SMA complex (three in pre-SMA, one in the border between pre-SMA and SMA, two in SMA) were recorded. Global cortical excitability via global mean field power (GMFP) was computed over three time-windows (10-50, 50-100, and 100-200 ms) and compared across targets. Normalized power in different frequency bands (, {beta}1, {beta}2, {gamma}) and natural frequency (NF) were also computed and compared across stimulation sites. Then, we calculated tractography-derived connectivity metrics to examine corticothalamic projections from each stimulation site. Apparent fiber density (AFD)-derived connection strength quantified intra-axonal fibre volume along pathways to the whole thalamus, whereas fibre bundle capacity (FBC) assessed the aggregate intra-axonal cross-sectional area at pathway endpoints, providing an estimate of information transmission capacity to each of the 23 thalamic nuclei. ResultsCortical excitability and oscillatory properties varied significantly across TMS targets. A gradient in cortical excitability was observed, where anterior and middle portions of pre-SMA exhibited significantly lower GMFP in the early post-stimulus phase (i.e., 10-50 ms) than SMA portions; interestingly, these differences were clearly dampened in a later time-window (50-100 ms) and disappeared after this period. Furthermore, higher and lower {beta}2 power were observed in anterior pre-SMA than posterior pre-SMA portions. No differences were observed in NF. Crucially, we also observed significant differences in whole- and single-nuclei thalamic structural connectivity. DiscussionThese findings provide direct human in vivo evidence for distinct neurophysiological and structural connectivity profiles of the pre-SMA and SMA, possibly reflecting their different cytoarchitectonic, myeloarchitectonic and connectivity properties. Variations in cortical excitability, oscillatory dynamics, and FBC along the rostro-caudal axis may mirror functional specialization, with pre-SMA supporting cognitive control processes and SMA engaging motor-related thalamocortical networks. Together, the results demonstrate the sensitivity of TMS-EEG and diffusion-weighted connectomics to measure fine-grained neurophysiological differences of corticothalamic projections, which can lead to individualized neuromodulation targeting.

neuroscience↗

bioRxiv2024

Transcranial electrical brain stimulation modulates the vestibular system

BackgroundTranscranial electrical stimulation (tES) techniques are widely used to modulate brain excitability, though their mechanisms remain unclear. ObjectiveWe aimed to demonstrate that an alternating current applied to several commonly used tES electrode montages can influence the vestibular system. MethodsBody-sway responses during alternating current stimulation at 4.6 or 4.8 Hz were measured in eight participants standing on a force platform. Sham and five active conditions, including common motor cortical tES electrode montages, were applied in a double-blind experiment. ResultsAll active conditions increased lateral body sway compared with sham at the stimulus frequency, with very large to huge effect sizes. The sway magnitude was proportional to the montage-specific electric field within the vestibular system, obtained from a computational model. The frequency and phase of the body sway were locked to the stimulus frequency and phase. An additional experiment in three participants showed that the sway response was the largest in the lower legs, similarly for both tES with a motor cortical electrode montage and an electrical vestibular stimulation montage. Together, the results suggest a direct effect of tES on the vestibular system. ConclusionUnwanted vestibular effects may interfere with the interpretation of tES studies across a wide range of typical tES frequencies and current intensities. HighlightsO_LIAlternating current stimulation produces a lateral body sway at the stimulation frequency. C_LIO_LILarger lateral electric field in the vestibular system produces a larger sway. C_LIO_LISeveral common tES montages can directly influence the vestibular system. C_LIO_LIUnwanted vestibular effects may complicate the interpretation of tES studies. C_LI

neuroscience↗

bioRxiv2019

Group-Level Analysis of Induced Electric Field in Deep Brain Regions by Different TMS Coils

Deep transcranial magnetic stimulation (dTMS) is a non-invasive technique used in treating depression. In this study, we computationally evaluate group-level dosage during dTMS with the aim of characterizing targeted deep brain regions to overcome the limitation of using individualized head models to characterize coil performance in a population.\n\nWe use an inter-subject registration method adapted to deep brain regions that enable projection of computed electric fields (EFs) from individual realistic head models (n = 18) to the average space of deep brain regions. The computational results showed consistent group-level hotspots of the EF in deep brain region with intensities between 20%-50% of the maximum EF in the cortex. Large co-activation in other brain regions was confirmed while half-value penetration depth from the cortical surface was smaller than 2 cm. The halo figure-8 assembly and halo circular assembly coils induced the highest EFs for caudate, putamen, and hippocampus.\n\nGeneralized induced EF maps of deep regions show target regions despite inter-individual difference. This is the first study that visualizes generalized target regions during dTMS and provides a method for making informed decisions during dTMS interventions in clinical practice.

biophysics↗

bioRxiv2018

Electric field dependent effects of motor cortical TDCS

Transcranial direct current stimulation (TDCS) can modulate motor cortical excitability. However, its after-effects are highly variable between individuals. Individual cranial and brain anatomy may contribute to this variability by producing varying electric fields in each subjects brain. Here we show that these fields are related to excitability changes following anodal TDCS of the primary motor cortex (M1). We found in two experiments (N=28 and N=9) that the after-effects of TDCS were proportional to the individual electric field in M1, calculated using MRI-based models. Individuals with the lowest and highest local electric fields in M1 tended to produce opposite changes in excitability. Furthermore, the effect was field-direction dependent and non-linear with stimulation duration or other experimental parameters. The electric field component pointing into the brain was negatively proportional to the excitability changes following 1 mA 20 min TDCS of right M1 (N=28); the effect was opposite after 1 mA 10 min TDCS of left M1 (N=9). Our results demonstrate that a large part of variability in the after-effects of motor cortical TDCS is due to inter-individual differences in the electric fields. We anticipate that individualized electric field dosimetry could be used to control the neuroplastic effects of TDCS, which is increasingly being explored as a treatment for various neuropsychiatric diseases.

neuroscience↗

bioRxiv2017

Assessing tACS-induced phosphene perception using closed-loop Bayesian optimization

Transcranial alternating current stimulation (tACS) can evoke illusory flash-like visual percepts known as phosphenes. The perception of phosphenes represents a major experimental challenge when studying tACS-induced effects on cognitive performance. Besides growing concerns that retinal phosphenes themselves could potentially have neuromodulatory effects, the perception of phosphenes may also modify the alertness of participants. Past research has shown that stimulation intensity, frequency and electrode montage affect phosphene perception. However, to date, the effect of an additional tACS parameter on phosphene perception has been completely overlooked: the relative phase difference between stimulation electrodes. This is a crucial and timely topic given the confounding nature of phosphene perception and the increasing number of studies reporting changes in cognitive function following tACS phase manipulations. However, studying phosphene perception for different frequencies and phases simultaneously is not tractable using standard approaches, as the physiologically plausible range of parameters results in a combinatorial explosion of experimental conditions, yielding impracticable experiment durations. To overcome this limitation, here we applied a Bayesian optimization approach to efficiently sample an exhaustive tACS parameter space. Moreover, unlike conventional methodology, which involves subjects judging the perceived phosphene intensity on a rating scale, our study leveraged the strength of human perception by having the optimization driven based on a subjects relative judgement. Applying Bayesian optimization for two different montages, we found that phosphene perception was affected by differences in the relative phase between cortical electrodes. The results were replicated in a second study involving new participants and validated using computational modelling. In summary, our results have important implications for the experimental design and conclusions drawn from future tACS studies investigating the effects of phase on cognition.

neuroscience↗