Biology subjects

Zetter, R.

Publications and source records attributed to Zetter, R..

5 recordsLinked to original sources

bioRxiv2025

Mapping of individual somatosensory representations - comparison of fMRI and TMS

Invasive brain mapping, functional magnetic resonance imaging (fMRI) and navigated transcranial magnetic stimulation (nTMS) results indicate that somatosensory representations vary between individuals. However, it is unknown how well somatosensory representations determined using nTMS and fMRI correspond. Here, we used single-pulse nTMS and fMRI in 17 right-handed subjects to determine the S1 representation of the tip of the right index finger stimulated mechanically with a Braille device. In an nTMS mapping experiment, the S1 site at which tactile sensation was blocked by nTMS, was considered the S1 representation site (S1HS) of the fingertip. The location of S1HS varied up to 36 mm between subjects. In the fMRI experiment, passive and oddball tactile tasks were employed. The location of the somatosensory peak fMRI activation varied up to 39 mm between subjects in the passive condition and up to 84 mm in the oddball condition. Within subjects, the mean distance between the S1HS and the peak fMRI activation was 10 + 2 mm (S.E.M.) in the passive condition and 14 + 3 mm in the oddball task. Taken together, our findings underscore the importance of multimodal approaches to brain mapping in future studies.

neuroscience↗

bioRxiv2019

Potential of on-scalp MEG: Robust detection of human visual gamma-band responses

Electrophysiological signals recorded intracranially show rich frequency content spanning from near-DC to hundreds of hertz. Noninvasive electromagnetic signals measured with electroencephalography (EEG) or magnetoencephalography (MEG) typically contain less signal power in high frequencies than invasive recordings. Particularly, noninvasive detection of gamma-band activity (> 30 Hz) is challenging since coherently active source areas are small at such frequencies and the available imaging methods have limited spatial resolution. Compared to EEG and conventional SQUID-based MEG, on-scalp MEG should provide substantially improved spatial resolution, making it an attractive method for detecting gamma-band activity.\n\nUsing an on-scalp array comprised of eight optically-pumped magnetometers (OPMs) and a conventional whole-head SQUID array, we measured responses to a dynamic visual stimulus known to elicit strong gamma-band responses. OPMs had substantially higher signal power than SQUIDs, and had a slightly larger relative gamma-power increase over the baseline. With only eight OPMs, we could obtain gamma-activity source estimates comparable to those of SQUIDs at the group level.\n\nOur results show the feasibility of OPMs to measure gamma-band activity. To further facilitate the noninvasive detection of gamma-band activity, the on-scalp OPM arrays should be optimized with respect to sensor noise, the number of sensors and inter-sensor spacing.

neuroscience↗

bioRxiv2018

Optical Co-registration of MRI and On-scalp MEG

Objective: To estimate the neural current distribution underlying magnetoencephalographic (MEG) signals and to link such estimates to brain anatomy, MEG data have to be co-registered with an anatomical image, typically an MR image. Optically-pumped magnetometers (OPMs) enable the construction of on-scalp MEG systems providing higher sensitivity and spatial resolution than conventional SQUID-based MEG systems. Here, we present a co-registration method that can be applied to on-scalp MEG systems, regardless of the number of channels. Methods: We apply a structured-light 3D scanner to create a surface mesh of the subject's head and the sensor array, which we fit to the MR image. To assess accuracy, we quantified the reproducibility of the surface mesh and localized current dipoles embedded in a phantom. Finally, we measured somatosensory evoked fields (SEF) to median nerve stimulation and compared their source estimates with those obtained with a SQUID-based MEG system. Results: The structured-light scanner reproduced the head surface with < 1 mm error. Phantom dipoles were localized with a mean error of 2.14 mm. The difference in SEF dipole positions between OPMs and SQUIDs were 5.0, 0.9, and 1.6 mm for N20m, P35m and P60m response peaks, respectively. Conclusion: The developed co-registration is inexpensive, fast and can easily be applied to on-scalp MEG. It is also more convenient to use than traditional co-registration methods while also being more accurate. Significance: We developed and validated a co-registration method that can be applied to on-scalp MEG systems. The method enables accurate source estimation with these novel MEG systems.

neuroscience↗

bioRxiv2018

On-scalp MEG system utilizing an actively shielded array of optically-pumped magnetometers

The spatial resolution of magnetoencephalography (MEG) can be increased from that of conventional SQUID-based systems by employing on-scalp sensor arrays of e.g. optically-pumped magnetometers (OPMs). However, OPMs reach sufficient sensitivity for neuromagnetic measurements only when operated in a very low absolute magnetic field of few nanoteslas or less, usually not reached in a typical magnetically shielded room constructed for SQUID-based MEG. Moreover, field drifts affect the calibration of OPMs. Static and dynamic control of the ambient field is thus necessary for good-quality neuromagnetic measurements with OPMs. Here, we describe an on-scalp MEG system that utilizes OPMs and external compensation coils that provide static and dynamic shielding against ambient fields.\n\nIn a conventional two-layer magnetically shielded room, our coil system reduced the maximum remanent DC-field component within an 8-channel OPM array from 70 to less than 1 nT, enabling the sensors to operate in the sensitive spin exchange relaxation-free regime. When compensating field drifts below 4 Hz, a low-frequency shielding factor of 22 dB was achieved, which reduced the peak-to-peak drift from 1.3 to 0.4 nT and thereby the standard deviation of the sensor calibration from 1.6% to 0.4%. Without band-limiting the field that is compensated, a low-frequency shielding factor of 43 dB was achieved.\n\nWe validated the system by measuring brain responses to electric stimulation of the median nerve. With dynamic shielding and digital interference suppression methods, single-trial somatosensory evoked responses could be detected. Our results advance the deployment of OPM-based on-scalp MEG in lighter magnetic shields.

neuroscience↗

bioRxiv2017

Requirements for coregistration accuracy in on-scalp MEG

Recent advances in magnetic sensing has made on-scalp magnetoencephalography (MEG) possible. In particular, optically-pumped magnetometers (OPMs) have reached sensitivity levels that enable their use in MEG. In contrast to the SQUID sensors used in current MEG systems, OPMs do not require cryogenic cooling and can thus be placed within millimetres from the head, enabling the construction of sensor arrays that conform to the shape of an individuals head. To properly estimate the location of neural sources within the brain, one must accurately know the position and orientation of sensors in relation to the head. With the adaptable on-scalp MEG sensor arrays, this coregistration becomes more challenging than in current SQUID-based MEG systems that use rigid sensor arrays.\n\nHere, we used simulations to quantify how accurately one needs to know the position and orientation of sensors in an on-scalp MEG system. The effects that different types of localisation errors have on forward modelling and source estimates obtained by minimum-norm estimation, dipole fitting, and beamforming are detailed.\n\nWe found that sensor position errors generally have a larger effect than orientation errors and that these errors affect the localisation accuracy of superficial sources the most. To obtain similar or higher accuracy than with current SQUID-based MEG systems, RMS sensor position and orientation errors should be < 4 mm and < 10{degrees}, respectively.

neuroscience↗