Biology subjects

Helms, G.

Publications and source records attributed to Helms, G..

4 recordsLinked to original sources

bioRxiv2024

Lifespan trajectory of chimpanzee brains characterized by magnetic resonance imaging histology

Chimpanzee brain maturation provides an invaluable framework for understanding the evolution of the human brain. We performed ultra-high resolution quantitative magnetic resonance imaging (qMRI) with histological validation on post mortem brains from captive and wild chimpanzees with a broad age range. We mapped developmental myelination and age-related iron accumulation across regions and layers of the neocortex. Compared to humans, chimpanzees showed more myelination and iron deposition in motor and premotor cortices, while the auditory cortex was more strongly myelinated in humans. Our model suggests that chimpanzees cortical myelination was largely completed by the age of nine years, while iron accumulation continued throughout the lifespan. The regions with highest adult levels of myelin and iron took the longest to mature, challenging the widespread assumption that highly myelinated regions complete their development first. The reported maps and developmental curves provide a foundation for comparative neuroscience research and understanding of human brain evolution.

neuroscience↗

bioRxiv2023

Cortical changes during the learning of sequences of simultaneous finger presses

The cortical alterations underpinning the acquisition of motor skills remain debated. In this longitudinal study in younger adults, we acquired performance and neuroimaging (7T MRI) measures weekly over the course of 6 weeks to investigate neural changes associated with learning sequences of simultaneous finger presses executed with the non-dominant hand. Both the intervention group (n = 33) and the control group (n = 30) showed general performance improvements, but performance improved more and became more consistent for sequences that were intensively trained by the intervention group, relative to those that were not. Brain activity for trained sequences decreased compared with untrained sequences in the bilateral parietal and premotor cortices. No training-related changes in the primary sensorimotor areas were detected. The similarity of activation patterns between trained and untrained sequences decreased in secondary, but not primary, sensorimotor areas, while the similarity of the activation patterns between different trained sequences did not show reliable changes. Neither the variability of activation patterns across trials, nor the estimates of brain structure displayed practice-related changes that reached statistical significance. Overall, the main correlate of learning configural sequences was a reduction in brain activity in secondary motor areas.

neuroscience↗

bioRxiv2022

B1+-correction of MT saturation maps optimized for 7T postmortem MRI of the brain

PurposeMagnetization transfer saturation (MTsat) is a useful marker to probe tissue macromolecular content and myelination in the brain. The increased [Formula] -inhomogeneity at [≥] 7T and significantly larger saturation pulse flip angles which are often used for postmortem studies exceed the limits where previous MTsat [Formula] correction methods are applicable. Here, we develop a calibration-based correction model and procedure, and validate and evaluate it in postmortem 7T data of whole chimpanzee brains. TheoryThe [Formula] dependence of MTsat was investigated by varying the off-resonance saturation pulse flip angle. For the range of saturation pulse flip angles applied in typical experiments on postmortem tissue, the dependence was close to linear. A linear model with a single calibration constant C is proposed to correct bias in MTsat by mapping it to the reference value of the saturation pulse flip angle. MethodsC was estimated voxel-wise in five postmortem chimpanzee brains. "Individual-based global parameters" were obtained by calculating the mean C within individual specimen brains and "group-based global parameters" by calculating the means of the individual-based global parameters across the five brains. ResultsThe linear calibration model described the data well, though C was not entirely independent of the underlying tissue and [Formula]. Individual-based and group-based global correction parameters (C = 1.2) led to visible, quantifiable reductions of [Formula]-biases in high resolution MTsat maps. ConclusionThe presented model and calibration approach effectively corrects for [Formula] in-homogeneities in postmortem 7T data.

neuroscience↗

bioRxiv2020

Non-negative least squares computation for in vivo myelin mapping using simulated multi-echo spin-echo T2 decay data

Multi-compartment T2-mapping has gained particular relevance for the study of myelin water in brain. As a facilitator of rapid saltatory axonal signal transmission, myelin is a cornerstone indicator of white matter development and function. Regularized non-negative least squares fitting of multi-echo T2 data has been widely employed for the computation of the myelin water fraction (MWF) and the obtained MWF maps have been histopathologically validated. MWF measurements depend upon the quality of the data acquisition, B1+-homogeneity and a range of fitting parameters. In this special issue article, we discuss the relevance of these factors for the accurate computation of multi-compartment T2 and MWF maps. We generated multi-echo spin-echo T2 decay curves following the approach of CarrPurcell-Meiboom-Gill for various myelin concentrations and myelin T2 scenarios by simulating the evolution of the magnetization vector between echoes based on the Bloch equations. We demonstrated that noise and imperfect refocusing flip angles yield systematic underestimations in MWF and intra-/extracellular water geometric mean (gm) T2. MWF estimates were more stable than myelin water gmT2 time across different settings of the T2 analysis. We observed that the lower limit of the T2 distribution grid should be slightly shorter than TE1. Both TE1 and the acquisition echo spacing also have to be sufficiently short to capture the rapidly decaying myelin water T2 signal. Among all parameters of interest, the estimated MWF and intra-/extracellular water gmT2 differed by approximately 0.13-4 percentage points and 3-4 ms, respectively, from the true values, with larger deviations observed in the presence of greater B1+-inhomogeneities and at lower signal-to-noise ratio. Tailoring acquisition strategies may allow to better characterize the T2 distribution, including the myelin water, in vivo.

biophysics↗