Nils Bodammer

Experience-dependent plasticity of white-matter microstructure extends into old age

Experience-dependent alterations in the human brains white-matter microstructure occur in early adulthood, but it is unknown whether such plasticity extends throughout life. We used cognitive training, diffusion-tensor imaging (DTI), and structural MRI to investigate plasticity of the white-matter tracts that connect the left and right hemisphere of the frontal lobes. Over a period of about 180 days, 20 younger adults and 12 older adults trained for a total of one hundred and one 1-h sessions on a set of three working memory, three episodic memory, and six perceptual speed tasks. Control groups were assessed at pre- and post-test. Training affected several DTI metrics and increased the area of the anterior part of the corpus callosum. These alterations were of similar magnitude in younger and older adults. The findings indicate that experience-dependent plasticity of white-matter microstructure extends into old age and that disruptions of structural interhemispheric connectivity in old age, which are pronounced in aging, are modifiable by experience and amenable to treatment.

Eddy current correction in diffusion-weighted imaging using pairs of images acquired with opposite diffusion gradient polarity.

In echo‐planar‐based diffusion‐weighted imaging (DWI) and diffusion tensor imaging (DTI), the evaluation of diffusion parameters such as apparent diffusion coefficients and anisotropy indices is affected by image distortions that arise from residual eddy currents produced by the diffusion‐sensitizing gradients. Correction methods that coregister diffusion‐weighted and non‐diffusion‐weighted images suffer from the different contrast properties inherent in these image types. Here, a postprocessing correction scheme is introduced that makes use of the inverse characteristics of distortions generated by gradients with reversed polarity. In this approach, only diffusion‐weighted images with identical contrast are included for correction. That is, non‐diffusion‐weighted images are not needed as a reference for registration. Furthermore, the acquisition of an additional dataset with moderate diffusion‐weighting as suggested by Haselgrove and Moore (Magn Reson Med 1996;36:960–964) is not required. With phantom data it is shown that the theoretically expected symmetry of distortions is preserved in the images to a very high degree, demonstrating the practicality of the new method. Results from human brain images are also presented. Magn Reson Med 51:188–193, 2004.

Differentiation of idiopathic Parkinson's disease, multiple system atrophy, progressive supranuclear palsy, and healthy controls using magnetization transfer imaging

The differentiation of multiple system atrophy (MSA) and progressive supranuclear palsy (PSP) from idiopathic Parkinsons disease (IPD) is difficult. Magnetization transfer imaging (MTI), a measure that correlates with myelination and axonal density, was employed in this study in the attempt to distinguish between these disorders. Measurements were carried out in 15 patients with IPD, 12 patients with MSA, 10 patients with PSP, and in 20 aged-matched healthy control subjects. The main finding was a change in the magnetization transfer ratio in the globus pallidus, putamen, caudate nucleus, substantia nigra, and white matter in IPD, MSA, and PSP patients, matching the pathological features of the underlying disorder. Furthermore, stepwise linear discriminant analysis provided a good classification of the individual patients into the different disease groups. All IPD patients and control subjects were correctly separated from the MSA and PSP cohort, and all PSP patients and 11 of 12 MSA patients were correctly separated from the IPD and control cohort. There was also a fairly good discrimination of IPD patients from control subjects and of MSA from PSP patients. In conclusion, MTI revealed degenerative changes in patients with different parkinsonian syndromes matching the underlying pathological features of the different diseases, underlining the high potential of this method in distinguishing MSA and PSP from IPD.

Growth of language-related brain areas after foreign language learning

The influence of adult foreign-language acquisition on human brain organization is poorly understood. We studied cortical thickness and hippocampal volumes of conscript interpreters before and after three months of intense language studies. Results revealed increases in hippocampus volume and in cortical thickness of the left middle frontal gyrus, inferior frontal gyrus, and superior temporal gyrus for interpreters relative to controls. The right hippocampus and the left superior temporal gyrus were structurally more malleable in interpreters acquiring higher proficiency in the foreign language. Interpreters struggling relatively more to master the language displayed larger gray matter increases in the middle frontal gyrus. These findings confirm structural changes in brain regions known to serve language functions during foreign-language acquisition.

Hippocampal Subfield Volumes: Age, Vascular Risk, and Correlation with Associative Memory

Aging and age-related diseases have negative impact on the hippocampus (HC), which is crucial for such age-sensitive functions as memory formation, maintenance, and retrieval. We examined age differences in hippocampal subfield volumes in 10 younger and 19 older adults, and association of those volumes with memory performance in the older participants. We manually measured volumes of HC regions CA1 and CA2 (CA1–2), sectors CA3 and CA4 plus dentate gyrus (CA3–4/DG), subiculum, and the entorhinal cortex using a contrast-optimized high-resolution PD-weighted MRI sequence. Although, as in previous reports, the volume of one region (CA1–2) was larger in the young, the difference was due to the presence of hypertensive subjects among the older adults. Among older participants, increased false alarm rate in an associative recognition memory task was linked to reduced CA3–4/DG volume. We discuss the role of the DG in pattern separation and the formation of discrete memory representations.

Spatial navigation training protects the hippocampus against age-related changes during early and late adulthood

It is unknown whether lifestyle, including mental stimulation, and appropriate training interventions, may directly improve spatial navigation performance and its underlying neural substrates. Here we report that healthy younger and older men performing a cognitively demanding spatial navigation task every other day over 4 months display navigation-related gains in performance and stable hippocampal volumes that were maintained 4 months after termination of training. In contrast, control groups displayed volume decrements consistent with longitudinal estimates of age-related decline. Hippocampal barrier density, as indicated by mean diffusivity estimated from diffusion tensor imaging, showed a quadratic shape of increased density after training followed by a return to baseline in the right hippocampus, but declined in the control groups and in the left hippocampus. We conclude that sustained experiential demands on spatial ability protect hippocampal integrity against age-related decline. These results provide the first longitudinal evidence indicating that spatial navigation experience modifies hippocampal volumes in humans, and confirm epidemiological results suggesting that mental stimulation may have direct effects on neural integrity.

Age-related water diffusion changes in human brain: A voxel-based approach

The aim of the present study is to investigate age-related changes in water self-diffusion in cerebral white matter by analysing diffusion-weighted MRI from a sample of 54 healthy volunteers. A voxel-based analysis of the relative anisotropy and the apparent diffusion coefficients was performed by applying an optimized normalization protocol. Linear regression analysis revealed significant correlations with age in the corpus callosum, prefrontal regions, the internal capsule, the hippocampal complex, and the putamen. However, in other regions, such as those surrounding the ventricles, the insula, or the inferior frontal plane, significant correlations between age and ADC were observed, presumably as a result of morphological age-related variations. A mask procedure was carried out in order to distinguish between morphological involvement and real age-related white matter changes. Our results indicate that in interpreting the changes in each significant region it is necessary to proceed with precaution because the voxel-based statistical analysis might yield a mixture of two effects: (i) morphological changes that remain after the normalization procedure and (ii) actual diffusivity parameter changes. Anatomically defined regions of interest may help us to minimize morphologic involvement and draw comparisons with findings previously published.

Comparing Manual and Automatic Segmentation of Hippocampal Volumes: Reliability and Validity Issues in Younger and Older Brains

We compared hippocampal volume measures obtained by manual tracing to automatic segmentation with FreeSurfer in 44 younger (20–30 years) and 47 older (60–70 years) adults, each measured with magnetic resonance imaging (MRI) over three successive time points, separated by four months. Retest correlations over time were very high for both manual and FreeSurfer segmentations. With FreeSurfer, correlations over time were significantly lower in the older than in the younger age group, which was not the case with manual segmentation. Pearson correlations between manual and FreeSurfer estimates were sufficiently high, numerically even higher in the younger group, whereas intra‐class correlation coefficient (ICC) estimates were lower in the younger than in the older group. FreeSurfer yielded higher volume estimates than manual segmentation, particularly in the younger age group. Importantly, FreeSurfer consistently overestimated hippocampal volumes independently of manually assessed volume in the younger age group, but overestimated larger volumes in the older age group to a less extent, introducing a systematic age bias in the data. Age differences in hippocampal volumes were significant with FreeSurfer, but not with manual tracing. Manual tracing resulted in a significant difference between left and right hippocampus (right > left), whereas this asymmetry effect was considerably smaller with FreeSurfer estimates. We conclude that FreeSurfer constitutes a feasible method to assess differences in hippocampal volume in young adults. FreeSurfer estimates in older age groups should, however, be interpreted with care until the automatic segmentation pipeline has been further optimized to increase validity and reliability in this age group. Hum Brain Mapp 35:4236–4248, 2014.

The dynamics of change in striatal activity following updating training

Increases in striatal activity have been suggested to mediate training‐related improvements in working‐memory ability. We investigated the temporal dynamics of changes in task‐related brain activity following training of working memory. Participants in an experimental group and an active control group, trained on easier tasks of a constant difficulty in shorter sessions than the experimental group, were measured before, after about 1 week, and after more than 50 days of training. In the experimental group an initial increase of working‐memory related activity in the functionally defined right striatum and anatomically defined right and left putamen was followed by decreases, resulting in an inverted u‐shape function that relates activity to training over time. Activity increases in the striatum developed slower in the active control group, observed at the second posttest after more than 50 days of training. In the functionally defined left striatum, initial activity increases were maintained after more extensive training and the pattern was similar for the two groups. These results shed new light on the relation between activity in the striatum (especially the putamen) and the effects of working memory training, and illustrate the importance of multiple measurements for interpreting effects of training on regional brain activity. Hum Brain Mapp, 2013.

High-Field fMRI Reveals Brain Activation Patterns Underlying Saccade Execution in the Human Superior Colliculus

Background The superior colliculus (SC) has been shown to play a crucial role in the initiation and coordination of eye- and head-movements. The knowledge about the function of this structure is mainly based on single-unit recordings in animals with relatively few neuroimaging studies investigating eye-movement related brain activity in humans. Methodology/Principal Findings The present study employed high-field (7 Tesla) functional magnetic resonance imaging (fMRI) to investigate SC responses during endogenously cued saccades in humans. In response to centrally presented instructional cues, subjects either performed saccades away from (centrifugal) or towards (centripetal) the center of straight gaze or maintained fixation at the center position. Compared to central fixation, the execution of saccades elicited hemodynamic activity within a network of cortical and subcortical areas that included the SC, lateral geniculate nucleus (LGN), occipital cortex, striatum, and the pulvinar. Conclusions/Significance Activity in the SC was enhanced contralateral to the direction of the saccade (i.e., greater activity in the right as compared to left SC during leftward saccades and vice versa) during both centrifugal and centripetal saccades, thereby demonstrating that the contralateral predominance for saccade execution that has been shown to exist in animals is also present in the human SC. In addition, centrifugal saccades elicited greater activity in the SC than did centripetal saccades, while also being accompanied by an enhanced deactivation within the prefrontal default-mode network. This pattern of brain activity might reflect the reduced processing effort required to move the eyes toward as compared to away from the center of straight gaze, a position that might serve as a spatial baseline in which the retinotopic and craniotopic reference frames are aligned.