Dricot, L. | Search bioRxiv

bioRxiv2023

MRI-assessed locus coeruleus contrast and functional response are not associated in young and late middle-aged individuals

The brainstem locus coeruleus (LC) influences a broad range of brain processes, including cognition. The so-called LC contrast is an accepted marker of the integrity of the LC that consists of a local hyperintensity on specific Magnetic Resonance Imaging (MRI) structural images. The small size of the LC has, however, rendered its functional characterization difficult in humans, including in aging. A full characterization of the structural and functional characteristics of the LC in healthy young and late middle-aged individuals is needed to determine to potential roles of the LC in different medical conditions. Here, we wanted to determine whether the activation of the LC in a mismatch negativity task changes in aging and whether the LC functional response was associated to the LC contrast. We used Ultra-High Field (UHF) 7-Tesla functional MRI (fMRI) to record brain response during an auditory oddball task in 53 healthy volunteers, including 34 younger (age: 22.15y {+/-} 3.27; 29 women) and 19 late middle-aged (age: 61.05y {+/-} 5.3; 14 women) individuals. Whole-brain analyses confirmed brain responses in the typical cortical and subcortical regions previously associated with mismatch negativity. When focusing on the brainstem, we found a significant response in the rostral part of the LC probability mask generated based on individual LC images. Although bilateral, the activation was more extensive in the left LC. Individual LC activity was not significantly different between young and late middle-aged individuals. Critically, while the LC contrast was higher in older individuals, the functional response of the LC was not associated with its contrast. These findings show that the age-related alterations of the LC structural integrity may not necessarily be related to changes in its functional response. The results further indicate that LC responses could remain stable in healthy individuals aged 20 to 70.

neuroscience↗

bioRxiv2022

Brain morphometric changes in congenitally blind subjects: a 7 Tesla MRI study

We used ultra-high field (7 Tesla) magnetic resonance imaging (MRI) at submillimeter resolution to assess structural brain changes in congenitally blind (CB) compared to matched normal sighted control (SC) subject groups. Region-of-interest analysis revealed grey matter (GM) volumetric reductions in the CB group in left cuneus and occipital pole, right posterior collateral sulcus and right occipito-temporal medial lingual sulcus. Non-visual areas with GM reductions in CB included the left central, postcentral and superior frontal gyri, and the right subcallosal gyrus. In contrast, there were no significant group differences in cortical thickness when using stringent statistical criteria. Regional differences in white matter (WM) showed an overall pattern similar as that of GM changes, characterized by volume reductions in occipital, parietal and temporal areas, but with additional reductions in precuneus and medial orbitofrontal cortex. Differences in cortical curvature were mostly situated in the occipital cortex and bore a close relationship with areas showing GM alterations; they may be indicative of increased cortico-cortical connectivity of the visually-deprived occipital cortex. The CB group had GM reductions in the basal ganglia, i.e., caudate nucleus, putamen, nucleus accumbens, globus pallidus, and thalamus. Within the cerebellum, GM and WM volumes were also reduced in the CB. Segmentation of the thalamus, hippocampus and amygdala into anatomic divisions revealed GM reductions in a number of thalamic nuclei, a few hippocampal regions, but not within amygdala. There were no findings of increased volume or cortical thickness in the CB group. Together, these data reveal a multitude of GM and WM reductions in CB, comprising not only the occipital cortex, but also temporal, parietal, and prefrontal cortices, as well as the basal ganglia and cerebellum. These findings in the CB may seem at odds with the large literature showing that the visually-deprived occipital cortex becomes a multimodal cortex responding to diverse non-visual sensory and cognitive inputs. The seeming mismatch between morphological atrophy and enhanced multimodality of occipital areas combined with superior performance by the CB in various non-visual tasks poses a challenge for our understanding of brain plasticity.

neuroscience↗