Volumetric magnetic resonance imaging studies have shown that intense learning can be associated with grey matter volume increases in the adult brain. The underlying mechanisms are poorly understood. Here we used monocular deprivation in rats to analyze the mechanisms underlying use-dependent grey matter increases. Optometry for quantification of visual acuity was combined with volumetric magnetic resonance imaging and microscopic techniques in longitudinal and cross-sectional studies. We found an increased spatial vision of the open eye which was associated with a transient increase in the volumes of the contralateral visual and lateral entorhinal cortex. In these brain areas dendrites of neurons elongated, and there was a strong increase in the number of spines, the targets of synapses, which was followed by spine maturation and partial pruning. Astrocytes displayed a transient pronounced swelling and underwent a reorganization of their processes. The use-dependent increase in grey matter corresponded predominantly to the swelling of the astrocytes. Experience-dependent increase in brain grey matter volume indicates a gain of structure plasticity with both synaptic and astrocyte remodeling.

Purpose: Bell's palsy, a unilateral, idiopathic facial nerve palsy, is a common disorder that is generally followed by a good recovery of function. The aim of this study was to investigate the impact of such a transiently decreased motor control (without deafferentation) on the functional reorganization of the brain. Methods: To address this issue, functional MRI was applied to 10 patients in the acute state of Bell's palsy and after their complete clinical recovery. The functional paradigm consisted of unilateral facial movements with the affected as well as the non-affected side. Results: We found an overactivity of several brain areas contralateral to the palsy that are related to error detection and sensory-motor integration in the acute stage and motor integration and control in the follow-up. Functional connectivity was disrupted in the affected cortical motor system during the acute stage of Bell's palsy compared to the follow-up. This altered connectivity was found mostly between motor areas in the hemisphere contralateral to the paretic side, whereas the interhemispherical connectivity remained largely stable. Conclusion: Our results indicate that a transient peripheral deefferentation causes functional reorganization in the brain that partly persists even after an apparently complete clinical recovery.

Background. Functional connectivity is defined as the temporal correlation between spatially remote neurophysiological events. This method has become particularly useful for studying neuroplasticity to detect changes in the collaboration of brain areas during cortical reorganization. Methods. In this article, the authors longitudinally studied voxel-based morphometry and resting state functional magnetic resonance imaging 10 times in 1 patient during the course of Bell palsy (idiopathic facial nerve palsy) up to complete clinical recovery. Results. Morphometric analysis revealed a significant alteration in the face area of the primary motor cortex (M1) contralateral to the paretic face, with an initial increase in gray matter concentration. Functional connectivity analysis between the M1 and other parts of the facial motor network revealed acutely disrupted intrahemispheric connectivity but unaltered interhemispheric connectivity. The disrupted functional connectivity was most pronounced on the day of the onset of symptoms, with a subsequent return toward normal during the course of recovery. This time course was found to differ between the selected parts of the facial motor network. However, the increase in functional connectivity strength preceded clinical recovery in all areas and reached a stable level before the patient fully recovered. Conclusion. These results demonstrate that recovery from facial nerve palsy is complemented by cortical reorganization, with pronounced changes of functional connectivity that precede clinical recovery.