Differences In Reorganization Research Articles

The Role of Visual Experience in Individual Differences of Brain Connectivity.

Visual cortex organization is highly consistent across individuals. But to what degree does this consistency depend on life experience, in particular sensory experience? In this study, we asked whether visual cortex reorganization in congenital blindness results in connectivity patterns that are particularly variable across individuals, focusing on resting-state functional connectivity (RSFC) patterns from the primary visual cortex. We show that the absence of shared visual experience results in more variable RSFC patterns across blind individuals than sighted controls. Increased variability is specifically found in areas that show a group difference between the blind and sighted in their RSFC. These findings reveal a relationship between brain plasticity and individual variability; reorganization manifests variably across individuals. We further investigated the different patterns of reorganization in the blind, showing that the connectivity to frontal regions, proposed to have a role in the reorganization of the visual cortex of the blind toward higher cognitive roles, is highly variable. Further, we link some of the variability in visual-to-frontal connectivity to another environmental factor—duration of formal education. Together, these findings show a role of postnatal sensory and socioeconomic experience in imposing consistency on brain organization. By revealing the idiosyncratic nature of neural reorganization, these findings highlight the importance of considering individual differences in fitting sensory aids and restoration approaches for vision loss.SIGNIFICANCE STATEMENT The typical visual system is highly consistent across individuals. What are the origins of this consistency? Comparing the consistency of visual cortex connectivity between people born blind and sighted people, we showed that blindness results in higher variability, suggesting a key impact of postnatal individual experience on brain organization. Further, connectivity patterns that changed following blindness were particularly variable, resulting in diverse patterns of brain reorganization. Individual differences in reorganization were also directly affected by nonvisual experiences in the blind (years of formal education). Together, these findings show a role of sensory and socioeconomic experiences in creating individual differences in brain organization and endorse the use of individual profiles for rehabilitation and restoration of vision loss.

Age-Related Differences in Reorganization of Functional Connectivity for a Dual Task with Increasing Postural Destabilization.

The aged brain may not make good use of central resources, so dual task performance may be degraded. From the brain connectome perspective, this study investigated dual task deficits of older adults that lead to task failure of a suprapostural motor task with increasing postural destabilization. Twelve younger (mean age: 25.3 years) and 12 older (mean age: 65.8 years) adults executed a designated force-matching task from a level-surface or a stabilometer board. Force-matching error, stance sway, and event-related potential (ERP) in the preparatory period were measured. The force-matching accuracy and the size of postural sway of the older adults tended to be more vulnerable to stance configuration than that of the young adults, although both groups consistently showed greater attentional investment on the postural task as sway regularity increased in the stabilometer condition. In terms of the synchronization likelihood (SL) of the ERP, both younger and older adults had net increases in the strengths of the functional connectivity in the whole brain and in the fronto-sensorimotor network in the stabilometer condition. Also, the SL in the fronto-sensorimotor network of the older adults was greater than that of the young adults for both stance conditions. However, unlike the young adults, the older adults did not exhibit concurrent deactivation of the functional connectivity of the left temporal-parietal-occipital network for postural-suprapostural task with increasing postural load. In addition, the older adults potentiated functional connectivity of the right prefrontal area to cope with concurrent force-matching with increasing postural load. In conclusion, despite a universal negative effect on brain volume conduction, our preliminary results showed that the older adults were still capable of increasing allocation of neural sources, particularly via compensatory recruitment of the right prefrontal loop, for concurrent force-matching under the challenging postural condition. Nevertheless, dual-task performance of the older adults tended to be more vulnerable to postural load than that of the younger adults, in relation to inferior neural economy or a slow adaptation process to stance destabilization for scant dissociation of control hubs in the temporal-parietal-occipital cortex.

Sensorimotor cortex reorganization in subacute and chronic stroke: A neuronavigated TMS study.

The integrity of the corticospinal system is an important biomarker for recovery from stroke. However, mapping the topography of the corticospinal system in subacute stroke is not trivial and how it changes over the course of recovery is poorly understood. We intend to use a transcranial magnetic stimulation (TMS) based mapping approach to quantify the topographic landscape of corticospinal activation in the ipsi- and contralesional sensorimotor cortices in the subacute and chronic phase of stroke. Mapping was conducted before (PRE) and after (POST), intervention in 10 chronic subjects and 8 subacute subjects. Reorganization was quantified in a unique way by dissociating reorganization attributed to changes in the expanse (area) of the sensorimotor territory, from that attributed to changes in the robustness of the activation (amplitude). In doing so, we observed differences in reorganization in the subacute and chronic stages indicating that recovery in different stages may not be guided by similar neurophysiological mechanisms of neuroplasticity.

Event-related potentials indicate bi-hemispherical changes in speech sound processing during aphasia rehabilitation

To investigate changes in brain activation related to tone and speech sound processing during aphasia rehabilitation. Longitudinal study investigating patients with stroke, subarachnoid hemorrhage and traumatic brain injury 3 and 7 months post-injury. Eight patients with aphasia, reflecting a wide range of auditory comprehension impairment. Token test and Norwegian Basic Aphasia Assessment were used to measure auditory comprehension function. Brain event-related potentials were recorded in passive paradigms with harmonically rich tones and syllables in order to obtain the mismatch negativity component that reflects automatic stimulus discrimination. In an active syllable discrimination paradigm, stimulus feature integration (N1), attended stimulus discrimination and classification (N2), and target detection (P3) were studied. Auditory comprehension scores improved approximately 10% during the observation period. Ipsilesional frontal P3- and N2-amplitude increased significantly. A significant shift in topographical distribution from the contralesional to the ipsilesional hemisphere was observed for the N2 component. The study of individual waveforms indicates inter-individual differences in reorganization after brain injury. Hemispherical distribution of brain activation correlating with speech sound processing in aphasia can change during the first months after brain injury. Event-related potentials are a potentially useful method for detecting individual activation patterns relevant to recovery in aphasia rehabilitation.

Residual Motor Control and Cortical Representations of Function Following Hemispherectomy: Effects of Etiology

Fifteen posthemispherectomy children were examined to assess residual motor function of the paretic side using the 74-point Fugl-Meyer Assessment of Motor Recovery scale. The degree of residual motor control differed for upper and lower extremities, with hand function being most severely impaired. Posthemispherectomy motor outcomes also differed as a function of etiology: cortical dysplasia, perinatal infarct, and Rasmussen's encephalitis. Children whose intractable seizures resulted from perinatal middle cerebral artery stroke demonstrated the most spared motor function. To detect cortical areas that represented motor control of the hemiparetic side, we focused on voluntary control of the affected lower extremity. Seven of our patients were able to carry out a foot dorsiflexion paradigm during functional magnetic resonance imaging, and these results were compared with activations found in normal controls. All children showed activations in the sensorimotor network ipsilateral to the affected side. The perinatal infarct group demonstrated greater activity in the cingulate cortex, whereas the Rasmussen's encephalitis group had significant activations in the insula, suggesting etiology-specific differences in reorganization. These findings are discussed in the framework of our understanding of mechanisms of cortical plasticity in the injured brain and its relevance to neurologic rehabilitation. We suggest that imaging techniques are important tools in identifying cortical regions underlying functional reorganization. Furthermore, detection of such areas might become a basis for specific training promoting the optimal reorganization of cortical networks to enhance motor control.

Plasticity in cerebellar tactile maps in the adult rat

We previously demonstrated that the fractured tactile cerebellar map within the crus IIa folia of the cerebellar hemispheres reorganizes after deafferentation of the upper lip in neonatal rats (postnatal day [PND] 1-30). The present study examined the capacity of this map to reorganize after deafferentation in adults and animals late in development (PND 30-89). Several months after cauterization of the infraorbital branch of the trigeminal nerve, the tactile map in the granule cell layer of crus IIa reorganized, with representations of intact structures expanding into the denervated area. The pattern of reorganization was similar to reorganization after neonatal lesions in that (1) all representations were from perioral structures, (2) the reorganized map maintained a fractured somatotopy, and (3) the denervated area was predominantly and consistently invaded by the upper incisor representation. We conclude that the spatial pattern of reorganization is essentially the same regardless of the age of deafferentation. However, we also observed developmental differences in reorganization. First, more areas of crus IIa were nonresponsive in animals lesioned later in development (PND 30-89). Second, we found a surprising degree of variability in the pattern of tactilely evoked cerebellar field potentials of PND 30-40 animals compared with neonates and adults, suggesting that this time period differs from other stages. The pattern of evoked potentials reflects the two primary inputs to the map. Our data show that, although both afferent pathways are capable of reorganization throughout development, their relative contribution to the map appears to differ, depending on the age at which lesion occurs.

Plasticity in cerebellar tactile maps in the adult rat

We previously demonstrated that the fractured tactile cerebellar map within the crus IIa folia of the cerebellar hemispheres reorganizes after deafferentation of the upper lip in neonatal rats (postnatal day [PND] 1–30). The present study examined the capacity of this map to reorganize after deafferentation in adults and animals late in development (PND 30–89). Several months after cauterization of the infraorbital branch of the trigeminal nerve, the tactile map in the granule cell layer of crus IIa reorganized, with representations of intact structures expanding into the denervated area. The pattern of reorganization was similar to reorganization after neonatal lesions in that (1) all representations were from perioral structures, (2) the reorganized map maintained a fractured somatotopy, and (3) the denervated area was predominantly and consistently invaded by the upper incisor representation. We conclude that the spatial pattern of reorganization is essentially the same regardless of the age of deafferentation. However, we also observed developmental differences in reorganization. First, more areas of crus IIa were nonresponsive in animals lesioned later in development (PND 30–89). Second, we found a surprising degree of variability in the pattern of tactilely evoked cerebellar field potentials of PND 30–40 animals compared with neonates and adults, suggesting that this time period differs from other stages. The pattern of evoked potentials reflects the two primary inputs to the map. Our data show that, although both afferent pathways are capable of reorganization throughout development, their relative contribution to the map appears to differ, depending on the age at which lesion occurs. J. Comp. Neurol. 413:583–592, 1999. © 1999 Wiley-Liss, Inc.

Age-dependent differences in reorganization of primary somatosensory cortex following low thoracic (T12) spinal cord transection in cats

The organization of primary somatosensory cortex was examined in chronic spinal cats that had sustained cord transection at T12 at 3 ages: 2 and 6 weeks of age, and as adults. Five months to 1 yr following transection, the deprived cortex was mapped electrophysiologically (multiunit recordings). The topographical organization found at each age was compared to that present in normal adults to study effects of developmental age on the ability of the somatosensory system to adjust to changes in afferent input. Cortical responses to deprivation of somatosensory input were age dependent. In animals cord transected at 2 weeks of age, the remaining somatic afferent input excited both its normal cortical area and the area normally reserved for the hindlimb. This resulted in 2 somatotopic maps of the rostral trunk and forelimb. In contrast, in cats spinalized at 6 weeks of age, there was only 1 map for the remaining somatosensory input that was distributed across the mediolateral axis of the primary somatosensory cortex. As a result, the remaining somatosensory input was shifted medially from its normal position and was narrower with respect to the rostrocaudal area driven by light tactile input. The amount of cortex that each body region could excite was essentially the same as in normal animals. In adults, a third response was observed; regions normally devoted to forelimb and trunk appeared to be unchanged, and the region previously serving the hindlimb responded only to a limited extent, and only to tactile stimulation of the trunk. In all cases, however, some sites in the cortex could be excited by parts of the body that in normal animals were served by cortical regions from 3 to 10 mm away, a distance much in excess of the maximum extent of reported thalamocortical overlap. We suggest that the various patterns of cortical organization observed at different ages reflect different developmental processes that are active at the time of transection. Further, we hypothesize that often, in major denervations such as spinal cord transection, a significant component of the reorganization occurs at synaptic levels below the cortex in young animals.