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H Johansen-Berg
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Functional MRI detects posterior shifts in primary sensorimotor cortex activation after stroke: evidence of local adaptive reorganization?
BACKGROUND AND PURPOSE: Further recovery from stroke can occur late, long after the end of the apparent evolution of pathological changes. This observation and evidence obtained from functional imaging for altered patterns of activation after brain injury suggest that cortical reorganization may contribute to recovery. Here, we have tested for potentially adaptive reorganization in the primary sensorimotor cortex. METHODS: We used functional MRI to study brain activation with dominant hand movement in right-handed healthy control subjects (n=20) and in patients after subcortical ischemic infarcts causing mild to moderate right hemiparesis (n=8). The numbers of pixels activated above threshold and the geometric centers of activation clusters were determined. RESULTS: Although random-effects analysis identified some differences in activation maxima, similar regions of the brain were activated with sequential finger tapping in the patient and control groups. However, consistent with the heterogeneity in the locations, sizes, and times after the infarcts, patterns and magnitudes of activation showed some heterogeneity between patients. Nonetheless, for the group as a whole, there was a decreased motor cortex lateralization index (-0.1+/-0.7 in patients and 0.7+/-0.3 in control subjects, P=0.05). The geometric center of activation of the primary sensorimotor cortex activation cluster contralateral to the affected hand in patients was also shifted posteriorly (mean 12 mm, P<0.04) relative to that of the control subjects. To confirm the latter observation, the activation response with a simple hand-tapping task was examined in some of the subjects. With this task, there was also a trend (mean 10 mm, P=0.07) toward a more posterior activation in patients. CONCLUSIONS: These results confirm altered patterns of activation in the contralateral and ipsilateral primary sensorimotor cortices after recovery from strokes causing hemiparesis. These (and other changes) suggest that modulation of widely distributed parts of the cortical network for motor control may contribute to adaptations leading to functional recovery after stroke.
The physiology and psychology of selective attention to touch.
In this chapter we review psychological and physiological experiments on selective attention to touch stimuli. We explore the role of selective attention in tactile target detection and search, determining those tasks that benefit from attention and those which can be effectively performed pre-attentively. We also try to determine the stage at which attentional selection occurs. We review electrophysiological and human brain imaging (PET, fMRI, MEG, SEP) studies to assess how early in the somatosensory processing pathway attentional modulation occurs. There is some evidence that the primary somatosensory cortex (S1) can show attentional effects. However, a number of studies have suggested that there is a hierarchy to attentional modulation in the somatosensory system, with the greatest effects being observed in secondary and association areas.
Attention to touch modulates activity in both primary and secondary somatosensory areas.
We used fMRI to establish whether attention to touch enhances somatosensory cortical activity. Subjects received somatosensory and visual stimulation and were instructed to attend selectively to one modality during alternating stimulus detection periods interspersed with rest periods during which no stimulus was delivered. The maximum signal change for each task versus rest was measured in anatomically defined regions of interest for each subject. Attended touch produced greater signal change than unattended touch in primary (S1) and secondary (S2) somatosensory cortex. In contrast to the conclusions of some previous studies, we found that the enhancement of activation with attention was at least as great in S1 as in S2. The attentional effect was unilateral in S1 and bilateral in S2 and the somatosensory insula.
The motor cortex shows adaptive functional changes to brain injury from multiple sclerosis.
Although multiple sclerosis (MS) is an inflammatory demyelinating disease, there can be substantial axonal injury and loss. We therefore hypothesized that adaptive cortical changes may contribute to limiting functional impairment, particularly in the early stages of the disease. To test our hypothesis, we used functional magnetic resonance imaging (MRI) to characterize the localization and volumes of activation in the motor cortex during simple flexion-extension finger movements. There were differences in the patterns of cortical activation with movement between the 12 MS patients and the 12 normal controls. All patients showed greater relative supplementary motor area activation than did the normal controls. The relative hemispheric lateralization of sensorimotor cortex (SMC) activation decreased in direct proportion to the total cerebral T2-weighted MRI hyperintense lesion load. This appeared to be due primarily to increases in ipsilateral SMC activation with increasing lesion load in white matter of the hemisphere contralateral to the limb moved. The center of activation in the contralateral SMC was shifted a mean of 8.8 mm posterior in patients relative to controls, providing additional evidence for cortical adaptive responses to injury. The magnitude of this posterior shift in the SMC activation increased with greater T2 lesion loads. These observations demonstrate that cortical recruitment for simple finger movements can change both quantitatively and qualitatively in the SMCs of MS patients, suggesting that cortical reorganization or "unmasking" of latent pathways can contribute to functional recovery. These adaptive changes are another factor potentially limiting the strength of the relationship between MRI measures of pathology and clinical measures of disability.
The effects of tetanus toxin-induced limbic epilepsy on the exploratory response to novelty in the rat.
PURPOSE: To investigate the exploratory response to novelty in rats that have recovered from experimental limbic epilepsy. METHODS: Epilepsy was induced in 12 male rats by injecting a minute amount of tetanus toxin into the ventral hippocampus (and buffer vehicle was injected into 12 controls). Eight weeks after the injection, when the animals appeared behaviourally normal (and previous work would indicate that their electroencephalograms also would have returned to normal), they were tested on the playground maze. In this, their exploratory response to a novel object introduced in the context of seven familiar objects is measured. Simultaneously, their locomotion and investigation of familiar objects is measured. RESULTS: Whereas the control animals showed a significant response to the novel object on both test days, in the toxin-injected rats the novelty response was not present. There was no difference between the groups on the locomotion measure, but the toxin rats explored the familiar as well as the novel objects less. CONCLUSIONS: The exploratory response to a novel object was abolished in the previously epileptic rats.
Parietal cortex and spatial-postural transformation during arm movements.
Cells in the parietal motor areas 5, MIP, and 7b have spatially tuned activity during movements. Lesions, however, do not disrupt visual reaching or learned nonspatial movement selection. The role of such parietal cells in sensorimotor coordinate transformations is unclear. The present experiment investigates whether the parietal motor areas are concerned with the following: 1) the transformation between the desired position in space of the hand and the limb's postural configuration during movement and 2) interjoint coordination. Six macaque monkeys were trained to reach in the dark. Spatial-postural transformations assume a simple form in the absence of vision and so may be most easily studied when animals reach in the dark. A lesion was placed in the parietal cortex that included areas 5, MIP, and 7b of three macaques. The simple relation between hand position and limb postural configuration seen in controls was disrupted after the lesion. The intercoordination of movements of the hand with those of the rest of the arm was also affected. The lesion did not affect the range or velocity of joint movements or the curvature of the hand's trajectory. The cell activity in parietal areas 5, MIP, and 7b may not be essential for the transformation between retinocentric representation of the target and shoulder centered representations of the desired position of the hand, but it is essential for both the subsequent transformation between desired hand position and the postural configuration of the arm and for interjoint integration.