Quantitative measures of the resting EEG in stroke: a systematic review on clinical correlation and prognostic value.

OBJECT: Quantitative electroencephalography (qEEG) has shown promising results as a predictor of clinical impairment in stroke. We systematically reviewed published papers that focus on qEEG metrics in the resting EEG of patients with mono-hemispheric stroke, to summarize current knowledge and pave the way for future research. METHODS: Following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines, we systematically searched the literature for papers that fitted our inclusion criteria. Rayyan QCRR was used to allow deduplication and collaborative blinded paper review. Due to multiple outcomes and non-homogeneous literature, a scoping review approach was used to address the topic. RESULTS: Or initial search (PubMed, Embase, Google scholar) yielded 3200 papers. After proper screening, we selected 71 papers that fitted our inclusion criteria and we developed a scoping review thar describes the current state of the art of qEEG in stroke. Notably, among selected papers 53 (74.3%) focused on spectral power; 11 (15.7%) focused on symmetry indexes, 17 (24.3%) on connectivity metrics, while 5 (7.1%) were about other metrics (e.g. detrended fluctuation analysis). Moreover, 42 (58.6%) studies were performed with standard 19 electrodes EEG caps and only a minority used high-definition EEG. CONCLUSIONS: We systematically assessed major findings on qEEG and stroke, evidencing strengths and potential pitfalls of this promising branch of research.

Cortico-muscular coherence as an index of fatigue in multiple sclerosis.

BACKGROUND: Highly common in multiple sclerosis (MS), fatigue severely impacts patients' daily lives. Previous findings of altered connectivity patterns led to the hypothesis that the distortion of functional connections within the brain-muscle circuit plays a crucial pathogenic role. OBJECTIVE: The objective of this paper is to identify markers sensitive to fatigue in multiple sclerosis. METHODS: Structural (magnetic resonance imaging with assessment of thalamic volume and cortical thickness of the primary sensorimotor areas) and functional (cortico-muscular coherence (CMC) from simultaneous electroencephalo- and surface electromyographic recordings during a weak handgrip task) measures were used on 20 mildly disabled MS patients (relapsing-remitting course, Expanded Disability Status Scale score ≤ 2) who were recruited in two fatigue-dependent groups according to the Modified Fatigue Index Scale (MFIS) score. RESULTS: The two groups were similar in terms of demographic, clinical and imaging features, as well as task execution accuracy and weariness. In the absence of any fatigue-dependent brain and muscular oscillatory activity alterations, CMC worked at higher frequencies as fatigue increased, explaining 67% of MFIS variance (p=.002). CONCLUSION: Brain-muscle functional connectivity emerged as a sensitive marker of phenomena related to the origin of MS fatigue, impacting central-peripheral communication well before the appearance of any impairment in the communicating nodes.

Early diagnosis of Alzheimer's disease: the role of biomarkers including advanced EEG signal analysis. Report from the IFCN-sponsored panel of experts.

Alzheimer's disease (AD) is the most common neurodegenerative disease among the elderly with a progressive decline in cognitive function significantly affecting quality of life. Both the prevalence and emotional and financial burdens of AD on patients, their families, and society are predicted to grow significantly in the near future, due to a prolongation of the lifespan. Several lines of evidence suggest that modifications of risk-enhancing life styles and initiation of pharmacological and non-pharmacological treatments in the early stage of disease, although not able to modify its course, helps to maintain personal autonomy in daily activities and significantly reduces the total costs of disease management. Moreover, many clinical trials with potentially disease-modifying drugs are devoted to prodromal stages of AD. Thus, the identification of markers of conversion from prodromal form to clinically AD may be crucial for developing strategies of early interventions. The current available markers, including volumetric magnetic resonance imaging (MRI), positron emission tomography (PET), and cerebral spinal fluid (CSF) analysis are expensive, poorly available in community health facilities, and relatively invasive. Taking into account its low cost, widespread availability and non-invasiveness, electroencephalography (EEG) would represent a candidate for tracking the prodromal phases of cognitive decline in routine clinical settings eventually in combination with other markers. In this scenario, the present paper provides an overview of epidemiology, genetic risk factors, neuropsychological, fluid and neuroimaging biomarkers in AD and describes the potential role of EEG in AD investigation, trying in particular to point out whether advanced analysis of EEG rhythms exploring brain function has sufficient specificity/sensitivity/accuracy for the early diagnosis of AD.

Sensorimotor integration in focal task-specific hand dystonia: a magnetoencephalographic assessment.

To obtain a direct sensorimotor integration assessment in primary hand cortical areas (M1) of patients suffering from focal task-specific hand dystonia, magnetoencephalographic (MEG) and opponens pollicis electromyographic (EMG) activities were acquired during a motor task expressly chosen not to induce dystonic movements in our patients, to disentangle abnormalities indicating a possible substrate on which dystonia develops. A simple isometric contraction was performed either alone or in combination with median nerve stimulation, i.e. when a non-physiological sensory inflow was overlapping with the physiological feedback. As control condition, median nerve stimulation was also performed at rest. The task was performed bilaterally both in eight patients and in 16 healthy volunteers. In comparison with results in controls we found that in dystonic patients: i) MEG-EMG coherence was higher; ii) it reduced much less during galvanic stimulation in the hemisphere contralateral to the dystonic arm, simultaneously with iii) stronger inhibition of the sensory areas responsiveness due to movement; iv) the cortical component including contributions from sensory inhibitory and motor structures was reduced and v) much more inhibited during movement. It is documented that a simultaneous cortico-muscular coherence increase occurs in presence of a reduced M1 responsiveness to the inflow from the sensory regions. This could indicate an unbalance of the fronto-parietal functional impact on M1, with a weakening of the parietal components. Concurrently, signs of a less differentiated sensory hand representation--possibly due to impaired inhibitory mechanisms efficiency--and signs of a reduced repertoire of voluntary motor control strategies were found.

Age dependence of primary motor cortex plasticity induced by paired associative stimulation.

OBJECTIVE: The increase of elderly population prompted growing research for the understanding of cerebral phenomena sustaining learning abilities, with inclusion of long-term potentiation (LTP)-like plasticity phenomena. Aim of the present study was to characterize LTP-like plasticity dependence on age and gender. METHODS: A LTP-like primary motor cortex plasticity inducing a potentiation of the motor evoked potential (MEP) to focal transcranial magnetic stimulation as a consequence of a paired associative stimulation (PAS) was induced in a 50 healthy subject cohort, equally distributed for gender and age groups (25 young subjects, mean age+/-SD=29.8+/-4.5 years; elderly 61.1+/-4.1 years). RESULTS: Resting motor thresholds' excitability level increased in the elderly group, the basal MEP did not depend on gender or age. The PAS-induced primary motor cortex (M1) plastic excitability modulation was similar in young females and males, while it decreased with age in females only. CONCLUSIONS: A reduction of the PAS-induced M1 plasticity in females after menopause was documented, possibly due to an impairment of intracortical excitatory network activity. SIGNIFICANCE: A LTP-like plasticity dependence on age was found in female only, suggesting caution in interpreting behavioural studies on learning abilities in dependence on age.

Neuroimaging experimental studies on brain plasticity in recovery from stroke.

Topographical cortical organization of sensorimotor area has been shown to be highly plastic, altering his configuration in response to training in different tasks in healthy controls and neurological patients. The term ''brain plasticity'' encompasses all possible mechanisms of neuronal reorganization: recruitment of pathways that are functionally homologous to, but anatomically distinct from, the damaged ones (eg, non-pyramidal corticospinal pathways), synaptogenesis, dendritic arborisation and reinforcement of existing but functionally silent synaptic connections (particularly at the periphery of core lesion). The study of neuroplasticity has clearly shown the ability of the developing brain--and of the adult and ageing brain--to be shaped by environmental inputs both under normal conditions (ie, learning) and after a lesion. Neuronal aggregates adjacent, or distant to a lesion in the sensorimotor area can progressively adopt the function of the injured area. Imaging studies indicate that recovery of motor function after a lesion (i.e. stroke) is associated with a progressive change of activation patterns in specific brain structures. Transcranial magnetic stimulation (TMS) and magnetoencephalography (MEG) can detect reshaping of sensorimotor areas; they have a high temporal resolution but have several limitations. TMS can only provide bidimensional scalp maps and MEG depicts three-dimensional spatial characteristics of virtual neural generators obtained by use of a mathematical model of the head and brain. However, the use of objective methods that assess brain reactivity to a physical stimulus (i.e., TMS) or to a sensory input (ie, electrical stimulation to hand and fingers) can integrate information from self-paced motor tasks, because the resolution of abnormal activation over time could be secondary to recovery. Functional MRI (fMRI) and positron emission tomography (PET), on their own, have insufficient time resolution to follow the hierarchical activation of relays within a neural network; however, because of their excellent spatial resolution, they can integrate the findings of TMS and MEG. An integrated approach constitutes, at present, the best way to assess the brain plasticity both under normal conditions and after a lesion.

Sensory-motor interaction in primary hand cortical areas: a magnetoencephalography assessment.

Movement control requires continuous and reciprocal exchange of information between activities of motor areas involved in the task program execution and those elaborating proprioceptive sensory information. Our aim was to investigate the sensorimotor interactions in the region dedicated to hand control in healthy humans, focusing onto primary sensory and motor cortices, by selecting the time window at very early latencies. Through magnetoencephalographic recordings, we obtained a simultaneous assessment of sensory cortex activity modulation due to movement and of motor cortex activity modulation due to sensory stimulation, by eliciting a galvanic stimulation to the nerve (the median nerve) innervating a muscle (the opponens pollicis), at rest or during voluntary contraction. The primary sensory and motor cortices activities were investigated respectively through excitability in response to sensory stimulation and the cortico-muscular coherence. The task was performed bilaterally. A clear reduction of the cortico-muscular coherence was found in the short time window following stimuli (between around 150-450 ms). In the same time period, the motor control of isometric contraction was preserved. This could suggest that cortical component of voluntary movement control was transiently mediated by neuronal firing rate tuning more than by cortico-muscular synchronization. In addition to the known primary sensory cortex inhibition due to movement, a more evident reduction was found for the component known to include a contribution from primary motor areas. Gating effects were lower in the dominant left hemisphere, suggesting that sensorimotor areas dominant for hand control benefit of narrowing down gating effects.

[Exposure to traffic noise and effects on attention]. / Effetti sull'attenzione per esposizione a rumore da traffico urbano.

This study was aimed to evaluate if workers exposed to environmental stressors, including the urban traffic noise, might show significant differences compared to a control group in neuro-psychological and emotional profile as well as neurophysiological functions. In particular if these differences could be evidenced by the application of the "oddball paradigm" for event related potential P300 component. The study consisted of the following examinations: (1) exposed workers vs. controls under the odd-ball paradigm and the Stroop test in baseline condition; (2) amplitude and latency ofP300 (in baseline condition and after administration of acute urban traffic noise and Stroop test). The research was carried on a sample of 81 volunteers: 39 workers exposed to environmental stressors and 42 controls. The phonometric measurements showed mean levels of noise due urban traffic like 74 dBAeq. In baseline condition significative differences in exposed workers vs. control were found in Raven's Matrices PM 38 (p = 0.002) and Arithmetic reasoning from WAIS-R (p = 0.0024). Attention capacities as measured by Digit Span Forward and Visual Search, emotional functioning as measured by state- and trait-anxiety test and mood profile were not different in the two groups. Either in baseline condition or after acute stimuli no significant changes were found in two groups concerning the odd-ball paradigm. Exposed workers showed a higher execution time at Stroop test compared to controls (p = 0.047). No differences were found in the number of errors at the Stroop test. Before the acute stimulus, P300 amplitude was significant higher in the exposed workers than in controls (p = 0.002) while the latency was not different between two groups. Both noise (p = 0.001) and Stroop test (p = 0.002) stimulation increased the P300 latency of the whole sample, without significant differences between exposed workers and controls. A significative decrease of P300 amplitude due noise both in the exposed workers (p = 0.001) and in controls (p = 0.012) was found, without significant difference between the two groups. These results are interpreted as follows: (1) there are chronic effects on cognitive functioning in the exposed group vs. controls in baseline condition, like showed by significant differences in Raven PM38 and WAIS-R; (2) the exposed workers have a smaller cognitive flexibility, as shown by the Stroop test results; (3) in baseline condition the greater P300 amplitude in exposed workers reflect a greater division of attentive resources vs. controls, probably linked to the chronic stimulation by environmental stressors, especially noise, to which these workers are exposed; (4) the effects on P300 latency and amplitude can document the physiological response both in the exposed and not exposed to the acute stimulus and that the lack of significative differences in P300 latency and amplitude may be due to adaptative response to acute stimuli in exposed too. Our results allow us to consider that in workers exposed to urban stressor, such as noise, there are effects on cognitive functioning, especially on attention, without auditory damages. The valuation of P300 might represent a valid diagnostic instrument to evaluate the effects on cognitive functions especially on attention, in workers chronically and acutely exposed to urban stressors.

Decrease of functional coupling between left and right auditory cortices during dichotic listening: an electroencephalography study.

The present study focused on functional coupling between human bilateral auditory cortices and on possible influence of right over left auditory cortex during dichotic listening of complex non-verbal tones having near (competing) compared with distant non-competing fundamental frequencies. It was hypothesized that dichotic stimulation with competing tones would induce a decline of functional coupling between the two auditory cortices, as revealed by a decrease of electroencephalography coherence and an increase of directed transfer function from right (specialized for the present stimulus material) to left auditory cortex. Electroencephalograph was recorded from T3 and T4 scalp sites, overlying respectively left and right auditory cortices, and from Cz scalp site (vertex) for control purposes. Event-related coherence between T3 and T4 scalp sites was significantly lower for all electroencephalography bands of interest during dichotic listening of competing than non-competing tone pairs. This was a specific effect, since event-related coherence did not differ in a monotic control condition. Furthermore, event-related coherence between T3 and Cz and between T4 and Cz scalp sites showed no significant effects. Conversely, the directed transfer function results showed negligible influence at group level of right over left auditory cortex during dichotic listening. These results suggest a decrease of functional coupling between bilateral auditory cortices during competing dichotic stimuli as a possible neural substrate for the lateralization of auditory stimuli during dichotic listening.

Personalizing the Electrode to Neuromodulate an Extended Cortical Region.

BACKGROUND: Among transcranial electric stimulation (tES) parameters, personalizing the electrode geometry might help overcome the individual variability of the induced effects. OBJECTIVE/HYPOTHESIS: To test the need for electrode personalization, instead of a universal electrode for everyone, to induce neuromodulation effects on the bilateral primary motor cortex (M1) devoted to upper and lower limb representation. METHODS: By an ad-hoc neuronavigation procedure, we shaped the personalized electrode and positioned it matching the projection on the scalp of the individual central sulcus by a 2 cm strip, with total area of 35 cm(2). The non-personalized electrode, i.e., equal for all subjects, was a 2 cm wide strip size-matched with the personalized electrode but shaped on a standard model fitting the curve passing through C3-CZ-C4 sites of the electroencephalographic (EEG) 10-20 International System. To test neuromodulation electrode-dependent efficacy, we induced a 20 Hz sinusoidal modulated current (transcranial alternating current stimulation, tACS) because it produces online effects. We simultaneously collected left and right hand and leg motor potentials (MEP) that were evoked by a rounded transcranial magnetic stimulation (TMS) coil. Through each electrode we delivered both real and sham stimulations. RESULTS: While cortical excitability during tACS increased during both the non-personalized and the personalized electrodes for the leg, the hand representation excitability enhancement was induced selectively when using the personalized electrode. The results were consistent bilaterally. CONCLUSIONS: We documented that by using a personalized electrode it is possible to induce the neuromodulation of a predetermined extended cortical target, which did not occur with a non-personalized electrode. Our findings can help in building neuromodulation methods that might compensate for individual alterations across specific brain networks.

Transcranial Direct Current Stimulation: Personalizing the neuromodulation.

The beneficial effects of transcranial direct current stimulation (tDCS) has been demonstrated, but the neuroscientific community is working to increase its efficiency. A promising line of advancement may be reducing the inter-individual variability of the response through the personalization of the stimulation, adapted to fit the structural and functional features of individual subjects. In this paper, we approach the personalization of stimulation parameters using modeling, a powerful tool to test montages enabling the optimization of brain's targeting.

Functional and structural balances of homologous sensorimotor regions in multiple sclerosis fatigue.

Fatigue in multiple sclerosis (MS) is a highly disabling symptom. Among the central mechanisms behind it, an involvement of sensorimotor networks is clearly evident from structural and functional studies. We aimed at assessing whether functional/structural balances of homologous sensorimotor regions-known to be crucial for sensorimotor networks effectiveness-decrease with MS fatigue increase. Functional connectivity measures at rest and during a simple motor task (weak handgrip of either the right or left hand) were derived from primary sensorimotor areas electroencephalographic recordings in 27 mildly disabled MS patients. Structural MRI-derived inter-hemispheric asymmetries included the cortical thickness of Rolandic regions and the volume of thalami. Fatigue symptoms increased together with the functional inter-hemispheric imbalance of sensorimotor homologous areas activities at rest and during movement, in absence of any appreciable parenchymal asymmetries. This finding supports the development of compensative interventions that may revert these neuronal activity imbalances to relieve fatigue in MS.

Coupling between "hand" primary sensorimotor cortex and lower limb muscles after ulnar nerve surgical transfer in paraplegia.

Previous neuroimaging evidence revealed an "invasion" of "hand" over "lower limb" primary sensorimotor cortex in paraplegic subjects, with the exception of a rare patient who received a surgical motor reinnervation of hip-thigh muscles by the ulnar nerve. Here, the authors show that a functional reorganization of cortico-muscular and cortico-cortical oscillatory coupling was related to the recovery of the rare patient, as a paradigmatic case of long-term plasticity in human sensorimotor cortex after motor reinnervation of paraplegic muscles. This conclusion was based on electroencephalographic and electromyographic data collected while the patient and normal control subjects performed isometric muscle contraction of the left hand or lower limb. Cortico-muscular and cortico-cortical coupling was estimated by electroencephalographic-electromyographic coherence and directed transfer function of a multivariate autoregressive model.

Functional localization of the sensory hand area with respect to the motor central gyrus knob.

The aim of this work was to investigate the topography of the primary sensory hand cortex with magnetoencephalography in order to define the functional anatomy of this area in healthy humans. Previous studies denoted an inverted Ohm or an horizontal epsilon shaped knob on the pre-central gyrus as a landmark for the motor hand area; therefore a systematic difference between the orientation of the source for thumb with respect to little finger should be observed. We found this systematic difference, but the direction of the sources activated during thumb and little finger stimulation did not converge, as would be expected if only the Ohm convexity is activated: in fact our results suggest that thumb sensory area also extends to the area lateral to this convexity.

Detection of fetal auditory evoked responses by means of magnetoencephalography.

MagnetoEncephaloGraphy (MEG) is proposed as a non-invasive technique to detect the physiological activity of fetal brain, due to its ability to record brain activity without direct contact with the head and the transparency of magnetic signals in passing through extracerebral fetal layers and the mother's abdomen. Healthy women with uncomplicated pregnancies and fetuses in breech presentation were examined; gestational ages at time of study ranged between 36 and 40 weeks. In order to evaluate fetal well-being, ultrasound and cardiotocographic data were assessed a few days before and after MEG recording sessions. The participating women were placed in a semi-reclining position in a magnetically shielded room; here the presentation of the fetus and precise region of the mother's abdomen corresponding to the fetal head were determined by ultrasound investigation in order to place the MEG detecting system as near as possible to the fetal brain. MEG recordings were performed by means of a 28-channel neuromagnetic system. Every MEG recording session was performed during the acoustic stimulation of fetuses, in order to detect the cerebral events evoked by peripheral stimuli. The auditory stimuli were delivered from a plastic tube placed on mother's abdomen, near the fetal head, and consisted of a 300 ms 103 dB pure tone at 500 and 1000 Hz, presented at a 0.4 c/s repetition rate. In six cases following accurate digital subtraction of maternal and fetal electrocardiographic (EKG) signals we remained with a stimulus-related response peaking at about 250 ms; this was considered to originate from the fetal brain. In favour of this in three cases a clear dipolar distribution was evident at the peak of brain response centered on the fetal head and consistent with a brain generator. Despite several technical problems requiring solution before a possible routine clinical application, MEG has been found to be suitable for the non-invasive exploration of the fetal brain.

Spatial properties and interhemispheric differences of the sensory hand cortical representation: a neuromagnetic study.

We performed a neuromagnetic investigation of the sensory hand cortical representation in the two hemispheres of 20 healthy volunteers. The localizations within the brain hemispheres of the cortical Equivalent Current Dipoles (ECDs) activated with the shortest latencies (N20 m and P30 m components) by separate stimulation of contralateral median nerve, thumb and little finger were analysed. The ECD spatial coordinates were in agreement with the known somatotopy of the sensory homunculus: little finger more medial and posterior, thumb more lateral and anterior, median nerve in-between. By considering the ECDs to thumb and little finger stimulation the boundaries of the hand cortical representation in primary sensory cortex, the 'hand extension' was evaluated as the distance between the two. This parameter was similar on the two hemispheres, the 'hand extension' being 17 mm and 12 mm for N20 m and P30 m components, respectively, with a standard deviation of 5 mm. We provide for the first time the ECDs localization of left and right median nerve, thumb and little finger, as well as the 'hand extension' values, and their interhemispheric differences as a normative data set describing the organization of primary sensory cortical areas reserved to the hand in the healthy population. This approach permits objective measurements of absolute values, as well as of interhemispheric differences, of the sensory hand area following a monohemispheric lesion as well as to non-invasively follow-up its reorganization during clinical recovery.

Changes in movement-related brain activity during transient deafferentation: a neuromagnetic study.

Neuromagnetic fields from the left cerebral hemisphere of three healthy, right-handed subjects were investigated preceding and during voluntary index finger movements performed every 8-15 s under two different experimental conditions: before (stage A) and during (stage B) anesthetic block of median and radial nerves at the wrist. The anesthesia caused blocking of cutaneous receptors and some of the proprioreceptors from a wide hand area, including the entire index finger. However, the index finger movements were not impaired because the muscles participating in the task were not anesthetized. The magnetic signals of the brain sources corresponding to the main components of the movement-related neuromagnetic fields (motor field, MF and movement-evoked field I, MEFI) were mapped and localized using a moving dipole model. In the three investigated subjects the MF and MEFI dipole sources were stronger (30% on average) during stage B than during stage A. No significant changes in spatial coordinates of the estimated dipole locations between stages A and B were observed. This was true for both MF and MEFI. The results show that the MEFI reflects not only proprioceptive input from the periphery but cutaneous inputs as well. In this way the results support the view that cutaneous inputs play a specific role in the cortical control of movement.

Neuromagnetic fields of the brain evoked by voluntary movement and electrical stimulation of the index finger.

Neuromagnetic fields from the left cerebral hemisphere of five healthy, right-handed subjects were investigated under two different experimental conditions: (1) electrical stimulation of the right index finger (task somatosensory evoked fields, task SEF's), and (2) voluntary movement of the same finger referred to as movement-related fields, (MRFs). The two conditions were, performed in random order every 5-8 s. In addition, the task SEF's were compared to control SEF's recorded at the beginning of the experiment in order to find the optimal dewar position for localizing the central sulcus. The magnetic signals of the sources corresponding to the main components of the somatosensory evoked fields (early ones at 24 ms and at 34 ms, and late ones after 50 ms) and movement-related fields (motor field, MF and movement-evoked field I-MEF I) were mapped and localized by means of a moving dipole model. In four out of five subjects the MEF I dipoles were found to be located deeper than the early task SEF dipoles. In addition, all of the task SEF's components were found to exhibit larger amplitudes than the control SEF's components. The results are discussed in respect to the ability to selectively analyze contributions of mainly proprioceptive (area 3a) and cutaneous (area 3b) areas in the primary somatosensory cortex using magnetoencephalography. An additional finding of the study was that all of the task SEF's components were found to exhibit larger amplitudes than the control SEF's components.

Short-term brain 'plasticity' in humans: transient finger representation changes in sensory cortex somatotopy following ischemic anesthesia.

Transient rearrangements of finger representation in primary somatosensory cortex induced by an anesthetic block of the sensory information from adjacent fingers have been shown invasively in animals. Such a phenomenon has been now replicated in seven healthy human volunteers. Somatosensory Evoked Fields (SEFs) have been recorded during separate electrical stimulation of the 1st, 3rd, or 5th finger. Recordings were obtained in control conditions (stage A), following complete ischemic anesthesia of the 4 non-stimulated fingers (stage B), and after regaining sensation (stage C). SEFs were recorded using a 28-channel DC-SQUID magnetometer; a single position of the sensor was enough to identify the source of N20m, P30m and following components using the Equivalent Current Dipole (ECD) model. The amount of afferent input during stages A through C was monitored with surface electrodes placed on the nerve at wrist and elbow. No variation of the nerve compound potential was observed during stages A through C. In stage A, the localizing algorithm was able to discriminate the individual finger representation in accordance with the somatotopic organisation of the sensory homunculus. It was observed that the ECDs responsible for the cortical responses from the unanesthetized finger were significantly changing following a relatively brief period of sensory deprivation from the adjacent fingers. Such changes of the ECDs with respect to the control conditions were characterized by an increase in strength and deepening for the middle finger, and by a shift on the coronal plane for the thumb and the little finger (medial for the former, lateral for the latter). Such changes became progressively evident in stage B, but were persisting in stage C.

On the reorganization of sensory hand areas after mono-hemispheric lesion: a functional (MEG)/anatomical (MRI) integrative study.

The topography of primary sensory cortical hand area following a monohemispheric lesion (sudden = stroke; progressive = neoplasm) was investigated in relationship with clinical recovery of sensorimotor deficits. Twenty seven patients with monohemispheric lesions were studied in a clinically stabilized condition. Functional informations from magnetoencephalography (MEG) were integrated with anatomical data from magnetic resonance imaging (MRI). MEG localizations of the neurons firing at early latencies in primary sensory cortex after separate stimulation of median nerve, thumb and little fingers of each hand were carried out. Characteristics of cerebral equivalent current dipoles (ECDs) activated by each contralateral stimulation, the 'hand extension' (i.e., the distance in millimetres between ECDs of first and fifth digits), as well as interhemispheric differences of the tested parameters were investigated. Finally, ECDs' locations were integrated with MRI. Lesions involving cortical (C) or subcortical (s.c.) areas receiving sensory input from the hand were often combined to increase interhemispheric asymmetry of the tested parameters (22% for C and 49% for s.c. lesions). This might be due to an activation of neuronal districts which in the affected hemisphere (AH) differ from those normally activated in the unaffected hemisphere (UH) and in the control population. Moreover, the 'hand extension' was enlarged on the AH--more frequently after a SC lesion--mainly due to a medial shift of the little finger ECD, combined to a tendency of both finger ECDs to shift frontally. After a C lesion, responses from the AH were often stronger than normal. Spatial reorganizations were also seen in the UH (7% of C and 14% of SC lesions). 'Hand extension' in the UH was selectively enlarged for the P30m only when combined with a similar enlargement in the AH. Significant interhemispheric asymmetries due to neuronal reorganization in the AH were associated with worse clinical outcomes compared to patients without asymmetries.