Franca Tecchio

Mapping distributed sources of cortical rhythms in mild Alzheimer's disease. A multicentric EEG study

The study aimed at mapping (i) the distributed electroencephalographic (EEG) sources specific for mild Alzheimers disease (AD) compared to vascular dementia (VaD) or normal elderly people (Nold) and (ii) the distributed EEG sources sensitive to the mild AD at different stages of severity. Resting EEG (10-20 electrode montage) was recorded from 48 mild AD, 20 VaD, and 38 Nold subjects. Both AD and VaD patients had 24-17 of mini mental state examination (MMSE). EEG rhythms were delta (2-4 Hz), theta (4-8 Hz), alpha 1 (8-10.5 Hz), alpha 2 (10.5-13 Hz), beta 1 (13-20 Hz), and beta 2 (20-30 Hz). Cortical EEG sources were modeled by low resolution brain electromagnetic tomography (LORETA). Regarding issue i, there was a decline of central, parietal, temporal, and limbic alpha 1 (low alpha) sources specific for mild AD group with respect to Nold and VaD groups. Furthermore, occipital alpha 1 sources showed a strong decline in mild AD compared to VaD group. Finally, distributed theta sources were largely abnormal in VaD but not in mild AD group. Regarding issue ii, there was a lower power of occipital alpha 1 sources in mild AD subgroup having more severe disease. Compared to previous field studies, this was the first investigation that illustrated the power spectrum profiles at the level of cortical (macroregions) EEG sources in mild AD patients having different severity of the disease with respect to VaD and normal subjects. Future studies should evaluate the clinical usefulness of this approach in early differential diagnosis, disease staging, and therapy monitoring.

Carpal tunnel syndrome modifies sensory hand cortical somatotopy: A MEG study

The adult somatosensory system has shown reorganizational abilities at cortical and subcortical levels after peripheral nerve lesions. In the present study the effects of carpal tunnel syndrome (CTS) are investigated as reflected on the somatotopy of the primary cortical hand representation. Position and intensity of cortical sources activated by the separate electrical stimulation of median nerve and Digits 1, 3, and 5 of both affected and non‐affected hands are evaluated by magnetoencephalographic (MEG) technique. Correlation of MEG results with patient‐, physician‐ and neurophysiological‐oriented evaluations of CTS was carried out. Patients showed changes in cortical hand somatotopy in strict relationship to self‐referred assessment of symptoms and hand disability in daily activities, including: 1) a more extended representation of the affected hand when paresthesias prevailed; and 2) a more restricted representation due to lateral shift of the little finger was observed when pain symptoms dominated the clinical picture. Contralateral to the side of CTS, the cortical sources activated by Digit 5‐stimulation appeared significantly enhanced with respect to contralateral ones from non‐affected hand. When comparing the amplitude of peripheral sensory nerve action potentials of median and ulnar nerves to that of cortical responses (i.e., ECD strengths of M20 and M30 components after stimulation of Digits 3 and 5), a significant selective amplification of M30 with respect to M20 and sensory nerve action potential (SNAP) appeared during Digit 3 stimulation compared to that observed for Digit 5. This has been interpreted as a central magnification mechanism in brain responsiveness, possibly revealing a safety factor enabling sensory perception despite the small peripheral signal due to nerve trunk dysfunction. Hum. Brain Mapping 17:28–36, 2002.

Linear inverse source estimate of combined EEG and MEG data related to voluntary movements

A method for the modeling of human movement‐related cortical activity from combined electroencephalography (EEG) and magnetoencephalography (MEG) data is proposed. This method includes a subjects multi‐compartment head model (scalp, skull, dura mater, cortex) constructed from magnetic resonance images, multi‐dipole source model, and a regularized linear inverse source estimate based on boundary element mathematics. Linear inverse source estimates of cortical activity were regularized by taking into account the covariance of background EG and MEG sensor noise. EEG (121 sensors) and MEG (43 sensors) data were recorded in separate sessions whereas normal subjects executed voluntary right one‐digit movements. Linear inverse source solution of EEG, MEG, and EEG‐MEG data were quantitatively evaluated by using three performance indexes. The first two indexes (Dipole Localization Error [DLE] and Spatial Dispersion [SDis]) were used to compute the localization power for the source solutions obtained. Such indexes were based on the information provided by the column of the resolution matrix (i.e., impulse response). Ideal DLE values tend to zero (the source current was correctly retrieved by the procedure). In contrast, high DLE values suggest severe mislocalization in the source reconstruction. A high value of SDis at a source space point mean that such a source will be retrieved by a large area with the linear inverse source estimation. The remaining performance index assessed the quality of the source solution based on the information provided by the rows of the resolution matrix R, i.e., resolution kernels. The i‐th resolution kernels of the matrix R describe how the estimation of the i‐th source is distorted by the concomitant activity of all other sources. A statistically significant lower dipole localization error was observed and lower spatial dispersion in source solutions produced by combined EEG‐MEG data than from EEG and MEG data considered separately (P < 0.05). These effects were not due to an increased number of sensors in the combined EEG‐MEG solutions. They result from the independence of source information conveyed by the multimodal measurements. From a physiological point of view, the linear inverse source solution of EEG‐MEG data suggested a contralaterally preponderant bilateral activation of primary sensorimotor cortex from the preparation to the execution of the movement. This activation was associated with that of the supplementary motor area. The activation of bilateral primary sensorimotor cortical areas was greater during the processing of afferent information related to the ongoing movement than in the preparation for the motor act. In conclusion, the linear inverse source estimate of combined MEG and EEG data improves the estimate of movement‐related cortical activity. Hum. Brain Mapping 14:197–209, 2001.

Auditory sensory processing in autism: a magnetoencephalographic study

BACKGROUND Patients with autism show clinical features suggestive of abnormal processing of auditory and other sensory information. We hypothesized that low-functioning autistic subjects present abnormalities in discriminating simple auditory stimuli at sensory system preconscious stages of cortical processing. METHODS To verify our hypothesis, we used magnetoencephalographic measurements of mismatch field (MMF), which reflects the detection of a change in the physical characteristics of a repetitive sound. Fourteen patients (aged 8-32 years) who met DSM-IV diagnostic criteria for autistic disorder participated in an auditory oddball experiment. Ten healthy participants matched for age and gender acted as control subjects. RESULTS Significant differences in cerebral responses between patients and control subjects were recorded. Whereas control subjects showed a clearly identifiable MMF, with distinct generators in the M100 brain wave with regard to latency, position, and strength, no identifiable MMF was present in the autistic group. CONCLUSIONS Our findings suggest that low-functioning autistic subjects present a dysfunction at preconscious stages of cortical auditory discrimination, playing a role in the abnormal processing of auditory sensory afferences. The attention independence of the MMF allows for exclusion of an effect related to impaired attention or task-related responses.

Inhibition of auditory cortical responses to ipsilateral stimuli during dichotic listening: evidence from magnetoencephalography

The present magnetoencephalography (MEG) study on auditory evoked magnetic fields (AEFs) was aimed at verifying whether during dichotic listening the contralateral auditory pathway inhibits the ipsilateral one, as suggested by behavioural and patient studies. Ten healthy subjects were given a randomized series of three complex tones (261, 293 and 391 Hz, 500 ms duration), which were delivered monotically and dichotically with different intensities [60, 70 or 80 dBA (audio decibels)]. MEG data were recorded from the right auditory cortex. Results showed that the M100 amplitude over the right auditory cortex increased progressively when tones of increasing intensity were provided at the ipsilateral (right) ear. This effect on M100 was abolished when a concurrent tone of constant intensity was delivered dichotically at the contralateral (left) ear, suggesting that the contralateral pathway inhibited the ipsilateral one. The ipsilateral inhibition was present only when the contralateral tone fundamental frequency was similar to the ipsilateral tone. It was proposed that the occlusion mechanism would be exerted in cortical auditory areas as the dichotic effects were observed at M100 but not M50 component. This is the first evidence showing a neurophysiological inhibition driven by the contralateral auditory pathway over the ipsilateral one during dichotic listening.

Somatosensory evoked potentials during the ideation and execution of individual finger movements

Scalp somatosensory evoked potentials (SEPs) were recorded in 10 volunteers after median nerve stimulation, in four experimental conditions of hand movements performance/ideation, and compared with the baseline condition of full relaxation. The experimental conditions were (a) self‐improvised hand‐finger sequential movements; (b) the same movements according to a read sequence of numbers; (c) mental ideation of finger movements; and (d) passive displacement of fingers in complete relaxation. Latencies and amplitudes of the parietal (N20, P25, N33, and P45) and frontal peaks (P20–22, N30, and P40) were analyzed. Latencies did not vary in any of the paradigms. Among the parietal complexes, only the P25‐N33 amplitude was significantly reduced in (a), (b), (c), and (d) and the N20‐P25 was reduced in (a) and (d); among frontal waves, N30 and P40 were significantly reduced (20–75%) in (a) and (b). Coronal electrodes showed amplitude decrements maximal at the frontal‐rolandic positions contralateral to the stimulated side.

Anodal Transcranial Direct Current Stimulation Enhances Procedural Consolidation

The primary motor cortex (M1) area recruitment enlarges while learning a finger tapping sequence. Also M1 excitability increases during procedural consolidation. Our aim was to investigate whether increasing M1 excitability by anodal transcranial DC stimulation (AtDCS) when procedural consolidation occurs was able to induce an early consolidation improvement. Forty-seven right-handed healthy participants were trained in a nine-element serial finger tapping task (SFTT) executed with the left hand. Random series blocks were interspersed with training series blocks. Anodal or sham tDCS was administered over the right M1 after the end of the training session. After stimulation, the motor skills of both trained and a new untrained sequential series blocks were tested again. For each block, performance was estimated as the median execution time of correct series. Early consolidation of the trained series, assessed by the performance difference between the first block after and the last block before stimulation normalized by the random, was enhanced by anodal and not by sham tDCS. Stimulation did not affect random series execution. No stimulation effect was found on the on-line learning of the trained and new untrained series. Our results suggest that AtDCS applied on M1 soon after training improves early consolidation of procedural learning. Our data highlight the importance of neuromodulation procedures for understanding learning processes and support their use in the motor rehabilitation setting, focusing on the timing of the application.

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 SEFs), 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 SEFs were compared to control SEFs 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 SEFs components were found to exhibit larger amplitudes than the control SEFs 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 SEFs components were found to exhibit larger amplitudes than the control SEFs components.

Brain activity preceding a 2D manual catching task.

We investigated the event-related desynchronization (ERD) and synchronization (ERS) properties of cortical EEG rhythms in regions of interest (ROI) during the preparation of a 2D task for manual catching of a moving object. EEG signals were recorded through a 32-channel system in eleven healthy subjects during the interception task consisting of 2D catching with the right hand of a handle moving at constant velocity (1.5 m/s) on a predefined straight trajectory. The first session of catching movements (CATCHING_PRE) was compared with a second session after 1 h with identical characteristics (CATCHING_POST) and with other two conditions, where the subjects had to reach and grasp the handle fixed in the medium of platform (REACHING) and they looked at the object moving without catching it (GAZE TRACKING). Changes of cortical rhythms were correlated with dynamic and kinematic indexes of motor performance in both catching sessions. Movements requiring different strategies (predictive versus prospective) are supported by specific changes of cortical EEG rhythms: in the CATCHING condition a more evident power decrease (ERD) in alpha 2 and beta band in the sensorimotor region contralateral to the catching hand was observed, while in the REACHING one a bilateral ERD in beta band was found. Motor learning and movement automatization were characterized by a significant reduction of theta ERS in the anterior cingulate cortex (ACC), a ROI linked to focused attention, and with a shift of neuronal activation in alpha 2 band from the bilateral superior parietal areas to the homologous area of the left hemisphere. Finally, our EEG findings are consistent with the role of supplementary motor (SMA), premotor and prefrontal areas in motor planning and preparation. In particular, theta ERS in left SMA significantly correlated with an improvement of motor performance, as evidenced by its correlation with the training-related reduction of interception time (IT).

Developmental Tuning and Decay in Senescence of Oscillations Linking the Corticospinal System

There is increasing evidence of the importance of synchronous activity within the corticospinal system for motor control. We compared oscillatory activity in the primary sensorimotor cortex [EEG of sensorimotor cortex (SMC-EEG)] and a motor neuronal pool [surface electromyogram of opponens pollicis (OP-EMG)], and their coherence in children (4–12 years of age), young adults (20–35 years of age), and elderly adults (>55 years of age). The ratio between lower (2–13 Hz) and higher (14–32 Hz) frequencies in both SMC-EEG and OP-EMG decreased with age, correlating inversely with motor performance. There was evidence for larger, more distributed cortical networks in the children and elderly compared with young adults. Corticomuscular coherence (CMC) was present in all age groups and shifted between frontal and parietal cortical areas. In children, CMC was smaller and less stationary in amplitude and frequency than in adults. Young adults had single peaks of CMC clustered near the modal frequency (23 Hz); multiple peaks with a broad spread of frequencies occurred in children and the elderly; the further the frequency of the maximum peak CMC was from 23 Hz, the poorer the performance. CMC amplitude was inversely related to performance in young adults but was not modulated in relation to performance in children and the elderly. We propose that progressive fine-tuning of the frequency coding and stabilization of the dynamic properties within and between corticospinal networks occurs during adolescence, refining the capacity for efficient dynamic communication in adulthood. In old age, blurring of the tuning between networks and breakdown in their integration occurs and is likely to contribute to a decrement in motor control.