Parietal gamma-band activity during auditory spatial precueing of motor responses.

Magnetoencephalographic gamma-band activity (GBA) was used to investigate synchronization of cortical networks in putative auditory dorsal stream areas during the transformation of auditory spatial information into motor preparation. GBA was compared between lateralized vowels precueing either ipsi- or contralateral responses in two experiments with randomized versus blocked task presentation. In both studies, parietal GBA was higher for the contralateral than the ipsilateral precues. Spectral amplitudes at 54-64 Hz were maximal at 120 ms post precue onset during randomized presentations, while in the blocked task 62-72 Hz activity was present at precue onset and less peaked. These findings suggest a fast activation of parietal networks when auditory spatial precues are used to plan contralateral responses.

Location changes enhance hemispheric asymmetry of magnetic fields evoked by lateralized sounds in humans.

Auditory mismatch negativity, the brain's change-detection response, has been shown to be more sensitive than other early auditory cortex responses to the hemispheric specialization of speech processing. The present study used magnetoencephalography to assess hemispheric differences in cortical evoked responses during auditory spatial processing. We compared N1m to lateralized vowels presented with equal probabilities with mismatch fields (MMNm) to rare lateralized noises interspersed in a sequence of frequent midline sounds. Both N1m and MMNm dipole amplitudes were higher in the hemisphere contralaterally to the side of sound lateralization, but this effect was about four times bigger in the mismatch paradigm. Moreover, only MMNm dipoles showed shorter latencies in the hemisphere contralaterally to stimulation. Apparently stimulus changes activate specialized auditory networks more strongly than non-deviant events.

Event-related beta desynchronization indicates timing of response selection in a delayed-response paradigm in humans.

Voluntary movements are preceded by event-related desynchronization (ERD) of alpha and beta activity. The present study used magnetoencephalography to investigate the relationship between motor preparation and the time course of beta ERD in a delayed response paradigm. Depending on the task, the required response (left or right finger lifting) was cued either spatially by the lateralization of a sound at trial onset, or verbally by the content of a midline auditory event. Beta ERD appeared over sensorimotor regions contralaterally to the response side about 200 ms earlier for the spatial than the verbal cue task. This suggests a close relationship between the latency of beta ERD onset and the duration of cognitive processes involved in selecting a motor response.

Early activation of the primary somatosensory cortex without conscious awareness of somatosensory stimuli in tumor patients.

The primary sensory cortex has usually been regarded as a necessary step in the information processing stream leading to conscious awareness. Recently, it has been proposed that that higher order associative areas rather than the primary sensory areas are the neural basis of conscious perception. In two patients with tumors near the central region we recorded magnetic somatosensory evoked fields. Magnetic source imaging revealed early (40 ms) neural activation in primary somatosensory cortex and absence of later (>60 ms) neural activation in the primary and associative areas in these patients. None of the patients showed conscious awareness of somatosensory stimuli applied to the corresponding body site although the first component of the evoked field was within normal limits. The time course of the magnetic responses and additional evidence on intensity ratings of somatosensory stimuli suggest that early activity in the primary somatosensory cortex is not sufficient for conscious experience to emerge.

Contralaterality of cortical auditory processing at the level of the M50/M100 complex and the mismatch field: a whole-head magnetoencephalography study.

Humans show a stronger cortical representation of auditory input at the opposite hemisphere each. To specify the temporal aspects of this contralaterality effect within the domain of speech stimuli, the present study recorded a series of evoked magnetic fields (M50, M100, mismatch field) subsequent to monaural application of stop consonant-vowel syllables using whole-head magnetoencephalography (MEG). The M50 components exhibited a skewed shape of cross-symmetrical distribution in terms of an initial maximum peak succeeded by a knot over the contralateral and a reversed pattern over the ipsilateral temporal lobe. Most presumably, this pattern of evoked fields reflects two distinct stages of central-auditory processing: (a) initial excitation of the larger contralateral and the smaller ipsilateral projection area of the stimulated ear; (b) subsequent transcallosal activation of the residual neurons, i.e. the targets of the non-stimulated ear, at either side. Previous studies using non-speech stimuli found contralaterality of central-auditory processing to extend to the M100 field. In contrast, a larger amplitude of ipsilateral M100 as compared to the respective opposite deflection emerged after stimulation of either ear. Finally, the computed magnetic analogues of mismatch negativity failed any significant laterality effects. These data provide first evidence for a distinct pattern of hemispheric differences at the level of the M50/M100 complex subsequent to monaural application of speech stimuli.

Reorganization of motor and somatosensory cortex in upper extremity amputees with phantom limb pain.

Phantom limb pain (PLP) in amputees is associated with reorganizational changes in the somatosensory system. To investigate the relationship between somatosensory and motor reorganization and phantom limb pain, we used focal transcranial magnetic stimulation (TMS) of the motor cortex and neuroelectric source imaging of the somatosensory cortex (SI) in patients with and without phantom limb pain. For transcranial magnetic stimulation, recordings were made bilaterally from the biceps brachii, zygomaticus, and depressor labii inferioris muscles. Neuroelectric source imaging of the EEG was obtained after somatosensory stimulation of the skin overlying face and hand. Patients with phantom limb pain had larger motor-evoked potentials from the biceps brachii, and the map of outputs was larger for muscles on the amputated side compared with the intact side. The optimal scalp positions for stimulation of the zygomaticus and depressor labii inferioris muscles were displaced significantly more medially (toward the missing hand representation) in patients with phantom limb pain only. Neuroelectric source imaging revealed a similar medial displacement of the dipole center for face stimulation in patients with phantom limb pain. There was a high correlation between the magnitude of the shift of the cortical representation of the mouth into the hand area in motor and somatosensory cortex and phantom limb pain. These results show enhanced plasticity in both the motor and somatosensory domains in amputees with phantom limb pain.

Investigation of brain dynamics in Parkinson's disease by methods derived from nonlinear dynamics.

EEGs were recorded from patients in early stages of Parkinson's disease (17 patients, 9 females) and healthy controls (12 subjects, 8 females) during rest and during execution/imagining of a complex motor task. The prediction that Parkinson's disease patients compared to controls would show more complex brain dynamics during performance of a complex motor task and imagination of the movements was confirmed by methods derived from nonlinear dynamics. In the resting state, analysis of correlation dimension of EEG time series revealed only slight topographical differences between the groups. During performance of a complex motor task, however, data from Parkinson's disease patients showed higher dimensionality than data from controls, indicating more complex EEG time series. The same difference was found when subjects did not perform any motor movements but imagined the complex movements they had just performed. The data are consistent with the hypothesis that the disturbances in Parkinson's disease result in the recruitment of superfluous cortical networks due to failed inhibition of alternative motor programs in the striatum and thus increase the complexity of cortical representation in motor conditions.

Neural correlates of duplex perception: a whole-head magnetencephalography study.

Simultaneous experience of the same acoustic stimulus in two distinct phenomenological modes, e.g. as a speech-like and as a non-speech event, is referred to as duplex perception (DP). The most widely investigated DP paradigm splits each of the stop consonant-vowel (CV) syllables /ga/ and /da/ into an isolated formant transient (chirp) and the remaining sound structure (base). The present study recorded mismatch fields in response to a series of dichotically applied base and chirp components using whole-head magnetencephalography (MEG). Preattentive mismatch fields showed larger amplitudes in response to contralateral deviants. During attention to the fused percept /da/, the left ear deviants chirps elicited an enhanced and posteriorly shifted dipole field over the ipsilateral hemisphere. These data provide first neurophysiological evidence that the integration of acoustic stimulus elements into a coherent syllable representation constitutes a distinct stage of left-hemisphere speech sound encoding.

Cortical reorganization after digit-to-hand replantation.

Functional recovery after digit-to-hand replantation depends on the interaction of various factors. In addition to peripheral mechanisms, cortical and subcortical reorganization of digit representation may play a substantial role in the recovery process. However, cortical processes during the first months after replantation are not well understood. In this 25-year-old man who had traumatically lost digits II to V (DII-V) on his right hand, the authors used magnetoencephalographic source imaging to document the recovery of somatosensory cortical responses after tactile stimulation at four sites on the replanted digits. Successful replantation of DIV and DV was accomplished at the original position of DIII and DIV with mixed innervation. Cortical evoked fields could be recorded starting from the 10th week after digit-to-hand replantation. Initially, signals from all sites showed decreased amplitudes and prolonged latencies. In the subsequent six recordings obtained between the 12th and 55th week postreplantation, a continuous increase in amplitude but only a slight recovery of latencies were observed. Components of the recorded somatosensory evoked fields were localized in the primary somatosensory cortex (SI). The localizations of the replanted DIV showed a gradual lateral-inferior shift in the somatosensory cortex over time, indicating cortical reorganization caused by altered peripheral input. The authors infer from this shift that the original cortical area of the missing finger (DII) was taken over by the replanted finger. From these data the authors conclude that magnetic source imaging might be a reliable noninvasive method to evaluate surgical nerve repair and that cortical reorganization of SI is involved in the regeneration process following peripheral nerve injury.

Simultaneous bilateral mismatch response to right- but not leftward sound lateralization.

Magnetoencephalography (MEG) was used to compare mismatch responses between hemispheres to changes in sound-source direction. Sixteen adults listened passively to two types of complex non-language sounds presented in separate blocks with midline standards and right- and left-lateralized deviants. Mismatch dipole amplitudes were larger contra- than ipsilaterally to the deviants. Both hemispheres processed right deviants simultaneously, whereas to left deviants, the left dipole peaked 20 ms later than the right dipole. A second experiment using the same standards but midline spectral deviants showed no interhemispheric differences. Here mismatch latencies were about 60 ms longer than in the location mismatch experiment. This suggested both fast, contralaterally dominant location mismatch responses and facilitated detection of auditory spatial deviance in the right hemifield.

Encoding of temporal speech features (formant transients) during binaural and dichotic stimulus application: a whole-head magnetencephalography study.

Spoken-word recognition depends upon the encoding of relevant 'information bearing elements' of the acoustic speech signal. For example, relatively rapid shifts of spectral energy distribution (formant transients) cue the perception of stop consonant-vowel (CV) syllables such as /ba/, /ga/, and /da/. A variety of data indicate left-hemisphere superiority with respect to the processing of formant transients. To further delineate the underlying neurophysiological mechanisms, evoked cortical fields in response to CV syllables (oddball design; frequent stimulus=binaural /ga/; four deviant constellations: Binaural /ba/, binaural /da/, left /da/ (left ear deviant)-right /ga/, right /da/ (right ear deviant)-left /ga/) were recorded by means of whole-head magnetencephalography (MEG; 151 channels) under two different conditions of attentional demands (visual distraction versus reaction to prespecified stimuli). (a) During binaural stimulus presentation attention toward target events resulted in a significantly enhanced mismatch field (MMNm, magnetic analogue to the mismatch negativity) over the left as compared to the right hemisphere. In contrast, preattentive processing of the CV syllables failed MMNm lateralization effects. (b) Dichotic application of /da/ elicited a larger contralateral MMNm amplitude in subjects with right ear advantage (REA) at behavioral testing. In addition, right ear deviants yielded a stronger ipsilateral response than the left ear cognates. Taken together, these data indicate bilateral preattentive processing and subsequent attention-related predominant left-hemisphere encoding of formant transients at the level of the supratemporal plane. Furthermore, REA during dichotic application of CV syllables seems to be linked to functional dissociation of the two hemispheres during auditory processing.

Right-hemisphere dominance for the processing of sound-source lateralization.

Cortical processing of change in direction of a perceived sound source was investigated in 12 human subjects using whole-head magnetoencephalography. The German word "da" was presented either with or without 0.7 msec interaural time delays to create the impression of right- or left-lateralized or midline sources, respectively. Midline stimuli served as standards, and lateralized stimuli served as deviants in a mismatch paradigm. Two symmetrically linked dipoles fitted to the mismatch fields showed stronger moments in the hemisphere contralateral to the side of the deviant. The right dipole displayed equal latencies to both left and right deviants, whereas left dipole latencies were longer for ipsilateral than contralateral deviants. Frequency analysis between 20-70 Hz and statistical probability mapping revealed increased induced gamma-band activity at 53+/-2.5 Hz to both types of deviants. Right deviants elicited spectral amplitude enhancements in this frequency range, peaking at latencies of 160 and 240 msec. These effects were localized bilaterally over the angular gyri and posterior temporal regions. Coherence analysis suggested the existence of two separate interhemispheric networks. For left-lateralized deviants, both spectral amplitude enhancements at 110 and 220 msec and coherence increases were restricted to the right hemisphere. In conclusion, both mismatch dipole latencies at the supratemporal plane and gamma-band activity in posterior parietotemporal areas suggested a right hemisphere engagement in the processing of bidirectional sound-source shifts. In contrast, left-hemisphere regions responded predominantly to contralateral events. These findings may help to elucidate phenomena such as unilateral auditory neglect.

Statistical probability mapping reveals high-frequency magnetoencephalographic activity in supplementary motor area during self-paced finger movements.

Investigations of both haemodynamic and electroencephalographic measures of brain activity have demonstrated supplementary motor area (SMA) involvement in self-paced finger movements. In contrast, analysis of magnetoencephalographic (MEG) signals in the time domain has usually failed to detect SMA activity in healthy individuals. We investigated oscillatory MEG activity in 12 normal adults during (a) a self-paced, complex sequence of finger movements and (b) a simple finger opposition task paced externally by tactile stimuli presented to the contralateral thumb. Statistical probability mapping revealed enhanced non-phase-locked spectral amplitudes in the 22-28 Hz range over bilateral frontal cortex during self-paced as compared to externally cued finger movements. This activity may reflect recruitment of cell assemblies in SMA during self-paced, complex movements.

Differential impact of periodic and aperiodic speech-like acoustic signals on magnetic M50/M100 fields.

Voiced and unvoiced sounds, characterized by a periodic or aperiodic acoustic structure, respectively, represent two basic information-bearing elements of the speech signal. Using whole-head magnetencephalography (MEG), magnetic fields (M50/M100) in response to synthetic vowel-like as well as noise-like signals matched in spectral envelope were recorded in 20 subjects. Aperiodic events gave rise to increased M50 concomitant with reduced M100 activity as compared to their periodic cognates. Attention toward the auditory channel enhanced the effects of signal periodicity. These data provide first evidence that speech-relevant acoustic features differentially affect evoked magnetic fields as early as the M50 component. Conceivably, the M50 field reflects an ongoing monitoring process whereas the M100 component is bound to more specific operations such as detection of signal periodicity.

[Not Available]. / Magnetenzephalographie in der Neurochirurgie Ein Routineprotokoll zur präoperativen Planung und intraoperativen Navigation : Ein Routineprotokoll zur präoperativen Planung und intraoperativen Navigation.

The combination of magnetoencephalography (MEG) and magnetic resonance imaging (MRI) allows the localization and functional characterization of the somatosensory and motor cortex. However, this method is technically pretentious and time consuming. Therefore, functional imaging is still not established as routine procedure in neurosurgery. The aim of the presented study was the development of a protocol, which allows the integration of functional MEG data into presurgical planning and intraoperative navigation, as a routine method.Forty-five patients with intracranial mass lesions within the central region were examined by MEG. For the somatosensory localization, the median and tibial nerves were electrically stimulated. The motor cortex was localized by tapping of the index finger. The coordinates of the functional dipole source were projected onto a 3D MRI data-set. Finally, the marked MR images were transferred to the neurosurgical navigation system. Following this strategy, after an average time of 3 hours, all data for presurgical planning and intraoperative navigation were available for the neurosurgeon.Our results show that MEG/MRI based functional neuronavigation can be used as a routine tool during daily neurosurgical practice.

Complexity of electrocortical dynamics in children: developmental aspects.

Dimensional complexity (DCx) is an EEG measure derived from nonlinear systems theory that can be indicative of the global dynamical complexity of electrocortical activity. This study examined developmental changes in DCx, as well as the effects of cognitive tasks, gender, and brain topography, and compared DCx with traditional spectral power measures. EEG was recorded in three groups of children at mean age of 7.5 (n = 37), 13.8 (n = 42), and 16.4 (n = 56) years at rest and during the performance of verbal and spatial cognitive tasks. DCx measured both at rest and during tasks increased with age. Specific effects of brain topography, condition, and gender became stronger with age, suggesting an increase in structural and functional differentiation of the cortex. Hemispheric asymmetry of DCx recorded during tasks also increased with age, with the task-induced DCx reduction being stronger in the left hemisphere. Gender differences in DCx suggested faster cerebral maturation in girls over late adolescence. Relationships between DCx and spectral power varied as a function of tasks and scalp locations, suggesting that these EEG measures can reflect different aspects of cortical functioning.

Preattentive processing of consonant vowel syllables at the level of the supratemporal plane: a whole-head magnetencephalography study.

A variety of clinical and experimental data indicate superiority of the left hemisphere with respect to the encoding of dynamic aspects of the acoustic speech signal such as formant transients, i.e., fast changes of spectral energy distribution across a few tens of milliseconds, which cue the perception of stop consonant vowel syllables. Using an oddball design, the present study recorded auditory evoked magnetic fields by means of a whole-head device in response to vowels as well as syllable-like structures. Both the N1m component (=the magnetic equivalent to the N1 response of the electroencephalogram (EEG)) and various difference waves between the magnetic fields to standard and respective rare events (MMNm=magnetic mismatch negativity) were calculated. (a) Vowel mismatch (/a/ against /e/) resulted in an enlarged N1m amplitude reflecting, most presumably, peripheral adaptation processes. (b) As concerns lateralized responses to syllable-like structures, only the shortest transient duration (=10 ms) elicited a significantly enhanced MMNm at the left side. Conceivably, the observed hemispheric difference contributes to prelexical parsing of the auditory signal rather than the encoding of linguistic categories.

Spatiotemporal organization of brain dynamics and intelligence: an EEG study in adolescents.

This study investigated relationships between global dynamics of brain electric activity and intelligence. EEG was recorded monopolarly from 10 symmetric leads (10-20 system) in 37 (17 males) healthy subjects (mean age 13.7 years) at rest and during performance of two visually presented cognitive tasks, verbal (semantic grouping) and spatial (mental rotation). On another occasion, the subjects were administered the Intelligence Structure Test (IST). Both total IST score and some individual subtests of specific abilities showed significant positive correlations with EEG coherence in the theta band and significant negative relationships with EEG dimension, a measure of complexity and unpredictability of neural oscillatory dynamics underlying the EEG time series. Furthermore, EEG coherence and dimensional complexity were inversely related. Taken together, these EEG metrics accounted for over 30% of the variability of the total IST score in this sample. No significant effects of the task type (spatial vs. verbal) or specific abilities were observed. Long-distance theta coherence between frontal and parieto-occipital areas showed the most consistent relationship with cognitive abilities. The results suggest that order to chaos ratio in task-related brain dynamics may be one of the biological factors underlying individual differences in cognitive abilities in adolescents.

Nouns and verbs in the intact brain: evidence from event-related potentials and high-frequency cortical responses.

Lesion evidence indicates that words from different lexical categories, such as nouns and verbs, may have different cortical counterparts. In this study, processing of nouns and verbs was investigated in the intact brain using (i) behavioral measures, (ii) stimulus-triggered event-related potentials and (iii) high-frequency electrocortical responses in the gamma band. Nouns and verbs carefully matched for various variables, including word frequency, length, arousal and valence, were presented in a lexical decision task while electrocortical responses were recorded. In addition, information about cognitive processing of these stimuli was obtained using questionnaires and reaction times. As soon as approximately 200 ms after stimulus onset, event-related potentials disclosed electrocortical differences between nouns and verbs over widespread cortical areas. In a later time window, 500-800 ms after stimulus onset, there was a significant difference between high-frequency responses in the 30 Hz range. Difference maps obtained from both event-related potentials and high-frequency responses revealed strong between-category differences of signals recorded above motor and visual cortices. Behavioral data suggest that these different physiological responses are related to semantic associations (motor or visual) elicited by these word groups. Our results are consistent with a neurobiological model of language representation postulating cell assemblies with distinct cortical topographies as biological counterparts of words. Assemblies representing nouns referring to visually perceived objects may include neurons in visual cortices, and assemblies representing action verbs may include additional neurons in motor, premotor and prefrontal cortices. Event-related potentials and high-frequency responses are proposed to indicate two different functional states of cell assemblies: initial full activation ('ignition') and continuous reverberatory activity.

Gamma-band MEG activity to coherent motion depends on task-driven attention.

We examined gamma-band magnetoencephalographic (MEG) activity in humans manipulating attention to visual stimuli by auditory distractors. After exposure to both visual and auditory noise (a baseline), subjects attended to the first of two stimuli (either regular motion of bars or a tone sequence) presented asynchronously, and responded to its offset. A spectral power analysis revealed an increased, relative to baseline, 40 Hz MEG response to attended coherent motion. The enhancement occurred within the initial 50-250 ms from motion onset over modality-specific (occipital) cortices. The increase was not observed when attention was captured by auditory distractors. Our findings suggest that 40 Hz activity in the human visual cortex is related to integration of featural information that is supported by attention.