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R Salmelin

Publications and source records attributed to R Salmelin.

At least 55 records · Page 3Linked to original sources
PubMed1997

Cortical control of human motoneuron firing during isometric contraction.

We recorded whole scalp magnetoencephalographic (MEG) signals simultaneously with the surface electromyogram from upper and lower limb muscles of six healthy right-handed adults during voluntary isometric contraction. The 15- to 33-Hz MEG signals, originating from the anterior bank of the central sulcus, i.e., the primary motor cortex, were coherent with motor unit firing in all subjects and for all muscles. The coherent cortical rhythms originated in the hand motor area for upper limb muscles (1st dorsal interosseus, extensor indicis proprius, and biceps brachii) and close to the foot area for lower limb muscles (flexor hallucis brevis). The sites of origin corresponding to different upper limb muscles did not differ significantly. The cortical signals preceded motor unit firing by 12-53 ms. The lags were shortest for the biceps brachii and increased systematically with increasing corticomuscular distance. We suggest that the motor cortex drives the spinal motoneuronal pool during sustained contractions, with the observed cortical rhythmic activity influencing the timing of efferent commands. The cortical rhythms could be related to motor binding, but the rhythmic output may also serve to optimize motor cortex output during isometric contractions.

Adult↗
PubMed1996

Information processing in the human brain: magnetoencephalographic approach.

Rapid progress in effective methods to image brain functions has revolutionized neuroscience. It is now possible to study noninvasively in humans neural processes that were previously only accessible in experimental animals and in brain-injured patients. In this endeavor, positron emission tomography has been the leader, but the superconducting quantum interference device-based magnetoencephalography (MEG) is gaining a firm role, too. With the advent of instruments covering the whole scalp, MEG, typically with 5-mm spatial and 1-ms temporal resolution, allows neuroscientists to track cortical functions accurately in time and space. We present five representative examples of recent MEG studies in our laboratory that demonstrate the usefulness of whole-head magnetoencephalography in investigations of spatiotemporal dynamics of cortical signal processing.

Auditory Perception↗
PubMed1996

Human cortical 40 Hz rhythm is closely related to EMG rhythmicity.

We recorded cortical neuromagnetic rhythms during self-paced index-finger movements from a subject previously reported to show prominent 40 Hz electroencephalographic activity during motor behavior. The 10 and 20 Hz components of the rolandic mu rhythm were bilaterally suppressed, whereas the contralateral 40 Hz (35-41 Hz) activity was slightly enhanced before both fast and slow movements and strongly enhanced during slow movements. The 40 Hz rhythm originated mainly in the hand motor cortex and was clearly correlated with the rhythmicity of the electromyogram from the extensor muscles, with a systematic time lag. In this subject motor preparation, and especially control of finger movements, may thus be associated with enhanced cortical rhythms near 40 Hz. The coherence of these rhythms with muscular firing patterns likely reflects communication between the sensorimotor cortex and the motor units.

Adult↗
PubMed1996

Odorants activate the human superior temporal sulcus.

The human olfactory pathways are well defined up to the level of the prepiriform cortex but the neocortical projections and their functional organization are still largely unknown. We recorded whole-scalp neuromagnetic signals to olfactory stimulation with boluses of phenylethyl alcohol, hydrogen sulphide, and vanillin. The main magnetic response peaked about 700 ms after the stimulus onset. The three odorants activated overlapping cortical areas around the superior temporal sulci of both hemispheres, revealing a neocortical area involved in olfactory processing.

Adult↗
PubMed1996

Impaired visual word processing in dyslexia revealed with magnetoencephalography.

Dyslexia is most often attributed to phonological impairments, manifested in abnormal activation of the left temporal and temporoparietal cortex in response to auditorily presented language and possibly associated with anomalies in the cytoarchitecture and hemispheric symmetry of the plana temporale. The immediate cortical correlate of the severely impaired reading process has, however, remained obscure. Here we report on the distinct time courses of cortical activation in dyslexic and control subjects during passive viewing of single words, tracked with whole-head magnetoencephalography. A striking difference was found in the left inferior temporo-occipital region where intracranial recordings have recently identified word-specific responses within 200 msec after stimulus onset: controls showed a sharp activation at about 180 msec after word presentation, whereas dyslexics failed to activate this area entirely, or showed a slowly increasing late response. Perception of words as specific units thus seems to be impaired in dyslexics.

Adolescent↗
PubMed1996

Neural net identification of thumb movement using spectral characteristics of magnetic cortical rhythms.

Neural nets have shown great promise as tools for reducing and examining multi-dimensional data. When carefully tuned with selected data sets of individual subjects neural nets have indisputable potential in identifying distinct stages of voluntary finger movements. However, robust, automatized data description methods would be needed to eventually extend the use of neural networks into visualization of brain activity during more complex, multimodal tasks where the cortical processes are not equally well understood. We explored the suitability of a self-organizing map (SOM) in the widely studied case of voluntary finger movements (left and right thumb), using as input such spectral characteristics that showed systematic task-dependent changes when averaged over repeated movements. SOMs constructed without individual fine-tuning and with generally chosen training parameters from these spectral features identified correctly 85% of the ongoing movements but, somewhat surprisingly, not the side of thumb movement. Even for this inclusive choice of input, the neural nets were sensitive to transient signals, but focused fine tuning, based on a priori known subgroups in the data, is clearly required for more detailed classification. Thus, a neural net visualization is likely not the most attractive first approach for characterization of cortical processing during complex multimodal tasks.

Brain Mapping↗
PubMed1996

Movement-related slow cortical magnetic fields and changes of spontaneous MEG- and EEG-brain rhythms.

Cortical activity was recorded from 5 healthy adults with a 122-channel whole-head magnetometer while the subjects performed during unilateral finger movements at self-paced intervals exceeding 6 s. The readiness field (RF) started over the contralateral somatomotor area 0.3-1 s prior to the movement onset in subjects (Ss) 1, 2, and 4, and culminated in the motor field (MF) 30 ms after it (Ss 1-4). These signals were followed by movement evoked fields MEFI (Ss 1-5) and MEFII (Ss 1-4) at 100-150 ms and 200-250 ms after the movement onset, respectively. One subject showed clear RF over the ipsilateral hemisphere as well. The contralateral dominance of the RF contrasted the more symmetric distribution of the simultaneously recorded electric Bereitschaftspotential (BP). The RF onset never preceded the BP onset. We suggest that BP receives contribution from the early bilateral activation of the crown of the precentral gyrus, whereas RF reflects later activity of the fissural motor cortex. Spontaneous oscillations in the background activity (spontaneous activity) of approximately 10 Hz started to dampen 2-3 s prior to the movement onset in the somatomotor areas of both hemispheres with contralateral predominance (S1 and S3), and returned to a steady level 0.8-2 s after the movement onset in all subjects. Higher frequency bands in the same area displayed a prominent rebound about 1 s after the movement onset in 4 subjects. Execution of self-paced movements is evidently expressed differently in the slow movement-related fields and in the cortical spontaneous activity.

Adult↗
PubMed1995

Tactile information from the human hand reaches the ipsilateral primary somatosensory cortex.

Neuromagnetic responses to median nerve stimulation were studied in six healthy right-handed subjects. In the rest condition, only the right and left median nerves were alternately stimulated at the wrists. In two other conditions, continuous superficial tactile stimulation was concurrently applied to either the left or right hand. Tactile stimulation of palm and fingers of one hand enhanced, in the ipsilateral primary somatosensory cortex (SI), responses to median nerve stimulation of the other hand. This effect was stronger in the left than the right SI. Our data provide evidence in humans for the access of cutaneous information from the hands to ipsilateral SI, probably via excitatory transcallosal pathways. This interhemispheric information transfer may represent a neurophysiological substrate of somatosensory fusion between the hands.

Adult↗
PubMed1995

Functional segregation of movement-related rhythmic activity in the human brain.

Multiple synaptic interconnections in the human brain support concerted rhythmic activity of a large number of cortical neurons, typically close to 10 and 20 Hz. Our present neuromagnetic data provide evidence for distinct functional roles of these spectral components in the somatomotor cortex. The sites of suppression during movement and the subsequent rebound of the 20-Hz rhythm followed, along the motor cortex, the representation of fingers, toes, and mouth, as opposed to the stable origin of the 10-Hz rhythms close to the hand somatosensory cortex. The 20-Hz activity appears to be a signature of active immobilization following movement, whereas the reactive 10-Hz signals likely reflect lack of relevant sensory input from the important upper limbs.

Adult↗
PubMed1995

Bilateral activation of the human somatomotor cortex by distal hand movements.

We recorded cortical magnetic signals, simultaneously over the whole scalp, from 6 healthy subjects during 3 motor tasks to track the varying proportion of contra- vs. ipsilateral activation. The subjects performed self-paced index finger flexions, simultaneous flexion of 4 fingers, and a sequence of rapid digit movements in different sessions. Index finger and 4-finger movements were associated with phasic bilateral dampening of spontaneous 10 and 20 Hz rhythms along the central sulcus, starting approximately 1 sec before the movement in the contralateral hemisphere. A rebound occurred within 1 sec after the index finger and 4-finger flexions; the rapid finger movements resulted in a persistent blocking of the rhythms. Averaging with respect to movement onset showed a slow bilateral frontal readiness field starting about 0.5 sec prior to motion onset. It was followed, within 200 msec after movement onset, by phasic movement-evoked fields (MEFs) which were bilateral during the tasks involving several fingers. The contra- vs. ipsilateral MEF amplitude ratio C/I decreased from 4.0 during index finger movements to 0.6 during rapid finger flexions, reflecting the enhanced activation of the ipsilateral primary somatomotor cortex with increasing complexity of movement.

Adult↗
PubMed1994

Dynamics of brain activation during picture naming.

The cerebral representation of language, deduced from observing patients with brain lesions and from stimulations and recordings performed during brain surgery, has been further clarified by recent positron emission tomography and functional magnetic resonance imaging measurements. We now expand this static view into the dynamics of cortical activation using the accurate spatiotemporal resolution of whole-head magnetoencephalography. During picture naming, the conversion from visual to symbolic representation progressed bilaterally from the occipital visual cortex towards temporal and frontal lobes. Overt naming elicited the most widespread cortical activation. Some language-related sites also reacted, though more weakly or after a longer delay, during covert naming and even passive viewing.

Adult↗
PubMed1994

Activation of the human posterior parietal cortex by median nerve stimulation.

We recorded somatosensory evoked magnetic fields from ten healthy, right-handed subjects with a 122-channel whole-scalp SQUID magnetometer. The stimuli, exceeding the motor threshold, were delivered alternately to the left and right median nerves at the wrists, with interstimulus intervals of 1, 3, and 5 s. The first responses, peaking around 20 and 35 ms, were explained by activation of the contralateral primary somatosensory cortex (SI) hand area. All subjects showed additional deflections which peaked after 85 ms; the source locations agreed with the sites of the secondary somatosensory cortices (SII) in both hemispheres. The SII responses were typically stronger in the left than the right hemisphere. All subjects had an additional source, not previously reported in human evoked response data, in the contralateral parietal cortex. This source was posterior and medial to the SI hand area, and evidently in the wall of the postcentral sulcus. It was most active at 70-110 ms.

Adult↗
PubMed1994

Characterization of spontaneous MEG rhythms in healthy adults.

We analyzed spontaneous MEG activity, measured with a whole-head neuromagnetometer in 7 healthy, relaxed adults. Rhythmic activity concentrated over the rolandic and parieto-occipital regions and contained spectral components in the 10 and 20 Hz ranges, each showing distinct reactivity. Sources of the rhythmic activity in different frequency ranges were localized within the brain and related to the sites of the auditory, visual, and somatosensory projection cortices. The sources of the rolandic mu rhythms concentrated in restricted areas of 3-5 cm in diameter, close to the hand projection area in the first somatomotor cortex. The posterior alpha activity received a significant contribution from the vicinity of the parieto-occipital fissure. Combination of spectral analysis, reactivity of the rhythms with respect to tasks or stimuli, and source localization will allow focusing on well-specified cortical rhythms and assist in gaining an understanding of the functional significance of cortical rhythms.

Adult↗
PubMed1994

Comparison of somatosensory evoked fields to airpuff and electric stimuli.

We recorded somatosensory evoked magnetic fields (SEFs) from 6 healthy subjects with a 122-channel whole-scalp SQUID gradiometer. In separate experiments, airpuff stimuli were delivered to the dorsum of the proximal phalanx of the middle finger, and electric stimuli were delivered to the median nerve at the wrist; the interstimulus interval was 3 sec and left and right hands were stimulated in subsequent sessions. Airpuffs evoked clear and reproducible responses in all subjects. First responses were recorded over the SI cortex. All subjects showed SII responses both to contra- and ipsilateral airpuffs. The posterior parietal source, identified previously to electric stimulation, was activated also by airpuffs, but only in the right hemisphere. The earliest responses from SI were smaller in amplitude and longer in latency to airpuffs than to electric stimuli; the long-latency responses arising from the other somatosensory areas did not differ significantly.

Adult↗
PubMed1994

Spatiotemporal characteristics of sensorimotor neuromagnetic rhythms related to thumb movement.

To assess the spatial extent and temporal behavior of rolandic rhythms we recorded neuromagnetic signals from four healthy subjects with a 24-channel magnetometer. The subjects performed self-paced thumb movements or the motions were triggered by electrical stimulation of the median nerve at the wrist. The main frequency components of the magnetic mu rhythm signals centered at 10 and 20 Hz. Both components were completely suppressed during the movement and increased substantially 0.5-2.5 s after it; the 20-Hz component reacted about 300 ms faster. The rebound was stronger after self-paced than after stimulated motion, and after contra- than after ipsilateral movement. The reactive source areas were identified for both frequency ranges, and they clustered on partly overlapping cortical areas of 6-8 cm2 wide along the course of the central sulcus. The 10-Hz rhythmic oscillations occurred predominantly at the primary somatosensory hand cortex; the sources of the 20-Hz signals were slightly more anterior. We hypothesize that the 10-Hz signal is a true somatosensory rhythm whereas the 20-Hz activity is essentially somatomotor in origin.

Adult↗
PubMed1994

Evidence of sharp frequency tuning in the human auditory cortex.

The frequency tuning of the human auditory cortex was studied by masking 100-ms tones of 1 and 2 kHz by continuous white-noise maskers with frequency notches around the tone frequencies. The subjects ignored the stimuli and concentrated on a reading task. The neuronal activity elicited by the test tones in the auditory cortex was measured with a 24-channel neuromagnetometer. The masker affected the amplitude and latency of the neuromagnetic N100m response, peaking about at 100 ms after stimulus onset, in a systematic way: the wider the notch, the shorter was the latency and the larger the amplitude. The source location of N100m in the auditory cortex did not depend on the notch width. Auditory filters at 1 and 2 kHz were modelled by a single-parameter rounded-exponential [Roex(p)] filter, based on the amplitude changes of N100m. The filters revealed sharp tuning of the auditory cortex, resembling that obtained in psychoacoustical masking studies. The results demonstrate that frequency tuning of the neurons or neuron ensembles in the human auditory cortex can be studied completely noninvasively. Moreover, since the stimuli were ignored by the subjects, the filter shape is not affected by the criterion adopted by the subject in the discrimination task.

Acoustic Stimulation↗
PubMed1993

Suppression of magnetic mu rhythm during parkinsonian tremor.

We recorded spontaneous magnetoencephalographic (MEG) activity and somatosensory-evoked fields (SEFs) with a 24-channel planar SQUIDgradiometer in five patients with hemiparkinsonism. The SEFs of the patients were within normal limits. During tremorless periods, the spontaneous activity over the somatomotor cortex had a frequency peak at approximately 10 Hz in all five patients and another at approximately 20 Hz in three. Tremor dampened the 10-Hz activity in all patients; in three the effect was bilateral. Tremor did not increase MEG activity at the tremor frequency. The suppression of the mu rhythm by the parkinsonian tremor resembled that occurring during voluntary movements in healthy subjects.

Adult↗