Riitta Salmelin

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.

Human cortical oscillations: a neuromagnetic view through the skull

The mammalian cerebral cortex generates a variety of rhythmic oscillations, detectable directly from the cortex or the scalp. Recent non-invasive recordings from intact humans, by means of neuromagnetometers with large sensor arrays, have shown that several regions of the healthy human cortex have their own intrinsic rhythms, typically 8-40 Hz in frequency, with modality- and frequency-specific reactivity. The conventional hypotheses about the functional significance of brain rhythms extend from epiphenomena to perceptual binding and object segmentation. Recent data indicate that some cortical rhythms can be related to periodic activity of peripheral sensor and effector organs.

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.

Involvement of Primary Motor Cortex in Motor Imagery: A Neuromagnetic Study

Functional brain imaging studies have indicated that several cortical and subcortical areas active during actual motor performance are also active during imagination or mental rehearsal of movements. Recent evidence shows that the primary motor cortex may also be involved in motor imagery. Using whole-scalp magnetoencephalography, we monitored spontaneous and evoked activity of the somatomotor cortex after right median nerve stimuli in seven healthy right-handed subjects while they kinesthetically imagined or actually executed continuous finger movements. Manipulatory finger movements abolished the poststimulus 20-Hz activity of the motor cortex and markedly affected the somatosensory evoked response. Imagination of manipulatory finger movements attenuated the 20-Hz activity by 27% with respect to the rest level but had no effect on the somatosensory response. Slight constant stretching of the fingers suppressed the 20-Hz activity less than motor imagery. The smallest possible, kinesthetically just perceivable finger movements resulted in slightly stronger attenuation of 20-Hz activity than motor imagery did. The effects were observed in both hemispheres but predominantly contralateral to the performing hand. The attempt to execute manipulatory finger movements under experimentally induced ischemia causing paralysis of the hand also strongly suppressed 20-Hz activity but did not affect the somatosensory evoked response. The results indicate that the primary motor cortex is involved in motor imagery. Both imaginative and executive motor tasks appear to utilize the cortical circuitry generating the somatomotor 20-Hz signal.

The neural basis of intermittent motor control in humans

The basic question of whether the human brain controls continuous movements intermittently is still under debate. Here we show that 6- to 9-Hz pulsatile velocity changes of slow finger movements are directly correlated to oscillatory activity in the motor cortex, which is sustained by cerebellar drive through thalamus and premotor cortex. Our findings suggest that coupling of 6- to 9-Hz oscillatory activity in the cerebello–thalamo–cortical loop represents the neural mechanism for the intermittent control of continuous movements.

An MEG Study of Picture Naming

The purpose of this study was to relate a psycholinguistic processing model of picture naming to the dynamics of cortical activation during picture naming. The activation was recorded from eight Dutch subjects with a whole-head neuromagnetometer. The processing model, based on extensive naming latency studies, is a stage model. In preparing a pictures name, the speaker performs a chain of specific operations. They are, in this order, computing the visual percept, activating an appropriate lexical concept, selecting the target word from the mental lexicon, phonological encoding, phonetic encoding, and initiation of articulation. The time windows for each of these operations are reasonably well known and could be related to the peak activity of dipole sources in the individual magnetic response patterns. The analyses showed a clear progression over these time windows from early occipital activation, via parietal and temporal to frontal activation. The major specific findings were that (1) a region in the left posterior temporal lobe, agreeing with the location of Wernickes area, showed prominent activation starting about 200 msec after picture onset and peaking at about 350 msec, (i.e., within the stage of phonological encoding), and (2) a consistent activation was found in the right parietal cortex, peaking at about 230 msec after picture onset, thus preceding and partly overlapping with the left temporal response. An interpretation in terms of the management of visual attention is proposed.

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.

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.

Modulation of human cortical rolandic rhythms during natural sensorimotor tasks

We studied modulation of cortical neuromagnetic rhythms in association with left and right median nerve stimulation, during rest, finger movements, and passive tactile hand stimulation, in seven healthy, right-handed adults. In the rest condition, the amplitude of the rhythmic sensorimotor activity decreased immediately after the median nerve stimuli and increased above the prestimulus level within 0.4 s afterward, especially in the 7- to 25-Hz band. The rebound occurred 100-300 ms earlier for 20 (7-15)-than for 10 (15-25)-Hz activity. Suppressions and rebounds were strongest in the contralateral sensorimotor hand area for the 20-Hz, but not for the 10-Hz, activity. The maximum rebound was on average 22-34% stronger in the left than in the right hemisphere. Active exploration of objects abolished rebounds of both 10- and 20-Hz signals in the contralateral hemisphere and markedly diminished them ipsilaterally. Finger movements without touching an object and passive tactile stimulation produced a weaker effect. The sensorimotor rhythms thus show a characteristic suppression and subsequent rebound after electrical median nerve stimulation. The rebound is left-hemisphere dominant in right-handed subjects and its suppression reveals bilateral cortical activation during both motor tasks and passive tactile stimulation, especially for explorative finger movements.

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.