Riitta Hari

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 Organization of the Human First and Second Somatosensory Cortices: a Neuromagnetic Study

Multichannel neuromagnetic recordings were used to differentiate signals from the human first (SI) and second (SII) somatosensory cortices and to define representations of body surface in them. The responses from contralateral SI, peaking at 20 – 40 ms, arose mainly from area 3b, where representations of the leg, hand, fingers, lips and tongue agreed with earlier animal studies and with neurosurgical stimulations and recordings on convexial cortex in man. Representations of the five fingers were limited to a cortical strip of ∼2 cm in length. Responses from SII peaked 100 – 140 ms after contra‐ and ipsilateral stimuli and varied considerably from one subject to another. Signs of somatotopical organization were seen also in SII. Responses of SII were not fully recovered at interstimulus intervals of 8 s.

Responses of the primary auditory cortex to pitch changes in a sequence of tone pips: Neuromagnetic recordings in man

Auditory evoked magnetic fields of the human brain were recorded with a four-channel 1st order gradiometer. Pitch deviance in a sequence of repetitive tone pips elicited magnetic evoked-response changes with a topography suggesting that a neuronal mismatch process to the deviant tones activates the primary auditory cortex.

Auditory evoked transient and sustained magnetic fields of the human brain localization of neural generators

SummaryA long auditory stimulus elicits a magnetic evoked response in the human brain, consisting of transient deflections followed by a sustained response. The distributions of the magnetic fields indicate that the auditory evoked transient response at a latency of 100 ms as well as the auditory sustained response are generated at and around the primary auditory cortex.

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.

Seeing speech: visual information from lip movements modifies activity in the human auditory cortex

Neuromagnetic responses were recorded over the left hemisphere to find out in which cortical area the heard and seen speech are integrated. Auditory stimuli were Finnish/pa/syllables presented together with a videotaped face articulating either the concordant syllable/pa/(84% of stimuli, V = A) or the discordant syllable/ka/(16%, V not equal to A). In some subjects the probabilities were reversed. The subjects heard V not equal to A stimuli as/ta/ or ka. The magnetic responses to infrequent perceptions elicited a specific waveform which could be explained by activity in the supratemporal auditory cortex. The results show that visual information from articulatory movements has an entry into the auditory cortex.

Somatosensory evoked cerebral magnetic fields from SI and SII in man.

We have recorded cerebral magnetic fields elicited by electrical stimulation of median and peroneal nerves. Field mapping indicates that the deflections at 30-80 and 150-180 msec are due to activity at SI. Additional activity at 90-125 msec is generated at SII, on the superior bank of the sylvian fissure. At SI, the source locations are in agreement with the known somatotopy. Only contralateral stimuli evoke responses at SI, whereas both ipsi- and contralateral stimuli elicit responses at SII.

Interstimulus interval dependence of the auditory vertex response and its magnetic counterpart: Implications for their neural generation

Auditory vertex responses elicited by short tone bursts were compared with their magnetic counter parts. Special attention was paid to the behaviour of the N100 deflection of the response. Electrical responses were recorded from scalp locations Fp2, Fz, Cz, Pz, C4 and T4 and the magnetic responses half way between P4 and T6, at a point where the response has one of its amplitude extrema. Different ISIs (from 1 to 16 sec) were applied in order to differentiate specific and nonspecific evoked potential components from each other. The main results were as follows: (1) The scalp distsribution of the electical vertex response depends on the ISI used: with frequent stimulation there are no marked differences in the amplitudes of N100 between frontal and central areas but with long ISIs the amplitude maxima move to the vertex. (2) The magnetic responses also show a clear ISI dependence. The magnetic counterpart of N100 saturates at shorter ISIs than N100 recorded from the vertex. Independent of the ISI the magnetic counterpart of P200 is constantly very small. On the basis of the different sensitivities of the EEG and MEG to current sources of different orientations it is concluded that the auditory vertex response contains both modality specific and non-specific components. Experimental conditions, especially the ISI used, determine the relative contributions of these components to the potential recorded on the scalp.

Altered central sensorimotor processing in patients with complex regional pain syndrome

&NA; Alterations in tactile sensitivity are common in patients with chronic pain. Recent brain imaging studies have indicated that brain areas activated by acute experimental pain partly overlap with areas processing innocuous tactile stimuli. However, the possible effect of chronic pain on central tactile processing has remained unclear. We have examined, both clinically and with whole‐head magnetoencephalography, six patients suffering from complex regional pain syndrome (CRPS) of the upper limb. The cortical somatosensory responses were elicited by tactile stimuli applied to the fingertips and the reactivity of spontaneous brain oscillations was monitored as well. Tactile stimulation of the index finger elicited an initial activation at 65 ms in the contralateral SI cortex, followed by activation of the ipsi‐ and contralateral SII cortices at about 130 ms. The SI responses were 25–55% stronger to stimulation of the painful than the healthy side. The distance between SI representations of thumb and little finger was significantly shorter in the hemisphere contralateral than ipsilateral to the painful upper limb. In addition, reactivity of the 20‐Hz motor cortex rhythm to tactile stimuli was altered in the CRPS patients, suggesting modified inhibition of the motor cortex. These results imply that chronic pain may alter central tactile and motor processing.