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Biomedical subjects

R Hari

Publications and source records attributed to R Hari.

At least 127 records · Page 7Linked to original sources
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

Reactivity of magnetic parieto-occipital alpha rhythm during visual imagery.

Spontaneous MEG signals were recorded during visual imagery from 13 healthy adults with a whole-scalp neuromagnetometer. The parieto-occipital 7-14 Hz alpha activity was suppressed strongly while subjects visualized and evaluated letters. The act of forming a visual image caused a smaller suppression than did inspection of the imaged pattern for a named property. The maximum suppression depended on the baseline alpha level and, for the majority of the subjects, occurred close to the area with the strongest alpha, showing no systematic hemispheric asymmetry. Sources for the alpha activity, modeled with equivalent current dipoles, clustered in the parietal and occipital lobes. The strongest suppression of the activity occurred near the parieto-occipital sulcus.

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↗
PubMed1995

MEG burst suppression in an anaesthetized dog.

Recording of the magnetic fields of the brain, magnetoencephalography (MEG), has proved to be a valuable method in neurophysiological research. In order to study the feasibility of MEG recording during anaesthesia we recorded magnetoecephalographic burst suppression in a dog during enflurane and propofol anaesthesia. The observed signal distribution implies a complex current distribution underlying the burst activity. This experiment also proves that an essentially artefact-free MEG recording can be obtained during respirator-assisted anaesthesia.

Anesthesia, Inhalation↗
PubMed1994

Neuromagnetic cortical signals in a patient with hydrocephalus.

We studied a patient with a communicating hydrocephalus using a whole-head neuromagnetometer. Auditory evoked fields (AEFs) were elicited with 600 ms tones, 0.25, 0.5, 1, 2 and 4 kHz in frequency. The source locations were more superficial and posterior in the patient than in controls, but still agreed with activation of the supratemporal cortex. Tonotopic organization of the AEF sources was similar in the patient and healthy control subjects. The amplitude spectra, source strengths and sites, and reactivity of the patient's spontaneous activity were normal. Despite apparent decrease of white matter, the patient's thalamocortical connections thus seem to be able to maintain normal evoked and spontaneous cortical activity.

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

Use of a computerized brain atlas in magnetoencephalographic activation studies.

A pilot study was carried out to test the feasibility of an adjustable computerized brain atlas, adapted to the individual anatomy for localizing current dipoles by means of magnetoencephalography (MEG). The atlas can be adapted to individual computed tomography (CT) or magnetic resonance (MR) images. Position information is transferred between these imaging methods and MEG using a stereotactic technique. For this purpose, a special non-magnetic helmet was designed to be used together with the ordinary head fixation system. It seems likely that the proposed combination of the brain atlas with MEG, CT and MRI methods will become a powerful tool in exploring different brain functions.

Anatomy, Artistic↗
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

Cortical reactivity in progressive myoclonus epilepsy.

We studied 4 patients with progressive myoclonus epilepsy (Unverricht-Lundborg disease; ULD). Somatosensory evoked fields (SEFs), auditory evoked fields (AEFs), and spontaneous activity over the somatomotor cortex were recorded with a 24-channel SQUID gradiometer. All patients had "giant" 20-45 msec median nerve SEFs at the first somatomotor cortex, with 2-6 times larger amplitudes than the healthy control subjects. Later deflections were not similarly enhanced. The dependence of SEF amplitudes on interstimulus interval (0.2-4 sec) and on successive ulnar-median nerve stimulation (stimulus interval 40 msec) was comparable to that in controls. Cortical AEFs were attenuated and delayed. In 3 patients, the spontaneous activity consisted of 6-8 Hz mu rhythm, which originated within 2 cm from the sources of SEFs and was abolished by clenching of the contralateral fist. Control subjects had major spectral peaks around 10 and 20 Hz. The SEF amplitudes and the strength of the 6-10 Hz mu correlated strongly, suggesting that some components of evoked and spontaneous activity obtain contributions from overlapping neuronal populations. The results imply that ULD is associated with thalamo-cortical hyperreactivity in the sensorimotor but not in the auditory system.

Acoustic Stimulation↗
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

Whole-head mapping of middle-latency auditory evoked magnetic fields.

We recorded middle-latency auditory evoked magnetic fields from 9 healthy subjects with a 122-channel whole-head SQUID gradiometer. The stimuli were click triplets, 2.5 msec in total duration, delivered alternately to the two ears once every 333 msec. Contralateral clicks elicited P30m responses in 16 and P50m responses in 12 out of 18 hemispheres studied; ipsilateral clicks did so in 7 and 13 hemispheres, respectively. The field patterns were satisfactorily explained by current dipoles in 16 and 4 hemispheres for contra- and ipsilateral P30m, and in 4 and 10 hemispheres for contra- and ipsilateral P50m. The peak latencies of P30m and P50m were not affected by stimulation side. The results show that middle-latency auditory evoked responses receive a strong contribution from auditory cortical structures, and that differences of input latency to cortical auditory areas, evaluated from MLAEF latencies, do not explain the latency differences seen in late auditory evoked fields to contralateral vs. ipsilateral stimulation.

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

Effect of interaural time differences on middle-latency and late auditory evoked magnetic fields.

To determine if interaural time differences (ITDs) in binaural stimuli affect the middle-latency auditory evoked fields (AEFs) in the same manner as they affect the N100m deflection, neuromagnetic responses were recorded over the whole head using a 122-channel SQUID magnetometer. Binaural stimuli were lateralized to three positions, left, midline, and right, on the basis of ITDs. The N100m was significantly larger to stimuli with contralaterally-leading ITDs than to stimuli with no, or with ipsilaterally-leading ITDs. Neither the P30m nor the P50m deflections of the middle-latency response were significantly affected by ITD, although the P30m showed a tendency, similar to but smaller than that of N100m, to be larger to stimuli with contralaterally-leading ITDs. In some subjects, the source location of the P50m was anterior and inferior to the sources of the P30m and N100m, which are generated in the superior surface of the temporal lobe. Sound-related muscular artifacts were seen in the posterior recording channels of one subject, and the contribution of this activity to the signals over the temporal area was determined.

Acoustic Stimulation↗
PubMed1994

Auditory cortical responses in humans with congenital unilateral conductive hearing loss.

We recorded auditory evoked magnetic fields from 6 patients with congenital unilateral conductive hearing disorder with a 122-channel whole-head neuromagnetometer. The stimuli were 50-ms 1-kHz tones delivered to the better ear at interstimulus intervals (ISIs) of 2 and 8 s at two different intensities (50 and 70 dB HL). As in normal-hearing subjects, the amplitudes of N100m, the 100-ms response, were larger in 5 patients and the latencies were shorter in 3 patients over the hemisphere contralateral to stimulation. However, in one patient N100m peaked already at 61 ms over the contralateral hemisphere and amplitudes were larger over the ipsilateral hemisphere, possibly reflecting reorganization of the auditory pathways. In 3 patients the latencies were shorter over the ipsilateral hemisphere. The effects of ISI and intensity were similar over both hemispheres and did not differ from those in controls. It seems that congenital unilateral conductive hearing loss does not necessarily lead to any gross disturbances in the human auditory cortex.

Acoustic Stimulation↗
PubMed1994

Human auditory cortical mechanisms of sound lateralisation: III. Monaural and binaural shift responses.

Neuromagnetic responses were recorded over the whole head with a 122-channel gradiometer. A pair of 150-ms 1-kHz tones separated by an interval of 150 ms was presented to one ear every 2 s. The other ear received either no input, an identical pair simultaneous to the first, an identical pair alternating with the first or a continuous 600-ms tone. The 'monaural shift' condition in which stimuli alternated between ears produced a clear perception of changing lateralisation, but the evoked response could be explained as merely the sum of simple monaural onset and offset responses; thus we found no evidence for a separate response to interaural intensity difference in this condition. The 'binaural shift' condition, in which intensity changed in one ear while the other received a continuous tone, evoked a transient response (N130m) at a latency of about 130 ms. N130m was larger over the hemisphere contralateral to the direction of shift, and larger than the corresponding monaural response, whether to an onset or an offset. We concluded that N130m also was not a separate directional response, but was analogous to a simple monaural response, the prolonged latency being due to masking and the enhanced amplitude to facilitation by the sustained response to the continuous tone.

Adult↗