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Biomedical subjects

R Hari

Publications and source records attributed to R Hari.

At least 199 records · Page 11Linked to original sources
PubMed1987

Neuromagnetic responses of the human auditory cortex to on- and offsets of noise bursts.

Cortical sources of neuromagnetic responses to noise bursts were compared in 7 healthy humans. The earliest response, P40m, peaking about 40 ms after the stimulus onset, was followed by a prominent deflection in the opposite direction at about 100 ms (N100m) and by another peak at 200 ms (P200m). A sustained field, seen near the end of the 400- to 550-ms stimuli, ended with an off-response of the same polarity as N100m. All deflections could be explained by cortical activity within the Sylvian fissure. The source of P200m was anterior to other sources of the on-response, except P40m. When noise bursts, or pauses of equal duration in a continuous noise were presented to the subject, striking similarities were found between the 100-ms deflections of the on- and off-responses: both peaked at about the same latency; their estimated sources were closed to each other in the supratemporal plane and their amplitudes depended in a similar way on the interstimulus interval (1.1-8.8 s). However, only the on-response was preceded by P40m, suggesting that P40m and N100m are not causally linked. N100m seems to reflect cortical activity related to any abrupt change in the auditory environment.

Auditory Cortex↗
PubMed1986

Activation of the auditory cortex by cochlear stimulation in a deaf patient.

We have recorded cerebral electric and magnetic responses to electric stimulation of the auditory nerve in a deaf patient with multichannel cochlear prosthesis. The electric response peaked with a vertex-negative deflection at 65 ms. Clear magnetic responses, coinciding with the vertex potential, were obtained only over the hemisphere ipsilateral to the stimulation; the field pattern indicated activation of the auditory cortex within the Sylvian fissure. The results suggest modification of the central auditory pathways in this patient deaf from early childhood.

Auditory Cortex↗
PubMed1986

Mixed and sensory nerve stimulations activate different cytoarchitectonic areas in the human primary somatosensory cortex SI. Neuromagnetic recordings and statistical considerations.

Magnetic responses evoked by stimulation of the mixed median nerve at the wrist and its cutaneous branches on the glabrous skin of the index and middle fingers were studied. The first responses to mixed nerve stimulation peaked at 19-24 ms, and those to cutaneous nerve stimulation about 4 ms later. The responses, up to a latency of 150 ms, reversed in polarity between the upper and lower parts of the rolandic fissure. Equivalent dipoles for the mixed nerve stimulation were stronger and they lay statistically significantly deeper from the scalp than those activated by the cutaneous nerve stimulation. It is suggested that mixed nerve stimulation activates areas 3a and 3b whereas cutaneous stimulation activates mainly area 3b at the human primary somatosensory cortex. Statistical procedures were developed for comparison of different field patterns and for determining confidence limits of source model parameters. For these purposes the quality and quantity of the noise were studied. The error caused by inaccuracies in the positioning of the magnetometer was found to be minimal in comparison with the signal noise which was estimated from the standard deviation of the averaged response.

Brain Mapping↗
PubMed1986

Cortical responses to painful CO2 stimulation of nasal mucosa; a magnetoencephalographic study in man.

Magnetic and electrical responses to CO2 stimulation of the nasal mucosa were recorded in 4 subjects. The magnetic responses peaked at 350-400 msec, coinciding with the negative vertex potential. The field pattern suggested neural activation in the upper bank of the sylvian fissure near the lateral end of the central sulcus, i.e., at or near SII. The results also suggest that this area contributes to the generation of the electrical vertex potential.

Adult↗
PubMed1986

Interpretation of neuromagnetic responses: two simple models for extended current sources in the human auditory cortex.

The usefulness of dipole models in interpreting neuromagnetic field patterns was studied. By statistical means it turned out to be difficult to differentiate between a point-like source and an extended source at the auditory cortex. A single dipole is an appropriate source model for localized cortical activity extending up to a few square centimeters. Additional information about the source configuration can be obtained by focusing the analysis on areas where the differences between patterns produced by different sources should theoretically be the largest.

Auditory Cortex↗
PubMed1986

Neuromagnetic responses to frequency modulation of a continuous tone.

Neuromagnetic responses to frequency modulation of a continuous tone were studied in nine subjects. The latencies of the transient responses increased and the amplitudes decreased with decreasing speed of modulation. The equivalent dipoles for modulation of a 1,000 Hz tone were slightly but statistically significantly anterior to the dipoles activated by modulation of a 500 Hz tone. The generation mechanisms of N100m are discussed.

Acoustic Stimulation↗
PubMed1986

Cerebral magnetic fields evoked by peroneal nerve stimulation.

We measured evoked magnetic fields and evoked potentials to peroneal nerve stimulation in healthy humans. The results were consistent with activation of the primary sensorimotor foot area on the mesial surface of the hemisphere during several deflections between 40 and 200 msec. A cortical origin is suggested for the electrical P40. Distinct magnetic field patterns were observed over the primary and secondary somatosensory cortices (SI and SII). At SII, responses to ipsilateral stimulation were weaker and had longer latencies than those to contralateral stimulation. Simulations were used to clarify the differences between electrical and magnetic patterns when SI and SII were simultaneously active.

Adult↗
PubMed1986

Cerebral magnetic fields.

A critical presentation is given about the state-of-the-art of neuromagnetism, the study of neural functions by the measurement of magnetic fields. First, an introduction is given about the neural origin of electromagnetic fields. The forward and inverse problems are then studied. The forward problem can be stated as follows: given electrical currents in the brain, calculate the magnetic field. Correspondingly, the inverse problem is to calculate currents in the brain from the magnetic field measured outside the head. The instrumentation necessary for successful neuromagnetic measurements is discussed, and the emphasis is on new developments. The review of experimental neuromagnetic results discusses studies on spontaneous cerebral magnetic fields in normal subjects and in epileptic patients. Furthermore, evoked-field studies of the various sensory modalities as well as studies of event-related fields are reviewed. The relevance of various efforts in neuromagnetism and prospects for the future are discussed.

Biomedical Engineering↗
PubMed1985

Magnetic fields produced by eye blinking.

Magnetic signals produced by voluntary eye blinking were recorded. The maximal signals were found over the posterior parts of the orbits. The polarity of the signal reversed over each eye and in the frontal midline. The amplitude increases with light and amplitude decreases with darkness were similar in the electrical and magnetic blink and eye movement signals. Three different simulation models were used in the interpretation of the results. It is concluded that the primary current source of the blink signal is the transretinal current density. Lid movements change the geometry of the volume conductor resulting in changes in the measured electrical and magnetic field patterns.

Adult↗
PubMed1985

Cerebral neuromagnetic responses evoked by short auditory stimuli.

We presented 30 msec sinusoidal tone bursts to the subject's left ear once every 1300 msec. The number of standard tones (1000 Hz) between deviants (1030 Hz) varied randomly from 3 to 15 (even distribution) so that the probability of the standards was 0.9 and that of the deviants 0.1. During stimulation the subject was reading a book. Magnetic responses to the standards and deviants were measured at the estimated ends of the sylvian fissure and electrical responses at Fpz, Fz and Cz. In addition, extensive field maps over the right hemisphere were made from 60 to 75 points in 3 subjects. A least-squares fit was performed to find out the parameters of the equivalent current dipole in a spherically symmetrical head model. Field maps suggested that the source of magnetic response at 100 msec (N100m) can be approximated by a current dipole at the supratemporal plane, possibly at the primary auditory cortex. In two subjects the location and/or the orientation of the equivalent dipole changed during N100m, possibly due to change in the size of the activated cortical area. The deviant stimuli elicited in addition to N100m a second deflection, MMNm, peaking at about 200 msec. This response was regarded as specific to stimulus change. On the basis of field maps it was also concluded that MMNm got a contribution from activity at the supratemporal plane.

Adult↗
PubMed1985

Neuromagnetic studies of somatosensory system: principles and examples.

A review is given about the basic principles of neuromagnetic recordings and somatically evoked magnetic fields with examples from the authors' own work. MEG provides good spatial resolution for activity in the fissural cortex. It has, for example, allowed differentiation of current sources in the primary and secondary somatosensory cortices. Some cortical areas activated by painful stimuli have also been localized by means of MEG.

Animals↗
PubMed1985

The effect of tourniquet-induced ischemia on somatically evoked cerebral magnetic fields in man.

We recorded somatically evoked magnetic fields (SEFs) during tourniquet-induced ischemia in healthy humans. The subjective intensities of the stimuli decreased earlier and more clearly than the amplitudes of the SEF deflections peaking at 60-80 ms. Mixed nerve and sensory nerve responses behaved differently during ischemia. The results indicate that these SEF deflections are transmitted through thick myelinated fibers. Muscle afferents might contribute considerably to mixed nerve SEFs.

Adult↗
PubMed1984

The activation of unmyelinated or myelinated afferent fibers by brief infrared laser pulses varies with skin type.

Brief infrared laser pulses were applied to 3 different skin areas in man. Reaction times indicated that A delta-fibers were activated in the dorsal and volar skin of the finger whereas only C-fibers were activated in the hairy skin of the forearm. The qualitative sensations were in line with this interpretation. We conclude that the activation of unmyelinated or myelinated afferent fiber populations with brief laser pulses varies with skin type.

Adult↗
PubMed1984

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.

Adult↗
PubMed1984

A four-channel SQUID magnetometer for brain research.

A 4-channel differential SQUID magnetometer has been built. Its design principles and construction are described. Proper matching of the detection coil to the SQUID input is discussed. Examples of auditory, dental pain and visual evoked response data are presented.

Electroencephalography↗
PubMed1983

Neuromagnetic localization of cortical activity evoked by painful dental stimulation in man.

We have recorded cerebral magnetic fields evoked by painful dental stimulation. The field pattern indicates a current source at the upper bank of the anterior Sylvian Fissure, corresponding to the anterior end of the secondary somatosensory cortex. This finding suggests cortical representation of tooth pulp in man. The neuromagnetic technique, allowing the investigation of this cortical area, thus provides a new non-invasive tool for pain research.

Adult↗