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Biomedical subjects

R Hari

Publications and source records attributed to R Hari.

12 recordsLinked to original sources
PubMed1992

Magnetoencephalographic localization of epileptic cortex--impact on surgical treatment.

A 24-channel, planar, superconducting quantum interference device gradiometer, sampling a fourth of the head surface over brain tissue, was used to determine the site of an epileptic focus in a 36-year-old woman with intractable complex partial epilepsy. The other presurgical findings appeared divergent: a large arachnoid cyst over the right parietal convexity, dissimilar interictal electroencephalographic patterns, and several neuropsychological dysfunctions. The equivalent current sources of magnetoencephalographic spikes were in the right posterior temporal region of the cortex, 4 cm apart from the cyst. Surgical exploration of the area pinpointed by magnetoencephalography revealed a pachygyric patch of cortex displaying focal discharges on the electrocorticogram. After resection, a dramatic reduction of seizures occurred. The good agreement between electrocorticography and magnetoencephalography warrants future investigation of multichannel magnetoencephalography as a potential alternative to invasive presurgical recordings.

Adult↗
PubMed1992

Cortical somatosensory magnetic responses in multiple sclerosis.

Somatosensory evoked magnetic fields (SEFs) to contralateral median and ulnar nerve stimulation were analyzed in 10 patients with multiple sclerosis and in 8 healthy controls. SEFs were recorded with a 24-channel SQUID gradiometer over both hemispheres. Seven patients showed abnormally large-amplitude SEF deflections at 60-80 msec; 5 of them had multiple lesions around lateral ventricles in magnetic resonance imaging. In 2 patients with plaques at the level of 3rd and 4th ventricles and medulla, the 30 msec responses were enlarged. The equivalent sources of 20 msec and 30-80 msec responses were in the primary hand sensorimotor cortex both in patients and in control subjects. The results suggest that early and middle-latency SEFs reflect parallel processing of somatosensory input. Recording of middle-latency evoked responses, electric or magnetic, may give additional information about the somatosensory function in multiple sclerosis.

Adult↗
PubMed1992

Neuromagnetic mismatch fields to single and paired tones.

In 8 subjects we recorded multichannel magnetic responses to pitch changes in single 50 msec tones and in tone pairs (oddball paradigm; interstimulus interval 745 msec). Either "A" (1 kHz) was standard (90%) and "B" (1.2 kHz) deviant (10%), or "AA" was standard and "AB" deviant in pairs with onset asynchrony of 75 msec. Subject did not pay attention to the tones. A mismatch field (MMF) was evident in responses to deviants with an equivalent source in the supratemporal auditory cortex, about 1 cm anterior to that for N100m. The MMF source was 3-fold stronger for tone pairs than for single tones.

Adult↗
PubMed1992

Interaction of afferent impulses in the human primary sensorimotor cortex.

We recorded somatosensory evoked magnetic fields (SEFs) over the hand area of the primary sensorimotor cortex (SMI) in 6 healthy adults in 2 sets of experiments to study interaction of afferent impulses. In experiment 1, SEFs were elicited by contralateral median nerve (MC) stimuli presented alone and 40 msec after a conditioning stimulus to the contralateral ulnar (UC), ipsilateral median (MI) or contralateral tibial (TC) nerve. N20m, P30m and P60m deflections to MC stimulation were markedly attenuated by preceding UC stimulation whereas N40m was enhanced, and a novel P80m emerged. In contrast, MI or TC stimulation did not affect the responses to MC. In experiment 2, the time course of recovery of N20m to median nerve stimuli was studied after stimulation of the adjacent ulnar and of the same median nerve. The recovery curves were similar for both conditioning stimuli with nearly full recovery of N20m at 120 msec. The results indicate marked interaction of impulses from ipsilateral median and ulnar nerves in human SMI, but no evidence was found of interaction from the two hands or from ipsilateral hand and foot.

Adult↗
PubMed1992

Generator sites of spontaneous MEG activity during sleep.

We have recorded spontaneous magnetoencephalographic (MEG) activity during overnight natural sleep in 4 healthy adults with a 24-channel SQUID gradiometer, mainly over the sides of the head. All sleep stages were obtained. The MEG wave forms resembled the EEG phenomena recorded simultaneously from the scalp midline, but the electric and magnetic signals did not always coincide. The source locations of different signals were studied by using a current dipole model. The equivalent sources of magnetic transients, resembling and often coinciding with the electric vertex waves and K-complexes, as well as the transients during REM sleep, were concentrated within a volume of 4 x 4 x 3 cm3 in the inferior parietal lobe. For spindles and slow waves, no such focal generators were found.

Adult↗
PubMed1992

Neuromagnetic auditory evoked responses after a stroke in the right temporal lobe.

Auditory evoked magnetic fields were recorded with a 24-SQUID magnetometer on a patient with a vascular lesion in the right auditory cortex and its vicinity. The contralateral transient 100 ms response (N100m) was absent over the damaged side, with no recovery within 16 months. Responses were normal over the healthy side. The responses to a binaural click train where interaural delays caused the perception of a shifting sound source; responses were again normal over the left side. Ipsilateral tones elicited N100m over the normal hemisphere but with smaller amplitude and longer latency than in healthy subjects. We suggest that ipsilateral responses are triggered by ipsilateral afferents but are significantly modified by excitatory callosal connections which did not work in our patient.

Auditory Cortex↗
PubMed1979

Auditory evoked transient and sustained potentials in the human EEG: I. Effects of expectation of stimuli.

The characteristics of auditory evoked transient and sustained potentials were recorded using trains of four-tone stimuli of 1-second duration (interstimulus interval = 1 second) presented once every minute. The subject either attentively expected the stimuli or ignored them while reading. The electroencephalogram was recorded from derivations Cz-Al and Fz-Al. Expectation of the stimuli was associated with increased amplitudes of the transient responses both at the first stimulus of the train and during stimulus repetition. In contrast, the sustained potential at the first stimulus of the train was unchanged or smaller when the subject expected the stimuli. During stimulus repetition, however, the amplitude of the potential was enhanced by expectation of the stimuli. The results support the hypothesis of two sustained potential components and stress the importance of stimulus repetition rate when sustained potentials are studied.

Acoustic Stimulation↗
PubMed1979

Auditory evoked transient and sustained potentials in the human EEG: II. Effects of small doses of ethanol.

The effect of small doses of ethanol (0.4 g/kg) on auditory evoked transient and sustained potentials was studied. Tones of 1-second duration were presented in trains of four stimuli (interstimulus interval = 1 second; intertrain interval = 1 minute). The electroencephalogram was recorded from derivation Cz-Al. Ethanol depressed the transient responses both at the first stimulus of the train and during repeated stimuli. The sustained potentials elicited by the first stimuli of the train were not affected by ethanol, whereas the sustained potentials elicited by repeated stimuli were larger in amplitude under the influence of ethanol than during control experiments. It is suggested that the decrease of the transient responses under the influence of ethanol is mainly due to depression of the reticular formation, whereas the increase of sustained potentials reflects ethanol-induced release of intracortical inhibition.

Acoustic Stimulation↗
PubMed1979

Biphasic effects of ethanol on behavioral and electrophysiological parameters can be explained by functional organization of neural networks.

The biphasic effect of ethanol on the CNS is usually explained by assuming that excitatory and depressant effects are mediated by different mechanisms. The present alternative explanation states that ethanol depresses the activity of all neurons to the same extent, but that the biphasic effect can result from the functional organization of the neural network concerned. Current experimental evidence is in accordance with both explanations.

Behavior↗
PubMed1978

Effect of stimulus repetition on negative sustained potentials elicited by auditory and visual stimuli in the human EEG.

Effect of repetition of auditory and visual stimuli on sensory-evoked sustained potentials was studied. The stimuli were tones and flashes of 1 sec duration presented in trains of six stimuli with an inter-stimulus-interval of 1 sec. The repetition rate of the trains was 1 train/min. The EEG was recorded from electrodes located at Cz, Pz and Oz. Both auditory and visual stimuli elicited negative sustained potentials which during the first stimuli of the trains were maximal in amplitude and shortest in latency at Cz. Repetition of the stimulus resulted in almost complete disappearance of the sustained potentials; for auditory stimulation a small negative shift was recorded only at Cz, whereas for visual stimulation a small sustained potential was seen only at Pz and Oz. The results indicate that the slow potentials evoked by short tones or lights are composed of at least two components. One of these components may reflect neural processing associated with orienting reflex. The other component seems to be related to specific neural processing of the stimulus.

Acoustic Stimulation↗