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Biomedical subjects

M Somasundaram

Publications and source records attributed to M Somasundaram.

11 recordsLinked to original sources
PubMed1994

The polarity of the induced electric field influences magnetic coil inhibition of human visual cortex: implications for the site of excitation.

Human perception of 3 briefly flashed letters in a horizontal array that subtends a visual angle of 3 degrees or less is reduced by a magnetic coil (MC) pulse given, e.g., 90 msec later. Either a round or a double square MC is effective when the lower windings or central junction region, respectively, are tangential to the skull overlying calcarine cortex and symmetrical across the midline. The modeled, induced electric field has peak amplitude at the midline, but the peak spatial derivatives lie many centimeters laterally. Thus, the foveal representation near the midline is closer to the peak electric field than to its peak spatial derivatives, i.e., excitation of calcarine cortex differs from excitation of a straight nerve. With an MC pulse that induces an electric field which is substantially monophasic in amplitude, the lateral-most letter (usually the right-hand letter) in the trigram is preferentially suppressed when the electric field in the contralateral occipital lobe is directed towards the midline. Inferences from using peripheral nerve models imply that medially located bends in geniculo-calcarine or corticofugal fibers are the relevant sites of excitation in visual suppression; end excitation of fiber arborizations or apical dendrites is considered less likely. This conclusion is supported by the fact that the induced electric field polarity in paracentral lobule for optimally eliciting foot movements is opposite to that for visual suppression, the major bends occurring at different portions of the fiber trajectories in the two systems.

Electromagnetic Fields↗
PubMed1991

Paraesthesias are elicited by single pulse, magnetic coil stimulation of motor cortex in susceptible humans.

A minority of normal humans experience paraesthesias (usually tingling) projected to the contralateral hand in response to individual transcranial magnetic coil (MC) pulses. The cortical source of the paraesthesias was sought by comparing their incidence with that of muscle responses to focal MC stimulation with either a figure 8 MC or with edge stimulation of a tilted round MC in 4 susceptible subjects. In all 4, paraesthesias were best felt with MC stimulation either at, or anterior to sites yielding movement, implying an initial source in precentral gyrus (and possible premotor cortex), rather than parietal cortex. In the two subjects exhibiting the strongest paraesthesias, the threshold for the paraesthesias was less than that for movement in the relaxed arm. The optimal site of the paraesthesias within the hand was usually in the digits, but differed among subjects. Motor responses and paraesthesias following a given stimulus occurred at different sites in the hand, implying that excitation of differing sets of motor cortical neurons subserved sensory and motor responses. In only one subject were the paraesthesias sufficiently reproducible to warrant interacting electrical digital and transcranial MC pulses. The data suggested that central processing of the response to the MC pulse is slowed by an antecedent digital stimulus, but the delay for perception of each type of stimulus does not greatly differ. The central sense of movement (Amassian et al., 1989a) elicited by MC stimulation of motor cortex is compared with the paraesthesias. Both are attributed to brief, high frequency discharge by motor cortical neurons accessing the perceptual system more readily than after excitation of post-central gyrus, which requires prolonged repetitive stimulation (Libet et al., 1964). Given also the normal pattern of muscle responses in the 4 subjects, their paraesthesias are best explained by a heightened sensitivity of the perceptual system to the motor cortical response to MC stimulation.

Action Potentials↗
PubMed1981

Inverse ocular bobbing.

A new form of abnormal spontaneous eye movement termed inverse ocular bobbing is described. It consists of a slow downward eye movement followed by delayed quick return upward to midposition. It is contrasted with other types of abnormal vertical eye movements.

Adult↗
PubMed1979

Subacute motor neuronopathy: a remote effect of lymphoma.

Ten patients developed a subacute lower motor neuron syndrome as a remote effect of Hodgkin's disease or other lymphoma. The illness usually followed a benign course independent of the activity of the underlying neoplasm. Seven of the patients improved spontaneously, and 3 became neurologically normal. Two patients died of intercurrent infections related to immunosuppression. Neuropathological examination of these 2 patients and 3 previously reported cases showed prominent neuronal degeneration restricted to the anterior horns of the spinal cord and mild posterior column demyelination. Demyelination was also present in the anterior roots of our autopsied patients and was accompanied by large, hyperchromatic Schwann cells. The cause of the illness is obscure, but both radiation therapy and opportunistic infection may be contributing factors. Attempts at virus isolation have been unsuccessful. The syndrome should be distinguished from the more common direct effects of lymphoma on the nervous system, since its identification spares the patient additional, potentially harmful therapy.

Adult↗