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Biomedical subjects

M Seyal

Publications and source records attributed to M Seyal.

27 records · Page 2Linked to original sources
PubMed1986

Effect of sleep on the electrographic manifestations of epilepsy.

Alterations in the level of arousal have profound predictable effects on the electrographic manifestations of epileptogenic abnormalities. The changes produced by sleep are dramatic in patients with generalized epilepsy as compared to patients with partial epilepsy. The epileptiform complexes associated with the generalized epilepsies show changes in spatial distribution, voltage, temporal sequencing, and waveform morphology. The changes are qualitatively similar in the different types of generalized seizure disorders but vary in the degree to which they are expressed in proportion to the "severity" of the seizure disorder. The presence of a generalized epileptogenic abnormality may result in alteration of K-complex waveform morphology. Multifocal spikes occurring in conjunction with bilateral synchronous discharges are frequently represented by a spatial field distribution consistent with a "horizontal dipole," during sleep especially in children.

Adult↗
PubMed1985

The human posterior tibial somatosensory evoked potential: synapse dependent and synapse independent spinal components.

Evidence has been obtained for the existence of two separate events occurring in the human spinal cord following posterior tibial nerve (PTN) stimulation. These events can be recorded on the surface in unanesthetized individuals. The first is an ascending wave which is conducted up to the cord at constant velocity and has a relatively short refractory period consistent with a compound nerve action potential. This represents the afferent volley traversing the lumbosacral plexus and the ascending dorsal columns. A second event, the N22/P22 complex, is surface negative on the back and surface positive anteriorly; its amplitude is maximal 5-15 cm above the level of the L4 spine and its peak latency remains constant at all levels. This activity has a relatively long refractory period. These characteristics of N22/P22 indicate that it is a localized synaptically dependent event conforming to a transverse dipole with dorsal negativity and a simultaneous anterior positivity. The N22/P22 is probably generated in the dorsal grey at the root entry zone. The N22/P22 is analogous to the stationary N13/P13 recorded over the neck following median nerve stimulation.

Adult↗
PubMed1984

Somatosensory evoked potentials following median nerve stimulation. I. The cervical components.

Median somatosensory evoked potentials were studied in 20 normal adult volunteers. Recording electrodes were positioned over posterior, anterior and lateral neck, as well as on the scalp. Three distinct cervical potentials were identified. Immediately after the afferent volley passes Erb's point, a travelling wave is recorded in the lateral cervical electrodes ipsilateral to the side of stimulation. This represents the afferent volley approaching the spinal cord in the proximal brachial plexus and cervical roots and has been designated the 'proximal plexus volley' (PPV). Following PPV, a second travelling wave is recorded which increases in latency from low to high cervical levels. It represents the afferent volley in the dorsal column, and has been designated the 'dorsal column volley' (DCV). Following DCV, a stationary potential, designated CERV N13/P13, is recorded with characteristics of a transverse midline dipole having maximal negativity posteriorly and maximal positivity anteriorly. This potential may be generated by interneurons in the dorsal grey of the cervical cord. Each of these cervical travelling waves is accompanied by a negative far-field potential recorded at the scalp. The PPV is accompanied by a negative scalp deflection with a nominal latency of 10 ms (N10), and the peak of DCV at SC1 is accompanied by a scalp negativity with a nominal latency of 12 ms (N12). In view of these observations, it is necessary to reexamine assumptions regarding the polarity of scalp-recorded potentials generated by remote neural events.

Adolescent↗
PubMed1983

Spinal and early scalp-recorded components of the somatosensory evoked potential following stimulation of the posterior tibial nerve.

Somatosensory evoked potentials (SEPs) were elicited by stimulation of the posterior tibial nerve (PTN) in 12 normal adults. Recording using both cephalic and non-cephalic references were obtained from multiple electrodes placed over the spine and scalp. Following PTN stimulation, the fastest recorded potentials of the afferent sensory volley proceeds up the spinal cord at constant velocity. After arrival of the volley at cervical cord levels, 3 widely distributed waves, P28, P31 and N34, are recorded from scalp electrodes. These 'far-field' potentials are followed by a localized positivity (P38) which has a peak voltage either at the vertex or just laterally toward the side of stimulation. A contralateral negativity (N38) was present in most individuals. We propose that P28 arises from medial lemniscus; that P31 is generated by ventrobasal thalamus; and that N34 is probably the result of further activity in thalamus and/or thalamocortical radiations. The P38/N38 complex represents the primary cortical response to PTN stimulation. Its most consistent characteristic is a positivity at the vertex or immediately adjacent scalp areas ipsilateral to the stimulated leg. The topography of the P38/N38 potential varies slightly from individual to individual in a manner consistent with a functional dipole situated in the leg and foot area on the mesial aspect of the postcentral gyrus, whose exact location and orientation changes in accordance with known variations in the location of the leg area.

Adolescent↗
PubMed1981

Visual evoked potentials and eye dominance.

The amplitudes of pattern-reversal VEPs in 25 healthy volunteers were significantly higher from the dominant eye than the non-dominant eye in right eye dominant subjects. The difference was present over both hemispheres and over the midline. Handedness did not appear to influence the amplitude asymmetry. A similar trend was noted in left eye dominant subjects, but the difference was significantly only at O2. The mean latency of the P100 peak was significantly shorter with stimulation of the dominant eye. These amplitude and latency disparities between dominant and non-dominant eyes provide electrophysiological evidence of lateralization in the nervous system.

Analysis of Variance↗
PubMed1978

Recurrent Guillain-Barré syndrome following influenza vaccine.

Two patients recovered from an attack of Guillain-Barré syndrome and then had a second attack of this disease, with a shorter latent period, following monovalent influenza vaccination. These cases suggest that an attack of Guillain-Barré syndrome may result in greater risk of future episodes of the syndrome in conjunction with exposure to influenza or other vaccinations.

Adult↗
PubMedDate not supplied

Short latency somatosensory evoked potentials following mechanical taps to the face. Scalp recordings with a non-cephalic reference.

Trigeminal somatosensory evoked potentials were recorded over the scalp using non-cephalic reference sites following mechanical taps to the face. A negative wave form, Nf17, was recorded bilaterally with its highest amplitude over the frontal scalp contralateral to the side of stimulation. A localized negative wave form, Np25, was recorded over the centro-parietal scalp contralateral to the side of stimulation. Np25 had an onset latency of 16.46 msec. The location and restricted spatial distribution of Np25 suggest that it represents the initial activation of the face area of the primary sensory cortex. The widespread bilateral nature of Nf17 and its latency of onset preceding that of Np25 suggest that Nf17 may be a 'far-field' potential reflecting activity in subcortical sensory pathways subserving the face.

Adult↗
PubMedDate not supplied

Enhancement of the amplitude of somatosensory evoked potentials following magnetic pulse stimulation of the human brain.

In this study we have demonstrated an enhancement of cortically generated wave forms of the somatosensory evoked potential (SEP) following magnetic pulse stimulation of the human brain. Subcortically generated activity was unaltered. The enhancement of SEP amplitude was greatest when the median nerve was stimulated 30-70 msec following magnetic pulse stimulation over the contralateral parietal scalp. We posit that the enhancement of the SEP is the result of synchronization of pyramidal cells in the sensorimotor cortex resulting from the magnetic pulse.

Brain↗