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V E Amassian

Publications and source records attributed to V E Amassian.

At least 37 records · Page 2Linked to original sources
PubMed1993

Rapid modulation of human cortical motor outputs following ischaemic nerve block.

The amplitudes of motor evoked potentials to transcranial magnetic stimulation from muscles immediately proximal to a temporarily anaesthetized (Bier's block) human forearm increase in minutes after the onset of anaesthesia and return to control values after the anaesthesia subsides. In order to determine the level at which the early modulation of human motor outputs takes place, we recorded maximal H reflexes, peripheral M responses, motor evoked potentials to transcranial magnetic stimulation, and motor evoked potentials to transcranial electrical stimulation and spinal electrical stimulation from a muscle immediately proximal to a limb segment made ischaemic by a pneumatic tourniquet. The amplitudes of motor evoked potentials to transcranial magnetic stimulation, but not to transcranial electrical stimulation and spinal electrical stimulation, were larger during ischaemia, implying that the site of change was in the motor cortex. The maximal H/M ratios were unaffected by ischaemia, indicating that alpha-motor neuron excitability to segmental Ia inputs remained unchanged. The map of cortical representation areas for this muscle obtained with transcranial magnetic stimulation was also enlarged. Taken together, our findings suggest that the temporary removal by ischaemic nerve block of myelinated afferent inputs reduces inhibition at the motor cortical level and that this disinhibition is responsible for the increased excitability of the corticospinal system.

Adult↗
PubMed1993

Magnetic coil stimulation of straight and bent amphibian and mammalian peripheral nerve in vitro: locus of excitation.

1. According to classical cable theory, a magnetic coil (MC) should excite a linear nerve fibre in a homogeneous medium at the negative-going first spatial derivative of the induced electric field. This prediction was tested by MC stimulation of mammalian phrenic and amphibian sciatic nerve and branches in vitro, immersed in Ringer solution within a trough, and identifying the sites of excitation by recording responses of similar latency to local electrical stimulation. Subsequently, the identified sites of excitation were compared with measurements of the induced electric field and its calculated first spatial derivative. A special hardware device was used to selectively reverse MC current direction and to generate predominantly monophasic- or polyphasic-induced pulse profiles whose initial phases were identical in polarity, shape and amplitude. When using the amphibian nerve preparation, a complication was excitation at low threshold points related to cut branches. 2. Reversal of monophasic current resulted in latency shifts corresponding approximately to the distance between induced cathode and anode. The location of each site of excitation was at, or very near, the negative-going first spatial derivative peaks of the induced electric field measured parallel to the straight nerve. Significantly, excitation of the nerve did not occur at the peak of the induced electric field above the centre of the 'figure of eight' MC junction. 3. A polyphasic pulse excited the nerve at both sites, by the negative-going first phase at one location, and approximately 150 microseconds later, by the reversed negative-going second phase at the other location. Polyphasic and monophasic pulses elicited responses with similar latency when the induced current flowed towards the recording electrode. 4. Straddling a nerve with non-coding solid lucite cylinders created a localized spatial narrowing and increase in the induced electric field, resulting in a lowered threshold of excitation. The corresponding closer spacing between first spatial derivative peaks was exhibited by a significant reduction in latency shift when MC current direction was reversed. 5. When a nerve is bent and the induced current is directed along the nerve towards the bend, the threshold of excitation is reduced there. Increasing the angle of the bend from 0 deg to more than 90 deg graded the decrease in threshold. 6. In a straight nerve the threshold was lowest when current was directed towards the cut end.(ABSTRACT TRUNCATED AT 400 WORDS)

Action Potentials↗
PubMed1992

Modelling magnetic coil excitation of human cerebral cortex with a peripheral nerve immersed in a brain-shaped volume conductor: the significance of fiber bending in excitation.

To help elucidate some basic principles of magnetic coil (MC) excitation of cerebral cortex, a model system was devised in which mammalian phrenic nerve, or amphibian sciatic nerve with its branches was suspended in appropriate Ringer's solution in a human brain-shaped volume conductor, an inverted plastic skull. The nerve was recorded monophasically out of the volume conductor. The site of nerve excitation by the MC was identified by finding where along the nerve a bipolar electrical stimulus yielded a similar action potential latency. MC excitation of hand-related corticospinal (CT) neurons was modelled by giving the distal end of nerve attached to the lateral skull an initial radial (perpendicular) trajectory, with subsequent bends towards the base and posterior part of the skull; this nerve was optimally excited by a laterally placed figure 8 or round MC when the induced electric field led to outward membrane current at the initial bend. By contrast, nerve given a trajectory modelling CT neurons related to the foot was optimally excited when the coil windings were across the midline, but again when membrane current flowed outward at the first bend. Corticocortical fibers were modelled by placing the nerve in the anteroposterior axis lateral to the midline; with the round MC vertex-tangentially orientated, optimal excitation occurred at the bend nearest the interaural line, i.e., near the peak electric field. The findings emphasize the importance of orientation and direction of current in the MC and fiber bends in determining nerve excitation. The findings in the peripheral nerve-skull model help explain (1) why lateral and vertex-tangentially orientated MCs preferentially excite arm-related CT neurons directly and indirectly (through corticocortical fibers), respectively, and (2) why the MC orientations for optimally exciting directly arm and leg-related CT neurons differ.

Action Potentials↗
PubMed1992

Cerebello-frontal cortical projections in humans studied with the magnetic coil.

Focal stimulation over human cerebellum with a figure 8 magnetic coil (MC) results in an evoked wave recorded from bipolar scalp electrodes on the interaural line and more anteriorly. In 3 subjects, the wave responses along the interaural line had latencies of 8.8-13.8 msec, lasted 17.4-29.0 msec and had a maximum amplitude of 14.4-26.8 microV. The responses were recorded more anteriorly from leads midway between the interaural line and frontal leads; responses recorded from frontal leads were up to 3.5 msec later. The evoked wave was preceded by a diphasic EMG response with a latency of 1.2-2.0 msec. Analysis of the averaged responses recorded by adjoining bipolar leads indicated that the response was predominantly surface positive and crossed. Control experiments eliminated eye movement and somatosensory input as explanations of the evoked response, thereby identifying it as a cortical response. The surface positive wave in humans was compared with the responses recorded in cat and monkey to cerebellar stimulation. The responses in humans could reflect dysfacilitation through MC activation of Purkinje cells, or feed-forward facilitation through transsynaptic or antidromic activation of dentate neurons. The latency of the surface positive wave exceeds that of cerebellar inhibition of MC elicited hand muscle responses, but the discrepancy is at least partly accounted for by the extra delay required to set up the indirect cortico-spinal component required for motoneuron discharge. Estimates made of the cerebello-frontal cortical and peripheral feedback loop times suggest that the central has less than one quarter the delay of the peripheral loop, which would be especially advantageous during fast skilled movements of the fingers.

Cerebellum↗
PubMed1991

Measurement of the electric field induced into inhomogeneous volume conductors by magnetic coils: application to human spinal neurogeometry.

We measured the electric fields induced by round and figure "8" magnetic coils (MCs) in homogeneous and inhomogeneous volume conductors. In homogeneous media, the round MC held tangential (i.e., flat) to the volume conductor induced an annular electric field. When the round MC was held on-edge (i.e., orthogonal) to the volume conductor, the induced electric field consisted of two loops mainly parallel to the surface of the volume conductor and which approximated each other directly under the contacting edge of the MC. The tangentially oriented figure "8" MC similarly induced two electric field loops which approximated one another maximally under the region of the junction in its long axis. In a complex inhomogeneous volume conductor, such as a segment of human cervical-thoracic vertebral spine located eccentrically within a large cylindrical tank and submerged in isotonic saline, the direction of electric fields within the spinal canal and across the intervertebral neuroforamina was similar to that observed in the homogeneous volume conductor. However, in and near a single neuroforamen, the electric field and especially its first spatial derivative were markedly elevated compared to that recorded within the long central axis of the vertebral canal. Motor unit and compound muscle action potentials elicited in limb muscles by MC stimulation of human cervical spine confirmed predictions derived from the physical model. The predictions included: (1) absence of spinal cord stimulation compared to relative ease of nerve root stimulation by current that is most likely concentrated at the neuroforamina. When stimulating current is directed towards the periphery, the most likely low threshold site of stimulation is inferred to be just distal to the neuroforamina. It is emphasized that with supramaximal stimulation, more distal sites of excitation may occur; (2) invariant latency shifts at threshold intensities when moving the MC along the rostrocaudal axis of the cervical vertebral column; (3) significant effect (on motor unit activation thresholds) of the direction of induced current flow across the neuroforamina; (4) reduced stimulation when the targeted nerve roots are close to the null point of the electric field, i.e., between locations of high electric field intensity, of opposite polarity; and (5) relatively focal nerve root stimulation by the junction of a transversely orientated figure "8" MC, i.e., parallel to the nerve roots.

Adult↗
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↗
PubMed1991

Stimulation of the human nervous system using the magnetic coil.

The magnetic coil (MC) is a unique probe that can be used to elucidate basic neurophysiological mechanisms in humans. Either by excitation or inhibition of responding neural elements, we have been able to investigate: (1) the distribution of the electric field induced within isotropic and anisotropic volume conductors by round and figure-eight MCs; (2) the theoretical relationship between electric field distribution and excitation of distal peripheral nerve, nerve root, cranial nerve, and motor cortex; (3) the effect of focal MC stimulation of motor and visual systems; (4) perturbation of sequential digit movements by MC stimulation of human premotor cortex; (5) activation of frontal motor areas related to speech; (6) elicitation of a sense of movement in an ischemic paralyzed limb by focal MC cortical stimulation; and (7) the effect of stimulation of the human visual system to (a) suppress and unmask visual perception using single MC stimuli and (b) prolong visual suppression using short trains of MC stimuli. In the future, prolongation of MC action by using repetitive stimuli should be useful in further investigating functions concerned with language, speech, and cognition.

Cerebral Cortex↗
PubMed1991

Magnetic coil stimulation of human visual cortex: studies of perception.

The effects of magnetic coil (MC) stimulation of human visual cortex on the foveal perception of briefly presented letter trigrams include: (1) letters were nearly always reported correctly at visual stimulus-MC pulse intervals less than 60-80 msec or greater than 120-140 msec. Thus, by 120-140 msec, information related to letter recognition is relayed from calcarine cortex. (2) Presentation of equiluminant chromatic stimuli (specifically green letters against a red background) results in suppression curves which commence at longer latencies than those obtained with achromatic stimuli. (3) At a stimulus-MC pulse interval of 100 msec, shifting the MC laterally or rostrally resulted in suppression of the contralateral or caudal-most letter respectively. This implies a focal, topographical effect on visual cortex. (4) Two trigram stimuli separated in time (e.g. 100 msec) resulted in classical backward masking in which S1 (the target) was suppressed by S2 (the mask), using an S2/S1 luminance-contrast ratio of 4:1. When the MC was subsequently discharged 80-100 msec after S2, and S2 was suppressed, the response to S1 was easily retrieved (unmasked). Presumably, by 160 msec, S1 has been transmitted to the next processing, extrastriate level. (5) The unmasking phenomenon has been used to track information flow from visual cortex to higher cortical centers (e.g. Wernicke's, Broca's, and related areas). (6) Using a prototype repetitive stimulator, a consecutive train of single MC pulses given 70, 143 and 216 msec following a brief alphabetic trigram stimulus elicited a significant reduction in letter perception. This notably contrasts with the absence of suppression when a single MC pulse was given 70 or 143 msec following presentation of the alphabetic trigram. The results with 3 pulses suggest that the first MC pulse (at 70 msec) delays but requires repetition to prevent processing and/or transmission of information from visual cortex.

Electroencephalography↗
PubMed1991

Matching focal and non-focal magnetic coil stimulation to properties of human nervous system: mapping motor unit fields in motor cortex contrasted with altering sequential digit movements by premotor-SMA stimulation.

Possible classifications of effects of magnetic coil (MC) stimulation are discussed, with the conclusion that the most useful is focal versus non-focal excitation. The mode of excitation of peripheral motor axons by the longitudinal-orthogonally orientated round MC is deduced from the insignificant latency shift in motor unit response when the current direction is reversed either by rotating the coil or by a switching device. A hypothesis is advanced of how peak membrane current entry and exit could occur only 1-3 nodes apart. The mode of excitation of cerebral cortex is more complex. Related to the orientation of the round MC, corticospinal neurons are: (1) directly excited and; or (2) indirectly excited through stimulation of corticocortical and other presynaptic inputs. Although the round MC can directly excite monkey corticospinal neurons at two sites, i.e. the initial segment and the node, the node is believed to be the main target in humans. The meaning of 'focality' of excitation is discussed as applied to MC stimulation of peripheral nerve and cerebral cortex. A potential conflict exists between focality and magnitude of response to MC excitation. However, by appropriately orientating the round MC, activation of all motor axons in one nerve (e.g., the median nerve at the wrist) can be achieved without coactivation of another (e.g., the ulnar nerve). By contrast, no orientation of the round MC, or use of a specially designed MC (e.g., double coil) over motor cortex permits all members of a defined set to be activated in isolation. Nevertheless, some members of the set can be activated in isolation with the MC over motor cortex. Response properties of individual motor units in the extensors of the digits when focally stimulating motor cortex with the figure '8' MC include: (1) Responses are variable to a given stimulus a little above threshold. Comparing responses by individual motor units with that of the population, or with other simultaneously recorded units revealed both coherent and independent sources of variability. (2) The scalp field from which the motor units could be driven by suprathreshold stimulation was of the order of 4-6 cm2. The fields were elongated in the antero-posterior axis, possibly related to the similar orientation of the junction region of the figure '8' MC. (3) Motor units initially excited by threshold MC stimulation were typically recruited early during voluntary contraction (confirming Hess et al. 1987).(ABSTRACT TRUNCATED AT 400 WORDS)

Action Potentials↗
PubMed1991

Mechanisms of peripheral nervous system stimulation using the magnetic coil.

We measured the distribution of the electric fields induced within isotropic and anisotropic volume conductors by figure '8' and round magnetic coils (MCs). For various MC orientations maximal electric fields were induced parallel to the inner surface of the volume conductor. Predictions from physical models roughly approximating human forearm and vertebral column were then tested by MC stimulation of distal median nerve at the wrist and nerve roots at the cervical spine, respectively. Predictions that were confirmed included: (1) Peripheral nerve is most easily excited by a tangential-edge round MC in which current is induced parallel to the long axis of the nerve. As a corollary, peripheral nerve is least excited when the round MC is geometrically bisected by the long axis of the nerve. (2) Peripheral nerve excitation most likely occurs at locations corresponding to the first spatial derivative of the electric field; i.e. where the electrical field is changing most rapidly over distance. For the figure '8' MC, this occurs distally when the anterior divergence of the junction is directed distally. (3) Nerve roots are excited at a relatively constant latency, low threshold location. This site most likely corresponds to the vicinity of the neuroforamina where the induced electric field is most intense and the first spatial derivatives peak. By contrast, the low value of the first derivative in the longitudinal axis of the vertebral canal implies that long tracts in the spinal cord are not likely to be excited using commercially available MCs. Our study also indicates that intracranial stimulation of facial nerve occurs at a selected low threshold location, probably reflecting focusing of induced current.

Action Potentials↗
PubMed1991

Mapping of motor cortex gyral sites non-invasively by transcranial magnetic stimulation in normal subjects and patients.

Transcranial stimulation offers the opportunity to replace a standard clinical method of invasive brain cortical mapping. A figure '8' magnetic coil with a 1-3-cm area of stimulation is combined with a new method of creating magnetic resonance imaging (MRI) scans which represent the cortical surface (mapping cuts) and thus establishing common landmarks between the MRI and the scalp for cortical mapping. This assigns gyral sights to scalp sites of transcranial stimulation. We report the development and evaluation of these techniques in 6 normals and 10 patients. Cortical maps were shown to enlarge during voluntary contraction. They were variable between individuals, as reported for direct cortical mapping. Maps showed a degree of short and longer term consistency within an individual. The maps were influenced by lesions. They also provided clinical information of the relation of functioning cortex to brain tumors that could influence clinical management including surgical decisions. A consideration of safety of the method in comparison to direct brain stimulation indicates that it falls within existing parameters for electrical stimulation of the brain, and could reduce the risks of the present standard by lowering charge density and the need for invasive methods. Broader application could lead to development of functional imaging methods, which would be a useful preoperative planning tool and investigative method.

Action Potentials↗
PubMed1990

Focal magnetic coil stimulation reveals motor cortical system reorganized in humans after traumatic quadriplegia.

A figure of '8' magnetic coil (MC) was used to stimulate focally the motor cortex of two adult, traumatic quadriplegics and three normal adults. The two patients were injured approximately 2 years previously and had intense physiotherapy, including biofeedback training of biceps and deltoid muscles, respectively, which were the most caudal muscles spared. The focal MC elicited compound motor action potentials (CMAPs) from these muscles from a much wider area of scalp than in the normal subjects. Latency of biceps and deltoid CMAPs were inversely related to CMAP amplitude. A reorganization of the motor cortical projection system is inferred, in which areas normally eliciting digit movements instead activate muscles in quadriplegics just above the spinal level. The reorganization applies also to the central sense of movement normally elicited by focal frontal cortex stimulation. Possible mechanisms of the reorganization and an implication for rehabilitation are discussed.

Action Potentials↗
PubMed1990

Corticospinal responses to electrical stimulation of motor cortex in the rat.

Direct and indirect corticospinal responses to electrical stimulation of motor cortex were identified in urethane-anesthetized rats. 'Killed-end' recordings were taken from the corticospinal tract at the level of the cervical cord (C1-C2) and from the medullary pyramid. The identities of direct (D) and indirect (I) corticospinal responses were confirmed by: (1) removing motor cortex to eliminate I activity, and (2) pharmacologically increasing neocortical excitability, prior to any lesions, to increase I activity. Our data indicate that the conduction velocity of the fastest corticospinal fibers is approximately 18 m/s. Our identification of the components of the corticospinal response will permit the interpretation of the more complicated surface or 'non-killed-end' depth recordings which have shown particular utility in evaluating spinal cord damage.

Action Potentials↗
PubMed1990

Spatial distribution of the electric field induced in volume by round and figure '8' magnetic coils: relevance to activation of sensory nerve fibers.

The electric fields induced in finite homogeneous volume conductors by a round and a figure '8' magnetic coil (MC) were measured and related to MC stimulation of the median nerve. The volume conductors, filled with isotonic saline, consisted of a large rectangular trough ('unrestricted') and a smaller trough, whose dimensions approximated human forearm ('restricted'). Various MC orientations were applied to the volume conductor. Bipolar recordings were obtained with a coaxial electrode, which measured the voltage gradient between the exposed edge of the cable shield and the central wire at its tip, 1 cm distant (a linear probe). The probe was moved in 3 dimensions, allowing computer reconstruction of the electric field as a function of the 3 spatial axes. When the probe was parallel to the plane of the round MC and tangential to the direction of current in its windings, the induced electric field was maximal; it tended towards zero when the probe was over the center of the MC, or when the probe, remaining parallel to the plane of the MC, was radial (i.e., perpendicular) to the direction of the current in the windings. For a variety of MC orientations, the electric field was consistently increased when the probe was adjacent and parallel to the edge of the trough, indicating the important effect of boundaries. The electric field was greatly increased focally when the round MC was applied orthogonally to the volume conductor, or when the figure '8' MC was applied tangentially (i.e., flat) to the volume conductor. With the figure '8' MC, a sharp central peak parallel to the long axis was bounded on each side by smaller (less than half amplitude) peaks. The findings from physical modeling led to correct predictions as to the most effective orientations of round and figure '8' MCs for eliciting sensory nerve action potentials (SNAPs) from the median nerve.

Action Potentials↗
PubMed1990

Brain stimulation revisited.

The results of the following selected studies using magnetic coil (MC) stimulation are presented: (1) evidence for focality of MC stimulation, (2) MC stimulation of frontal areas related to speech, (3) transcallosal responses evoked by MC stimulation, and (4) suppression of visual perception with MC stimulation over occipital cortex. The authors served as subjects, and in most studies a standard Cadwell stimulator and round MC were used. Using a more vertical, rather than tangential, MC orientation and threshold stimulation, nearly isolated movements of individual digits were elicited implying focal cortical excitation. MC stimulation of frontal areas of either hemisphere elicited electromyography in contralateral laryngeal muscles. The shortest latency responses that were often accompanied by arm movement were thought to be elicited from intermediate areas of precentral gyrus and longer latency responses, from near Broca's area, extreme lateral precentral gyrus, and the supplementary motor area. MC stimulation over the occipital cortex resulted in suppression of visual perception of letters briefly flashed on a screen. The topography of suppression implicated the geniculocalcarine system as the site of MC effect. Focal MC stimulation of posterior frontal cerebral cortex elicited a transcallosal response from contralateral homologous cortex with a latency similar to that obtained with focal anodic stimulation but with considerably less excitation of cranial muscles.

Brain↗
PubMed1989

A sense of movement elicited in paralyzed distal arm by focal magnetic coil stimulation of human motor cortex.

Two magnetic coils (MCs) of special design (Cadwell Laboratories) were used to elicit movements predominantly of one or a few digits by percutaneous stimulation of human motor cortex. When cortically elicited movements were ischemically blocked, the MC still elicited a discrete sense of digit movement; a sense of movement map was constructed by stimulating at different scalp sites. Our findings support the existence of corollary discharge.

Arm↗