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V E Amassian

Publications and source records attributed to V E Amassian.

69 records · Page 4Linked to original sources
PubMed1974

Periodic components in steady-state activity of cuneate neurones and their possible role in sensory coding.

1. Individual cuneate touch, touch-hair and proprioceptive neurones often display periodic components in their steady-state resting or evoked discharges. The periods may be visible in the interspike interval distribution or may be revealed only by the expectation density function. Several levels of complexity were identified; from one to four mutually prime, periodic components may be present, with or without an aperiodic component.2. The periodic components usually depend on the peripheral input, as shown by their introduction or modification by peripheral stimulation and by their disappearance following deafferentation.3. The coefficient of variation of periodic discharges by touch and touch-hair neurones was 0.024-0.23 (aperiodic, driven discharges usually had CV's of 0.64-0.77).4. The occasional recording extracellularly of a periodically occurring, unitary prepotential preceding the spike and the intracellular recording of a periodic, ;giant', unitary e.p.s.p. imply that an individual periodic impulse in an afferent fibre may elicit a post-synaptic discharge. Even with threshold stimulation, the timing of post-synaptic discharge was quite precise.5. The periodicity did not appear to reflect a cycle of subnormality intrinsic to the post-synaptic neurone, because a current pulse injected through a micro-electrode lying just external to the membrane could block the next expected periodic discharge, but did not continuously vary its waiting time.6. A mathematical model that fits some discharge patterns predicts the interval distribution from an over-all probability of response to the periodic input: each periodic afferent impulse is treated as an independent trial resulting in success (post-synaptic discharge) or failure. Some other interval distributions are fitted by a ;conditioned' model in which a success reduces the probability of response at the next periodic trial.7. The discussion includes a hypothesis of tactile discrimination, in which the output of a sensory relay, e.g. cuneate neurone, signals both the number of afferent fibres converging upon it and the afferent period in each.

Animals↗
PubMed1966

The effect of (+)-tubocurarine chloride and of acute hypotension on electrocortical activity of the cat.

1. The effects of I.V. injection of tubocurarine (3 mg/kg) and of acute hypotension upon electrocortical activity of cats under sodium pentobarbitone were analysed. Technical artifacts associated with changes in brain volume were minimized.2. Initially, the superficial cortical response was reduced by tubocurarine either insignificantly or by 34% or less. Cats bled before injection of tubocurarine showed initially a similar reduction but, occasionally, a severe delayed reduction was found. Any fall in the superficial cortical response started during the secondary rise in B.P., many seconds after the B.P. had reached its lowest initial level. By contrast, spontaneous electrocortical activity was reduced approximately when the peak initial fall of B.P. occurred and it started to recover during the secondary rise in B.P. Reduction of the superficial cortical response could still be elicited after spindling had been suppressed by deep barbiturate anaesthesia. The initial thalamocortical afferent component in somatosensory area I, which was evoked by dorsal column stimulation, was less reduced by injection of tubocurarine than were the post-synaptic positive-negative components.3. The depressant effects of injected tubocurarine upon the superficial cortical response and spindle activity were approximated by rapid arterial bleeding.4. The superficial cortical response was reduced following virtual arrest of the cerebral circulation by acute exsanguination or by acute oil embolization. Such reduction most likely does not depend on systemic transport of inhibitory substances to the brain.5. After injection of tubocurarine the rate of outflow from the superior sagittal sinus was variably reduced during the initial fall in B.P., and often showed a variable, secondary increase. A net deficit in cerebral blood flow was usually present during the period of 80 sec after injection.6. The variability in the effects of injected tubocurarine upon electrocortical activity is attributed to the variability of the net deficit in cerebral blood flow.

Animals↗
PubMedDate not supplied

Long loop participation of red nucleus in contact placing in the adult cat with facilitation by tactile input at the spinal level.

This report focuses on the question 'Why does stimulation of a few hairs of a cat's unsupported paw lead to biceps activation and contact placing (CP) while stimulation of the supported paw does not?' The rubral system is used as the model; individual rubral neurons (RN) are recorded in the awake cat during CP and rubrospinal tract neurons (RTN) are identified by demonstrating antidromic invasion. Classes of RN were identified corresponding to each of the main stages of CP. In particular, a contingent, contact-locked category was identified, responding only if placing followed the contact. Another category, the pre-delayed movement-locked, responded 40-160 ms prior to delayed placing. Both of these categories imply that the rubral system dynamically participates (with other higher motor neurons) in CP, a conclusion strengthened by the positive relationship between RN discharge and movement parameters under varying load conditions. Physiological stimulation of the supported paw may fail to excite RN, which respond strongly to gentle contact of the unsupported paw (which leads to CP). Cooling or making permanent lesions in nucleus interpositus and dentatus drastically reduces resting and evoked RN and especially RTN activities in the awake cat. Some evidence of recovery was present by 4-6 weeks after the lesions. The effect of behavioral context was also studied on the biceps responses in the awake cat to a train of 3-4 pulses at 500 Hz delivered either to the red nucleus or to the rubrospinal tract (RT). The biceps responses were enhanced when the forepaw was removed from a support (the pre-condition for CP).(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMedDate not supplied

Focal stimulation of human cerebral cortex with the magnetic coil: a comparison with electrical stimulation.

Percutaneous stimulation of human motor cortex electrically (focal anode) and with magnetic coils (MCs) of various designs is compared. The theoretical prediction was confirmed that positioning the standard round MC laterally and orientating it more towards the vertical induces an electric field appropriate for directly exciting corticospinal neurons (cf., the conventional tangential orientation at the vertex). Thus, during voluntary contraction, minimal latency compound motor action potentials (CMAPs) in contralateral arm were elicited both by focal anodic and appropriately orientated MC stimulation. Conduction time from motor cortex to motoneuron was estimated by subtracting peripheral conduction time and monosynaptic delay at the motoneuron from the overall CMAP latency, yielding an estimated corticospinal conduction velocity as high as 66 m/sec. Discontinuous latency variations observed in population CMAPs or individual motor units approximated mono- or polysynaptic cortical synaptic delays and, therefore, are attributed to the intervals between direct and early, or late indirect corticospinal discharges. A TV computer system was used to track movements of individual digits and the hand following MC stimulation. An appropriately orientated MC readily elicited movements predominantly of a single digit, implying focal activation of motor cortex. A double square and a small pointed MC proved especially convenient for eliciting reproducibly single digit movements. Stronger stimulation revealed a topographical gradient in the responses of the different digits. Responses to a given MC stimulus a little above threshold were variable in amplitude, which could not be explained by the relationship of stimulus to phase of the cardiac or respiratory cycle. Overall, our findings indicate the importance of appropriately orientating a standard round MC and using a specially designed MC to obtain the various types of motor response to stimulation of cerebral cortex.

Action Potentials↗
PubMedDate not supplied

Comparison of human transcallosal responses evoked by magnetic coil and electrical stimulation.

Human transcallosal responses (TCRs) were elicited by focal magnetic coil (MC) stimulation of homologous sites in contralateral frontal cortex and compared with those to focal anodic stimulation. With MC stimulation, the TCR consisted of an initially positive wave with an onset latency of 8.8-12.2 msec, a duration of 7-15 msec, and an amplitude which reached up to 20 microV, sometimes followed by a broad low amplitude negative wave. With anodic stimulation, a similar response was obtained in which the positive wave was similar in latency and maximum amplitude, but had a greater duration. With anodic stimulation, not only was the TCR threshold below that for contralateral movement, but it reached substantial size at intensities below motor threshold. With MC stimulation, contralateral arm movement and scalp corticomotor potentials were observed when the MC was displaced posteriorly towards the central sulcus. Unlike with anodic stimulation, the MC evoked TCR was usually not preceded by a prominent EMG potential from temporalis muscle and was not associated with subject discomfort. The TCR provides unique information concerning the functional integrity of callosal projection neurons, their axons and transsynaptic processes in recipient cortex. This information may prove useful in the evaluation of intrinsic cerebral mechanisms and in establishing cortical viability.

Corpus Callosum↗
PubMedDate not supplied

Suppression of visual perception by magnetic coil stimulation of human occipital cortex.

Magnetic coil (MC) stimulation percutaneously of human occipital cortex was tested on perception of 3 briefly presented, randomly generated alphabetical characters. When the visual stimulus-MC pulse interval was less than 40-60 msec, or more than 120-140 msec, letters were correctly reported; at test intervals of 80-100 msec, a blur or nothing was seen. Shifting the MC location in the transverse and rostro-caudal axes had effects consistent with the topographical representation in visual cortex, but incompatible with an effect on attention or suppression from an eyeblink. The MC pulse probably acts by eliciting IPSPs in visual cortex. The neural activity subserving letter recognition is probably transmitted from visual cortex within 140 msec of the visual stimulus.

Humans↗
PubMedDate not supplied

A comparison of corticospinal activation by magnetic coil and electrical stimulation of monkey motor cortex.

The effects of different orientations of a Cadwell round magnetic coil (MC) were compared with each other and with surface electrical stimulation of motor cortex in monkeys anesthetized with pentobarbital or urethane. Recordings were made from within the lateral corticospinal tract, either from axonal populations or with a microelectrode from individual axons. A lateral-sagittally orientated MC directly excited corticospinal neurons at lower stimulus intensity than was required for indirect, i.e., transsynaptic excitation via inputs to corticospinal neurons. By contrast, in 2 out of 3 macaques tested, a vertex-tangential orientation could excite corticospinal neurons indirectly at lower intensities than were required for direct excitation; at higher intensities, direct excitation also occurred. The site of direct corticospinal excitation by a lateral-sagittally orientated MC was inferred by comparing the response variability and latency to MC and surface electrical stimuli. Cathodal stimuli elicited more variable corticospinal population responses and later individual axonal responses than were obtained with anodal stimuli. The variability in response is attributed to interaction between nearby, on-going synaptic bombardment and the stimulus, implying that surface cathodal stimuli directly activate corticospinal neurons at the spike trigger zone (presumably the initial segment). By contrast, the consistency and reduced latency of the corticospinal responses to surface anodal stimuli are attributed to the direct excitation of corticospinal fibers within the white matter. When the stimulus intensity is clearly above threshold, surface anodal and cathodal stimuli can activate corticospinal neurons both directly and indirectly. Direct corticospinal excitation by the MC can resemble the effects of either surface anodal or surface cathodal stimuli. We conclude that the MC can activate corticospinal neurons at the spike trigger zone or their fibers deeper in white matter. The findings in the monkey are used to interpret the effects of different MC orientations in the human.

Action Potentials↗