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Biomedical subjects

J Victor

Publications and source records attributed to J Victor.

At least 91 records · Page 5Linked to original sources
PubMed1986

Hyperacuity in cat retinal ganglion cells.

Cat X retinal ganglion cells that can resolve sine gratings of only 2.5 cycles per degree can nevertheless respond reliably to displacements of a grating of approximately 1 minute of arc. This is a form of hyperacuity comparable in magnitude to that seen in human vision. A theoretical analysis of this form of hyperacuity reveals it to be a result of the high gain and low noise of ganglion cells. The hyperacuity expected for the best retinal ganglion cells is substantially better than that observed in behavioral experiments. Thus the brain, rather than improving on the retinal signal-to-noise ratio by pooling signals from many ganglion cells, is unable to make use of all the hyperacuity information present in single ganglion cell responses.

Animals↗
PubMed1986

Source geometry and dynamics of the visual evoked potential.

A method is developed for the analysis of the steady-state visual evoked potential (VEP) recorded at an array of electrodes. The VEP is assumed to result from the sum of a number of independent mechanisms. Each mechanism proves to have a fixed intracerebral source (independent of stimulus paradigm) and dynamics which depend on the stimulus. The steps in the analysis are: Fourier analysis of the responses and retention of the first 4 even harmonics; factor analysis in the frequency domain; an invariant representation ('the invariant fingerprint') of the geometric information contained in the factor analysis; and interpretation of the invariant fingerprint in terms of a simple biophysical model. This analysis is applied to responses elicited by a contrast-reversing checkerboard. The dependence of the midline VEP wave form on reversal frequency, check size, and area of stimulation (upper, lower, and full-field) reflects a change in the dynamics of the generators, rather than a change in their geometry.

Evoked Potentials, Visual↗
PubMed1985

[Anti-arrhythmia effects of magnesium salts. 4 cases].

Known since 1935, the antiarrhythmic effects of magnesium salts are periodically forgotten. They are illustrated here by 4 cases with entirely different clinical and biological features. Of unquestionable therapeutic value was the use of magnesium for the treatment of 2 episodes of severe ventricular extrasystoles with "torsades de pointe". These antiarrhythmic properties are observed whether blood magnesium levels are normal or low and cannot therefore be ascribed to correction of magnesium deficiency. Experimental data are in favour of interferences with transcellular ion flows. Magnesium is thought to ensure normal potassium gradient and also to reactivate the fast sodium channel inhibited by hypokalaemia. Above all, the magnesium ion appears to act as a calcium inhibitor, which would explain its antiarrhythmic properties irrespective of the state of magnesium body stores.

Adult↗
PubMed1985

[Influence of intravenous clonidine on normal and pathological sinus and atrioventricular nodes and carotid sinus sensitivity].

A number of clinical observations have suggested that clonidine may be responsible for dizziness and even syncope. The aim of this study was to assess the effects of this drug on normal and pathological sinus and AV nodes and on carotid sinus sensitivity. 19 patients were investigated (average age: 73 years). 14 patients complained of dizziness or syncope, including 3 patients with spontaneous sinus node dysfunction. 5 patients were asymptomatic; 3 were investigated for severe sinus bradycardia (1 on clonidine); 1 patient had sinoatrial block and 1 patient underwent pre-operative assessment for intraventricular block. The sinus node was studied using Mandel's method at 100, 120 and 150/min; the AV node was studied by the extrastimulus method with fixed atrial cycle of 600 ms. The following parameters were measured: Wenckebach point, AH interval in spontaneous and paced cycle length of 600 ms, effective refractory periods. Carotid sinus sensitivity was tested by right and left carotid sinus massage. These parameters were measured under basal conditions and 15 and 30 minutes after IV injection of 0.150 mg of clonidine. Two groups of patients were identified from the results under basal conditions: group 1:11 patients with corrected post-stimulation pauses less than 525 ms, and group 2:8 patients with at least one corrected post-stimulation pause of over 525 ms. Clonidine influenced the post-stimulation pauses significantly in both groups. However, the number of pathological pauses increased much more in group 2 than in group 1.(ABSTRACT TRUNCATED AT 250 WORDS)

Aged↗
PubMed1984

[Plasma catecholamine concentrations during exercise in the untrained subject and in the sportsman].

Plasma noradrenaline (NA) and adrenaline (A) concentrations were measured by high liquid pressure chromatography in five untrained subjects and in five well-trained rowers during an exercise test on bicycle ergometer. Blood samples were collected, via a venous catheter, at rest, after standing 5 minutes, at maximal work level and at 5, 10, 20 minutes of post-exercise. Plasma NA and A concentrations at rest and after 5 minutes of standing were similar in the two groups. At maximal work load (340 +/- 38 for rowers and 220 +/- 11 W for untrained subjects), for a same pulse rate (178.0 +/- 7.6 for rowers and 176.6 +/- 4.7 beats/minutes-1 for untrained subjects), NA and A concentrations were higher in athletes (NA: 5.57 +/- 1.32 microgram/l; A: 0.95 +/- 0.31 microgram/l) than in untrained subjects (NA: 2.13 +/- 0.80 microgram/l; A: 0.40 +/- 0,34 microgram/l). During recovery, no significant difference was observed between the two groups.

Adult↗
PubMed1982

Embarrassment as the aura of a complex partial seizure.

A man with a grade II astrocytoma in the medial frontal lobe exhibited complex partial seizures, with the aura being a feeling of extreme embarrassment. This previously unreported ictus suggests that an electric discharge in the prefrontal cortex may produce the opposite effect of prefrontal lesions, in which there is a lack of insight and an absence of the capacity for embarrassment.

Adult↗
PubMed1980

A method of nonlinear analysis in the frequency domain.

A method is developed for the analysis of nonlinear biological systems based on an input temporal signal that consists of a sum of a large number of sinusoids. Nonlinear properties of the system are manifest by responses at harmonics and intermodulation frequencies of the input frequencies. The frequency kernels derived from these nonlinear responses are similar to the Fourier transforms of the Wiener kernels. Guidelines for the choice of useful input frequency sets, and examples satisfying these guidelines, are given. A practical algorithm for varying the relative phases of the input sinusoids to separate high-order interactions is presented. The utility of this technique is demonstrated with data obtained from a cat retinal ganglion cell of the Y type. For a high spatial frequency grafting, the entire response is contained in the even-order nonlinear components. Even at low contrast, fourth-order components are detectable. This suggests the presence of an essential nonlinearity in the functional pathway of the Y cell, with its singularity at zero contrast.

Animals↗
PubMed1978

[Intimal coronary fibrous dysplasia and Prinzmetal's angina].

It is currently considered as exceptional for there to be arterial pathology other than atheroma in Prinzmetal angina. The authors have found five cases of coronary fibrous dysplasia in the literature. They add a case, in a woman, of intimal fibrous dysplasia diffusely involving the three main coronary vessels. Coronary arteriography showed that spasm of the right coronary artery was the cause of attacks with the clinical and electrocardiographic features of Prinzmetal angina. The whole coronary tree was otherwise normal angiographically and also macroscopically at autopsy. The dysplasia was only discovered by histological study of serial sections of the three coronary vessels. Histological examination showed no evidence of an atheromatous lesion. The authors place these coronary artery lesions within the general classification of arterial fibrous dysplasias. It remains hypothetical whether there is a relationship between the occurrence of spasm and this very special histological feature of the coronary artery wall.

Angina Pectoris↗
PubMed1975

[Phentolamine in treatment of acute left ventricular insufficiancies].

Phentolamine (Regitine) at the dose of 0.3 mg/mn behaves as an arterial and, above all venous, vasodilatator agent, resulting in a marked and early lowering of the pulmonary pressures in acute oedema of the lung and in cardiac asthma. It was used alone in 47 attacks of acute severe left ventricular failure with very favourable results in 43 cases, as proved by the rapid improvement of the haemodynamic status and of the aicd-base balance. Under strict observation, tolerance has been excellent. This therapeutic method seems of great interest in the cases of acute pulmonary oedema with a maintained blood pressure level, and in the forms with severe arterial hypertension which might tolerate larger doses.

Acid-Base Equilibrium↗