Biomedical subjects
Publications and source records attributed to C W Burckhardt.
The design of a robot for functional stereotactic surgery which is under construction in our laboratory is described. The main features include operation inside the computed tomographic (CT) scanner, the possibility of intraoperative CT scanning and complete handling of the stereotactic probes by the robot.
It has been reported that psychophysical flicker sensitivity, electroretinogram (ERG), pattern electroretinogram (P-ERG), and visual evoked cortical potential (VECP) responses are affected by ocular hypertension and glaucoma. Moreover, it has also been revealed that artificially augmented intraocular pressure (IOP) impairs electrophysiologic responses. In this investigation the authors varied the levels of IOP of ten normal subjects and determined their flicker sensitivities at these levels. Temporarily elevated IOPs were produced by exerting pressure on the eyeball through the eyelids by a new type of ophthalmodynamometer. The results obtained showed that increasing IOP produced loss of flicker sensitivity. For an IOP of 27.2 +/- 2.1 mm Hg, significant losses occurred in the range of frequencies lower than 20 Hz. For a higher IOP (40.7 +/- 2.1 mm Hg) the loss was significantly at frequencies higher than 5 Hz. However, the critical flicker frequency was not affected significantly at either level of IOP. The authors also discovered a phenomenon, called "curtain phenomenon," in that for a critical level of IOP, a high flicker frequency stimulus could completely fade out. The authors' results suggest that the psychophysical flicker sensitivity measurement is a more sensitive technique for the investigation of the effect of the variation of the IOP and that the curtain phenomenon constitutes an interesting indication for additional study of the ocular hypertension mechanism.
1 The sensitivity to flickering light at various light frequencies (DeLange curve) was determined in 20 controls and 45 patients receiving maintenance doses of digoxin. 2 Flicker thresholds (mean percentage of maximal light modulation +/- s.d.; F 30 Hz) were 7.6 +/- 1.7 in controls and 9.4 +/- 1.7 in patients with optimal plasma digoxin levels (0.5-1.9 ng/ml), but they rose to 15.5 +/- 1.9 at subtoxic levels (2.0-3.0 ng/ml), and to 21.8 +/- 2.6 at toxic levels (above 3.0 ng/ml). 3 Flicker sensitivity was inversely correlated with plasma digoxin levels and returned to baseline values when the administration of digoxin was interrupted. 4 The DeLange curve seems to be a valuable tool to measure the toxic effects of digitalis on the visual system.
The distribution of extracellular and intracellular potential in the sucrose gap apparatus, previously established for a single fiber using the cable equations for a core conductor model (Jirounek and Straub, Biophys. J., 11:1, 1971), is obtained for a multifiber preparation. The exact equation is derived relating the true membrane potential change to the measured potential differences across the sucrose gap, the junction potentials between sucrose and physiological solution, the membrane potential in the sucrose region, and the electrical parameters of the preparation in each region of the sucrose gap. The extracellular potential distribution has been measured using a modified sucrose gap apparatus for the frog sciatic nerve and the rabbit vagus nerve. The results indicate a hyperpolarization of the preparations in the sucrose region, of 60--75 mV. The hyperpolarization is independent of the presence of junction potentials. The calculation of the correction terms in the equation relating the actual to the measured potential change is illustrated for the case of complete depolarization by KC1 on one side of the sucrose gap. The correction terms in the equation are given for various experimental conditions, and a number of nomographic charts are presented, by means of which the correction factors can be rapidly evaluated.
An ocular trauma may be followed only by a decrease in visual acuity without any tissular alteration. The measurement of the flicker sensitivity permits to hypothesize that the origin of the disturbance is in the retina itself. We postulate, therefore, the existence of a trauma of the lateral inhibition and we propose the term contusive intraretinal syndrome.
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