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Biomedical subjects

M Korth

Publications and source records attributed to M Korth.

At least 109 records · Page 6Linked to original sources
PubMed1984

Effect of stimulus intensity and contrast on the pattern ERG.

Stripe patterns of varying spatial frequency were presented in the onset-offset mode. At a high contrast the amplitude of the onset response showed a spatial selectivity which occurred with a high intensity at a higher spatial frequency than with a low intensity. The offset response showed only a monotonic decrease in amplitude at both intensities. Contrast sensitivity functions were derived at different levels of contrast. The spatial selectivity of the onset response occurred at a contrast spatial frequency and was more pronounced with low contrasts. The amplitude of the offset response was spatially selective only at low contrasts; with increasing contrast a gradual shift to a non-selective behavior was noted.

Electroretinography↗
PubMed1984

Nasopharyngeal recordings separate retinal from optic nerve potentials.

Electrical potentials in response to single flashes and to pattern-reversal stimuli presented in Maxwellian view were recorded from several intranasal locations by withdrawing a nasopharyngeal electrode from the epipharynx through the nose in steps of 1 cm. From the anterior parts of the nose a waveform could be obtained resembling the corneal electroretinogram. In the epipharynx an ERG was recorded which was inverted in polarity. This reversal in polarity was explained by assuming an electrical dipole of the retina oriented parallel to the electrode path. When the electrode was below and behind the posterior pole of the eye ball a high-frequency activity of increased amplitude was observed the origin of which could not be explained by the retinal dipole. It is suggested that these increased oscillations originate from the optic nerve. Their behavior is explained by a hypothetical electrical dipole of the optic nerve oriented vertical to the electrode path.

Electroretinography↗
PubMed1983

Pattern-evoked responses and luminance-evoked responses in the human electroretinogram.

Electrical potentials were recorded from the cornea of the human eye in response to the onset and offset of square-wave stripe patterns varying in spatial frequency, luminance and contrast. Under high retinal illumination (50119 photopic td) the response to pattern onset was a positive wave (pattern-onset wave) followed by a negative after-potential. As long as the pattern was presented, a steady potential of positive polarity (plateau potential) was observed. The response to pattern offset was a biphasic negative-positive wave followed by a negative after-potential. A comparison between wave forms obtained with pattern onset-offset stimuli and luminance increase-decrease stimuli suggests: (a) the pattern-onset wave is the result of an interaction between a luminance-increase and a luminance-decrease response; (b) the plateau potential is mainly an additive superposition of two receptor processes originating from retinal areas that receive increases and decreases in local luminance; (c) the negative-positive potential is an a-wave and a b-wave originating from retinal areas that receive increases in local luminance. The amplitude of the pattern-onset wave was greatest at a spatial frequency around 3-4 c/deg. This behaviour was closely correlated with contrast sensitivity determined psychophysically by previous investigators. Therefore, the pattern-onset wave seems to be a pattern-evoked response. The amplitude of the b-wave following pattern offset showed a monotonic decrease with increasing spatial frequency. It is mainly a luminance-evoked response. Under low retinal illumination (457 photopic td) the pattern-onset wave and the b-wave at pattern offset were smaller. However, the pattern-onset wave had its maximum amplitude at a lower spatial frequency. When increases or decreases in space-average luminance were combined with pattern onset or offset, transitions between pattern- and luminance-evoked responses could be observed. The results suggest that a decrease in local luminance is the essential stimulus for evoking a pattern-related response while increases in local light intensity generate luminance-evoked responses.

Cornea↗
PubMed1980

Information coding in the visual system: a new hypothesis.

Experiments on subjective color sensations using a special signal generator and a standard TV monitor (black and white) led to a new hypothesis on information coding in the visual system. According to this hypothesis a trigger signal is required which restores the phase correlation of the pulse-coded spiking of the neurons transmitting visual information from the eye to the brain. The "shift effect" ("periphery effect"), either artificially induced or initiated by microsaccades, might act as the postulated trigger.

Color Perception↗
PubMed1980

Intra- and extracellular K+ and Na+ activities and resting membrane potential in sheep cardiac purkinje strands.

K+- and Na+-selective liquid ion-exchanger microelectrodes were used to measure intracellular K+ activity (aK i) and intracellular Na+ activity (aNa i) of sheep cardiac Purkinje strands in different solutions. In Tyrode's solution with an extracellular K+ concentration ([K+]o) of 5.4 mM, aK i was between 80 and 140 mM and averaged 109.6 +/- 4.0 mM (mean +/- SE, 20 strands). The measured aK i was closely correlated with the resting membrane potential, so that the K+ equilibrium potential was always about 10 mV more negative. When [K+]o was lower than 5.4 mM, aK i fell, and when [K+]o was higher than 5.4 mM it increased, aNa i was between 4 and 12 mM, and averaged 6.6 +/- 0.6 mM (14 strands). Its variation was also correlated with resting potential. Over a wide range of [K+]o and extracellular Na+ concentrations ([Na+]o), the aiNa changes were such that Na+ equilibrium potential remained between +70 and +80 mV. The quiescent membrane behaved as a K+-electrode when [K+]o was higher than 5.4 mM. When [K+]o was low and [Na+]o was zero, then Ca2+ and perhaps Cl- contributed to the resting potential.

Animals↗
PubMed1980

Luminosity functions of human electroretinogram wavelets evoked with pattern-reversal stimuli.

Concomitant determinations of the spectral sensitivity of the b-wave and of the wavelets which are superimposed on the ascending slope of the b-wave of the human electroretinogram were made in two subjects with the use of alternating colored checkerboard patterns. The method of constant criterion response was applied to the luminance curves of the b-wave and of the wavelets relating the peak latency of the response to the stimulus intensity. The spectral sensitivity of the b-wave showed a Purkinje shift and was of mesopic activity at those adaptation levels at which the wavelets could be observed for the first time. The spectral sensitivity of the wavelets could be described best by the curve of the peripheral cones at all intensity levels associated with the appearance oing photopic retinal activity unaffected by the contributions of the scotopic system.

Dark Adaptation↗
PubMed1979

[The influence of vigilance on DC response in human visual evoked potential (author's transl)].

Visual evoked cortical potentials elicited by long-lasting stimuli of red light were examined in eight subjects under different conditions of vigilance. The DC components of the evoked potentials were compared with the delta and theta activity (0-7.5 Hz) of the EEG by means of frequency power spectra. The results are: 1) A proportional relationship exists between the amplitude of the negative DC component of the evoked potential and the theta and delta activity in the EEG. 2) Under conditions of low vigilance the negative DC potential does not return immediately to the baseline after the off-effect. Instead it may reach a second peak. 3) Under conditions of high vigilance, the DC potential may reverse its polarity. Its return to the baseline following the off-effect is immediate. 4) No dependence of the cortical DC potential on the intensity of the light stimulus could be demonstrated. 5) Since the negative potential outlasts the visual stimulus and could be recorded at the occiput and the vertex, it may be regarded as a nonspecific arousal reaction. Furthermore, since the positive DC potential can only be recorded at the occiput, it is regarded as a specific stimulus-correlated response.

Adult↗
PubMed1979

Stimulus alternation and fast retinal potentials: photopic anc scotopic contributions.

Human retinal oscillatory potentials in response to an alternating checker-board pattern stimulus were studied in two subjects over an intensity range of 7.5 log units. Under scotopic conditions, two wavelets (S1 and S2) could be recorded. At an intensity of 1.9 photopic log Td four high frequency oscillations (O1--O4) were noticed and a discontinuity was observed in the corresponding luminance curve of the b-wave together with a sudden decrease in the magnitude of the standard deviation of the amplitude measures. The oscillations were noticed only on the ascending slope of the b-wave. With increasing stimulus intensity, their latency decreased at a slower rate than that of the b-wave and their number decreased. Each wavelet had an amplitude maximum at a certain stimulus intensity level. It was suggested that O1--O4 were generated by the activation of the photopic system and that S1 and S2 were of scotopic origin.

Action Potentials↗
PubMed1977

[The D.C. response in the human electroretinogram under different degrees of wakefulness (author's transl)].

A report is given on the D.C. component of the human ERG following stimulation with red light of varying intensities and under different conditions of vigilance. The degree of the subject's wakefulness can be followed by looking at certain frequency components of the power spectrum of the occipital EEG. 1. The ERG remains uninfluenced by the degree of the subject's vigilance. 2. Red light is especially suited for eliciting separately photopic an scotopic responses in the on-effect of the ERG, possibly in the off-effect also. The D.C. potential is regarded as a component of the scotopic b-wave. The photopic x-wave can be recorded with very low stimulus intensities and stays clearly separated from the D.C. component.

Electroretinography↗
PubMed1975

Influence of glyceryl trinitrate on force of contraction and action potential of guinea-pig myocardium.

1. The inotropic effect of glyceryl trinitrate (GTN) was studied in guinea-pig papillary muscles and atrial strips by analysing the isometric contraction curve and the monophasic action potential (AP). 2. GTN, 7X10(-5)M in papillary muscles and at 1.4x10(-4)M in atrial strips. The maximum of the contractile force was reached in both preparations at 2x10(-4)M GTN. Positive inotropic effects were transitory (3--5 min) and were followed by marked negative inotropic effects. 3. In the presence of GTN, only 15 of 26 papillary muscles showed a positive inotropic response and there was a great variance in its intensity. Prior exposure of papillary muscles to a low GTN concentration, which by itself reduced force of contraction (like every single GTN application), was the prereqch by itself reduced force of contraction (like every single GTN application), was the prerequisite for the positive inotropic effect of a subsequent higher GTN concentration. In atrial strips the positive inotropic action was consistent and uniform. The maximum force of contraction in response to single applications of GTN was only about 50% of that in response to cumulatively increased GTN concentrations. 4. In the presence of 5x10(-4)M GTN, the tyramine concentration-effect curve was shifted to the left (by one log unit at the ED50 level). 5. Beta-Adrenoceptor blockade by(+/-)-propranolol (5x10(-6)M) or noradrenaline depletion by pretreatment of the animals with reserpine (5 mg/kg, 18--22 hrs prior to the experiment)prevented the positive inotropic effects of GTN in both preparations. Hence, the GTN-induced increase in contractile force is induced by the liberation of noradrenaline and an inhibitory effect on the monoamine oxidase (MAO) of sympathetic nerve endings might be involved. 6. In atrial preparations exposed to 5x10(-4)M GTN, time to peak force (tu) and relaxation time(t2) were shortened by 12% and 33%, respectively. Pretreatment of the animals with reserpine prevented the shortening og t1 and changed the shortening of t2 from 33% to 19%. 7. In papillary muscles, 5x10(-4) M GTN shortened t1 by 10%, while t2 was prolonged by 17% in noradrenaline-depleted, and by 36% in control muscles. Prolongation of t2 at 5x10(-4)M GTN was accompanied by an increase in the duration of the monophasic action patential (AP) in reserpine-pretreated as well as in control muscles by 12% and 26%, respectively (measured at 90% repolarization). The same GTN concentration slowed the maximum rate of depolarisiation by 32%. After 35 min the AP returned to approximately the control value. In the presence of 5x10(-4) M GTN, noradrenalin (1x10(-5)M) lengthened the AP by 38% in both, control muscles and noradrenaline-depleted preparations.

Action Potentials↗