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Biomedical subjects

R Srebro

Publications and source records attributed to R Srebro.

48 records · Page 3Linked to original sources
PubMed1980

Visually evoked potentials to pseudorandom binary sequence stimulation. Preliminary clinical trails.

We report results using a new stimulus for clinical testing of visually evoked potentials (VEPs). The stimulus was the modulation of the luminance of a large unpatterned field by a temporal pseudorandom binary sequence. The stimulus was similar to Gaussian white noise in that a large number of sinusoidal harmonics were presented concurrently. It has the important advantage that conventional signal averaging techniques can be used to analyze the VEP. Abnormal VEP responses were obtained when the stimulus was applied to patients with minimal macular and optic nerve disease.

Evoked Potentials↗
PubMed1978

The visually evoked response. Binocular facilitation and failure when binocular vision is disturbed.

The visually evoked response (VER) caused by the horizontal sinusoidal movement of a display consisting of a checkerboard with 15-minute checks of relatively low contrast oscillating at 6.3 Hz is itself sinusoidal with a frequency of 12.6 Hz. When viewed binocularly, the VER is 25% to 30% greater in amplitude than the sum of the amplitudes for monocular viewing. This binocular faciltation may be a VER correlate of normal binocular single vision. It is lost in small-angle esotropes and in normals whose binocular function is disturbed by a vertical prism placed over one eye. Characteristic curves relating VER amplitude and phase angle to frequency to oscillation of the checker-board display suggest that two "systems" carry information to the visual cortex: a long and a short latency system. In amblyopia, the long latency system may be selectively impaired.

Adult↗
PubMed1976

Calcium and the control of discrete wave latency in the ventral photoreceptor of Limulus.

1. Discrete, transient depolarization (discrete waves) of the ventral photoreceptor of the horseshoe crab, Limulus, occur spontaneously in the dark adapted photoreceptor and are also evoked by light. They form the basic events which comprise the receptor potential. A brief, low energy flash of light evokes variable numbers of discrete waves which have variable latencies. Evidence suggesting that discrete wave latency reflects the kinetics of the chemical reactions of phototransduction is reviewed. 2. The concentration of extracellular Ca influences both the average discrete wave latency and its variability. Lowering extracellular Ca prolongs the latency and increases its variability. Increasing extracellular Ca has the opposite effect. 3. Changes in discrete wave latency caused by changes in extracellular Ca require 10--15 min to become fully manifest, whereas when the concentration of extracellular K is increased the photoreceptor achieves a steady-state depolarization in 10-15 sec. 4. Iontophoresis of the Ca-chelating agent EGTA into the photoreceptor increases both the average discrete wave latency and its variability. Iontophoresis of Ca-EGTA mixtures may either increase or decrease discrete wave latency and its variability depending upon the proportion of Ca mixed with EGTA. 5. It is suggested that the concentration of intracellular rather than extracellular ionized Ca is the prime factor indicating discrete wave latency. The effects of changing extracellular Ca can be explained if the photoreceptor is permeable to Ca in the dark and if it maintains a low intracellular Ca concentration by virtue of active metabolic processes (a pump-leak system). 6. Lowering the temperature of the photoreceptor also has the dual effect of increasing discrete wave latency and its variability. However, effects of lowering temperature and Ca simultaneously are greater than the sum of the two effects in individually. This suggests that Ca may be a reactant in the chemical process of phototransduction. 7. Changing the concentration of extracellular Ca does not change the quantum efficiency of discrete wave production. A previous study showed that quantum efficiency is not changed by temperature. Thus, once initiated by the absorption of light, the reactions that subserve phototransduction may be forced to completion. Ca probably exerts its influence by changing one or more rate constants in the reaction sequence.

Action Potentials↗
PubMed1974

Light adaptation in the ventral photoreceptor of Limulus.

Light adaptation in both the ventral photoreceptor and the lateral eye photoreceptor is a complex process consisting of at least two phases. One phase, which we call the rapid phase of adaptation, occurs whenever there is temporal overlap of the discrete waves that compose a light response. The recovery from the rapid phase of adaptation follows an exponential time-course with a time constant of approximately 75 ms at 21 degrees C. The rapid phase of adaptation occurs at light intensities barely above discrete wave threshold as well as at substantially higher light intensities with the same recovery time-course at all intensities. It occurs in voltage-clamped and unclamped photoreceptors. The kinetics of the rapid phase of adaptation is closely correlated to the photocurrent which appears to initiate it after a short delay. The rapid phase of adaptation is probably identical to what is called the "adapting bump" process. At light intensities greater than about 10 times discrete wave threshold another phase of light adaptation occurs. It develops slowly over a period of (1/2) s or so, and decays even more slowly over a period of several seconds. It is graded with light intensity and occurs in both voltage-clamped and unclamped photoreceptors. We call this the slow phase of light adaptation.

Adaptation, Ocular↗
PubMed1974

Discrete waves and phototransduction in voltage-clamped ventral photoreceptors.

Discrete waves in the voltage-clamped photoreceptor of Limulus are remarkably similar in all essential properties to those found in an unclamped cell. The latency distribution of discrete waves is not affected by considerable changes in the holding potential in a voltage-clamped cell. Both large and small waves occur in voltage-clamped and unclamped cells and in approximately the same proportion. Large and small waves also share the same latency distributions and spectral sensitivity. We suggest that small waves may result from the activation of damaged membrane areas. Large waves have an average amplitude of approximately 5 nA in voltage-clamped photoreceptors. It probably requires several square microns of cell membrane to support this much photo-current. Thus the amplification inherent in the discrete wave process may involve spatial spread of activation from unimolecular dimensions to several square microns of cell membrane surface. Neither local current flow, nor pre-packaging of any transmitter substance appears to be involved in the amplification process. The possible mechanisms of the amplification are evaluated with relationship to the properties of discrete waves.

Animals↗
PubMed1972

Light adaptation of discrete waves in the Limulus photoreceptor.

Light adaptation affects discrete waves in two ways. It reduces their average size and decreases the probability that a photon incident at the cornea causes a discrete wave. There is no effect of light adaptation on the latency of discrete waves, or on their time-course.

Adaptation, Ocular↗
PubMed1972

The thermal origin of spontaneous activity in the Limulus photoreceptor.

1. Discrete depolarizations of the photoreceptor cell membrane called discrete waves occur spontaneously and in response to illumination in the eye of the horseshoe crab, Limulus. Each light induced discrete wave is caused by the absorption of a single photon.2. The frequencies of spontaneous and light induced discrete waves were studied at different temperatures from 0 to 25 degrees C using a new method of counting them to avoid errors due to their temporal overlap.3. The frequency of spontaneous discrete waves followed the Arrhenius relationship with activation energy equal to 48.6 kcal.4. The frequency of the discrete waves caused by a fixed level of steady illumination was not significantly changed when the temperature of the cell was changed.5. The relationship of the frequency of spontaneous discrete waves to temperature was compared to a prediction based on the relationship of the quantum relative spectral sensitivity of the Limulus eye to the temperature of the eye. The prediction was in good agreement with observation and suggests that spontaneous discrete waves result from thermally induced cis to trans isomerizations of visual pigment molecules.

Adaptation, Ocular↗
PubMed1971

A stochastic model for discrete waves in the Limulus photoreceptor.

A stochastic model that links the absorption of a photon to the production of a discrete wave in the photoreceptor of the lateral eye of Limulus is proposed. By separating a discrete wave into an initial component due directly to the absorption of a photon, and a second quasi all-or-nothing component, a mathematical description of the latencies of discrete waves is deduced and some important features of their time courses are suggested. The predictions of the model are compared to observations from 60 different ommatidia.

Animals↗
PubMed1970

Stochastic properties of discrete waves of the limulus photoreceptor.

In the dark-adapted photoreceptor of the horseshoe crab, Limulus, transient discrete depolarizations of the cell membrane, discrete waves, occur in total darkness and their rate of occurrence is increased by illumination. The individual latencies of the discrete waves evoked by a light stimulus often cannot be resolved because the discrete waves overlap in time. The latency of the first discrete wave that follows a stimulus can be determined with reasonable accuracy. We propose a model which allows us to make an estimate of the distribution of the latencies of the individual light-evoked discrete waves, and to predict the latency distribution of the first discrete wave that follows a stimulus of arbitrary intensity-time course from the latency distribution of the first discrete wave that follows a brief flash of light. For low intensity stimuli, the predictions agree well with the observations. We define a response as the occurrence of one or more discrete waves following a stimulus. The distribution of the peak amplitudes of responses suggests that the peak amplitude of individual discrete waves sometimes has a bimodal distribution. The latencies of the two types of discrete waves, however, follow similar distributions. The area under the voltage-time curve of responses that follow equal energy long (1.25 sec) and short (10 msec) light stimuli follows similar distributions, and this suggests that discrete waves summate linearly.

Animals↗
PubMed1966

A thermal component of excitation in the lateral eye of Limulus.

1. The temperature dependence of the relative spectral sensitivity of the excised lateral eye of Limulus was examined using its electrical response to light.2. At wave-lengths longer than 625 mmu lowering the temperature from 27 to 7 degrees C reduced the relative spectral sensitivity, while no effect was measurable at shorter wave-lengths.3. The reduction in relative sensitivity increased linearly with decreasing wave number.4. The observations support the hypothesis that a critical amount of energy (activation energy) must be supplied to the photopigment molecule in order that it excite the photoreceptor. Quanta with energy lower than the activation energy are effective only if the thermal energy of the molecule can supply the deficit.5. The lower limit of the activation energy for the photoreceptor of the lateral eye of Limulus determined from the results of this study is 44 kcal mole(-1).

Animals↗