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Biomedical subjects

J M Valeton

Publications and source records attributed to J M Valeton.

10 recordsLinked to original sources
PubMed1988

Effects of pictorial noise interfering with visual detection.

In order to evaluate the potential gain of noise-suppression techniques, visual detection of sine-wave gratings in images with additive fine- and coarse-grain pictorial noise was investigated. An important part of the threshold elevations caused by pictorial noise can be explained in terms of masking. The threshold elevation can be reduced by about a factor of 7 through intelligent image processing. In addition to masking by similar noise components, the detection of low-spatial-frequency target detail can also be hindered by high-spatial-frequency noise components. Correspondingly, the detection of lower spatial frequencies can be improved moderately by blurring.

Humans↗
PubMed1983

Light adaptation of primate cones: an analysis based on extracellular data.

Extracellular photo-cone responses were isolated in the intact rhesus monkey eye by fractional recording across the outersegment layer in the fovea with a bipolar microelectrode. The cone response vs intensity function was determined in the presence of adapting backgrounds up to 10(6) td. Increment and decrement responses, as well as the response to the steady backgrounds were recorded. All steady state and transient response data could be described with the equation V/Vm = In/(In + sigma n) with n = 0.74 and V and I representing total response and total incident light intensity. This invariant response function is shifted both along the intensity and the response axis with increasing background intensity. The decrease in sensitivity, corresponding to these shifts, could be attributed to cellular adaptation (sigma-adaptation), pigment bleaching and response compression. An analysis of the threshold vs intensity function shows how each of these mechanisms contributes to produce Weber behaviour.

Adaptation, Ocular↗
PubMed1983

Photoreceptor light adaptation models: an evaluation.

Two current descriptions of photoreceptor responses in the presence of adapting background illumination are evaluated. Both descriptions are based on the Michaelis-Menten equation V/Vm = In/(In + sigma n), but they use this equation in different ways: (1) to describe increment responses to light increments on a background (the "increment-light model") and (2) to describe the total receptor response as a function of total incident light intensity, i.e. increment plus background, (the "incident-light model"). It is shown that the incident-light model gives the best and most comprehensive description of photoreceptor responses. This conclusion is based on theoretical considerations and is supported with examples using rod and cone response data taken from the literature.

Adaptation, Ocular↗
PubMed1982

Fractional recording and component analysis of primate LERG: separation of photoreceptor and other retinal potentials.

The local electroretinogram, as obtained with a single micro electrode in the rhesus monkey fovea is known to be the sum of a number of components. This paper shows how the receptor component may be isolated from the LERG by using a bipolar micro electrode that records the potential across the outer segments of the foveal cones. A component analysis of the single electrode LERG, performed at a number of electrode depths in the retina, yields four components: a receptor-, b-, dc- and slow component. The latter has not been reported previously in the monkey and it is proposed that this is the slow PIII of the primate retina. The amplitude-depth profiles of the four components show why the receptor component may be isolated by fractional recording. The recording area of a bipolar electrode is 10 times smaller than that of a monopolar electrode. Contribution of rods to the responses is often present in the dark adapted foveal LERG but is absent in fractional records from the central fovea. The infusion of sodium aspartate in the eye abolishes the b-component, but does not affect the slow component. Aspartate produces, however, such further complexities that the method seems not suitable for isolation of the receptor potential in the intact primate eye.

Animals↗
PubMed1982

Intraretinal recordings of slow electrical responses to steady illumination in monkey: isolation of receptor responses and the origin of the light peak.

The onset of steady illumination of the mammalian retina initiates a series of complex electrical responses. Corneal recordings in man show a relatively fast ERG, followed by a much slower "light response": a "light peak" followed by a series of damped slow oscillations that may last for 1 or 2 hr. The latter oscillations are a prominent feature of the EOG. We recorded the response to steady illumination with micro electrodes in the intact rhesus monkey eye. The intraretinal "light response" was analyzed by simultaneous recordings at different depths, using a bipolar microelectrode. We found that a steady photoreceptor component can be isolated by fractional recording across the cone outer segment layer in the fovea. Further, through simultaneous recordings at the retinal and choroidal sides of the retinal pigment epithelium, we found that this structure is most probably the generating site of the "light peak" and subsequent oscillations of the standing potential.

Animals↗
PubMed1981

Two types of bipolar microelectrodes for intraretinal use.

This paper describes in some detail the construction of two types of bipolar microelectrodes that were used in intraretinal recordings from the intact rhesus monkey eye. The first type consists of a glass micropipette with a partly insulated silver paint coating, yielding a recording site located 120 micrometers from the tip of the glass pipette. The second type consists of two coaxial glass micropipettes with a tip distance of 50 micrometers and tip diameter of 10 and 1 micrometer, respectively. The coaxial glass electrode has a better electrical stability than the silver paint electrode and is mechanically stronger. An example of a recording is shown.

Animals↗
PubMed1981

Compensation for cross-talk and high frequency attenuation of bipolar microelectrodes.

Bipolar glass microelectrodes have a capacitance between the two recording channels and each channel has a capacitance to ground. These capacitances cause respectively cross-talk and high frequency attenuation. A new and general method for compensation of both distortions is developed. In this "inverse filter' method, the compensation circuit is derived from the equivalent circuit of the electrode. The method is applicable to all kinds of bi-and multipolar electrodes. The performance of the new method is compared with the existing "feedback' method (Freygang and Frank, 1959; Tomita, 1962) for glass microelectrodes. Detailed analysis shows that methods perform well for frequencies below 500 Hz, but the inverse filter method has good characteristics to at least 5 kHz. The poor high frequency behaviour of the feedback method is caused by preamplifier phase lag.

Electrochemistry↗