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W Skrandies

Publications and source records attributed to W Skrandies.

At least 19 recordsLinked to original sources
PubMed1992

Differential effects of mild hypoglycemia on proximal and distal retinal structures in man as revealed by electroretinography.

We investigated the effect of blood glucose concentration on the human visual system. Blood sugar levels were modified by intravenous infusion with insulin or glucose. Luminance electroretinograms (L-ERG) were elicited with colored flashes, and pattern ERGs (P-ERG) evoked by checkerboard reversal stimuli were obtained simultaneously with cortical visually evoked potential (VEP). With glucose levels above or below the baseline values the b-wave of the L-ERG was significantly larger than with normal glucose levels, whereas the P-ERG displayed maximal amplitudes at normal glucose levels and a significant amplitude decrease with glucose levels below or above normal. Since L-ERG and P-ERG activity is generated by different retinal structures, this indicates that distal and proximal layers of the human retina are affected differently by changing glucose levels probably due to regional differences in retinal circulation.

Adult↗
PubMed1991

Contrast and stereoscopic visual stimuli yield lateralized scalp potential fields associated with different neural generators.

The use of dynamic random-dot stereograms (RDS) allows to investigate evoked potential components generated exclusively by cortical structures. We analyzed the scalp distribution of stereoscopically evoked or contrast evoked potential field by recording electrical brain activity in 20 channels simultaneously from an electrode array covering the occipital scalp areas. Evoked brain activity was obtained from 13 healthy adults with dynamic RDS stimuli presented as a stereoscopic checkerboard pattern in the center, or in the right or left visual half-field. Such stereoscopically evoked scalp potential distributions were compared to those elicited by a conventional 2-dimensional checkerboard reversal stimulus of the same mean luminance and retinal extent. We found that the latencies of the major evoked components were similar for contrast and stereoscopic stimuli, while significant differences were observed when we compared the strength of the evoked potential fields or the topographical pattern elicited by lateralized stereoscopic and contrast stimuli. The functional relation of evoked electrical brain activity to the retinal stimulus location was significantly different for stereoscopic and contrast stimuli. We present evidence that stereoscopic perception relies on the activation of cortical structures in the human visual system that are different from those activated by comparable contrast stimuli, supporting the conclusions derived from our earlier electrophysiological experiments on stereoscopic vision. These data on the physiological correlates of processing of stereoscopic information in humans are in line with the results obtained with single neuron recordings from the cat and monkey visual cortex.

Adolescent↗
PubMed1991

[Retinal and cortical electrical activity in man: physiologic bases and clinical applications].

Noninvasive electrophysiological recordings allow to investigate various stages of human visual information processing separately. Luminance and pattern electroretinograms (ERG) contain components originating from anatomically and physiologically different retinal structures while visual evoked potentials (VEP) reflect activity in more central parts of the visual system. Simultaneous recordings of ERGs and VEPs to carefully selected visual stimuli help to relate physiological processes to neural structures, and thus may be employed as an important tool also in clinical investigations. Corresponding experimental data from both basic physiological research as well as from clinical studies are reviewed.

Electroretinography↗
PubMed1990

Global field power and topographic similarity.

Multichannel recordings are commonly presented as topographic maps series displaying the change of the potential distribution over time. When reviewing a sequence of potential maps it becomes obvious that there are epochs with only little activity (few field lines; small extrema values) while at other times the fields display high peaks and deep troughs with steep gradients. The measure of global field power (GFP) corresponds to the spatial standard deviation, and it quantifies the amount of activity at each time point in the field considering the data from all recording electrodes simultaneously resulting in a reference-independent descriptor of the potential field. Global field power is plotted as a function of time, and the occurrence times of GFP maxima are used to determine the latencies of evoked potential components. The topographical change occurring in subsequent potential field distributions may also be quantified by computing an index of global dissimilarity. Global field power and global dissimilarity show a complementary behavior over time: in general, high GFP is associated with similar fields while during periods between GFP peaks the topographic patterns of successive field distributions change rapidly accompanied by high dissimilarity values. The topographic changes, however, are best recognized by a segmentation procedure that considers field structure independent of GFP and global dissimilarity. The principles and practical applications of GFP computation, component latency determination and global dissimilarity of potential field distributions as well as a topographical time segmentation procedure will be illustrated with multichannel data evoked by visual stimuli.

Brain↗
PubMed1989

Cortical and retinal refractory periods in the human visual system.

Refractory periods of the visual system were investigated in 12 healthy subjects by simultaneously recording retinal (ERG) and cortical (VEP) evoked electrical activity. Double-flash stimuli were presented at different interstimulus intervals, and response components evoked by the second flash were analyzed in detail, and related to psychophysical detection thresholds. With short interstimulus intervals ERG b-wave peak latencies were increased and b-wave amplitudes were significantly reduced, while P100 component latencies of the VEP were significantly influenced only at long interstimulus intervals. Regression analysis of the individual data as well as analysis of the retinocortical transmission times showed that the cortical latency changes were not simply caused by changes on the level of the retina. Additional influences of the interstimulus interval on nonretinal structures of the human visual system must be assumed. The subjective psychophysical detection thresholds were significantly higher than the threshold values at which reliable electrical or cortical response components could be elicited.

Adult↗
PubMed1988

Dopamine and serotonin in cat retina: electroretinography and histology.

Anatomical structures of the cat retina were related to functional changes induced by the application of dopaminergic and serotonergic substances. We report on the contribution of dopaminergic and serotonin accumulating retinal neurones to retinal activity as reflected by the electroretinogram. The effect of dopaminergic neurones was investigated by the application of the neurotoxin 6-hydroxy-dopamine (6-OHDA) which is known to destroy dopaminergic neurones, and injections of either serotonin (5-HT) or the analogue 5,6-dihydroxy-tryptamine (5,6-DHT) were used to monitor the effects of indoleamines. In control experiments aminophosphonobutyric acid (APB), an agonist of glutamate transmission, was injected. Conventional immunohistochemical methods identified dopaminergic and serotonin accumulating neurones, and the electrophysiological data obtained from the same animals were related to the anatomical structures influenced by the respective substances. Destruction of dopaminergic amacrine cells by 6-OHDA increased the ERG b-wave amplitude. Accumulation of indoleamines by certain amacrine cells also caused an increase of the ERG b-wave. However, intra-vitreal injection of APB completely blocked the b-wave. The data show that ERG mass responses can be used to monitor transmitter-specific effects on retinal circuitry.

5,6-Dihydroxytryptamine↗
PubMed1988

Time range analysis of evoked potential fields.

Potentials evoked by contrast reversing grating stimuli of different spatial frequency and orientation were recorded in 16 channels from twelve healthy adults. The amount of electrical brain activity was quantified independent of the reference electrode by the computation of global field power (GFP). Maxima of the GFP function over times, determined component latencies which turned out to be influenced by spatial frequency and orientation. Both effects were statistically significant. Analysis of GFP at component latencies demonstrated the significant influence of spatial frequency on the amount of activity in the potential fields, whereas different stimulus orientations yielded brain activity of similar strength. Component location on the scalp determined at P100 latency showed no systematic variation with spatial frequency or orientation of the grating pattern. A reference-free topographic segmentation procedure based on the statistical recognition of stable potential field configurations disregarding amplitude characteristics, was used for all subjects and stimulus conditions. Segments were identified over the whole recording epoch and were interpreted as time epochs with identical potential field configurations, that were compatible with identical neuronal generators. The sequence of such segments was further analysed and compared statistically between subjects and stimulus conditions. The results showed topographical differences that were not observed when only selected time points at component latency were considered.

Adult↗
PubMed1988

Pattern ERGs and VEP topography evoked by lateral eccentric pattern reversal stimulation.

In ten healthy subjects we recorded simultaneously pattern-ERGs and multichannel VEPs evoked by checkerboard reversal stimuli presented in the center, and at three different locations on the nasal and temporal retina. In the near periphery significantly larger pattern-ERG amplitudes were found for the nasal retina. Farther out in the periphery this effect reversed, and stimuli presented to the temporal retina yielded larger pattern-ERG amplitudes. Component latencies showed also differences with larger latencies for nasal than temporal retinal stimuli in the near periphery. In addition, pattern-ERG amplitudes increased with check size while peak latencies decreased. Retinal stimulus location had a significant influence on the VEP topography reflected by potential distributions lateralized over occipital scalp areas contralateral to the hemiretina stimulated. There were no differences between VEP activity evoked by nasal or temporal retinal stimuli. The results relate to differences in local retinal organization reflected by the distribution of photoreceptors and other neural elements across the retina as well as to functional differences in visual information processing between different retinal areas.

Adult↗
PubMed1987

Alteration of visual evoked potentials and electroretinograms in Parkinson's disease.

A group of 24 patients with Parkinson's disease (PD) with normal fundi and normal visual acuities was examined electrophysiologically. Checkerboard reversal VEPs and ERGs (P-ERGs) at various contrast levels as well as photopic and scotopic luminance ERGs were recorded and compared with an age-matched group of controls. Earlier reported latency increases of the VEPs of the patients were confirmed for patterns of high contrast only. Scotopic and photopic luminance ERGs of the patients showed normal latencies, but at all light intensities the amplitudes of the scotopic and photopic b wave, as well as the amplitudes of the photopic a waves, were significantly reduced, P-ERG amplitudes were reduced at 50% contrast. Identical results were obtained in patients under dopaminergic treatment (n = 17) and in patients who did not receive any treatment (n = 7). These results suggest that alterations occur already at the retinal level where dopamine receptors have been found. Thus the reported changes of the VEP are not caused by the visual cortex alone.

Adult↗
PubMed1987

Visual persistence of stereoscopic stimuli: electric brain activity without perceptual correlate.

Dynamic random-dot stereograms (RDS) were used to study cortical neuronal mechanisms related to visual persistence in nine subjects. Electric brain potential components evoked by stereoscopic stimuli were compared to those evoked by conventional checkerboard stimuli with contrast borders. In a pattern-onset/offset presentation visual persistence thresholds were significantly lower for stereoscopic than for contrast stimuli: with temporally modulated patterns stereoscopic stimuli appeared to visually persist at much shorter interstimulus intervals than contrast stimuli. With stereoscopic stimuli all subjects reported not seeing changes of the RDS pattern, while the corresponding evoked potentials showed components related to the stimulus changes indicating a consistent discrepancy between psychophysical and electrophysiological data. The electrical brain activity was not caused by vergence eye movements elicited by the stereoscopic stimulus. In addition, for dynamic RDS stimuli a significant inverse linear relationship between temporal modulation frequency and evoked potential amplitude was found which was not observed with comparable contrast stimuli.

Adult↗
PubMed1986

The standing potential of the human eye reflects differences between upper and lower retinal areas.

In 12 healthy subjects the "light peak" of the electrooculogram was measured following localized stimulation of various retinal locations. Significant differences in "light peak" amplitudes were found between central and peripheral stimulation, and at 10 deg eccentricity the "light peak" amplitudes were significantly larger following upper retinal stimulation than those elicited by lower retinal stimuli. In addition, the "light peak" amplitude produced by upper or lower retinal stimulation behaved differently when test light intensity increased. The upper retinal areas showed consistently a higher sensitivity to light intensity changes than the lower retinal areas. The "light peak" of the EOG is believed to index the rate of retinal metabolism elicited by light stimuli. Our findings show that upper retinal areas display a higher level of light-induced activity reflecting the interaction between the photoreceptors and the retinal pigment epithelium than lower retinal areas. The results are interpreted as a superiority of the upper over the lower retina and are related to other electrophysiological and functional differences between upper and lower retinal areas of man.

Action Potentials↗
PubMed1986

Alterations of visual contrast sensitivity in Parkinson's disease.

Contrast sensitivity functions were determined in a population of 18 patients suffering from Parkinson's disease, and compared with the data obtained in an age-matched group of healthy controls. The controls were more sensitive at all spatial frequencies tested than the patients. The statistical comparisons were highly significant, indicating general differences between the PD patients and the controls not related to individual spatial frequency channels. When comparing the sensitivity loss between low and high spatial frequencies no significant differences were found suggesting that the decrease in contrast sensitivity is a global effect. We controlled for effects of age and cerebral atrophy, and our findings cannot be accounted for by these factors. In addition, the amount of contrast sensitivity loss was not correlated with the severity of the disease. These global functional alterations appear to be related to the reduction of dopamine at various sites of the visual system.

Adult↗
PubMed1985

Stereoscopic stimuli activate different cortical neurones in man: electrophysiological evidence.

Dynamic random dot stimuli offer the possibility to study cortical neuronal mechanisms related to depth perception in man because these stimuli operationally skip all stages prior to the activation of cortical binocular disparity neurones. In the present paper, the scalp topography of brain potential components evoked by stereoscopic stimuli are compared with those evoked by similar binocular contrast stimuli, and our results present evidence that different neuronal populations of the visual cortex are activated in these two stimulus conditions.

Adult↗
PubMed1985

[Stereovision in random dot pattern VECP: normal findings and clinical use].

Cortical potentials evoked by dynamic random dot stereograms were recorded in 15 normal subjects and 19 patients with impaired stereoscopic vision. The potentials obtained were compaired with those evoked by similarly arranged checkerboard contrast stimuli, binocularly presented. The two modes of stimulation yielded potentials of similar component latencies but the stereoscopically evoked component amplitudes were significantly lower. In nine-channel recordings the stereoscopically evoked potentials were found to be located farther anterior topographically, suggesting a different neural generating mechanism. Patients with impaired stereoscopic vision had lower amplitudes and longer peak latencies than normal subjects. In addition, larger disparities had to be used to elicit a response in these patients. There was a high degree of correlation between the disparity thresholds determined by evoked cortical potentials and those obtained by sensory measurements.

Depth Perception↗
PubMed1985

Critical flicker fusion and double flash discrimination in different parts of the visual field.

The temporal sensitivity of the visual system was investigated in fifteen healthy subjects by determining the critical flicker fusion frequency (CFF) and the performance in a double flash discrimination (DFD) task. Measurements were obtained using a small test light of 2.5 degrees diameter in the fovea and at eight different perifoveal retinal locations along the horizontal and vertical meridians. Statistically significant differences were found for both the CFF and the DFD task showing a higher temporal sensitivity of the nasal and upper than the temporal and lower retinal areas. The functional significance of the findings is discussed and related to electrophysiological and behavioral data which describe a functional superiority of the upper over the lower hemiretinal system.

Adult↗
PubMed1985

Human contrast sensitivity: regional retinal differences.

Contrast sensitivity functions of foveal and of perifoveal upper and lower hemiretinal regions were measured in a population of twenty subjects. Foveal stimuli yielded consistently higher contrast sensitivities as well as a shift of the maximal sensitivity towards higher spatial frequencies as compared to perifoveal stimuli. The upper hemiretinal area was more sensitive at all spatial frequencies tested than the corresponding lower hemiretinal area. The statistical comparisons were highly significant, indicating not only regional retinal differences in visual acuity as reflected by the different contrast threshold levels at the highest spatial frequency, but also global differences between the upper and lower hemiretina systems not restricted to certain spatial frequency channels.

Adult↗
PubMed1979

Multichannel evoked potential fields show different properties of human upper and lower hemiretina systems.

Scalp potential fields in human subjects were evoked by checkerboard reversals to the upper and lower hermiretinae, using 1.6 and 3.2 reversals/s. Averaged fields were sampled along a saggital midline row of electrodes (field profiles) in 20 subjects, and from a 47 electrode array (field maps) in five subjects. In five subjects, profile peaks and troughs between 84 and 128 ms latency resided within the recorded electrode row surrounded by lesser potential values, and thus met the evaluation criteria. Response latency defined as maximal voltage difference between two electrodes within the profiles was significantly shorter (medians 12 and 11 ms, respectively) for upper than for lower hemiretina stimuli at both frequencies. There was a significant difference between latencies to 3.2 and 1.6 stimuli/s in the upper but not in the lower hemiretina system, suggesting different system behavior of the two retinal halves. Pertinent anatomical, electrophysiological, and behavioral data are reviewed.

Adult↗