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W Skrandies

Publications and source records attributed to W Skrandies.

At least 37 records · Page 2Linked to original sources
PubMed1997

Local luminance and pattern reversal stimuli yield different visual evoked potential topography.

We studied how the stimulation of quadrants of the visual field affect brain potential topography, and we compared activity elicited by conventional pattern reversal or by local luminance stimuli. The method of quasi-simultaneous stimulation of many small visual field elements by binary m-sequences allowed us to reconstruct the potentials evoked at each of 54 visual field locations independently. Data from all field elements within each quadrant and in the whole stimulation field were summed and compared to those elicited by checkerboard reversal stimuli presented in the four quadrants or as full field stimuli. In twenty-two healthy adults evoked brain activity was recorded in 30 channels with an electrode array densely spaced over the occipital brain areas. With local flash stimuli as well as with checkerboard reversal the topographical distributions of cortical activation changed significantly with retinal stimulus location. Analysis of three components occurring between 50 and 240 ms revealed significant differences between pattern reversal and local luminance evoked brain activity. Reversal stimuli yielded not only larger amplitudes but also a completely different component structure and topography. Our results illustrate that different neuronal generators are activated by pattern reversal and local luminance stimuli although visual field location of the stimuli was identical indicating that the same retinal and cortical structures respond in a different way depending on stimulation mode.

Adult↗
PubMed1996

Instantaneous frequency maps, dipole models and potential distributions of pattern reversal-evoked potential fields for correct recognition of stimulated hemiretinae.

Lateral hemifield pattern-reversal visual evoked potential (PVEP) field data were evaluated using potential distributions, dipole modelling and distributions of Hilbert transformation-based instantaneous frequency in order to determine the stimulated hemisphere. Twenty channel records were collected from 35 normal volunteers in two laboratories using similar stimulus conditions (11-20.5 degrees target, 60-75 min checks, 2/s reversal, 500 ms analysis epoch). P100 latency was determined in each average by the global field power maximum between 90 and 120 ms. Using the data from O1 and O2 at P100 latency, the stimulated hemisphere was identified by maximal potential or minimal instantaneous frequency on the stimulus-contralateral side, or, using the 20-electrodes data at P100 by the ipsilateral lateralization of the dipole model. Correct classification of the stimulated 70 hemiretinae was achieved by potential distribution in 44 cases, by dipole modelling in 54 cases and by instantaneous frequencies in 68 cases. Errors in the classification by potential distribution and dipole location were twice as frequent for decisions based on expected locations over the left than over the right hemisphere. This finding might be caused by the relatively larger size of the left occipital lobe. We conclude that a single value of instantaneous frequency which implies a massive data reduction can serve as a robust parameter for the characterization of the input conditions of hemifield PVEP (i.e. the stimulated hemiretina). It is more successful than potential distribution or dipole modelling, probably because instantaneous frequency incorporates considerably more information than the other two measures. It is suggested to explore instantaneous frequency as a parameter to recognize small retinal area stimuli in perimetry studies.

Adult↗
PubMed1996

Sensory thresholds and neurophysiological correlates of human perceptual learning.

The repeated presentation of visual targets yields significant improvements of psychophysical thresholds, and in the present paper corresponding alterations in neurophysiological activity of the human brain are reported. A group of 23 healthy adults was investigated in a psychophysical experiment in which hyperacuity thresholds were determined repeatedly for up to about 25 min with vertically and horizontally oriented targets. Evoked brain activity was recorded during the same experimental session from 30 electrodes over the occipital brain areas. For each subject mean potentials were computed offline for two conditions corresponding to "before learning' (i.e. the first half of the experiment) and "after learning' (i.e. the second half of the experiment), and visual evoked brain activity was compared between these conditions. In the psychophysical experiments vernier thresholds decreased significantly with time, and this improvement in performance could be confirmed in a control session after an interval of 9 weeks. All changes in discrimination performance were specifically bound to stimulus orientation. Learning effects were also observed in electrical brain activity, which displayed significantly smaller component latencies and larger amplitudes as a function of training time. In addition, learning affected the topography of the evoked potential fields suggesting that cortical neurons are activated in a different way before and after perceptual learning.

Adolescent↗
PubMed1995

Visual information processing: topography of brain electrical activity.

Multichannel recordings allow the non-invasive assessment of the electrical fields of the brain. Topographical analysis of EEG and EPs should not be restricted to the qualitative graphical display of maps at many time points instead of time series at many recording points. It is mandatory that quantitative methods are applied to EEG and evoked potential data in order to extract relevant information from such series of maps. For spontaneous EEG data the spatial analysis of frequency bands is of major interest while evoked potential fields are analyzed in terms of latencies and amplitudes of individual components. The computation of global field power, global dissimilarity as well as the location of potential maxima, minima or centroids enable a meaningful data reduction and the statistical evaluation of the effects of different experimental conditions and the comparison of various clinical populations. In a similar way multivariate statistical techniques may be employed in order to detect characteristic potential field configurations. Topographical methods to analyze brain electrical activity are illustrated with multichannel data evoked by localized visual stimuli and by stereoscopic stimulation as well as with evoked potential data obtained during perceptual learning experiments.

Brain↗
PubMed1995

The perception of temporal structure and auditory evoked brain activity.

The processing and perception of auditory signals depends on the temporal structure of stimulus characteristics. We studied 26 healthy subjects who participated in psychophysical experiments and in electrophysiological recordings of auditory evoked potentials from C2, C3, C4, T3 and T4. Stimuli consisted of tone series presented binaurally as tones or gaps with a base duration of 100 ms. In the psychophysical experiments, difference thresholds as indicators of temporal discrimination performance were significantly lower for tones than for gaps. In the electrophysiological recordings, gaps often failed to elicit N100 components. Tones produced shortest component latencies with largest amplitudes. In addition, brain activity was strongest at C2, and showed a symmetrical fall-off over both hemispheres. N100 components had significantly longer latencies and smaller amplitudes when they were evoked by the end of the gap (i.e. with the continuation of the tone) than by tones. Our data illustrate how the temporal structure of auditory stimuli affects neuronal responses of the brain. Similar effects were observed in psychophysical and electrophysiological experiments, and we were able to demonstrate a direct relationship between subjective sensory thresholds and auditory evoked brain activity.

Adult↗
PubMed1994

Neurophysiological correlates of perceptual learning in the human brain.

Rapid learning processes are crucial for human object recognition. We report here on the alterations in neurophysiological activity in the human brain induced by repeated presentation of visual stimuli. In psychophysical experiments the percentage of correct responses increased significantly within less than 30 minutes in untrained observers. This stimulus-specific improvement was not carried over to differently oriented stimuli. Similar learning effects were observed in component latencies of evoked potential field distributions. The occurrence of specific potential field configurations reflected perceptual learning. A spatio-temporal activation pattern with steep gradients over the primary visual cortex appeared to be correlated with plasticity in the human visual system.

Adult↗
PubMed1994

An electrophysiological correlate of learning in motion perception.

We investigated learning in a motion-detection task using both psychophysical and neurophysiological methods in normal humans. A total of 20 naive observers had to discriminate between a small motion to the left versus to the right (jump displacement) or between a motion upward versus downward. Their performance improved significantly within less than 30 min in discriminating between directions in the psychophysical jump-displacement task. The improvement of performance with practice was very specific and did not transfer to the same stimulus rotated by 90 degrees. After training for the same task, multichannel evoked-potential recordings changed significantly in component latency and in the distribution of field potentials. This indicates that neuronal ensembles rather than single cells are involved in perceptual learning. Significant differences between the potential distributions occur for potentials at latencies of less than 100 ms over the occipital pole, suggesting an involvement of and plasticity in the primary visual cortex of human adults.

Adult↗
PubMed1993

Monocular and binocular neuronal activity in human visual cortex revealed by electrical brain activity mapping.

In the present study, we investigated topographical differences between monocularly and binocularly evoked potential fields related to the retinal location and spatial frequency of grating stimuli. Electrical brain activity was recorded in 18 healthy adults using an array of 21 electrodes over the occipital areas. Vertical black-and-white grating patterns of different spatial frequencies were presented with central fixation or lateralized to the left or right hemiretina. Computation of global field power determined component latency. Topographic characteristics of the field distributions were examined at the individual component latency for each subject using statistical comparisons between experimental conditions. The strength of the potential fields was significantly larger with binocular stimuli, whereas no effects were observed when comparing component latencies. Pronounced differences occurred in the spatial distribution of electrical brain activity: with 2.5 cycles/deg, large, significant topographic differences between monocularly and binocularly evoked activity were obtained. The potential fields showed a more anterior and more lateralized component distribution with binocular than monocular stimuli. In addition, when the gratings were presented binocularly, significant topographic differences were observed when low and high spatial frequency stimuli were compared. Our results suggest that the relationship between the topography of evoked components and retinal stimulus location and spatial frequency is different for monocular and binocular stimuli, indicating that binocular information processing triggers different neuronal processes in the human visual cortex.

Adult↗
PubMed1993

EEG/EP: new techniques.

The topographic analysis of electrical brain activity consists of the extraction of quantitative features which adequately describe the scalp recorded electrical fields of the brain. In the beginning of brain electrical activity mapping most methods centered mainly around the graphical display of multichannel EEG and evoked potential data. Meanwhile quantitative analysis strategies have been developed, and such methods are applied to topographic EEG and evoked potential data enabling the statistical evaluation of the effects of different experimental conditions as well as the comparison of various clinical populations. Major new analysis techniques comprise the computation of global field power and global dissimilarity for determination of components of evoked potential fields, the segmentation of map series by topographical features, time range analysis, FFT approximation for the spatial analysis of EEG frequency bands as well as correlation analysis and spatial principal components analysis (Spatial PCA). Data from experiments dealing with evoked brain activity will illustrate the application of these quantitative methods that also can be used for the analysis of the spontaneous EEG.

Brain↗
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↗