Biomedical subjects

W Skrandies

Publications and source records attributed to W Skrandies.

At least 19 recordsLinked to original sources

PubMed2003

Topography of spectral EEG and late VEP components in patients with benign rolandic epilepsy of childhood.

We studied 11 patients (7-15 years of age) with benign rolandic epilepsy of childhood. Spontaneous EEG and flash evoked potentials were recorded from 19 channels. Following visual stimulation, five of the patients displayed a late sustained component over temporal and central scalp regions (mean latency: 194.8 ms). The occurrence of this component was related to the spectra of spontaneous EEG and clinical parameters. Significant topographical differences were observed between the EEG amplitude spectra of patients with and without a N200 component. The effects were most evident in the lower alpha (7.5-9.5 Hz) and beta band (13-15 Hz) when temporal, parietal and occipital regions of the left and right hemisphere were compared. There was also a tendency that children without N200 displayed more hypersynchroneous activity in the spontaneous EEG than children with N200. This suggests a protective role of focal interictal spikes, and probably coincides with a benign prognosis of epilepsy.

Adolescent↗

PubMed2003

Dimensions of affective semantic meaning--behavioral and evoked potential correlates in Chinese subjects.

The affective meaning of words can be quantified statistically by the 'semantic differential technique'. We studied a total of 55 Chinese adults in two experiments: first, 210 nouns were rated by 32 subjects. Factor analysis on these data yielded three independent semantic dimensions. Semantically unique words were used in electrophysiological experiments in another group of 23 healthy right-handed adults. Words of similar physical appearance belonging to different semantic classes were presented visually in random order. The electroencephalogram [corrected] was recorded in 32 channels. Evoked activity was computed for each semantic class. Significant differences in electrical brain activation between semantic word classes were seen as early as 80 ms after stimulus onset confirming related findings in German subjects. These results illustrate similar early neural activation in subject groups of different language and culture.

Adult↗

PubMed2003

Rapid extraction of emotional expression: evidence from evoked potential fields during brief presentation of face stimuli.

Although the emotional expression of faces is believed to be accessed rapidly, previous ERP studies hardly found correlates of these processes. Here, we report findings from a study that investigated dichoptic binocular interaction using emotional face stimuli. Thirty-one subjects were briefly presented with schematic normal and scrambled faces (of neutral, positive, or negative expression) that occurred simultaneously in the left and right visual fields. Stimuli for both eyes could be congruent (control) or incongruent (dichoptic). Subjects decided which of the superimposed images in both hemi-fields appeared more "face-like" and during this task, the EEG was recorded from 30 channels. VEPs were analysed topographically according to the influence of the different experimental conditions (defined by presentation form, emotional expression, and location). Behavioural responses to the ambiguous dichoptic stimuli demonstrated a functional eye dominance not related to visual acuity and conventional eye preference. Electrophysiological data revealed three components with mean latencies of 85, 160, and 310 ms. Topography of the second component (equivalent to the face-related N170) differed in left-right and anterior-posterior direction compared with simple checkerboard stimuli. Dichoptic presentation caused reduced field strength of all three, and increased latency of the first component. Faces with negative expression yielded largest field strength of the second and third components. Besides that, emotional expression affected topography not only of late, but also the first component. This provides new evidence about the timing of perceptual processes related to facial expression, indicating that already VEP components occurring at 80-90 ms are sensitive to emotional content.

Adult↗

PubMed2001

Perceptual learning: psychophysical thresholds and electrical brain topography.

We studied perceptual learning by determining psychophysical discrimination thresholds for visual hyper acuity targets (vernier stimuli) as a function of stimulus orientation. One aim was to relate perceptual improvements to changes of electrophysiological activity of the human brain. A group of 43 healthy adults participated in a psychophysical experiment where vernier thresholds for vertical and horizontal vernier targets were compared. In 16 subjects thresholds were measured for each orientation twice at an interval of 25 min. Between threshold estimations, evoked brain activity was recorded from 30 electrodes over the occipital brain areas while the subjects observed appearance and disappearance of supra-threshold vernier offsets. Mean evoked potentials were computed for the first and second 600 stimulus presentations, and the scalp topography of electrical brain activity was analyzed. Vertically oriented stimuli yielded significantly better performance than horizontal targets, and thresholds were significantly lower in the second half of the experiment, i.e. after prolonged viewing of stimuli. The improvements in discrimination performance were specific for stimulus orientation and did not generalize. Learning effects were also observed with electrical brain activity, and field strength of the potentials increased significantly as a function of time. Scalp topography of the evoked components was significantly affected indicating a shift of activation between different neuronal elements induced by perceptual learning.

Adult↗

PubMed2001

Effects of temporal gaps between successive fixation targets on discrimination performance and evoked brain activity.

Planning and executing of action in real-world conditions require continuous sensory input from many modalities. At the same time, sensory functions depend on reafferent and efference-copy information flow as imposed by motor actions. We studied how a specific oculomotor task influences afferent visual processing. Twenty healthy adults performed visually guided saccades. Between the offset of a fixation light and the onset of a new visual target a temporal gap of a duration of about 200 ms was introduced. This time structure is known from previous studies to elicit saccades at express latencies. In a control condition, 'no gap' was used. During eye movements one of four visual patterns with different orientations was presented, triggered by the horizontal electro-oculogram. We analyzed discrimination performance and the simultaneously recorded multichannel EEG activity. In the gap condition, shorter saccadic latencies were accompanied by significant more correct perceptual judgments. However, brain activity, as quantified by global field power, evoked component latency and topographical descriptors (centers of gravity or centroids) were not affected by the gap. This contrasts the notion that parieto-occipital areas are the most important sites of sensorimotor integration. Furthermore, the presence of a visual masking stimulus did not degrade discrimination performance, demonstrating that local retinal afterimages were not used for perceptual decisions. We conclude that intra-saccadic visual processing is influenced by pre-saccadic events. Under the short-time constraints prevalent in the saccadic task, fixation target cues are not only used for motor planning but also influence the visibility of visual patterns presented during the eye movement.

Adult↗

PubMed2001

The processing of stereoscopic information in human visual cortex: psychophysical and electrophysiological evidence.

Three-dimensional depth perception relies in part on the binocular fusion of horizontally disparate stimuli presented to the left and right eye. The mammalian visual system offers a unique possibility to study electrophysiologically cortical neuronal mechanisms: since the input of the two eyes remains separated up to the level of the visual cortex, evoked potential components that are generated exclusively by cortical structures may be explored when dynamic random-dot stereograms (dRDS) are presented. In a series of independent studies, we determined the scalp topography of dRDS evoked brain activity in different groups of healthy subjects, and we found consistent results. Major differences between stereoscopic and contrast evoked brain activity are seen in the strength of the potential fields as well as in their topography. Our findings suggest that there are fewer neurons in the human visual cortex that are responsive to horizontal disparity, and that higher visual areas like V2 are more engaged with stereoscopic processing than the primary visual cortex. On the other hand, component latencies of evoked brain activity show no effect signifying that the binocular information flow to the visual cortex has a similar time course for both the processing of contrast information and of dRDS stimuli. We could also verify that healthy subjects can learn to perceive 3D structure contained in dRDS. Changes in perceptual ability as measured with psychophysical tests are paralleled by systematic alterations in the topography of stereoscopically evoked potential fields. Stereoscopic VEP recordings may also be of clinical use: in patients with selectively disturbed depth perception but normal visual acuity there is a high correlation between clinical symptoms, perceptual deficiency, and altered VEP amplitudes and latencies.

Brain Mapping↗

PubMed2000

Neural correlates of reafference: evoked brain activity during motion perception and saccadic eye movements.

The ability to perceive a stable visual environment despite eye movements and the resulting displacement of the retinal image is a striking feature of visual perception. In order to study the brain mechanism related to this phenomenon, an EEG was recorded from 30 electrodes spaced over the occipital, temporal and parietal brain areas while stationary or moving visual stimuli with velocities between 178 degrees/s and 533 degrees/s were presented. The visual stimuli were presented both during saccadic eye movements and with stationary eyes. Stimulus-related potentials were measured, and the effects of absolute and relative stimulus velocity were analyzed. Healthy adults participated in the experiments. In all 36 subjects and experimental conditions, four potential components were found with mean latencies of about 70, 140, 220 and 380 ms. The latency of the two largest components between 100 and 240 ms decreased while field strength increased with higher absolute stimulus velocity for both stationary and moving eyes, whereas relative stimulus velocity had no effect on amplitude, latency and topography of the visual evoked potential (VEP) components. If the visual system uses retinal motion information only, we would expect a dependence upon relative velocity. Since field strength and latency of the components were independent of eye movements but dependent upon absolute stimulus velocity, the visual cortex must use extraretinal information to extract stimulus velocity. This was confirmed by the fact that significant topographic changes were observed when brain activity evoked during saccades and with stationary eyes was compared. In agreement with the reafference principle, the findings indicate that the same absolute visual stimulus activates different neuronal elements during saccades than during fixation.

Adult↗

PubMed2000

Electrophysiological correlates of human intrasaccadic processing.

Visual discrimination performance is thought to be suppressed during saccades in order to contribute to space constancy. However, under certain experimental conditions, visual inhibition may not take place, suggesting a more complex underlying mechanism. We tested the discrimination ability of 20 healthy subjects during visually guided horizontal saccades and recorded simultaneously the evoked brain activity from 30 channels over occipital, parietal, and temporal areas. During the execution of saccadic eye movements, visual stimuli were presented for 30 ms. In order to prevent retinal afterimages, stimuli were followed by a visual mask. In a control condition, the same stimuli were presented with stationary eyes. Electro-oculogram (EOG) and electroencephalogram (EEG) signals were recorded continuously together with information about the stimuli and the subject's response. Evoked potentials were computed offline, and component latency, field strength (global field power), and topography were compared between conditions. During saccades, subjects showed only slightly reduced discrimination performance which remained very high above the chance level; thus, there was no evidence for strong saccadic suppression with the supra-threshold stimuli employed. However, the cortical activation patterns exhibited large alteration when a physically identical stimulus was presented during the eye movement: around 130 ms latency, field strength was significantly smaller than when stationary targets were processed, and scalp topography was also different. These effects on evoked field distributions may be attributed to neural interactions of an efference copy signal (linked to the oculomotor command) with the afferent excitation following the visual stimulus.

Adult↗

PubMed2000

Associative learning in humans--conditioning of sensory-evoked brain activity.

A classical conditioning paradigm was employed in two experiments performed on 35 human volunteers. In nine subjects, the presentation of Landolt rings (conditioned stimuli, CS + ) was paired with an electric stimulus (unconditioned stimuli, UCS) applied to the left median nerve. Neutral visual control stimuli were full circles (CS -) that were not paired with the UCS. The skin conductance response (SCR) was determined in a time interval of 5 s after onset of the visual stimuli, and it was measured in the acquisition and test phase. Associative learning was reflected by a SCR occurring selectively with CS +. The same experiment was repeated with another group of 26 adults while electroencephalogram (EEG) was recorded from 30 electrodes. For each subject, mean evoked potentials were computed. In 13 of the subjects, a conditioning paradigm was followed while the other subjects served as the control group (non-contingent stimulation). There were somatosensory and visual brain activity evoked by the stimuli. Conditioned components were identified by computing cross-correlation between evoked somatosensory components and the averaged EEG. In the visual evoked brain activity, three components with mean latencies of 105.4, 183.2, and 360.3 ms were analyzed. Somatosensory stimuli were followed by major components that occurred at mean latencies of 48.8, 132.5, 219.7, 294.8, and 374.2 ms latency after the shock. All components were analyzed in terms of latency, field strength, and topographic characteristics, and were compared between groups and experimental conditions. Both visual and somatosensory brain activity was significantly affected by classical conditioning. Our data illustrate how associative learning affects the topography of brain electrical activity elicited by presentation of conditioned visual stimuli.

Adult↗

PubMed2000

An early antecedent to modern random dot stereograms --'the secret stereoscopic writing' of Ramón y Cajal.

The use of computerized random dot stimuli in modern neuroscience was introduced by Julesz in the 1960s. This method made it possible to study exclusively cortical processing of binocular information by disparity-sensitive neurons, and it has attained widespread use among neuroscientists and psychologists. It is now largely forgotten that in the last century, the famous neuroanatomist Ramón y Cajal had worked on random dot stereograms as a means of encoding written information. A brief note was finally published in a Spanish journal on photography in 1901. We present a translation of this text and summarize the early ideas on random dot stereograms, and we also supply a brief historical account on stereoscopic perception.

Depth Perception↗

PubMed1999

Electroencephalographic cortical oscillations and saccadic eye movements in humans.

A model predicting different types of saccades has suggested that the presence of rhythmic brain activity determines whether a subject will produce regular or express saccades. We studied cortical oscillations preceding saccadic eye movements. Brain electrical activity was recorded in nine healthy adults continuously from 30 electrodes while subjects performed saccades. In a so-called gap condition multimodal latency distributions resulted. Express saccades were preceded by different oscillatory activity than regular saccades. This was a highly significant finding restricted to the alpha and beta bands of the EEG. Step-wise discriminant analysis showed that cortical oscillations measured from only few electrode sites allowed to predict reliably which type of saccade a subject will make. These findings support the notion that stimulus-induced oscillations of the human EEG may modulate thresholds for triggering saccades.

Adult↗

PubMed1999

Learning to see 3-D: psychophysics and brain electrical activity.

We investigated human perceptual learning with stereoscopic stimuli presented below threshold. Different visual patterns were shown as dynamic random dot stereograms in a forced-choice design in order to determine the psychophysical thresholds of 16 adults. Brain electrical activity was recorded from 30 electrodes over parieto-occipital areas while stereograms were presented with horizontal disparities below threshold. During the observation of sub-threshold stimuli, we tested repeatedly whether implicit perceptual learning occurred. More than half of the subjects learned to see stereoscopic targets. This was accompanied by topographic changes in the pattern of activation of neural assemblies in the visual cortex where the center of activity shifted towards the right hemisphere. Subjects who did not improve in perception, displayed no such effects.

Adult↗

PubMed1999

Topography of evoked brain activity during mental arithmetic and language tasks: sex differences.

We studied visual information processing using two different tasks in a group of 10 female and 10 male healthy, right-handed adults. Subjects solved arithmetic tasks shown sequentially on a computer monitor, and they also compared words presented as anagrams. The experimental design allowed us to compare the effects of reading or actively processing a given stimulus. Task difficulty was varied in three steps ('easy', 'medium', 'hard') after an independent group of 81 young adults had judged the stimulus material according to difficulty by answering questionnaires. Brain activity was recorded from an array of 30 electrodes extending from the inion to 5% anterior of F2. For each subject mean potentials were averaged off-line after screening the EEG for artifacts. Components were determined quantitatively as epochs of stable topography resulting in 10 independent components occurring within 1200 ms after stimulus onset. Significant effects were seen with field strength and scalp topography: simply reading the stimuli yielded significantly smaller amplitudes than when the subjects actively processed the same stimuli. Females had consistently larger global field power than males, and they also displayed different scalp field topography of various components. In addition, processing anagrams was accompanied by larger field strength than mental arithmetic. The scalp field distributions were also affected by sex, task type and difficulty indicating the activation of different neuronal assemblies during visual information processing of males and females. Many effects were seen at short latencies in the order of 70-120 ms indicating very early selective processing of visual stimuli where specific differences were introduced by sex and task parameters.

Adult↗

PubMed1998

Scalp distribution components of brain activity evoked by visual motion stimuli.

We analyzed the scalp distribution of electrical brain activity elicited by visual motion stimuli in 14 healthy adults. Stimuli were square-wave gratings of high or low contrast moving with a velocity of 4.9 deg/s on a computer monitor. Adaptation to motion was varied by changing the so-called duty cycle of stimulus presentation (i.e., the relation of motion to total presentation time) in order to enhance motion-related activity. Data obtained with motion stimuli were compared with checkerboard pattern reversal evoked activity. Spatial principal components analysis revealed four latent topographical components that accounted for 92.05% of the variance. Two components showed occipital extreme values surrounded by steep potential gradients while another two components displayed lateralized activity. Analysis of the contribution of these spatial components to the observed potential fields revealed significant differences between activity evoked by pattern reversal and that evoked by motion. The topographical patterns of cortical activation changed rapidly within 240 ms after motion onset. Our results confirm the sequential and parallel activation of different neuronal generators selectively sensitive to physical stimulus parameters of motion stimuli. The converging evidence of specialized quality-specific streams of sensory processing stemming from single-unit recordings in monkeys and imaging methods is supplemented by our electrophysiological results reflecting the activation of different brain areas.

Adaptation, Physiological↗

PubMed1998

Evoked potential correlates of semantic meaning--A brain mapping study.

According to the 'semantic differential technique' the affective meaning of words can be quantified in statistically defined, independent dimensions where every word is uniquely located on the three dimensions evaluation ('good-bad'), potency ('strong-weak'), and activity ('active-passive'). Two experiments were performed on a total of 52 adults: first, 162 nouns were rated by 30 subjects. All words had a comparable number of letters and frequency of occurrence in the German language. A factor analysis followed by varimax rotation on the ratings yielded three semantic dimensions, and for each dimension up to 20 words were selected which scored highly positive or highly negative on one of the three dimensions, and had small scores on the others. This resulted in six semantic word classes which were then used in electrophysiological experiments performed on another group of 22 healthy right-handed adults. Stimuli were presented sequentially on a computer monitor in a randomized order, and the EEG was recorded in 30 channels and continuously stored on hard disk. A checkerboard reversal stimulus was used in a control condition. Evoked potentials were computed off-line for each semantic class. Comparison of the factor structure revealed highly similar semantic dimensions and classification of all words used. In the electrophysiological data, specific brain activity occurred that was related to semantic processing. These components, however, showed distinctive differences to brain activity elicited by contrast reversing checkerboard patterns as was evident from significant differences in component latency, amplitude, and scalp topography. Significant differences in scalp topography, latency and field strength between semantic word classes were not restricted to late 'cognitive' components, but brain activity at small latencies was affected by semantic meaning of the stimuli. Our data show how visually evoked brain activity is modulated by the meaning of the stimuli at early processing stages without reflecting hemispheric differences.

Adult↗

PubMed1998

Functional perimetry combined with topographical VEP analysis.

The processing of visual input depends on the position of the visual stimuli in the visual field. Based on the anatomical structure of the retina and the cortex, the function and perception vary with the location in the visual field. Due to the low signal-to-noise ratio, electrophysiological recordings in human subjects commonly have to use large stimuli and, therefore, yield poor spatial resolution. The combination of the method of quasi-simultaneous stimulation of many small (1.5 degrees x 1.5 degrees squares) visual field elements by binary m-sequences and topographical recordings allowed us to reconstruct the potential maps elicited at each of 54 visual field locations independently. Twenty-two normal subjects participated in the experiments and observed monocularly a stimulation field of 13.5 degrees x 9 degrees filled with the 54 squares. Mean luminance was 6.5 cd/m2 and contrast was 95%. The EEG was recorded in 30 channels with a dense array of electrodes over the occipital brain areas. Individual noise levels of the subjects were estimated and significant signals were analyzed quantitatively. We determined three components between 90 ms and 220 ms latency. Both global field power (GFP) and topography of the components were affected by retinal stimulus location, showing a significant decline of GFP with retinal eccentricity. Our data demonstrate that even small retinal targets may evoke brain activity which can be recorded simultaneously. Scalp field topography depends critically on the exact stimulus location within the foveal and parafoveal retinal areas while response strength mainly depends on eccentricity.

Adult↗

PubMed1997

Electrophysiological evidence for direction-specific rotary evoked potentials in human subjects--a topographical study.

The spatio-temporal characteristics of rotary evoked potentials are unknown up to now. Transient motions with sinusoidal velocity profile (60 degrees , 47.12 degrees /s, 74.02 degrees /s2, duration 2 s) were alternately applied (rightward/leftward) to 12 healthy subjects. Fixation of a target-cross moving with them suppressed the vestibulo-ocular reflex. Quasi-DC-scalp potentials were recorded from a total of 21 equidistant (3 cm) locations (single sweep: 5 s, 0.016-100 Hz). Brain activity evoked by rotary stimulation is dominated by a late, long-lasting component within a mean peak latency of about 1800 ms after motion onset. Topographic distribution over both hemispheres specifically depends upon the direction of rotation and is mirror-symmetric with respect to the sagittal midline. The gradient of the potential field obtained shows its maximum along a lateral orientation corresponding to the temporo-parietal orientation of vestibular cortical projection areas.

Adult↗

PubMed1997

Topography of visually evoked brain activity during eye movements: lambda waves, saccadic suppression, and discrimination performance.

Eye movement-related brain activity was studied in 14 subjects by recording EEG topographically in 16 channels over the occipital brain areas. Potential fields obtained with or without the simultaneous presentation of a visual stimulus during the time course of horizontal saccades were compared. Without visual stimulation, eye movements were followed at a mean latency of about 65 ms by a lateralized occipital dominant component whose topography was determined by the direction of the saccade but whose latency was independent of the time course of the eye movements. This component was reminiscent of lambda waves, however, it could also be elicited in complete darkness. When stimuli were presented during saccades, component latencies increased significantly, and there were also topographic changes in the evoked potential fields. Negative centroids were located more anteriorly and positive ones more posteriorly on the scalp when compared to brain activity recorded with stable eye positions and visual stimulation. All subjects reported no suppression of visual stimuli when presented during saccades occurred. This was confirmed by testing the discrimination performance of an independent group of 27 subjects. Our data show that the execution of saccades elicits electrophysiological patterns of activation in the visual cortex even without visual input. The increase of component latency observed during saccades as well as topographical differences suggest that visual information is processed by different neuronal elements during saccadic eye movements.

Adult↗