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W Skrandies

Publications and source records attributed to W Skrandies.

58 records · Page 4Linked to original sources
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↗
PubMedDate not supplied

Data reduction of multichannel fields: global field power and principal component analysis.

Electroencephalographic data recorded for topographical analysis constitute multidimensional observations, and the present paper illustrates methods of data analysis of multichannel recordings where components of evoked brain activity are identified quantitatively. The computation of potential field strength (Global Field Power, GFP) is used for component latency determination. Multivariate statistical methods like Principal Component Analysis (PCA) may be applied to the topographical distribution of potential values. The analysis of statistically defined components of visually elicited brain activity is illustrated with data sets stemming from different experiments. With spatial PCA the dimensionality of multichannel data is reduced to only three components that account for more than 90% of the variance. The results of spatial PCA relate to experimental conditions in a meaningful way, and this method may also be used for time segmentation of topographic potential maps series.

Action Potentials↗
PubMedDate not supplied

Depth perception and evoked brain activity: the influence of horizontal disparity and visual field location.

The perception of dynamic random-dot stereograms (RDS) depends on the physiological fusion of horizontally disparate binocular visual input. Thus, the use of RDS offers the possibility to study selectively cortical processing of visual information in man. We investigated the influence of horizontal disparity on the scalp topography of RDS evoked brain activity in 33 healthy subjects. Stereoscopic checkerboard patterns were presented in the center or lateralized in the left or right visual field with horizontal disparities changing at temporal frequencies of six or eight depth reversals/s using different disparity values ranging from 3.5 to 28 min of arc. In 11 subjects evoked potential fields were recorded from 16 electrodes, and 21 subjects participated in 30-channel recordings with electrodes located over the parietal and occipital brain areas. Stimulation frequency-related brain activity was obtained with all disparity values; however, with large or small disparities the potential field strength decreased significantly while largest responses were obtained with intermediate disparities. Significant differences were observed in RDS evoked brain activity when central and lateralized stimulus locations were compared. With lateral stimuli (extending from the fovea to 17.1-deg eccentricity) maximal amplitudes were obtained at larger disparities than with central stimuli. In addition there were pronounced differences between brain activity evoked with stimuli presented in the left or right visual field; however, there were very similar evoked potential signals recorded from electrodes located over the left and right hemispheres. Our findings indicate that the processing of disparity information with lateralized stimuli is different from the processing in the center of the visual field. In addition, lateralized stimulation yields a significant disparity tuning mainly with stereoscopic targets occurring to the right from the fixation point (but not with stimuli to the left) suggesting a functional difference between the visual half-fields.

Adult↗