Topographical Subcomponents of Electrical Brain Activity Allow to Identify Semantic Learning.

We investigated the change of event-related brain activity elicited by reading meaningful or meaningless Japanese symbols in 20 healthy German adults. In a learning phase of about 20 min, subjects acquired the meaning of 20 Kanji characters. As control stimuli 20 different Kanji characters were presented. Electrical brain activity was obtained before and after learning, The mean learning performance of all subjects was 92.5% correct responses. EEG was measured simultaneously from 30 channels, artifacts were removed offline, and the data before and after learning were compared. We found five spatial principal components that accounted for 83.8% of the variance. A significant interaction between training time (before/after learning) and stimulus (learning/control) illustrates a significant relation between successful learning and topographical changes of brain activity elicited by Kanji characters. Effects that were induced by learning were seen at short latencies in the order of 100 ms. In addition, we present evidence that differences in the weighted combination of spatial components allow to identify experimental conditions successfully by linear discriminant analysis using topographical ERP data of a single time point. In conclusion, semantic meaning can be aquired rapidly and it is associated with specific changes of ERP components.

Total variation for the analysis of event-related potentials.

BACKGROUND: Event-related potential waveforms are often analysed in the time-domain for changes of striking morphological features, like amplitudes or latencies of extrema, at the expense of missing less obvious changes in overall morphology. NEW METHOD: The measure of total variation can capture a variety of changes in curve morphology. We show analytical examples, and the application to two sets of EEG data (n1=41, n2=19) difficult to analyse with more traditional methods. RESULTS: Total variation can be used to identify the effects of experimental manipulations on event-related potential waveforms, and can additionally be used to identify the respective time windows by means of hierarchical subdivision of longer signals. COMPARISON WITH EXISTING METHODS: The ANOVA of total variation provided additional insights into effects already hinted at by the ANOVA of global field power in the first experiment, and identified a number of interactions missed by an ANOVA of amplitude as well as a topographic ANOVA in the second one. CONCLUSIONS: The analysis of total variation can be an interesting complement to more traditional analyses, especially when changes are hard to assess with traditional methods, e.g. in the absence of pronounced extrema, or the presence of noise or large interindividual variations of latency.

Reconstruction of electroencephalographic data using radial basis functions.

OBJECTIVE: In this paper we introduce a new interpolation method to use for scalp potential interpolation. The predictive value of this new interpolation technique (the multiquadric method) is compared to commonly used interpolation techniques like nearest-neighbour averaging and spherical splines. METHODS: The method of comparison is cross-validation, where the data of one or two electrodes is predicted by the rest of the data. The difference between the predicted and the measured data is used to determine two error measures. One is the maximal error in one interpolation technique and the other is the mean square error. The methods are tested on data stemming from 30 channel EEG of 10 healthy volunteers. RESULTS: The multiquadric interpolation methods performed best regarding both error measures and have been easier to calculate than spherical splines. CONCLUSION: Multiquadrics are a good alternative to commonly used EEG reconstruction methods. SIGNIFICANCE: Multiquadrics have been widely used in reconstruction on sphere-like surfaces, but until now, the advantages have not been investigated in EEG reconstruction.

Olfactory and gustatory functions and its relation to body weight.

In the present study we investigated the influence of body weight as defined by BMI on gustatory and olfactory perception. A total of 66 healthy adults (41 females; 25 males) participated in psychophysical measurements using the "Sniffin' Sticks" test and "Taste Strips" test. Odor thresholds as well as discrimination and identification performance were determined. Tests of gustatory function involved the identification and thresholds of sweet, sour, salty, or bitter taste. In this study, all subjects were healthy participants in a middle age range (between 20 and 56 years of age). Persons with an extreme BMI value were excluded. Subjects were classified according to their BMI in four groups: (1) 15-19.9 kg/m, (2) 20-24.9 kg/m, (3) 25-29.9 kg/m, and (4) >30 kg/m. We did not observe an overall effect of BMI on general sensory sensitivity. There was a significant influence of BMI on olfactory thresholds (F(3,62)=2.79; p<0.047) which increased with increasing BMI. In a similar line, the gustatory thresholds for "salty" were significantly higher with higher BMI (F(3,62)=3.06; p<0.035). Olfactory discrimination and identification was not affected by BMI. Thresholds for odor and sweet or salty taste were also correlated. Our data show that body weight influences gustatory and olfactory perception in healthy adults. Increasing BMI is associated with a decrease in olfactory and taste sensitivity. These findings may have implications for the understanding of pathophysiological mechanisms in patients.

Brain activity and learning of mathematical rules--effects on the frequencies of EEG.

We investigated the change of evoked EEG frequencies induced by learning to solve mathematical tasks by applying divisibility rules. The performance on easy (divisibility by 2, 3, or 5) and hard tasks (divisibility by 9 or by 11) was compared. In a behavioral experiment on 52 adults we found a significant increase in performance from 67% to 90% correct responses induced by rule learning. Subsequently, the EEG data recorded from 30 additional volunteers were analyzed. EEG recordings were performed in two parts: First, subjects had to solve 200 tasks without knowing the divisibility rules. Then the rules were explained, followed by another set of 200 tasks. EEG was measured simultaneously in 30 channels, artifacts were removed offline, and the data before and after rule learning were compared. A wavelet transformation with the Morlet-5 wavelet was computed, and the scalp topography of the maximal frequency and its occurrence time was compared. Largest effects were observed with frequencies between about 6 and 18 Hz. In the frequency band between 12 and 30 Hz maximal frequencies were significantly different after successful learning over frontal and centro-parietal scalp areas of the right hemisphere. These changes were paralleled by decreased response times. In summary, our data illustrate a significant relation between successful learning divisibility rules and changes in the frequency content of the task-related EEG. Significant effects were observed after a very short training period of less than 10 min.

Electrophysiological correlates of connotative meaning in healthy children.

The affective, connotative meaning of words can be statistically quantified by the semantic differential technique. Words that are located clearly on one of the three dimensions called "Evaluation", "Potency", and "Activity" were used as visual stimuli in a topographic event related potential study (ERP). Stimuli had been statistically defined in a group of 249 children (Skrandies, Jpn Psychol Res 53: 65-76, 2011). We investigated electrical brain activity in 19 healthy children with normal intelligence and reading skills between 11 and 15 years of age. Words that belonged to different semantic classes were presented at random on a monitor, and EEG was measured from 30 channels. Evoked potentials were computed offline for each semantic class. In the ERP data we observed significant effects of word class on component latency, field strength and topography. Similar as with adult subjects such effects occurred at small latency of about 115 ms after word presentation. The language-evoked components in children were similar but not identical to those reported previously for various groups of adults. Our data show that visually evoked brain activity is modulated by connotative meaning of the stimuli at early processing stages not only in adults but also in children.

Topographic changes in event-related potentials because of learning of meaningful Kanji characters.

Japanese Kanji constitutes meaningful logograms, and its processing shows interhemispheric features. In the present study, human semantic learning of Kanji characters in 18 healthy native German adults was examined. Twenty Kanji characters were presented before and after a learning phase of about 20 min, and the electroencephalographic activity was recorded from 30 electrodes and averaged for each condition. Twenty different Kanji characters served as control stimuli. Successful learning was observed in all participants. The evoked potential maps showed the largest component occurring over occipital areas at latencies between 100 and 130 ms. Significant differences in the field strength (global field power) were observed for this component before and after learning. After learning, the distribution between the left and the right hemispheres significantly changed the negative centroid location from the left to the right hemisphere and from the posterior to the anterior area in each hemisphere. These effects were observed only after successful learning, and our findings suggest that the acquisition of meaning of Kanji characters following intensive short-term learning is related to neurophysiological changes at an early stage of processing. The topographical changes in electrical brain activity reflect plasticity presumably in primary sensory areas during learning of meaningful materials that is related to top-down information processing.

Evaluation of multisensory stimuli--dimensions of meaning and electrical brain activity.

The semantic differential technique is used to statistically define connotative dimensions of meaning. The brain depends on these dimensions to process words. Earlier studies demonstrated that stimuli of the different semantic classes led to differences in neuronal processing. We investigated the influence of connotative meaning on multisensory processing (food words strongly related to odor, taste, vision or somatosensory texture). A group of 795 subjects rated 197 food words on the basis of 11 pairs of adjectives with opposite meanings. Factor analysis revealed three dimensions (Evaluation, Potency and Texture). Words with high positive or negative scores, and low scores on the other dimensions, were used as stimuli in an ERP experiment. EEG was recorded in 40 healthy adults from 30 channels and averaged according to semantic stimulus class. Component latency, global field power and topography were influenced by semantic meaning. These experiments determined that very early effects at 107 ms after stimulus presentation where latency and GFP were affected by stimulus class. When mapped topographically, different stimulus classes led to different scalp topography of evoked brain activity in sagittal direction already at an early state of processing (around 107 ms). The extent of lateralization of potential fields' centers of gravity was influenced by stimulus class around 304 ms. In summary, semantic dimensions influence neuronal processing of words related to multisensory perception. Such effects suggest a rapid and complex way of processing multisensory stimuli.

A reliable statistical method to detect eyeblink-artefacts from electroencephalogram data only.

For data preprocessing and artefact removal in an ERP experiment we were confronted with the question how blink artefacts can be detected reliably, even in the absence of usable electrooculogram (EOG) data. We propose an objective and quantitative method for the automatic detection of eyeblink artefacts from raw data using extreme value statistics, with a p-value acting as a threshold parameter. For testing the method, we used 29 channel electroencephalogram recordings of 55 healthy subjects. A total 7,700 s of EEG were analysed. The proposed method was found to detect blink artefacts reliably, showing that extreme value statistics can be employed to detect blink artefacts, even in the absence of EOG recordings.

Assessment of depth perception using psychophysical thresholds and stereoscopically evoked brain activity.

Dynamic random-dot stereograms (dRDS) elicit brain activity generated exclusively by cortical neurons sensitive to binocular horizontal disparity. We studied 20 adults with stereovision deficiency but otherwise normal vision. Psychophysical thresholds were determined with static RDS and with the three-rod experiment. VEP was recorded from seven occipital channels. Stimuli were presented on a monitor by dRDS as stereoscopic checkerboard patterns that moved in depth with 8 depth reversals per second. Horizontal disparity ranged from 7 to 24.5 min of arc. Stimuli were displayed at the center, or in the left or right half field. We determined electrophysiological thresholds as well as the disparity where largest responses occurred. Subjective and electrophysiological thresholds showed a significant positive correlation. In addition, the right visual field was more sensitive to dRDS stimuli than other locations. Squint angle was related to the disparity thresholds. Our data illustrate correlations between clinical symptoms, perceptual deficiency, and VEP parameters.

The effect of stimulation frequency and retinal stimulus location on visual evoked potential topography.

The activity of cortical neurons is influenced by retinal stimulus location and temporal modulation. We investigated how reversal frequency of black-and-white checkerboard patterns presented in different parts of the visual field affects evoked potential topography. Visual evoked potentials were recorded from an array of 16 electrodes over the occipital cortex in 12 healthy adults. A checkerboard reversal stimulus (40' check size) was presented with frequencies between 1.95 reversals/s and 7.81 reversals/s in the center or in the left or right hemiretina. Evoked potential fields displayed the well-known components of pattern reversal evoked activity. Computation of FFT and wavelets displayed electrical brain responses directly related to stimulation frequency. Further analysis showed that both retinal stimulus location and stimulation frequency affected visual evoked activity. Field strength as well as scalp field topography changed significantly with different reversal frequency. In addition, the pattern of lateralization of components also depended on temporal frequency of stimulation. Electrical brain activity elicited by visual stimuli shows globally similar features which are modulated by stimulus location and frequency. Our results indicate that--at least partly--different neuronal assemblies are activated by stimuli of different temporal characteristics.

Conventional and wavelet coherence applied to sensory-evoked electrical brain activity.

The use of coherence is a well-established standard approach for the analysis of biomedical signals. Being entirely based on frequency analysis, i.e., on spectral properties of the signal, it is not possible to obtain any information about the temporal structure of coherence which is useful in the study of brain dynamics, for example. Extending the concept of coherence as a measure of linear dependence between realizations of a random process to the wavelet transform, this paper introduces a new approach to coherence analysis which allows to monitor time-dependent changes in the coherence between electroenecphalographic (EEG) channels. Specifically, we analyzed multichannel EEG data of 26 subjects obtained in an experiment on associative learning, and compare the results of Fourier coherence and wavelet coherence, showing that wavelet coherence detects features that were inaccessible by application of Fourier coherence.

ERP topography and human perceptual learning in the peripheral visual field.

We studied human perceptual learning in the peripheral visual field in 16 healthy adults. Horizontal or vertical vernier stimuli were presented simultaneously at 8 locations at an eccentricity of 4 degrees . One of the stimuli displayed an offset, and subjects were asked to detect the target offset. Training was performed with either vertical or horizontal stimuli by the repeated presentation of stimuli. Discrimination performance was also measured with the untrained stimuli. Before and after the psychophysical experiment, EEG was recorded from 30 electrodes over the occipital areas (between the inion and Cz) while targets were presented at all locations as vernier onset/offset stimuli. The EEG was averaged for each orientation separately. Improvement in discrimination performance was observed in about 70% of the subjects with the trained orientation only. The evoked potential maps displayed three components occurring between 80 and 160, 180 and 260, and 280 and 340 ms. The potential field topography of the first and third component showed significant differences before and after learning. In addition, field strength (global field power) of the second and third component increased with learning. No effects were seen with the untrained stimuli in the psychophysical and electrophysiological experiments. Our findings suggest that perceptual learning in the peripheral visual field is specifically related to neurophysiological changes induced by training, and it is not caused by unspecific changes of spatial attention. The changes of electrical brain activity reflect short-term plasticity related to human perceptual learning.

Brain mapping of visual evoked activity--topographical and functional components.

Electroencephalographic (EEG) and evoked activity can be recorded non-invasively in order to monitor human brain activation online. Electrical fields are generated by large intracranial neural populations and spread to the scalp through volume conduction. All measured signals depend on the location of the recording and reference electrodes. Simultaneous recordings from many scalp positions allow for a topographical assessment of the complete electrical fields of the brain and avoid problems typically seen with time series analysis. The present contribution will illustrate the fundamentals of topographic mapping of human electrophysiological brain activity as well as quantitative analysis methods. Visual evoked potential data obtained in a group of 12 healthy adults are presented. We will focus on the definition and identification of evoked components that represent steps in visual information processing. In addition, the application of statistical data reduction techniques are described, and the results of functional components which are related to experimental variations are discussed.

The processing of semantic meaning in Chinese words and evoked brain topography.

The connotative meaning of words can be quantified statistically by the "semantic differential technique" resulting in statistically defined, independent dimensions where every word is uniquely located on the three dimensional evaluation. In an earlier study on German subjects we demonstrated that there are electrophysiological correlates of these meaning dimensions. Here a group of 55 Chinese adults was investigated in two experiments: first, 210 nouns were rated by 32 subjects, and factor analysis on the questionnaire data yielded three independent semantic dimensions. Semantically unique words stemming from these results were then used as stimuli 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 EEG was recorded from 29 channels, and evoked brain activity was computed for each semantic class. Significant differences in electrical brain activation between these semantic word classes were observed as early as 80 ms after stimulus onset, confirming earlier reports on similar findings in German subjects. Further similarities were revealed by a direct comparison of the topographical distribution of potential components elicited by words, and by the results of spatial PCA performed on both sets of data. These results illustrate similar early neural activation based on semantic class, in subject groups of different language and culture.

Human perceptual learning in the peripheral visual field: sensory thresholds and neurophysiological correlates.

Perceptual learning in the peripheral visual field was studied in 24 adults using vernier targets. The aim was to relate perceptual improvements to changes of electrical brain activity. Thresholds were measured before, during, and after training, and on the next day. During training, the subjects passively looked at suprathreshold targets, and EEG activity was recorded from 30 electrodes over the occipital brain areas. Mean evoked potentials were computed for the first and second block of 1200 stimulus presentations, and the scalp topography of visual evoked potential (VEP) activity was analysed. Only for the stimulated area, training resulted initially in increased thresholds that, however, decreased significantly after consolidation. Electrical brain activity displayed smaller field strength and altered topography after training. Some of the effects were caused by habituation or adaptation to the training stimuli resulting in less efficient neurophysiological processing. The topographical changes indicate that different neuronal elements were activated after perceptual learning.