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Biomedical subjects

C S Herrmann

Publications and source records attributed to C S Herrmann.

18 recordsLinked to original sources
PubMed2002

Auditory novelty oddball allows reliable distinction of top-down and bottom-up processes of attention.

An auditory novelty-oddball task, which is known to evoke a P3 event-related potential (ERP) in a target condition and a novelty-P3 ERP in response to task-irrelevant unique environmental sounds, was repeatedly applied to healthy participants (n = 14) on two separate recording sessions, 7 days apart. Both target-P3 and novelty-P3 were internally consistent and test-retest reliable. Interestingly, novelty-P3 amplitude declined from the first to the second half of each recording session, whereas no systematic alteration between both sessions occurred. The target-P3 showed the opposite pattern, i.e. a reduced amplitude from the first to the second session, but no systematic change within each session. These findings suggest that novelty-P3 amplitude changes reflect habituation, whereas target-P3 session effects may indicate the adjusted amount of processing resources invested into the task. In general, the results support the interpretation of the novelty-P3 as indicating automatic, bottom-up related aspects of attention, whereas the target-P3, in the present paradigm, seems to reflect voluntary, top-down related aspects of attention.

Acoustic Stimulation↗
PubMed2001

Gestalt perception modulates early visual processing.

We examined whether early visual processing reflects perceptual properties of a stimulus in addition to physical features. We recorded event-related potentials (ERPs) of 13 subjects in a visual classification task. We used four different stimuli which were all composed of four identical elements. One of the stimuli constituted an illusory Kanizsa square, another was composed of the same number of collinear line segments but the elements did not form a Gestalt. In addition, a target and a control stimulus were used which were arranged differently. These stimuli allow us to differentiate the processing of colinear line elements (stimulus features) and illusory figures (perceptual properties). The visual N170 in response to the illusory figure was significantly larger as compared to the other collinear stimulus. This is taken to indicate that the visual N170 reflects cognitive processes of Gestalt perception in addition to attentional processes and physical stimulus properties.

Adult↗
PubMed2001

Perception of pain coincides with the spatial expansion of electroencephalographic dynamics in human subjects.

The dynamics of cortex driven by painful median nerve stimulation were investigated in event-related oscillation (ERO). We applied a wavelet time-frequency analysis to differentiate the brain dynamics between painful and non-painful somatosensory stimulation. The observed pattern to pain-induced effects exhibited a stepwise decrease of frequencies over time, starting around 26 ms over somatosensory cortex at 80 Hz, intermediate oscillations at 40 and 20 Hz around 40 ms, and reaching down to 10 Hz after 160 ms. This step-wise frequency decrease of ERO, coincident with spatial shift from the contralateral somatosensory area at 80 Hz to the centro-frontal brain at 40/20 Hz and final spatial expansion to the large region of centro-parietal areas at 10 Hz, may represent the cortical processes necessary to transfer sensory information from perceptual stages to subsequent cognitive stages in consciousness.

Adult↗
PubMed2001

Hemodynamic and electroencephalographic responses to illusory figures: recording of the evoked potentials during functional MRI.

The feasibility of recording event-related potentials (ERP) during functional MRI (fMRI) scanning using higher level cognitive stimuli was studied. Using responses to illusory figures in a visual oddball task, evoked potentials were obtained with their expected configurations and latencies. A rapid stimulation scheme using randomly varied trial lengths was employed, and class-wise characteristics of the hemodynamic response were obtained by a nonlinear analysis of the fMRI time series. Implications and limitations of conducting combined ERP-fMRI experiments using higher level cognitive stimuli are discussed. EEG/fMRI results revealed a sequential activation of striate and extrastriate occipital cortex along the ventral path of object processing for Kanizsa figures. Interestingly, Kanizsa figures activated the human motion area MT. Targets resulted in activations of frontal and parietal cortex which were not activated for standard stimuli.

Adult↗
PubMed2001

Human EEG responses to 1-100 Hz flicker: resonance phenomena in visual cortex and their potential correlation to cognitive phenomena.

The individual properties of visual objects, like form or color, are represented in different areas in our visual cortex. In order to perceive one coherent object, its features have to be bound together. This was found to be achieved in cat and monkey brains by temporal correlation of the firing rates of neurons which code the same object. This firing rate is predominantly observed in the gamma frequency range (approx. 30-80 Hz, mainly around 40 Hz). In addition, it has been shown in humans that stimuli which flicker at gamma frequencies are processed faster by our brains than when they flicker at different frequencies. These effects could be due to neural oscillators, which preferably oscillate at certain frequencies, so-called resonance frequencies. It is also known that neurons in visual cortex respond to flickering stimuli at the frequency of the flickering light. If neural oscillators exist with resonance frequencies, they should respond more strongly to stimulation with their resonance frequency. We performed an experiment, where ten human subjects were presented flickering light at frequencies from 1 to 100 Hz in 1-Hz steps. The event-related potentials exhibited steady-state oscillations at all frequencies up to at least 90 Hz. Interestingly, the steady-state potentials exhibited clear resonance phenomena around 10, 20, 40 and 80 Hz. This could be a potential neural basis for gamma oscillations in binding experiments. The pattern of results resembles that of multiunit activity and local field potentials in cat visual cortex.

Adult↗
PubMed2001

Adaptive frequency decomposition of EEG with subsequent expert system analysis.

We present a hybrid system for automatic analysis of clinical routine EEG, comprising a spectral analysis and an expert system. EEG raw data are transformed into the time-frequency domain by the so-called adaptive frequency decomposition. The resulting frequency components are converted into pseudo-linguistic facts via fuzzification. Finally, an expert system applies symbolic rules formulated by the neurologist to evaluate the extracted EEG features. The system detects artefacts, describes alpha rhythm by frequency, amplitude, and stability and after artefact rejection detects pathologic slow activity. All results are displayed as linguistic terms, numerical values and maps of temporal extent, giving an overview about the clinical routine EEG.

Adolescent↗
PubMed2001

Mechanisms of human attention: event-related potentials and oscillations.

Electrophysiological and hemodynamical responses of the brain allow investigation of the neural origins of human attention. We review attention-related brain responses from auditory and visual tasks employing oddball and novelty paradigms. Dipole localization and intracranial recordings as well as functional magnetic resonance imaging reveal multiple areas involved in generating and modulating attentional brain responses. In addition, the influence of brain lesions of circumscribed areas of the human cortex onto attentional mechanisms are reviewed. While it is obvious that damaged brain tissue no longer functions properly, it has also been shown that functions of non-lesioned brain areas are impaired due to loss of modulatory influence of the lesioned area. Both early (P1 and N1) and late (P3) event-related potentials are modulated by excitatatory and inhibitory mechanisms. Oscillatory EEG-correlates of attention in the alpha and gamma frequency range also show attentional modulation.

Attention↗
PubMed2000

The loci of oscillatory visual-object priming: a combined electroencephalographic and reaction-time study.

The detection of reaction-times (RTs) to a target Kanizsa-type square (an illusory square defined by the colinear arrangement of 90 degrees corner junctions) within a matrix of distractor junctions are expedited when the target display is preceded by a 40-Hz flickering display of premask crosses presented prior to, and at the locations subsequently occupied by the junctions of the target display. Priming effects were obtained when four crosses (which together matched the Gestalt arrangement of the target) were presented at the display locations subsequently occupied by the junctions forming the target Kanizsa square (Elliott and Müller, 1998, 2000). The present study was conducted with the aim of replicating the 40-Hz RT priming effects, while simultaneously recording the observers EEG in order to establish the presence and location of Gestalt priming in the brain. The statistical pattern obtained in the RT data corresponded well with previous studies and was matched by the pattern of target P300 latencies across bilateral central and posterior electrodes. Planned analyses focused upon the evoked 40-Hz activity that co-occurs with the P300, revealing a more specific pattern of 40-Hz priming over the visual cortex. A subsequent series of cross-correlational analyses examined the cortical distribution and timing of Gestalt-prime generation during and subsequent to premask-display presentation. Correlations were revealed between stimulus related 40-Hz activity over a range of cortical loci, including the right temporal lobe, which is considered important for figure coding. Taken together, these findings not only support the role of a distributed 40-Hz mechanism during Gestalt-figure priming, but also suggest that patterns of oscillatory brain activity may be directly influenced by, and interpretable in terms of equivalent temporal patterns of stimulus activity.

Adult↗
PubMed2000

Magnetoencephalographic responses to illusory figures: early evoked gamma is affected by processing of stimulus features.

We examined evoked and induced responses in event-related fields and gamma activity in the magnetoencephalogram (MEG) during a visual classification task. The objective was to investigate the effects of target classification and the different levels of discrimination between certain stimulus features. We performed two experiments, which differed only in the subjects' task while the stimuli were identical. In Experiment 1, subjects responded by a button-press to rare Kanizsa squares (targets) among Kanizsa triangles and non-Kanizsa figures (standards). This task requires the processing of both stimulus features (colinearity and number of inducer disks). In Experiment 2, the four stimuli of Experiment 1 were used as standards and the occurrence of an additional stimulus without any feature overlap with the Kanizsa stimuli (a rare and highly salient red fixation cross) had to be detected. Discrimination of colinearity and number of inducer disks was not necessarily required for task performance. We applied a wavelet-based time-frequency analysis to the data and calculated topographical maps of the 40 Hz activity. The early evoked gamma activity (100-200 ms) in Experiment 1 was higher for targets as compared to standards. In Experiment 2, no significant differences were found in the gamma responses to the Kanizsa figures and non-Kanizsa figures. This pattern of results suggests that early evoked gamma activity in response to visual stimuli is affected by the targetness of a stimulus and the need to discriminate between the features of a stimulus.

Adolescent↗
PubMed2000

Noise affects auditory and linguistic processing differently: an MEG study.

We investigated the influence of noise on brain responses to spoken sentences in MEG. Sixteen subjects had to listen to acoustically presented sentences and judge their syntactic correctness. Sentences were either presented on a silent background or with noise. Noise had differential effects on early auditory and syntactic processes. While noise affected early auditory processes only in the right hemisphere, noise had a general effect on syntactical processes. The evoked responses to syntactic violations compared with correct sentences, namely an early left anterior negativity, were significantly suppressed when noise was present The noise suppression effect, however, was not lateralized.

Acoustic Stimulation↗
PubMed2000

Localization of early syntactic processes in frontal and temporal cortical areas: a magnetoencephalographic study.

Previous electrophysiological studies had found an early anterior negativity often with a maximum over the left hemisphere to correlate with the early detection of an error in the syntactic structure of a sentence. In this paper, the cortical structures involved in such early syntactic parsing processes were localized using MEG. Subjects were presented with acoustic sentences and asked to judge their syntactic correctness. The subjects' brain responses to syntactic violations were recorded with a 148-channel whole-head magnetometer. Dipole source localization was performed using a realistically shaped standard volume conductor model with fMRI constraints. The results show that the early syntactic parsing processes are supported by temporal regions, possibly the planum polare, as well as by fronto-lateral regions. As indicated by the resultant dipole strengths, these regions are activated bilaterally with a dominance in the left hemisphere for four out of the five subjects. The contribution of the left temporal regions to the early syntactic processes seems to be larger than that of the left fronto-lateral regions.

Adolescent↗
PubMed2000

Recording of the event-related potentials during functional MRI at 3.0 Tesla field strength.

The feasibility of recording event-related potentials (ERP) during functional MRI (fMRI) scanning was studied. Using an alternating checkerboard stimulus in a blocked presentation, visually evoked potentials were obtained with their expected configuration and latencies. A clustered echoplanar imaging protocol was applied to observe the hemodynamic response due to the visual stimulus interleaved with measuring ERPs. Influences of the electrode/amplifier set up on MRI scanning and the scanning process on the recording of electrophysiological signals are reported and discussed. Artifacts overlaid on the electrophysiological recordings were corrected by post hoc filtering methods presented here. Implications and limitations of conducting combined ERP/fMRI experiments using higher-level cognitive stimuli are discussed. Magn Reson Med 44:277-282, 2000.

Adult↗
PubMed2000

Sleep stage dependant changes of the high-frequency part of the somatosensory evoked potentials at the thalamus and cortex.

OBJECTIVES: It is known that the high-frequency oscillations (above 400 Hz) of the somatosensory evoked potentials (SEPs) diminish during sleep while the N20 persists (Neurology 38 (1988) 64; Electroenceph clin Neurophysiol 70 (1988) 126; Electroenceph clin Neurophysiol 100 (1996) 189). We investigated possible differential effects of sleep on the 600 Hz SEPs at the thalamus and cortex. METHODS: SEPs from 10 subjects were recorded using 64 channels following electric stimulation at the wrist during awake state and sleep stages II, IV and REM. Dipole source analysis was applied to separate brain-stem, thalamic and cortical activity in the low-frequency (20-450 Hz) and the high-frequency (450-750 Hz) part of the signal. RESULTS: The low-frequency SEPs showed a non-significant increase of the latency of the N20 and a bifid change of the waveform in 3 subjects. The high-frequency SEPs showed a significant decrease of their amplitude at the level of the thalamus and cortex but not at the brain-stem. This decrease in amplitude at the thalamus and cortex were significantly correlated. There was no effect on the latency of the signal. In addition, at the cortex, differential effects on early and late parts of the 600 Hz oscillations were found by time-frequency analysis using a wavelet transformation. CONCLUSIONS: Sleep dependent decrease of the high-frequency SEPs were first observed at the thalamus pointing to the known function of the reticular thalamic nucleus regulating arousal. The results presented here provide further evidence for a thalamic origin of the 600 Hz oscillations. In addition, on the basis of the differential effects on early (up to the N20 peak) and late (between 20 and 25 ms) parts of the signal, at least one intracortical generator of these oscillations is proposed. In general, the high-frequency SEPs (600 Hz oscillations) are supposed to reflect activity of a somatosensory arousal system.

Adult↗
PubMed1999

Gamma responses and ERPs in a visual classification task.

OBJECTIVE: We examined event-related potentials (ERPs) and gamma range EEG activity in a visual classification task to assess which variables affect these responses. METHODS: Ten subjects silently counted the occurrence of rare Kanizsa squares (targets) among Kanizsa triangles and non-Kanizsa figures (standards). By applying a time-frequency analysis to the data and selectively calculating topographical maps of certain frequencies. RESULTS: We were able to find 3 different types of gamma responses to Kanizsa figures: an early phase-locked gamma response at 40 Hz in the N100 time range, late phase-locked gamma activity (200-300 ms) at 40 Hz and a continuous phase-locked gamma response at 80 Hz due to the monitor refresh frequency. The two 40 Hz responses were significantly higher for Kanizsa figures than for non-Kanizsa figures and within the Kanizsa figures were higher for the target figure than for the non-target. CONCLUSION: The phase-locking of these two responses, previously found also as non-phase-locked activity, could be synchronized due to the monitor flicker frequency. Also, our findings suggest that the gamma responses are not solely associated with the binding of stimulus features, but reflect some processes related to target processing.

Adult↗