Biomedical subjects

Andreas Keil

Publications and source records attributed to Andreas Keil.

24 records · Page 2Linked to original sources

PubMed2003

Temporal stability of high-frequency brain oscillations in the human EEG.

Temporal stability of a given measurement within individual participants is a desirable property of psychophysiological measures. The present study aims to examine the reliability of high-frequency oscillatory activity in the human electroencephalogram (EEG) across 4 sessions. Convolution of the EEG time series with Morlet wavelets yielded time-frequency representations of the signal for each session. Stability of both topography and time course of gamma-band activity (GBA) was determined for two participants performing a feature-based selective attention task in four separate sessions, spaced at weekly intervals. We found high temporal stability of non-phase-locked GBA typically occurring in time ranges between 200 and 500 ms following presentation of a stimulus, both in terms of topography and time course. Early phase-locked GBA (80-120 ms) showed higher variability with respect to topography, but was consistent in terms of time course. We conclude that measures of high-frequency oscillatory activity as used in the cognitive neurosciences meet stability requirements necessary for meaningful interpretation of this parameter of brain function.

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PubMed2003

Early modulation of visual perception by emotional arousal: evidence from steady-state visual evoked brain potentials.

Allocation of processing resources to emotional picture stimuli was examined using steady-state visual evoked brain potentials (ssVEPs). Participants viewed a set of 60 colored affective pictures from the International Affective Picture System, presented in a flickering mode at 10 Hz in order to elicit ssVEPs. Phase and amplitude of the 10-Hz ssVEP were examined for six picture categories: threat and mutilation (unpleasant), families and erotica (pleasant), and household objects and persons (neutral). Self-reported affective arousal and hedonic valence of the picture stimuli were assessed by means of subjective ratings. Viewing affectively arousing (unpleasant and pleasant) pictures was associated with enhanced ssVEP amplitude at parieto-occipital recording sites, as compared with neutral stimuli. Phase information suggested increased coactivation of right occipitotemporal and frontotemporal sources during processing of affectively arousing stimuli. These findings are consistent with reentrant modulation of early visual processing by distributed networks including subcortical and neocortical structures according to a stimulus's motivational relevance.

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PubMed2002

Modulation of induced gamma band responses in a perceptual learning task in the human EEG.

Fragmented pictures of an object, which appear meaningless when seen for the first time, can easily be identified after the presentation of an unfragmented version of the same picture. The neuronal mechanism for such a rapid perceptual learning phenomenon is largely unknown. Recently, induced gamma band responses (GBRs) have been discussed as a possible physiological correlate of activity in cell assemblies formed by learning. The present study was designed to investigate the modulation of induced GBRs in a perceptual learning task by using a 128-channel EEG montage. In the first sequence of the experiment, fragmented pictures from the Snodgrass and Vandervart inventory were presented. The fragmentation of the pictures was selected that subjects were unable to identify them. In the second experimental sequence - the perceptual learning sequence - half of the pictures were displayed in their unfragmented version. In the third sequence, all pictures were presented again in the fragmented version. Now, subjects had to rate whether or not they could identify the images. Results showed an increase in spectral gamma power at parietal electrode sites for identified pictures. In addition, neural activity in the gamma band was highly synchronized between posterior electrodes. For pictures not presented in their complete version, we found no such pattern in the third sequence. From our results, we concluded that induced GBRs might represent a signature of synchronized neural activity in a Hebbian cell assembly, activated by the fragmented picture after perceptual learning took place. No difference between identified and unidentified pictures was found in the visual evoked potential in the same time range and in the evoked GBR in the same frequency range as the induced response.

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PubMed2002

The influence of response competition on cerebral asymmetries for processing hierarchical stimuli revealed by ERP recordings.

It is widely accepted that the left and right hemispheres differ with respect to the processing of global and local aspects of visual stimuli. Recently, behavioural experiments have shown that this processing asymmetry strongly depends on the response competition between the global and local levels of a stimulus. Here we report electrophysiological data that underline this observation. Hemispheric differences for global/local processing were mainly observed for response-incompatible stimuli and were most prominent between 320 and 400 ms after stimulus onset. These results underpin the idea that hemispheric differences are more likely to show up when a more elaborated stimulus representation is needed for triggering the response, that is, when a response conflict has to be resolved.

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PubMed2002

High resolution EEG indicators of pain responses in relation to hypnotic susceptibility and suggestion.

In this report we use a dense array (129 electrodes) EEG procedure to examine the effects of hypnotic susceptibility and hypnotic suggestions on electrocortical and self-report measures of painful stimuli. Self-report and event-related potential measures of six high and six low hypnotic susceptible individuals in response to pain were examined during an initial baseline condition and following a standard hypnotic induction under suggestions to either increase (hyperalgesia) or decrease (hypoalgesia) the painful stimulation. Our results show that high and low hypnotically susceptible individuals: (1) show few self-report or psychophysiological differences in response to baseline pain stimuli; (2) report differential pain experiences depending on hypnotic suggestions and (3) display differential psychophysiological indicators following an hypnotic induction with a suggestion of hypoalgesia. Overall, the findings suggest that hypnotic suggestions with high susceptible individuals modulate the later components of the evoked potential in a global manner and point up the importance of using both high and low hypnotically susceptible individuals preceding and following an hypnotic induction.

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PubMed2002

Large-scale neural correlates of affective picture processing.

Hemodynamic and electrophysiological studies indicate differential brain response to emotionally arousing, compared to neutral, pictures. The time course and source distribution of electrocortical potentials in response to emotional stimuli, using a high-density electrode (129-sensor) array were examined here. Event-related potentials (ERPs) were recorded while participants viewed pleasant, neutral, and unpleasant pictures. ERP voltages were examined in six time intervals, roughly corresponding to P1, N1, early P3, late P3 and a slow wave window. Differential activity was found for emotional, compared to neutral, pictures at both of the P3 intervals, as well as enhancement of later posterior positivity. Source space projection was performed using a minimum norm procedure that estimates the source currents generating the extracranially measured electrical gradient. Sources of slow wave modulation were located in occipital and posterior parietal cortex, with a right-hemispheric dominance.

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