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Andreas K Engel

Publications and source records attributed to Andreas K Engel.

At least 19 recordsLinked to original sources
PubMed2006

Improved quality of auditory event-related potentials recorded simultaneously with 3-T fMRI: removal of the ballistocardiogram artefact.

EEG signals recorded simultaneously with fMRI are massively compromised by severe artefacts, among them the cardiac cycle-related ballistocardiogram (BCG) artefact. Different methods have been proposed to remove the BCG artefact focusing on channel-wise template subtraction procedures or spatial filtering approaches such as independent component analysis (ICA). Here we systematically compared the performance of the optimal basis set (OBS), a channel-wise correction approach, with ICA and a recently proposed combination of both (OBS-ICA). The three different procedures were applied to 60-channel EEG data from 12 subjects recorded during fMRI acquisition in a 3-T scanner. In addition to examination of the residual BCG artefact, the signal-to-noise ratio (SNR) and the topography of the resulting auditory evoked potential component N1 were compared. Whereas all three approaches led to a significant artefact reduction, the ICA procedure resulted in a significantly reduced N1 SNR and amplitude when compared to BCG-uncorrected data, indicating a rather poor performance. In contrast to ICA, OBS and OBS-ICA corrected data substantially improved the SNR of the N1. The quality of the auditory evoked potential N1 topography was investigated by means of equivalent current dipole modelling. On a descriptive level, all three correction procedures led to a reduced localization error when compared to BCG-uncorrected data. This improvement was significant for OBS-ICA. We conclude that OBS and OBS-ICA can efficiently remove BCG artefacts and substantially improve the quality of EEG signals recorded inside the scanner, a prerequisite for the successful integration of simultaneously recorded EEG and fMRI.

Adult↗
PubMed2006

Single-trial EEG-fMRI reveals the dynamics of cognitive function.

Two major non-invasive techniques in cognitive neuroscience, electroencephalography (EEG) and functional magnetic resonance imaging (fMRI), have complementary advantages with regard to their spatial and temporal resolution. Recent hardware and software developments have made it feasible to acquire EEG and fMRI data simultaneously. We emphasize the potential of simultaneous EEG and fMRI recordings to pursue new strategies in cognitive neuroimaging. Specifically, we propose that, by exploiting the combined spatiotemporal resolution of the methods, the integration of EEG and fMRI recordings on a single-trial level enables the rich temporal dynamics of information processing to be characterized within spatially well-defined neural networks.

Brain↗
PubMed2006

High-frequency activity in human visual cortex is modulated by visual motion strength.

A central goal in systems neuroscience is to understand how the brain encodes the intensity of sensory features. We used whole-head magnetoencephalography to investigate whether frequency-specific neuronal activity in the human visual cortex is systematically modulated by the intensity of an elementary sensory feature such as visual motion. Visual stimulation induced a tonic increase of neuronal activity at frequencies above 50 Hz. In order to define a functional frequency band of neuronal activity, we parametrically investigated which frequency band displays the strongest monotonic increase of responses with strength of visual motion. Consistently in all investigated subjects, this analysis resulted in a functional frequency band in the high gamma range from about 60 to 100 Hz in which activity reliably increased with visual motion strength. Using distributed source reconstruction, we found that this increase of high-frequency neuronal activity originates from several extrastriate cortical regions specialized in motion processing. We conclude that high-frequency activity in the human visual motion pathway may be relevant for encoding the intensity of visual motion signals.

Adult↗
PubMed2005

Trial-by-trial coupling of concurrent electroencephalogram and functional magnetic resonance imaging identifies the dynamics of performance monitoring.

Goal-directed behavior requires the continuous monitoring and dynamic adjustment of ongoing actions. Here, we report a direct coupling between the event-related electroencephalogram (EEG), functional magnetic resonance imaging (fMRI), and behavioral measures of performance monitoring in humans. By applying independent component analysis to EEG signals recorded simultaneously with fMRI, we found the single-trial error-related negativity of the EEG to be systematically related to behavior in the subsequent trial, thereby reflecting immediate behavioral adjustments of a cognitive performance monitoring system. Moreover, this trial-by-trial EEG measure of performance monitoring predicted the fMRI activity in the rostral cingulate zone, a brain region thought to play a key role in processing of response errors. We conclude that investigations of the dynamic coupling between EEG and fMRI provide a powerful approach for the study of higher order brain functions.

Adult↗
PubMed2005

EEG gamma-band activity in rapid serial visual presentation.

Evidence is available that oscillatory activity in the gamma frequency range (>30 Hz) might be related to the attentional selection of target items. Rapid serial visual presentation (RSVP) paradigms are instrumental in addressing cognitive functions such as visual attention, and they are increasingly combined with the measurement of electrical brain activity. In the present study, gamma-band responses for target and standard stimuli were investigated in an RSVP oddball paradigm. In a first study, stimuli were presented at a frequency of 10 Hz, the stimulus sequence consisted of rare colored letters (targets) and frequent black letters (standards). In addition, stimulus size was varied across experimental blocks. Significant target modulations were observed for the P3 ERP and induced (i.e., not phase-locked) gamma-band responses. Besides this late activation, no further gamma-band responses were observed. A second study aimed at replicating these findings by employing a reduced stimulus presentation rate of 7.1 Hz. Again, besides the P3 ERP a late increase in induced gamma-band activity was observed. However, as compared to Study 1, this induced response was less pronounced. The induced gamma-band response observed in the present studies might reflect utilization of information derived from previous processing steps for behavioral performance or memory storage as suggested in the 'match-and-utilization-model' of gamma activity.

Adult↗
PubMed2005

Cortical correlates of false expectations during pain intensity judgments--a possible manifestation of placebo/nocebo cognitions.

We investigated the effects of expectation on intensity ratings and somatosensory evoked magnetic fields and electrical potentials following painful infrared laser stimuli in six healthy subjects. The stimulus series contained trials preceded by different auditory cues which either contained valid, invalid or no information about the upcoming laser intensity. High and low intensities occurred equally probable across cue types. High intensity stimuli induced greater pain than low intensity across all cue types. Furthermore, laser intensity significantly interacted with cue validity: high intensity stimuli were perceived less painful and low intensity stimuli more painful following invalid compared to valid cues. The amplitude of the evoked magnetic field localized within the contralateral secondary somatosensory cortex (SII) at about 165 ms after laser stimuli varied also both with stimulus intensity and cue validity. The evoked electric potential peaked at about 300 ms after laser stimuli and yielded a single dipole source within a region encompassing the caudal anterior cingulate cortex and posterior cingulate cortex. Its amplitude also varied with stimulus intensity, but failed to show any cue validity effects. This result suggests a priming of early cortical nociceptive sensitivity by cues signaling pain severity. A possible contribution of the SII cortex to the manifestation of nocebo/placebo cognitions is discussed.

Acoustic Stimulation↗
PubMed2005

What is novel in the novelty oddball paradigm? Functional significance of the novelty P3 event-related potential as revealed by independent component analysis.

To better understand whether voluntary attention affects how the brain processes novel events, variants of the auditory novelty oddball paradigm were presented to two different groups of human volunteers. One group of subjects (n=16) silently counted rarely presented 'infrequent' tones (p=0.10), interspersed with 'novel' task-irrelevant unique environmental sounds (p=0.10) and frequently presented 'standard' tones (p=0.80). A second group of subjects (n=17) silently counted the 'novel' environmental sounds, the 'infrequent' tones now serving as the task-irrelevant deviant events. Analysis of event-related potentials (ERPs) recorded from 63 scalp channels suggested a spatiotemporal overlap of fronto-central novelty P3 and centro-parietal P3 (P3b) ERP features in both groups. Application of independent component analysis (ICA) to concatenated single trials revealed two independent component clusters that accounted for portions of the novelty P3 and P3b response features, respectively. The P3b-related ICA cluster contributed to the novelty P3 amplitude response to novel environmental sounds. In contrast to the scalp ERPs, the amplitude of the novelty P3 related cluster was not affected by voluntary attention, that is, by the target/nontarget distinction. This result demonstrates the usefulness of ICA for disentangling spatiotemporally overlapping ERP processes and provides evidence that task irrelevance is not a necessary feature of novelty processing.

Acoustic Stimulation↗
PubMed2005

Invasive recordings from the human brain: clinical insights and beyond.

Although non-invasive methods such as functional magnetic resonance imaging, electroencephalograms and magnetoencephalograms provide most of the current data about the human brain, their resolution is insufficient to show physiological processes at the cellular level. Clinical approaches sometimes allow invasive recordings to be taken from the human brain, mainly in patients with epilepsy or with movement disorders, and such recordings can sample neural activity at spatial scales ranging from single cells to distributed cell assemblies. In addition to their clinical relevance, these recordings can provide unique insights into brain functions such as movement control, perception, memory, language and even consciousness.

Brain↗
PubMed2005

The anterior ectosylvian visual area of the ferret: a homologue for an enigmatic visual cortical area of the cat?

The present paper describes the results of electrophysiological mapping experiments focused on the anterior limb of the ectosylvian gyrus of the ferret (Mustela putorius). The aim was to determine if the ferret possessed a homologous cortical area to the anterior ectosylvian visual area (AEV) of the domestic cat, but not clearly delineated in any other mammal studied to date. We were able to gather data on the visuotopic organization of a region that we consider to be a homologue of cat AEV. The visual map in this area showed a distinct visuotopic organization and covered a large extent of the visual field. Within the ferret AEV there were clusters of bimodal recording sites (somato-visual and audio-visual) that were located adjacent to surrounding unimodal cortical areas (such as the second somatosensory area and primary and secondary auditory areas). The ferret AEV, like that of the cat, was topographically isolated from other visual cortical areas by intervening auditory and somatosensory areas. Taken together these features suggest that the region described here as AEV in the ferret is indeed a direct homologue of the previously described cat AEV.

Animals↗
PubMed2004

Neural correlates of conscious perception in the attentional blink.

If attending to a target in a rapid stream of visual stimuli within the next 400 ms or so, a second target in the stream is frequently not detected by an observer. This so-called attentional blink can provide a comparison of neural signals elicited by identical stimuli that, in one condition, reach conscious awareness and, in the other, fail to be selected for awareness. In the present study, using event-related functional magnetic resonance imaging (fMRI), differences of neural activation were studied in an attentional blink experiment in order to identify brain regions putatively involved in controlling the access of information to consciousness. Subjects viewed a rapid stream of black letters in which the second target (T2) was either presented within or outside the attentional blink period, or not at all. We observed an increase in activation for detected as compared to missed T2 presented during the attentional blink in frontal and parietal cortices. In contrast, in occipitotemporal regions activation was increased for missed as compared to detected T2. Furthermore, in several frontal and parietal areas, missed targets were associated with increased activity if compared to the condition in which no second target was presented. Finally, a selective decrease in activation for detected T2 presented during the attentional blink was observed in areas associated with emotional and predominantly automatic processing. While activations in occipitotemporal regions might mainly reflect duration of attentive search, the frontoparietal areas seem to be involved in a highly distributed network controlling visual awareness.

Adult↗
PubMed2004

Amplitude and direction of saccadic eye movements depend on the synchronicity of collicular population activity.

Synchronization of neuronal discharges has been observed in numerous brain structures, but opinions diverge regarding its significance in neuronal processing. Here we investigate whether the motion vectors of saccadic eye movements evoked by electrical multisite stimulation of the cat superior colliculus (SC) are influenced by varying the degree of synchrony between the stimulus trains. With synchronous activation of SC sites, the vectors of the resulting saccades correspond approximately to the averages of the vectors of saccades evoked from each site alone. In contrast, when the pulses of trains applied to the different sites are temporally offset by as little as 5-10 ms, the vectors of the resulting saccades come close to the sum of the individual vectors. Thus saccade vectors depend not only on the site and amplitude of collicular activation but also on the precise temporal relations among the respective spike trains. These data indicate that networks within or downstream from the SC discriminate with high temporal resolution between synchronous and asynchronous population responses. This supports the hypothesis that information is encoded not only in the rate of neuronal responses but also in the precise temporal relations between discharges.

Action Potentials↗
PubMed2004

Size matters: effects of stimulus size, duration and eccentricity on the visual gamma-band response.

OBJECTIVE: The effects of stimulus size, duration and eccentricity on the visual gamma-band response (GBR) in human EEG were investigated and compared to visual evoked potentials (VEPs) in order to differentiate in future (and past) experiments whether changes in GBRs are due to stimulus-related (exogenous) or cognitive effects. METHODS: EEG was recorded from 23 subjects while they performed a simple choice reaction time task requiring discrimination of squares and circles. In separate blocks stimulus size, duration, and eccentricity were manipulated. EEG was recorded from 64 electrodes. A wavelet transform based on Morlet wavelets was employed for the analysis of gamma-band activity. RESULTS: Amplitude of the GBR was diminished for small and peripheral stimuli. With short stimulus durations ON and OFF responses of the GBR merged into one peak. In comparison, VEP amplitudes were less susceptible to stimulus features. In contrast to VEP latencies, however, GBR latency did not show a lateralization for eccentric stimuli. CONCLUSIONS: In addition to previous experiments which have shown a modulation of the GBR by various cognitive processes, the present results demonstrate the susceptibility of the GBR in human EEG to exogenous factors, as numerous intracortical recordings in non-human primates have shown before. The results suggest that the human GBR resides in early visual areas. SIGNIFICANCE: The demonstration of the susceptibility of the GBR to stimulus properties implies that studies aimed at exploring the involvement of the GBR in information processing have to be designed carefully. It also constrains the localization of the human GBR.

Adult↗
PubMed2004

Cognitive functions of gamma-band activity: memory match and utilization.

Oscillatory neural activity in the gamma frequency range (>30Hz) has been shown to accompany a wide variety of cognitive processes. So far, there has been limited success in assigning a unitary basic function to these oscillations, and critics have raised the argument that they could just be an epiphenomenon of neural processing. We propose a new framework that relates gamma oscillations observed in human, as well as in animal, experiments to two underlying processes: the comparison of memory contents with stimulus-related information and the utilization of signals derived from this comparison. This model attempts to explain early gamma-band responses in terms of the match between bottom-up and top-down information. Furthermore, it assumes that late gamma-band activity reflects the readout and utilization of the information resulting from this match.

Animals↗
PubMed2003

Features of neuronal synchrony in mouse visual cortex.

Synchronization of neuronal discharges has been hypothesized to play a role in defining cell assemblies representing particular constellations of stimulus features. In many systems and species, synchronization is accompanied by an oscillatory response modulation at frequencies in the gamma-band. The cellular mechanisms underlying these phenomena of synchronization and oscillatory patterning have been studied mainly in vitro due to the difficulty in designing a direct in vivo assay. With the prospect of using conditional genetic manipulations of cortical network components, our objective was to test whether the mouse would meet the criteria to provide a model system for the study of gamma-band synchrony. Multi-unit and local field potential recordings were made from the primary visual cortex of anesthetized C57BL/6J mice. Neuronal responses evoked by moving gratings, bars, and random dot patterns were analyzed with respect to neuronal synchrony and temporal patterning. Oscillations at gamma-frequencies were readily evoked with all types of stimuli used. Oscillation and synchronization strength were largest for gratings and decreased when the noise level was increased in random-dot patterns. The center peak width of cross-correlograms was smallest for bars and increased with noise, yielding a significant difference between coherent random dot patterns versus patterns with 70% noise. Field potential analysis typically revealed increases of power in the gamma-band during response periods. Our findings are compatible with a role for neuronal synchrony in mediating perceptual binding and suggest the usefulness of the mouse model for testing hypotheses concerning both the mechanisms and the functional role of temporal patterning.

Action Potentials↗
PubMed2003

Top-down attentional processing enhances auditory evoked gamma band activity.

In contrast to animal studies, relatively little is known about the functional significance of the early evoked gamma band activity in humans. We investigated whether evoked and induced 40 Hz activity differentiate automatic, bottom-up aspects of attention from voluntary, top-down related attentional demands. An auditory novelty-oddball task was applied to 14 healthy subjects. As predicted, more evoked gamma was found for the target condition than in the two task-irrelevant conditions. Since gamma band activity was not enhanced for novel stimuli, the evoked gamma response cannot be explained with a simple concept of stimulus arousal. Neither induced gamma nor the degree of 40 Hz phase-locking were different between the experimental conditions. Taken together, our data emphasize the role of evoked gamma band activity for top-down attentional processing.

Acoustic Stimulation↗
PubMed2003

Event-related potential correlates of the attentional blink phenomenon.

The attentional blink phenomenon results from a transitory impairment of attention that can occur during rapid serial stimulus presentation. A previous study on the physiological correlates of the attentional blink employing event-related potentials (ERPs) suggested that the P3 ERP component for target items presented during this impairment is completely suppressed. This has been taken to indicate that the target-related information does not reach working memory. To reevaluate this hypothesis, we compared ERPs evoked by detected and missed targets in the attentional blink paradigm. Eighteen subjects performed a rapid serial visual presentation (RSVP) task in which either one target (control condition) or two targets had to be detected. ERPs elicited by the second target were analyzed separately for trials in which the target had been detected and missed, respectively. As predicted, detected targets did elicit a P3 during and after the attentional blink period. No clear P3 was found for detected targets presented before the attentional blink, that is, at lag 1. In contrast, missed targets generally did not evoke a P3. Our results provide evidence that targets presented during the attentional blink period can reach working memory. Thus, these findings contribute to evaluating theories of the attentional blink phenomenon.

Adult↗
PubMed2002

Oscillatory neuronal synchronization in primary visual cortex as a correlate of stimulus selection.

Spike and local field potential activity were recorded simultaneously from multiple sites in primary visual cortex of strabismic cats, while monocular stimulation alternated with dichoptic stimulation, inducing interocular rivalry. During interocular rivalry, there is competition between the two nonfusible stimuli presented to the two eyes, and only one stimulus is selected at any time. We biased this competition in three different ways: (1) we exploited the condition that in strabismic cats there is often one dominant eye that is selected for most of the time. (2) We presented the two stimuli with a temporal offset, which biases competition in favor of the newly appearing stimulus. (3) We presented the two stimuli with highly different contrasts, which biases competition in favor of the stimulus with higher contrast. Whenever competition was biased in favor of the stimulus activating the recorded neurons, gamma-frequency synchronization of the respective responses was enhanced, and vice versa. Firing rates showed some differences between stimulation conditions. However, when present, these changes were inversely related to a competitive advantage of the respective stimulus. We hypothesize that enhanced gamma-frequency synchronization in primary visual cortex is a correlate of stimulus selection. Synchronization is likely to be translated into firing rate changes at later processing stages.

Action Potentials↗