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ABSTRACT: Growing evidence from electrophysiological data in animal and human studies suggests that multisensory interaction is not exclusively a higher-order process, but also takes place in primary sensory cortices. Such early multisensory interaction is thought to be mediated by means of phase resetting. The presentation of a stimulus to one sensory modality resets the phase of ongoing oscillations in another modality such that processing in the latter modality is modulated. In humans, evidence for such a mechanism is still sparse. In the current study, the influence of an auditory stimulus on visual processing was investigated by measuring the electroencephalogram (EEG) and behavioral responses of humans to visual, auditory, and audiovisual stimulation with varying stimulus-onset asynchrony (SOA). We observed three distinct oscillatory EEG responses in our data. An initial gamma-band response around 50 Hz was followed by a beta-band response around 25 Hz, and a theta response around 6 Hz. The latter was enhanced in response to cross-modal stimuli as compared to either unimodal stimuli. Interestingly, the beta response to unimodal auditory stimuli was dominant in electrodes over visual areas. The SOA between auditory and visual stimuli--albeit not consciously perceived--had a modulatory impact on the multisensory evoked beta-band responses; i.e., the amplitude depended on SOA in a sinusoidal fashion, suggesting a phase reset. These findings further support the notion that parameters of brain oscillations such as amplitude and phase are essential predictors of subsequent brain responses and might be one of the mechanisms underlying multisensory integration.
Journal of Neuroscience 05/2011; 31(21):7729-36. · 7.11 Impact Factor
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ABSTRACT: Previous studies showed higher gamma-band responses (GBRs, ≈40 Hz) of the electroencephalogram (EEG) for moving compared to stationary stimuli. However, it is unclear whether this modulation by motion reflects a special responsiveness of the GBR to the stimulus feature "motion," or whether GBR enhancements of similar magnitude can be elicited also by a salient change within a static stimulus that does not include motion. Therefore, we measured the EEG of healthy subjects watching stationary square wave gratings of high contrast that either started to move or reversed their black and white pattern shortly after their onset. The strong contrast change of the pattern reversal represented a salient but motionless change within the grating that was compared to the onset of the stationary grating and the motion onset. Induced and evoked GBRs were analyzed for all three display conditions. In order to assess the influence of fixational eye movements on the induced GBRs, we also examined the time courses of microsaccade rates during the three display conditions. Amplitudes of both evoked and induced GBRs were stronger for pattern reversal than for motion onset. There was no significant amplitude difference between the onsets of the stationary and moving gratings. However, mean frequencies of the induced GBR were ~10 Hz higher in response to the onsets of moving compared to stationary gratings. Furthermore, the modulations of the induced GBR did not parallel the modulations of microsaccade rate, indicating that our induced GBRs reflect neuronal processes. These results suggest that, within the gamma-band range, the encoding of moving gratings in early visual cortex is primarily based on an upward frequency shift, whereas contrast changes within static gratings are reflected by amplitude enhancement.
NeuroImage 03/2011; 55(2):808-17. · 5.89 Impact Factor
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ABSTRACT: In everyday life we often encounter situations in which we can expect a visual stimulus before we actually see it. Here, we study the impact of such stimulus anticipation on the actual response to a visual stimulus. Participants were to indicate the sex of deer and cattle on photographs of the respective animals. On some trials, participants were cued on the species of the upcoming animal whereas on other trials this was not the case. Time frequency analysis of the simultaneously recorded EEG revealed modulations by this cue stimulus in two time windows. Early [Formula: see text] spectral responses [Formula: see text] displayed strongest stimulus-locking for stimuli that were preceded by a cue if they were sufficiently large. Late [Formula: see text] responses displayed enhanced amplitudes in response to large stimuli and to stimuli that were preceded by a cue. For late responses, however, no interaction between cue and stimulus size was observed. We were able to explain these results in a simulation by prestimulus gain modulations (early response) and by decreased response thresholds (late response). Thus, it seems plausible, that stimulus anticipation results in a pretuning of local neural populations.
Cognitive Neurodynamics 07/2008; 2(2):89-100. · 0.99 Impact Factor
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ABSTRACT: Reactions to sensory events sometimes require quick responses whereas at other times they require a high degree of accuracy-usually resulting in slower responses. It is important to understand whether visual processing under different response speed requirements employs different neural mechanisms.
We asked participants to classify visual patterns with different levels of detail as real-world or non-sense objects. In one condition, participants were to respond immediately, whereas in the other they responded after a delay of 1 second. As expected, participants performed more accurately in delayed response trials. This effect was pronounced for stimuli with a high level of detail. These behavioral effects were accompanied by modulations of stimulus related EEG gamma oscillations which are an electrophysiological correlate of early visual processing. In trials requiring speeded responses, early stimulus-locked oscillations discriminated real-world and non-sense objects irrespective of the level of detail. For stimuli with a higher level of detail, oscillatory power in a later time window discriminated real-world and non-sense objects irrespective of response speed requirements.
Thus, it seems plausible to assume that different response speed requirements trigger different dynamics of processing.
PLoS ONE 02/2008; 3(2):e1675. · 4.09 Impact Factor
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ABSTRACT: Physical properties of visual stimuli affect electrophysiological markers of perception. One important stimulus property is spatial frequency (SF). Therefore, we studied the influence of SF on human alpha (8-13 Hz) and gamma (>30 Hz) electroencephalographic (EEG) responses in a choice reaction task. Since real world images contain multiple SFs, an SF mixture was also examined. Event related potentials were modulated by SF around 80 and 300 ms. Evoked gamma responses were strongest for the low SF and the mixture stimulus; alpha responses were strongest for high SFs. The results link evoked and induced alpha and evoked gamma responses in human EEG to different modes of stimulus processing.
Vision Research 07/2007; 47(15):2086-98. · 2.41 Impact Factor
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ABSTRACT: Phase-locked gamma oscillations have so far mainly been described in relation to perceptual processes such as sensation, attention or memory matching. Due to its very short latency ( approximately 90 ms) such oscillations are a plausible candidate for very rapid integration of sensory and motor processes.
We measured EEG in 13 healthy participants in a speeded reaction task. Participants had to press a button as fast as possible whenever a visual stimulus was presented. The stimulus was always identical and did not have to be discriminated from other possible stimuli. In trials in which the participants showed a fast response, a slow negative potential over central electrodes starting approximately 800 ms before the response and highly phase-locked gamma oscillations over central and posterior electrodes between 90 and 140 ms after the stimulus were observed. In trials in which the participants showed a slow response, no slow negative potential was observed and phase-locked gamma oscillations were significantly reduced. Furthermore, for slow response trials the phase-locked gamma oscillations were significantly delayed with respect to fast response trials.
These results indicate the relevance of phase-locked gamma oscillations for very fast (not necessarily detailed) integration processes.
BMC Neuroscience 02/2007; 8:27. · 3.04 Impact Factor
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ABSTRACT: Evoked, phase-locked gamma oscillations of the electroencephalogram (EEG) have been demonstrated to be modulated by both bottom-up as well as top-down factors. However, to date the test-retest reliability of these oscillations has not been studied systematically.
We recorded EEG activity of 12 healthy volunteers in response to stimuli of different sizes. Each participant took part in two sessions separated by two weeks in time. To obtain an estimate of the reliability of evoked gamma band responses (GBRs), we compared frequency and magnitude of phase-locked EEG oscillations between sessions.
In response to large stimuli magnitude and frequency of the evoked GBR yielded significant reliability. However, this was not the case for stimuli which were too small to evoke detectable GBRs.
The results are in accordance with studies demonstrating a dependence of gamma oscillations on stimulus parameters.
The current findings suggest that using appropriate stimulation, the evoked gamma response has sufficient test-retest reliability for use in assessing clinical changes in neurophysiological status.
Clinical Neurophysiology 02/2007; 118(1):221-7. · 3.41 Impact Factor
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ABSTRACT: Abstract
Background
Phase-locked gamma oscillations have so far mainly been described in relation to perceptual processes such as sensation, attention or memory matching. Due to its very short latency (≈90 ms) such oscillations are a plausible candidate for very rapid integration of sensory and motor processes.
Results
We measured EEG in 13 healthy participants in a speeded reaction task. Participants had to press a button as fast as possible whenever a visual stimulus was presented. The stimulus was always identical and did not have to be discriminated from other possible stimuli. In trials in which the participants showed a fast response, a slow negative potential over central electrodes starting approximately 800 ms before the response and highly phase-locked gamma oscillations over central and posterior electrodes between 90 and 140 ms after the stimulus were observed. In trials in which the participants showed a slow response, no slow negative potential was observed and phase-locked gamma oscillations were significantly reduced. Furthermore, for slow response trials the phase-locked gamma oscillations were significantly delayed with respect to fast response trials.
Conclusion
These results indicate the relevance of phase-locked gamma oscillations for very fast (not necessarily detailed) integration processes.
BMC Neuroscience. 01/2007;
