[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] ABSTRACT: Evoked gamma-band responses (GBRs) were shown to be involved in different aspects of human cognition and behavior. They have been linked to the integration and processing of incoming information leading to an adequate behavioral outcome. Consequently, altered evoked GBRs have been associated with impaired cognitive and behavioral states present in a variety of psychiatric disorders. However, to the best of our knowledge, there are no reports directly comparing evoked GBRs of different clinical groups in the same experimental setting. Thus, the purpose of the present study was to shed light on the question, whether evoked GBRs, as a kind of a neurophysiological biomarker of pathological states, might serve for characterization and distinguishing of groups suffering from diverse psychiatric disorders. We measured EEG during a passive auditory oddball-paradigm. Participants were patients diagnosed with schizophrenia, mood disorder, and personality disorders as well as a fourth group consisting of healthy participants. Our results indicate that evoked GBRs significantly differed from healthy participants only in schizophrenic patients whereas no difference could be observed for the other clinical groups. Our findings support the notion that early evoked GBRs could be indeed a trait variable of schizophrenia and are not a general marker of pathological brain states.
No preview · Article · Jan 2011 · International journal of psychophysiology: official journal of the International Organization of Psychophysiology
[Show abstract][Hide abstract] ABSTRACT: Neurophysiological studies yield contrary results whether attentional problems of patients with attention-deficit/hyperactivity disorder (ADHD) are related to early visual processing deficits or not. Evoked gamma-band responses (GBRs), being among the first cortical responses occurring as early as 90ms after visual stimulation in human EEG, have been assigned a pivotal role in early visual processing. In particular, they are involved in memory matching processes and are enhanced when known stimuli are processed. The current study examined whether evoked GBR patterns during early memory matching processes could be indicative of an early visual processing deficit in ADHD patients. EEG was recorded from 13 young ADHD patients as well as 13 age-matched healthy participants. Both groups performed a simple forced choice reaction task employing line drawings of either known real-world items with representations in long-term memory or physically similar unknown items without such representations. Evoked GBRs of ADHD patients did not differentiate between known and unknown items. However, in healthy children, evoked GBRs were enhanced when stimuli matched a representation stored in memory. This finding indicates disadvantages at early visual processing stages in ADHD patients: In contrast to healthy participants, ADHD children lack an early memory based classification, possibly resulting in an impaired ability to rapidly reallocate attentional resources to relevant stimuli. These findings suggest that impaired early automatic stimulus classification in ADHD patients could be involved in deficits of selective and sustained attention.
No preview · Article · Mar 2010 · Neuropsychologia
[Show abstract][Hide abstract] ABSTRACT: For efficient and fast encoding of our complex acoustic environment, not only aspects of bottom-up processing are significant, but rather top-down influences such as attention, memory, and anticipation promote specific behavior and perception. Neural oscillatory activity in the gamma-range (30-80 Hz) is discussed as a conceivable candidate to represent very rapid modulations of top-down factors. We investigated effects of anticipation on early gamma-band responses (GBRs) of the EEG and event-related potentials (ERPs) in response to tone sequences. These sequences were composed of six sinusoidal tones, which could be either regularly ascending or descending in frequency. Thus, the sequences reflected a good continuation of pitch, which also resulted in the buildup of strong expectancies for the upcoming stimulus within the sequence. However, some of the tone sequences contained a violation of the good continuation of pitch at the third or fifth tone position. The early phase-locked portion of the gamma-band activity was significantly increased when tones were in line with the good continuation of sequences compared to deviant tones. Further, a pronounced early negative ERP response, starting at 150 ms, was elicited by deviant tones at the third and fifth position. Our results support the notion that gamma-band oscillations reflect perceptual grouping processes of concurrent sounds and anticipatory top-down modulation, which involves some of the first stages of auditory information processing.
[Show abstract][Hide abstract] ABSTRACT: The time course of local field potentials (LFPs) displaying typical discharge frequencies in the gamma frequency range highly correlates with the blood oxygen level dependent (BOLD) signal in response to rotating checkerboard stimuli in animals. In humans, oscillatory gamma-band responses (GBRs) show strong inter-individual variations in frequency and amplitude but considerable intra-individual reliability indicating that individual gamma activity reflects a personal trait. While the functional role of these GBRs is still debated, investigations combining electroencephalography-functional magnetic resonance imaging (EEG-fMRI) measurements provide a tool to obtain further insights into the underlying functional architecture of the human brain and will shed light onto the understanding of the dynamic relation between the BOLD signal and the properties of the electrical activity recorded on the scalp. We investigated the relation between the hemodynamic response and evoked gamma-band response (eGBR) to visual stimulation. We tested the hypothesis that the amplitude of human eGBRs and BOLD responses covary intra-individually as a function of stimulation as well as inter-individually as a function of gamma-trait. Seventeen participants performed visual discrimination tasks during separate EEG and fMRI recordings. Results revealed that visual stimuli that evoked high GBRs also elicited strong BOLD responses in the human V1/V2 complex. Furthermore, inter-individual variations of BOLD responses to visual stimuli in the bilateral primary (Area 17) and secondary (Area V5/MT) visual cortex and the right hippocampal formation were correlated with the individual gamma-trait of the subjects. The present study further supports the notion that neural oscillations in the gamma frequency range are involved in the cascade of neural processes that underlie the hemodynamic responses measured with fMRI.
Full-text · Article · Feb 2009 · Frontiers in Human Neuroscience
[Show abstract][Hide abstract] ABSTRACT: The central focus of the current thesis is to compare the visual with the auditory system at different stages of information processing. Our perception is always an active process and therefore appears to involve an interaction between bottom-up information and feedback connections from higher-order cortical areas. Within the scope of studying such mechanisms of information processing in the visual and auditory modality, fast oscillatory activity in the frequency range between 30 and 80 Hz have received particular interest, since their relationship to sensory coding processes as well as to higher cognitive brain functions is frequently reported. The current thesis presents two experiments of low-level perception as well as one experiment of high-level perception, using the electroencephalogram (EEG). The first two experiments concentrated on bottom-up processing in the visual and auditory modality. I have presented grating stimuli with different visual contrast levels and further sinusoidal tones with different sound intensities to study the influence of this stimulus manipulation on the gamma-band response (GBR). Both experiments yielded similar findings: High contrast stimuli or tones with a high sound intensity evoked the largest GBRs (visual: at ~90 ms, auditory: at ~60 ms), while this increase in evoked activity is mainly caused by stronger phase-locking. Therefore, studies examining rather cognitive aspects of information processing should carefully match low-level attributes of their stimulus material to avoid confounding GBR modulations. Numerous impressions within our environment occur simultaneously, which appears to be a very complex process. That requires a fast perceptual analysis and organization to produce adequate behavioral reactions. We often anticipate events and are therefore able to prepare a faster and more accurate behavior. The third study aimed to investigate, in which way anticipation affects the auditory stimulus processing as an analog to a visual EEG-experiment (Fründ et al., 2008). As a result, I observed an enhancement in the early evoked GBR (at ~50 ms) that reflects memory matching and anticipatory processes. The findings suggest that anticipatory top-down modulation involves some of the first stages of auditory information processing. The findings are in line with anticipatory processing of visual stimuli. In conclusion, early processing of stimulus features is equally reflected in the evoked GBR occurring with a latency between 60-100 ms after stimulus onset with generators in predominantly sensory cortices. There is further convincing evidence for similar operating top-down processing concerning memory matching and anticipatory mechanisms reflected in the evoked GBR.
[Show abstract][Hide abstract] ABSTRACT: Gamma-band responses (GBRs) are associated with Gestalt perception processes. In the present EEG study, we investigated the effects of perceptual grouping on the visual GBR in the perimetrically intact visual field of patients with homonymous hemianopia and compared them to healthy participants. All observers were presented either random arrays of Gabor elements or arrays with an embedded circular arrangement. For the hemianopic patients, the circle was presented in their intact hemifield only. For controls, the hemifield for the circle presentation was counterbalanced across subjects. The participants were instructed to detect the circle by pressing a corresponding button. A wavelet transform based on Morlet wavelets was employed for the calculation of oscillatory GBRs. The early evoked GBR exhibited a larger amplitude and shorter latency for the healthy group compared to hemianopic patients and was associated with behavioral measures. The late total GBR between 200 and 400ms after stimulus onset was significantly increased for Gestalt-like patterns in healthy participants. This effect was not manifested in patients. The present findings indicate deficits in the early and late visual processing of Gestalt patterns even in the intact hemifield of hemianopic patients compared to healthy participants.
[Show abstract][Hide abstract] ABSTRACT: Previous electrophysiological as well as imaging research has contributed to the understanding of impairments in attention, executive functions, and memory in patients with attention-deficit/hyperactivity disorder (ADHD). However, there is a lack of studies investigating ADHD related differences in the gamma range of human electroencephalogram (EEG), although gamma activity is strongly associated with cognitive processes impaired in ADHD patients and is also modulated by dopamine polymorphisms linked with ADHD. To close this gap, the present study compared gamma activity in ADHD children with that of healthy controls and correlated it with memory performance. EEG was recorded from 13 ADHD patients as well as 13 healthy control subjects during the encoding phase of a visual memory paradigm. In a subsequent recognition test, participants had to judge pictures as being old or new. Analysis of evoked gamma-band responses (GBRs) during stimulus encoding revealed a strong task-related enhancement for ADHD patients in parieto-occipital areas. Interestingly, this augmentation was not associated with recognition performance, whereas healthy subjects exhibited a strong positive correlation between evoked gamma activity during stimulus encoding and subsequent recognition performance. We interpret this finding as evidence of enhanced excitation levels and unspecific activation of processing resources in ADHD patients. Furthermore, enhanced GBRs in ADHD could also indicate a decrease of neuronal signal-to-noise ratio, partially caused by the genetic variations within the dopaminergic pathway of ADHD patients. The involved genetic polymorphisms have been shown to modulate evoked GBRs, which therefore could be a possible marker of impaired neurotransmission in ADHD.
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] ABSTRACT: The simultaneous recording of EEG and fMRI offers the advantage of combining precise spatial information about neuronal processing obtained by fMRI data with the high temporal resolution of EEG data. One problem for the analysis of auditory processing, however, is the noisy environment during fMRI measurements, especially when EPI sequences are employed. While EEG studies outside an MRI scanner repeatedly demonstrated a clear sound level-dependent increase of N1 amplitude, this finding was less obvious in simultaneous recordings inside a scanner. Based on the assumption that this inconsistency might be due to the confounding effect of the rather loud EPI noise, we employed a low-noise fMRI protocol. This method was previously used to reveal level-dependent fMRI activation in auditory cortex areas. We combined this method with simultaneous EEG recordings to investigate the effect of different sound intensities on the auditory evoked potentials. Eight participants without hearing deficits took part in our experiment. Frequency modulated tones (FM) were presented monaurally with two sound intensities (60 and 80 dB HL). The task of the participants was to categorize the FM-direction (rising vs. falling). Our results inside the scanner replicate the sound level dependence of AEPs from previous EEG studies outside the scanner. The data analysis revealed a significant shortening of N1 latency and an increase in the N1-P2 peak-to-peak amplitude for the higher sound intensity. On a descriptive level, the 80 dB HL stimulation yielded more activated voxels in fMRI and stronger activations. This effect was pronounced over the right hemisphere. Our results suggest that low-noise sequences might be advantageous for the examination of auditory processing in simultaneous EEG and fMRI recordings.
No preview · Article · Apr 2008 · International Journal of Psychophysiology
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] ABSTRACT: We studied the effect of different contrast levels on the visual evoked gamma-band response (GBR) in order to investigate whether the GBR is modulated in a similar manner as previously reported for visual evoked potentials. Previous studies showed that the GBR can be modulated by individual characteristics (age) and experimental conditions (task difficulty, attention). However, stimulus properties, such as size and spatial frequency, also have a large impact on the GBR, which necessitates identification and control of relevant stimulus properties for optimal experimental setups. Twenty-one healthy participants were investigated during a forced-choice discrimination task. Sinusoidal gratings were presented at three contrast levels with a constant spatial frequency of 5 cycles per degree visual arc (cpd). The present data replicate the results reported for visual evoked potentials and exhibit a contrast dependent modulation of the GBR. Gamma activity is increased for higher contrast levels. These results demonstrate the importance of stimulus contrast for evoked gamma activity. Thus, it appears meaningful to control the contrast of stimuli in experiments investigating the role of gamma activity in perception and information processing.
Full-text · Article · Nov 2007 · International Journal of Psychophysiology
[Show abstract][Hide abstract] ABSTRACT: Auditory perception comprises bottom-up as well as top-down processes. While research in the past has revealed many neural correlates of bottom-up processes, less is known about top-down modulation. Memory processes have recently been associated with oscillations in the gamma-band of human EEG (30 Hz and above) which are enhanced when incoming information matches a stored memory template. Therefore, we investigated event-related potentials (ERPs) and gamma-band activity in 17 healthy participants in a Go/NoGo-task. They listened to four frequency-modulated (FM) sounds which varied regarding the frequency range traversed and the direction of frequency modulation. One sound was defined as target and required a button press. The results of ERPs (N1, P2, N2, and P3) were consistent with previous studies. Analysis of evoked gamma-band responses yielded no significant task-dependent modulation, but we observed a stimulus dependency, which was also present in a control experiment: The amplitude of evoked gamma responses showed an inverted U-shape as a function of stimulus frequency. Investigation of total gamma activity revealed functionally relevant responses at high frequencies (90 Hz to 250 Hz), which showed significant modulations by matches with STM: Complete matches led to the strongest enhancements (starting around 100 ms after stimulus onset) and partial matches resulted in intermediate ones. The results support the conclusion that very high frequency oscillations (VHFOs) are markers of active stimulus discrimination in STM matching processes and are attributable to higher cognitive functions.
[Show abstract][Hide abstract] ABSTRACT: We studied the effect of different sound intensities on the auditory evoked gamma-band response (GBR). Previous studies observed oscillatory gamma activity in the auditory cortex of animals and humans. For the visual modality, it has been demonstrated that the GBR can be modulated by top-down (attention, memory) as well as bottom-up factors (stimulus properties). Therefore, we expected to find a sound intensity modulation for the auditory GBR. 21 healthy participants without hearing deficits were investigated in a forced-choice discrimination task. Sinusoidal tones were presented at three systematically varied sound intensities (30, 45, 60 dB hearing level). The results of the auditory evoked potentials were predominantly consistent with previous studies. Furthermore, we observed an augmentation of the evoked GBR with increasing sound intensity. The analysis indicated that this intensity difference in the GBR amplitude most likely arises from increased phase-locking. The results demonstrate a distinct dependency between sound intensity and gamma-band oscillations. Future experiments that investigate the relationship between auditory evoked GBRs and higher cognitive processes should therefore select stimuli with an adequate sound intensity and control this variable to avoid confounding effects. In addition, it seems that gamma-band activity is more sensitive to exogenous stimulus parameters than evoked potentials.
Full-text · Article · Sep 2007 · International Journal of Psychophysiology
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] ABSTRACT: In recent years the cognitive functions of human gamma-band activity (30-100 Hz) advanced continuously into scientific focus. Not only bottom-up driven influences on 40 Hz activity have been observed, but also top-down processes seem to modulate responses in this frequency band. Among the various functions that have been related to gamma activity a pivotal role has been assigned to memory processes. Visual experiments suggested that gamma activity is involved in matching visual input to memory representations. Based on these findings we hypothesized that such memory related modulations of gamma activity exist in the auditory modality, as well. Thus, we chose environmental sounds for which subjects already had a long-term memory (LTM) representation and compared them to unknown, but physically similar sounds. 21 subjects had to classify sounds as 'recognized' or 'unrecognized', while EEG was recorded. Our data show significantly stronger activity in the induced gamma-band for recognized sounds in the time window between 300 and 500 ms after stimulus onset with a central topography. The results suggest that induced gamma-band activity reflects the matches between sounds and their representations in auditory LTM.
Full-text · Article · May 2007 · International Journal of Psychophysiology
[Show abstract][Hide abstract] 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.
Full-text · Article · Feb 2007 · Clinical Neurophysiology
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] ABSTRACT: The early visual gamma-band response is an oscillatory signal evoked approximately 100 ms after stimulation. While some studies have found effects of various cognitive processes on this signal, such effects could not be replicated in other studies. Accordingly, some authors have claimed that evoked gamma-band activity reflects merely sensory functions. To resolve these conflicting positions, we conducted a target detection experiment in which the feature that defined the target could be distributed over a large or a small part of the entire stimulus. Only targets covering a larger area of the entire stimulus evoked stronger gamma-band activity than standards although the over-all stimulus size was identical for all stimuli. This increase in evoked activity resulted from stronger oscillatory power and not exclusively from stronger phase-locking. In contrast, N1 and P3 amplitudes were larger for target stimuli irrespective of the distribution of the relevant stimulus feature. These results are consistent with the notion that early gamma-band activity is generated by feature-selective neural assemblies the activity of which can in fact be modulated by top-down processes. This interaction, however, may be only detectable in scalp-recorded EEG if it affects a sufficient number of neural assemblies.