State-Dependent Visual Processing

The temporal dynamics and anatomical correlates underlying human visual cognition are traditionally assessed as a function of stimulus properties and task demands. Any non-stimulus related activity is commonly dismissed as noise and eliminated to extract an evoked signal that is only a small fraction of the magnitude of the measured signal. We review studies that challenge this view by showing that non-stimulus related activity is not mere noise but that it has a well-structured organization which can largely determine the processing of upcoming stimuli. We review recent evidence from human electrophysiology that shows how different aspects of pre-stimulus activity such as pre-stimulus EEG frequency power and phase and pre-stimulus EEG microstates can determine qualitative and quantitative properties of both lower and higher-level visual processing. These studies show that low-level sensory processes depend on the momentary excitability of sensory cortices whereas perceptual processes leading to stimulus awareness depend on momentary pre-stimulus activity in higher-level non-visual brain areas. Also speed and accuracy of stimulus identification have likewise been shown to be modulated by pre-stimulus brain states

Right Parietal Brain Activity Precedes Perceptual Alternation of Bistable Stimuli

Momentary fluctuations of baseline activity have been shown to influence responses to sensory stimulation both behaviorally and neurophysiologically. This suggests that perceptual awareness does not solely arise from physical stimulus properties. Here we studied whether the momentary state of the brain immediately before stimulus presentation indicates how a physically unique but perceptually ambiguous stimulus will be perceived. A complex Necker cube was intermittently presented and subjects indicated whether their perception changed with respect to the preceding presentation. EEG was recorded from 256 channels. The prestimulus brain-state was defined as the spatial configuration of the scalp potential map within the 50 ms before stimulus arrival, representing the sum of all momentary ongoing brain processes. Two maps were found that doubly dissociated perceptual reversals from perceptual stability. For EEG sweeps classified as either map, distributed inverse solutions were computed and statistically compared. This yielded activity confined to a region in right inferior parietal cortex that was significantly more active before a perceptual reversal. In contrast, no significant topographic differences of the evoked potentials elicited by stable vs. reversed Necker cubes were found. This indicates that prestimulus activity in right inferior parietal cortex is associated with the perceptual chang

EEG-Meta-Microstates: Towards a More Objective Use of Resting-State EEG Microstate Findings Across Studies.

Over the last decade, EEG resting-state microstate analysis has evolved from a niche existence to a widely used and well-accepted methodology. The rapidly increasing body of empirical findings started to yield overarching patterns of associations of biological and psychological states and traits with specific microstate classes. However, currently, this cross-referencing among apparently similar microstate classes of different studies is typically done by "eyeballing" of printed template maps by the individual authors, lacking a systematic procedure. To improve the reliability and validity of future findings, we present a tool to systematically collect the actual data of template maps from as many published studies as possible and present them in their entirety as a matrix of spatial similarity. The tool also allows importing novel template maps and systematically extracting the findings associated with specific microstate maps from ongoing or published studies. The tool also allows importing novel template maps and systematically extracting the findings associated with specific microstate maps in the literature. The analysis of 40 included sets of template maps indicated that: (i) there is a high degree of similarity of template maps across studies, (ii) similar template maps were associated with converging empirical findings, and (iii) representative meta-microstates can be extracted from the individual studies. We hope that this tool will be useful in coming to a more comprehensive, objective, and overarching representation of microstate findings

Whole-scalp EEG mapping of somatosensory evoked potentials in macaque monkeys

High-density scalp EEG recordings are widely used to study whole-brain neuronal networks in humans non-invasively. Here, we validate EEG mapping of somatosensory evoked potentials (SSEPs) in macaque monkeys (Macaca fascicularis) for the long-term investigation of large-scale neuronal networks and their reorganisation after lesions requiring a craniotomy. SSEPs were acquired from 33 scalp electrodes in five adult anaesthetized animals after electrical median or tibial nerve stimulation. SSEP scalp potential maps were identified by cluster analysis and identified in individual recordings. A distributed, linear inverse solution was used to estimate the intracortical sources of the scalp potentials. SSEPs were characterised by a sequence of components with unique scalp topographies. Source analysis confirmed that median nerve SSEP component maps were in accordance with the somatotopic organisation of the sensorimotor cortex. Most importantly, SSEP recordings were stable both intra- and interindividually. We aim to apply this method to the study of recovery and reorganisation of large-scale neuronal networks following a focal cortical lesion requiring a craniotomy. As a prerequisite, the present study demonstrated that a 300-mm2 unilateral craniotomy over the sensorimotor cortex necessary to induce a cortical lesion, followed by bone flap repositioning, suture and gap plugging with calcium phosphate cement, did not induce major distortions of the SSEPs. In conclusion, SSEPs can be successfully and reproducibly recorded from high-density EEG caps in macaque monkeys before and after a craniotomy, opening new possibilities for the long-term follow-up of the cortical reorganisation of large-scale networks in macaque monkeys after a cortical lesion

Electrophysiological Insights into Timing Aspects of Discourse Processing in Aphasic Patients

In the present thesis, three ERP experiments were used to investigate timing aspects of discourse processing in two groups of healthy subjects and two groups of aphasic patients. In two experiments, subjects were auditorily presented with sentence pairs. The two factors Context (Discourse, Sentence) and Coherence (Coherent, Incoherent) were crossed: in the Discourse condition, the second sentence continued the context introduced by the first in a meaningful way; in the Sentence condition, the two sentences were unrelated in meaning. For both the Sentence and the Discourse condition, a coherent and an incoherent version were created by means of the respective fit of the final word of the second sentence to which ERPs were acquired. In Experiment 1, semantic relationships between the critical word (CW) and at least one content word in the respective preceding context determined the respective fit/misfit of the CW (CW in CAPS, semantic match underlined; Discourse: Bob covered his pancakes with maple syrup. He likes them very SWEET / SPICY. Sentence: We went to see the famous performer. The gardener has mowed the LAWN / HAIR). Note that this introduced a semantic match / violation for the Coherent and Incoherent conditions, respectively.For the control groups, the hypothesis of interactive processing of discourse information was confirmed by the absence of onset differences of the N400 effects in both conditions which shows that both local (sentence) and global (discourse) coherence are processed concurrently and in parallel rather than serially.For the groups of aphasic patients, the nature of their processing deficit should be further determined, i.e. whether their comprehension deficits were due to impaired lexical activation or lexical integration. Furthermore, the hypothesis of heuristic use of semantic information was investigated. Patients were classified as either High or Low Comprehenders based on their auditory comprehension scores on the Boston Diagnostic Aphasia Examination (BDAE). ERP evidence was obtained which confirmed the hypothesis of an underlying integration rather than an activation deficit. ERP evidence reveals an integration deficit which varies as a function of both the severity of the comprehension deficit and the amount of information that has to be integrated: High Comprehenders show a stronger delay of the N400 component in the Discourse than the Sentence condition, and Low Comprehenders do not show any effects in either context condition. The hypothesis of heuristic use of semantic information could not be answered with the present design, i.e. it could not be determined whether the presence of a semantic relationship facilitated processing of the CW or whether the semantic violation impaired processing.A second ERP experiment with a similar design was used to further investigate this aspect of aphasic comprehension. In this experiment, the fit of the CW was to be derived from the respective context in the absence of lexical relationships (Discourse: Joel s office is forty miles away. He does not like the long COMMUTE / CURTAINS. Sentence: Helen reached up to dust the chandelier. While skiing, Randy broke his LEG / NOSE.). This experiment confirmed the results for the control groups from Experiment 1, and for the patients, it showed that rather from profiting from the semantic relations, they suffered from the semantic violations. Yet, did the integration impairment vary as a function of the severity of the deficit. High Comprehenders did show integration with a normal time course in the Sentence and delayed integration in the Discourse condition. Low Comprehenders showed a tendency toward a delay in the Sentence and no signs of successful integration in the Discourse condition.Finally, in a three-stimulus-oddball experiment, the language specificity of the N400 effects was confirmed: control subjects show a frontally distributed P3a component and a centro-posteriorly distributed P3b component. High Comprehenders do not show a P3a component, but they show a P3b component. Low Comprehenders do show both a P3a and a P3b component which rules out that the absence of N400 effects can be attributed to brain damage per se but that they are language specific

Errors can be related to pre-stimulus differences in ERP topography and their concomitant sources

Much of the variation in both neuronal and behavioral responses to stimuli can be explained by pre-stimulus fluctuations in brain activity. We hypothesized that also errors are the result of stochastic fluctuations in pre-stimulus activity and investigated the temporal dynamics of the scalp topography and their concomitant intracranial generators of stimulus- and response-locked high-density event-related potentials (ERPs) to errors and correct trials in a Stroop task. We found significant differences in ERP map topography and intracranial sources before the onset of the stimulus and after the initiation of the response but not as a function of stimulus-induced conflict. Before the stimulus, topographic differences were accompanied by differential activity in lateral frontal, parietal and temporal areas known to be involved in voluntary reorientation of attention and cognitive control. Differential post-response activity propagated both medially and laterally on a rostral-caudal axis of a network typically involved in performance monitoring. Analysis of the statistical properties of error occurrences revealed their stochasticity

Insights from intermittent binocular rivalry and EEG.

Novel stimulation and analytical approaches employed in EEG studies of ambiguous figures have recently been applied to binocular rivalry. The combination of intermittent stimulus presentation and EEG source imaging has begun to shed new light on the neural underpinnings of binocular rivalry. Here, we review the basics of the intermittent paradigm and highlight methodological issues important for interpreting previous results and designing future experiments. We then outline current analytical approaches, including EEG microstates, event-related potentials, and statistically based source estimation, and propose a neural model of the sequence of brain events that may underlie different aspects of binocular rivalry. Finally, we discuss the advantages and limitations of using binocular rivalry as a tool to investigate the neural basis of perceptual awareness