Slow cortical potential shifts modulate P300 amplitude and topography in humans

Neural Mechanisms Determining the Duration of Task-free, Self-paced Visual Perception

Humans spend hours each day spontaneously engaging with visual content, free from specific tasks and at their own pace. Currently, the brain mechanisms determining the duration of self-paced perceptual behavior remain largely unknown. Here, participants viewed naturalistic images under task-free settings and self-paced each image's viewing duration while undergoing EEG and pupillometry recordings. Across two independent data sets, we observed large inter- and intra-individual variability in viewing duration. However, beyond an image's presentation order and category, specific image content had no consistent effects on spontaneous viewing duration across participants. Overall, longer viewing durations were associated with sustained enhanced posterior positivity and anterior negativity in the ERPs. Individual-specific variations in the spontaneous viewing duration were consistently correlated with evoked EEG activity amplitudes and pupil size changes. By contrast, presentation order was selectively correlated with baseline alpha power and baseline pupil size. Critically, spontaneous viewing duration was strongly predicted by the temporal stability in neural activity patterns starting as early as 350 msec after image onset, suggesting that early neural stability is a key predictor for sustained perceptual engagement. Interestingly, neither bottom-up nor top-down predictions about image category influenced spontaneous viewing duration. Overall, these results suggest that individual-specific factors can influence perceptual processing at a surprisingly early time point and influence the multifaceted ebb and flow of spontaneous human perceptual behavior in naturalistic settings.

Simultaneous transcranial direct current stimulation (tDCS) and whole-head magnetoencephalography (MEG): assessing the impact of tDCS on slow cortical magnetic fields

Transcranial direct current stimulation (tDCS) can influence cognitive, affective or motor brain functions. Whereas previous imaging studies demonstrated widespread tDCS effects on brain metabolism, direct impact of tDCS on electric or magnetic source activity in task-related brain areas could not be confirmed due to the difficulty to record such activity simultaneously during tDCS. The aim of this proof-of-principal study was to demonstrate the feasibility of whole-head source localization and reconstruction of neuromagnetic brain activity during tDCS and to confirm the direct effect of tDCS on ongoing neuromagnetic activity in task-related brain areas. Here we show for the first time that tDCS has an immediate impact on slow cortical magnetic fields (SCF, 0-4Hz) of task-related areas that are identical with brain regions previously described in metabolic neuroimaging studies. 14 healthy volunteers performed a choice reaction time (RT) task while whole-head magnetoencephalography (MEG) was recorded. Task-related source-activity of SCFs was calculated using synthetic aperture magnetometry (SAM) in absence of stimulation and while anodal, cathodal or sham tDCS was delivered over the right primary motor cortex (M1). Source reconstruction revealed task-related SCF modulations in brain regions that precisely matched prior metabolic neuroimaging studies. Anodal and cathodal tDCS had a polarity-dependent impact on RT and SCF in primary sensorimotor and medial centro-parietal cortices. Combining tDCS and whole-head MEG is a powerful approach to investigate the direct effects of transcranial electric currents on ongoing neuromagnetic source activity, brain function and behavior.

One size fits all? Slow cortical potentials neurofeedback: a review

OBJECTIVE

The intent of this manuscript was to review all published studies on slow cortical potentials (SCP) neurofeedback for the treatment of ADHD, with emphasis on neurophysiological rationale, study design, protocol, outcomes, and limitations.

METHOD

For review, PubMed, MEDLINE, ERIC, and Google Scholar searches identified six studies and six subsequent publications. In addition to five studies focusing on children with Diagnostic and Statistical Manual of Mental Disorders (4th ed.; DSM-IV)-diagnosed ADHD, one study reports on adults.

RESULTS

SCP protocols utilize unipolar-electrode placement at Cz, randomized bidirectional signal regulation, feedback/transfer trials, and discrete feedback/rewards. Results demonstrated learning of SCP self-regulation, moderate to large within group effect sizes for core ADHD symptom reduction, and enhancement of event-related potentials/electroencephalogram components. Neurophysiological and session variables were predictive of treatment outcome, but open questions of specific and nonspecific effects remain. Study limitations and future directions are discussed.

CONCLUSION

SCP is an efficacious and standardized neurofeedback protocol that addresses behavioral and neurophysiological deficits in ADHD.

The fMRI signal, slow cortical potential and consciousness

As functional magnetic resonance imaging (fMRI) has become a driving force in cognitive neuroscience, it is crucial to understand the neural basis of the fMRI signal. Here, we discuss a novel neurophysiological correlate of the fMRI signal, the slow cortical potential (SCP), which also seems to modulate the power of higher-frequency activity, the more established neurophysiological correlate of the fMRI signal. We further propose a hypothesis for the involvement of the SCP in the emergence of consciousness, and review existing data that lend support to our proposal. This hypothesis, unlike several previous theories of consciousness, is firmly rooted in physiology and as such is entirely amenable to empirical testing.

Novelty P3 and P3b in first-episode schizophrenia and chronic schizophrenia

The objective of this study was to evaluate P3b and novelty P3 responses in patients with first-episode schizophrenia (FES) and chronic schizophrenia (CS). P3b is consistently reported to be reduced in CS. However, novelty P3 results in CS are controversial. Novelty P3 is not studied, and there are only a few P3b studies in patients with FES. Subject groups comprised 31 patients with FES and 36 younger control subjects, and 26 patients with CS and 35 older control subjects. Automatically elicited auditory novelty P3 and effortfully elicited auditory P3b potentials were assessed. P3b amplitudes were reduced in both patients with FES and CS relative to their controls. CS and FES patients did not differ in P3b amplitude. Novelty P3 amplitude was reduced in patients with CS. Novelty P3 amplitude in patients with FES did not differ from their controls. P3b amplitude reduction may be a trait marker of schizophrenia and may not progress over the course of illness, although this can only be definitively determined by longitudinal studies. Novelty P3 amplitude reduction present in patients with CS, is not found at the onset of illness. Novelty P3 seems unaffected early in the disease process.

Alpha rhythm of the EEG modulates visual detection performance in humans

The effects of the changes in the frequency spectrum of the electroencephalogram (EEG) on the perception of near-threshold visual stimuli and on the event-related potentials (ERPs) produced by these stimuli were investigated on 12 healthy volunteers. The stimulus intensity, at which each subject could detect 50% of the presented stimuli, was defined as the sensory threshold for that subject. Single ERP trials were separated into two groups: trials with detected and undetected stimuli. The ERPs and the average power spectra of the 1 s prestimulus periods were computed for both conditions. P300 amplitudes of the ERPs, and total power and relative band powers of the delta (0.5-4 Hz), theta (4-7.5 Hz), alpha (7.5-13 Hz), beta (13-30 Hz), and gamma (30-70 Hz) frequency bands of the prestimulus power spectra were measured. Between the two conditions, a specific difference was observed in the relative power of the alpha band, which was significantly lower before detected stimuli (p < 0.01) in line with significantly higher amplitudes of the ERPs (p < 0.001). These results show that short-lasting changes in brain's excitability state are reflected the relative alpha power of the EEG, which may explain significant variability in perceptual processes and ERP generation especially at boundary conditions such as sensory threshold.

Decomposition of event-related brain potentials into multiple functional components using wavelet transform

Event related brain potential (ERP) waveforms consist of several components extending in time, frequency and topographical space. Therefore, an efficient processing of data which involves the time, frequency and space features of the signal, may facilitate understanding the plausible connections among the functions, the anatomical structures and neurophysiological mechanisms of the brain. Wavelet transform (WT) is a powerful signal processing tool for extracting the ERP components occurring at different time and frequency spots. A technical explanation of WT in ERP processing and its four distinct applications are presented here. The first two applications aim to identify and localize the functional oddball ERP components in terms of certain wavelet coefficients in delta, theta and alpha bands in a topographical recording. The third application performs a similar characterization that involves a three stimulus paradigm. The fourth application is a single sweep ERP processing to detect the P300 in single trials. The last case is an extension of ERP component identification by combining the WT with a source localization technique. The aim is to localize the time-frequency components in three dimensional brain structure instead of the scalp surface. The time-frequency analysis using WT helps isolate and describe sequential and/or overlapping functional processes during ERP generation, and provides a possibility for studying these cognitive processes and following their dynamics in single trials during an experimental session.

Slow cortical potential shifts modulate the sensory threshold in human visual system

The relationship between the spontaneous slow cortical potential shifts and the detection of visual stimuli at sensory threshold were investigated. The mean slow cortical potentials preceding the detected stimuli were more negative than those preceding the missed stimuli. Accordingly, the stimulus detection performance was higher during negative compared to positive cortical potential shifts. These findings demonstrate that the cortical negativity reflects increased excitability of neural networks, thereby facilitates the detection of threshold stimuli, in contrast to cortical positivity. Therefore, at near-threshold stimulus intensities one reason for detecting the stimulus in one trial but missing it in another could be the change in the EEG baseline between the trials.

A model for P300 generation based on responses to near-threshold visual stimuli

Near-threshold and suprathreshold visual ERPs and their frequency components were compared with the aim to obtain further information on the generation mechanism of the P300 wave. Decrease of the stimulus energy from suprathreshold to near-threshold level resulted in an increase of the P300 amplitude specifically in the occipital region. This finding was in contrast with the P300 amplitude decrease in central and frontal regions and its constancy in parietal area. Delta and theta responses showed a similar distribution pattern, whereas alpha responses decreased in all regions as the stimulus energy decreased. We conclude that P300 wave may correspond to a delta oscillation during a widespread, transient interruption of afferent inputs from subcortical structures to the cortical neurons including those in the visual sensory area and simultaneous increase of the cortico-cortical interactions. If visual inputs are of suprathreshold strength, they override this effect specifically in the primary visual area and disrupt the cortico-cortical interactions and the emergence of P300 in the occipital cortex.