Scalp-recorded direct current potential shifts induced by hypocapnia and hypercapnia in humans

BOLD-contrast functional MRI signal changes related to intermittent rhythmic delta activity in EEG during voluntary hyperventilation-simultaneous EEG and fMRI study

Differences in the blood oxygen level dependent (BOLD) signal changes were studied during voluntary hyperventilation (HV) between young healthy volunteer groups, (1) with intermittent rhythmic delta activity (IRDA) (N = 4) and (2) controls (N = 4) with only diffuse arrhythmic slowing in EEG (normal response). Subjects hyperventilated (3 min) during an 8-min functional MRI in a 1.5-T scanner, with simultaneous recording of EEG (successful with N = 3 in both groups) and physiological parameters. IRDA power and average BOLD signal intensities (of selected brain regions) were calculated. Hypocapnia showed a tendency to be slightly lighter in the controls than in the IRDA group. IRDA power increased during the last minute of HV and ended 10-15 s after HV. The BOLD signal decreased in white and gray matter after the onset of HV and returned to the baseline within 2 min after HV. The BOLD signal in gray matter decreased approximately 30% more in subjects with IRDA than in controls, during the first 2 min of HV. This difference disappeared (in three subjects out of four) during IRDA in EEG. BOLD signal changes seem to depict changes, which precede IRDA. IRDA due to HV in healthy volunteers represent a model with a clearly defined EEG pattern and an observable BOLD signal change.

Millivolt-scale DC shifts in the human scalp EEG: evidence for a nonneuronal generator

Slow shifts in the human scalp-recorded EEG, including those related to changes in brain CO(2) levels, have been generally assumed to result from changes in the level of tonic excitation of apical dendrites of cortical pyramidal neurons. We readdressed this issue using DC-EEG shifts elicited in healthy adult subjects by hypo- or hypercapnia. A 3-min period of hyperventilation resulted in a prompt negative shift with a rate of up to 10 microV/s at the vertex (Cz) and an extremely steep dependence (up to 100 microV/mmHg) on the end-tidal Pco(2). This shift had a maximum of up to -2 mV at Cz versus the temporal derivations (T3/T4). Hyperventilation-like breathing of 5% CO(2)-95% O(2), which does not lead to a significant hypocapnia, resulted in a near-complete block of the negative DC shift at Cz. Hypoventilation, or breathing 5% CO(2) in air at normal respiratory rate, induced a positive shift. The high amplitude of the voltage gradients on the scalp induced by hyperventilation is not consistent with a neuronal origin. Instead, the present data suggest that they are generated by extracortical volume currents driven by a Pco(2)-dependent potential difference across epithelia separating the cerebrospinal fluid and blood. Since changes in respiratory patterns and, hence, in the level of brain Pco(2), are likely to occur under a number of experimental conditions in which slow EEG responses have been reported (e.g., attention shifts, preparatory states, epileptic seizures, and hypoxic episodes), the present results call for a thorough reexamination of the mechanisms underlying scalp-recorded DC-EEG responses.

Non-invasive single-trial monitoring of human movement-related brain activation based on DC-magnetoencephalography

Neuroimaging techniques, such as fMRI, PET and near-infrared spectroscopy, monitor task-related neuronal activations in the brain indirectly through the associated neurovascular/metabolic responses. To assess the primary neuronal activations directly, magnetoencephalography was combined here with a mechanical modulation of the head-to-sensor position and signal separation via independent component analysis. In all of five subjects this approach allowed to monitor the time evolution of DC fields (<0.1 Hz) over the left hemisphere related to complex finger movements of the right hand alternating with rest periods (30 s each). Throughout the recording period of 30 min, stable task-related DC fields were recordable in a single-trial mode, i.e. without any averaging. DC-MEG opens up the possibility of analysing non-invasively cortical DC-activity also in stroke, migraine or epilepsy patients.

Hyperventilation-induced human cerebral magnetic fields non-invasively monitored by multichannel 'direct current' magnetoencephalography

Self-paced hyperventilation (HV) induces slow cerebral magnetic field changes which were monitored and mapped continuously over 15 min using 49-channel DC-coupled ('direct current') magnetoencephalography (DC-MEG) based on a modulation technique. In nine/nine healthy subjects HV caused an increase (range: 1.1-6.2 pT) of the mean global DC-MEG field strength which slowly decayed after HV termination (mean time constant: 2 min). The complex HV-related field patterns were distinctly different from mainly dipolar somatosensory evoked field maps (N20m) in four/four subjects. Thus, current sources in the primary somatosensory cortex need not regularly dominate DC-field changes as had been previously considered. Rather, DC-MEG enabled the monitoring of a widely distributed HV-induced enhanced cortical excitability which may serve as model to study epileptic or post-anoxic cerebral hyperexcitability.

Hypoxia- and hypercapnia-induced DC potential shifts in rat at the scalp and the skull are opposite in polarity to those at the cerebral cortex

In anaesthetized and artificially ventilated rats DC (direct current) potential shifts induced by hypoxia or hypercapnia for 10 min were monitored at the surface of the skin, of the skull and of the cerebral cortex. Hypoxia was induced by decreasing the inspiratory O2 content from 20 to 10 or to 6% O2 in N2. Hypercapnia was induced by applying gas mixtures with CO2 contents from 0 to 5, 10, 20 or 30% CO2 in O2. DC potentials were recorded with non-polarizing electrodes filled with 150 mM NaCl solution. Hypercapnia evoked a negative DC shift (1.2-1.9 mV) epicranially and a large positive DC deflection (8.8-17.1 mV) epidurally. In contrast, hypoxia elicited a positive DC shift in skull recordings with a negative DC shift (2.6-3.1 mV) overshooting the baseline during recovery. DC shifts at the skin were positive, but smaller in amplitude (up to 3.1 mV). At the surface of the dura hypoxic DC shifts were negative and smaller (1.5-2.2 mV) than at the skull. The results show that a negative DC shift induced by hypercapnia recorded non-invasively from the skin or the skull is not always reflecting an increased cortical activation state and, vice versa, a positive DC shift does not always reflect a decreased state of neuronal activation. The electrogenesis of gas content-induced potentials could be due to the electrochemical diffusion of ions through the selective permeability barrier between the blood and tissue compartment. These observations are relevant for any application of non-invasive DC electroencephalography in the human cortex.