Roles of calcium- and voltage-sensitive potassium currents in the generation of neuromagnetic signals and field potentials in a CA3 longitudinal slice of the guinea-pig

Invariance in current dipole moment density across brain structures and species: physiological constraint for neuroimaging

Although anatomical constraints have been shown to be effective for MEG and EEG inverse solutions, there are still no effective physiological constraints. Strength of the current generator is normally described by the moment of an equivalent current dipole Q. This value is quite variable since it depends on size of active tissue. In contrast, the current dipole moment density q, defined as Q per surface area of active cortex, is independent of size of active tissue. Here we studied whether the value of q has a maximum in physiological conditions across brain structures and species. We determined the value due to the primary neuronal current (q primary) alone, correcting for distortions due to measurement conditions and secondary current sources at boundaries separating regions of differing electrical conductivities. The values were in the same range for turtle cerebellum (0.56-1.48 nAm/mm(2)), guinea pig hippocampus (0.30-1.34 nAm/mm(2)), and swine neocortex (0.18-1.63 nAm/mm(2)), rat neocortex (~2.2 nAm/mm(2)), monkey neocortex (~0.40 nAm/mm(2)) and human neocortex (0.16-0.77 nAm/mm(2)). Thus, there appears to be a maximum value across the brain structures and species (1-2 nAm/mm(2)). The empirical values closely matched the theoretical values obtained with our independently validated neural network model (1.6-2.8 nAm/mm(2) for initial spike and 0.7-3.1 nAm/mm(2) for burst), indicating that the apparent invariance is not coincidental. Our model study shows that a single maximum value may exist across a wide range of brain structures and species, varying in neuron density, due to fundamental electrical properties of neurons. The maximum value of q primary may serve as an effective physiological constraint for MEG/EEG inverse solutions.

Recursive artifact windowed-single tone extraction method (RAW-STEM) as periodic noise filter for electrophysiological signals with interfering transients

The single tone extraction method (STEM) is a well developed algorithm for estimating the frequency, amplitude, and phase of one periodic signal or a single tone in complex temporal signals. This method is useful in neuroscience research since it provides an efficient simple means to remove line frequency noise present in many types of signal measurements. However, the method encounters problems when the signal contains transients such as a stimulus artifact which distort the estimation of power line parameters. Here we report a modification of STEM that overcomes this limitation. In this new method we call recursive artifact windowed (RAW)-STEM, the line frequency noise is removed for each single epoch by estimating the three parameters (frequency, amplitude and phase) of each line frequency after windowing the time period containing an interfering transient and iteratively applying the STEM. In a simulation study we evaluated its performance for electrophysiological data with a stimulus artifact and demonstrated advantages of the RAW-STEM over the classic STEM. The RAW-STEM is able to efficiently extract the 60 Hz parameter, requiring less than five iterations, with a precision of 0.007-2% depending on the parameters. It does not suffer from the problem of ringing following the stimulus artifact or distortion of electrophysiological signals. It is fast enough to be used for single trial analyses in electrophysiological studies. The RAW-STEM may be widely useful for the removal of periodic noise since it can be applied even when there are multiple interfering transients in the recording.

Magnetic fields in the human hippocampal area evoked by a somatosensory oddball task

We recorded neuromagnetic fields evoked by a somatosensory oddball task requiring subjects to discriminate target stimuli from nontarget stimuli, which are different in stimulation intensity, with a mental count of the target stimuli. A whole head type 80-channel magnetoencephalography (MEG) system with a 50-mm baseline gradiometer array was used. Targets and nontargets were somatosensory stimuli with an electrical current intensity of twice and three times the sensory threshold, respectively. The source current locations of the evoked magnetic fields in three dimensions and the equivalent current dipole (ECD) moments were calculated by using a single dipole model, assuming the brain as a sphere. In 28 of 63 recording sessions for 7 subjects, the loci of neuronal activities were observed in the left and/or right hippocampal areas. The latency of left hippocampal activity (293 +/- 49 ms) was significantly shorter than that of the right (333 +/- 45 ms) (P = 0.027, non-paired t-test). In view of previous studies that have shown the time window of sensory integration as approximately 200-300 ms, sensory information may be transferred to hippocampal areas following sensory integration.

Contribution of ionic currents to magnetoencephalography (MEG) and electroencephalography (EEG) signals generated by guinea-pig CA3 slices

A mathematical model was used to analyse the contributions of different types of ionic currents in the pyramidal cells of longitudinal CA3 slices to the magnetic fields and field potentials generated by this preparation. Murakami et al. recently showed that a model based on the work of Traub et al. provides a quantitatively accurate account of the basic features of three types of empirical data (magnetic fields outside the slice, extracellular field potentials within the slice and intracellular potentials within the pyramidal neurons) elicited by stimulations of the soma and apical dendrites. This model was used in the present study to compute the net current dipole moment (Q) due to each of the different voltage- and ligand-gated channels in the cells in the presence of fast GABAA inhibition. These values of Q are proportional to the magnetic field and electrical potential far away from the slice. The intrinsic conductances were found to be more important than the synaptic conductances in determining the shape and magnitude of Q. Among the intrinsic conductances, the sodium (gNa) and delayed-rectifier potassium (gK(DR)) channels were found to produce sharp spikes. The high-threshold calcium channel (gCa) and C-type potassium channel (gK(C)) primarily determined the overall current waveforms. The roles of gCa and gK(C) were independent of small perturbations in these channel densities in the apical and basal dendrites. A combination of gNa, gK(DR), gCa, and gK(C) accounted for most of the evoked responses, except for later slow components, which were primarily due to synaptic channels.

Ionic channels and long-range electrical signals: a probabilistic interaction

The synchronized activation of ionic channels generates long-range electrical signals that can be detected on the surface using EEG and MEG. We propose that the spatial and temporal properties of these signals during rhythmic activity are associated with the probabilistic attributes of the ionic channels. Specifically: (1) The EEG-signals present a phase relationship with the open probability of the channels that drives the rhythm, (2) The oscillatory preferences of the signals--the 'EEG spectral profile'--are determined by the location probability (concentration) of the channels. In order to test and apply the hypothesis, it is suggested to correlate open and location probabilities with changes in the structure and dynamic of water molecules, water protons and pH using novel MRI-techniques.

Physiological origins of evoked magnetic fields and extracellular field potentials produced by guinea-pig CA3 hippocampal slices

This study examined whether evoked magnetic fields and intra- and extracellular potentials from longitudinal CA3 slices of guinea-pig can be interpreted within a single theoretical framework that incorporates ligand- and voltage-sensitive conductances in the dendrites and soma of the pyramidal cells. The 1991 CA3 mathematical model of R. D. Traub is modified to take into account the asymmetric branching patterns of the apical and basal dendrites of the pyramidal cells. The revised model accounts for the magnitude and waveform of the bi- and triphasic magnetic fields evoked by somatic and apical stimulations, respectively, in the slice in the absence of fast inhibition (blocked by 0.1 mM picrotoxin). The revised model also accounts for selective effects of 4-aminopyridine (4-AP) and tetraethylammonium (TEA), which block the potassium channels of A and C type, respectively, on the slow wave of the magnetic fields. Furthermore, the model correctly predicts the laminar profiles of field potential as well as intracellular potentials in the pyramidal cells produced by two classes of cells - those directly activated and those indirectly (synaptically) activated by the applied external stimulus. The intracellular potentials in this validated model reveal that the spikes and slow waves of the magnetic fields are generated in or near the soma and apical dendrites, respectively. These results demonstrate that a single theoretical framework couched within the modern concepts of cellular physiology provides a unified account of magnetic fields outside the slice, extracellular potentials within the slice and intracellular potentials of the pyramidal cells for CA3.