Laminar interactions in rat motor cortex during cyclical excitability changes of the penicillin focus

Uncovering population contributions to the extracellular potential in the mouse visual system using Laminar Population Analysis

The local field potential (LFP), the low-frequency part of the extracellular potential, reflects transmembrane currents in the vicinity of the recording electrode. Thought mainly to stem from currents caused by synaptic input, it provides information about neural activity complementary to that of spikes, the output of neurons. However, the many neural sources contributing to the LFP, and likewise the derived current source density (CSD), can often make it challenging to interpret. Efforts to improve its interpretability have included the application of statistical decomposition tools like principal component analysis (PCA) and independent component analysis (ICA) to disentangle the contributions from different neural sources. However, their underlying assumptions of, respectively, orthogonality and statistical independence are not always valid for the various processes or pathways generating LFP. Here, we expand upon and validate a decomposition algorithm named Laminar Population Analysis (LPA), which is based on physiological rather than statistical assumptions. LPA utilizes the multiunit activity (MUA) and LFP jointly to uncover the contributions of different populations to the LFP. To perform the validation of LPA, we used data simulated with the large-scale, biophysically detailed model of mouse V1 developed by the Allen Institute. We find that LPA can identify laminar positions within V1 and the temporal profiles of laminar population firing rates from the MUA. We also find that LPA can estimate the salient current sinks and sources generated by feedforward input from the lateral geniculate nucleus (LGN), recurrent activity in V1, and feedback input from the lateromedial (LM) area of visual cortex. LPA identifies and distinguishes these contributions with a greater accuracy than the alternative statistical decomposition methods, PCA and ICA. The contributions from different cortical layers within V1 could however not be robustly separated and identified with LPA. This is likely due to substantial synchrony in population firing rates across layers, which may be reduced with other stimulus protocols in the future. Lastly, we also demonstrate the application of LPA on experimentally recorded MUA and LFP from 24 animals in the publicly available Visual Coding dataset. Our results suggest that LPA can be used both as a method to estimate positions of laminar populations and to uncover salient features in LFP/CSD contributions from different populations.

Uncovering population contributions to the extracellular potential in the mouse visual system using Laminar Population Analysis

The local field potential (LFP), the low-frequency part of the extracellular potential, reflects transmembrane currents in the vicinity of the recording electrode. Thought mainly to stem from currents caused by synaptic input, it provides information about neural activity complementary to that of spikes, the output of neurons. However, the many neural sources contributing to the LFP, and likewise the derived current source density (CSD), can often make it challenging to interpret. Efforts to improve its interpretability have included the application of statistical decomposition tools like principal component analysis (PCA) and independent component analysis (ICA) to disentangle the contributions from different neural sources. However, their underlying assumptions of, respectively, orthogonality and statistical independence are not always valid for the various processes or pathways generating LFP. Here, we expand upon and validate a decomposition algorithm named Laminar Population Analysis (LPA), which is based on physiological rather than statistical assumptions. LPA utilizes the multiunit activity (MUA) and LFP jointly to uncover the contributions of different populations to the LFP. To perform the validation of LPA, we used data simulated with the large-scale, biophysically detailed model of mouse V1 developed by the Allen Institute. We find that LPA can identify laminar positions within V1 and the temporal profiles of laminar population firing rates from the MUA. We also find that LPA can estimate the salient current sinks and sources generated by feedforward input from the lateral geniculate nucleus (LGN), recurrent activity in V1, and feedback input from the lateromedial (LM) area of visual cortex. LPA identifies and distinguishes these contributions with a greater accuracy than the alternative statistical decomposition methods, PCA and ICA. Lastly, we also demonstrate the application of LPA on experimentally recorded MUA and LFP from 24 animals in the publicly available Visual Coding dataset. Our results suggest that LPA can be used both as a method to estimate positions of laminar populations and to uncover salient features in LFP/CSD contributions from different populations.

waveCSD: A method for estimating transmembrane currents originated from propagating neuronal activity in the neocortex: Application to study cortical spreading depression

BACKGROUND

Recent years have witnessed an upsurge in the development of methods for estimating current source densities (CSDs) in the neocortical tissue from their recorded local field potential (LFP) reflections using microelectrode arrays. Among these, methods utilizing linear arrays work under the assumption that CSDs vary as a function of cortical depth; whereas they are constant in the tangential direction, infinitely or in a confined cylinder. This assumption is violated in the analysis of neuronal activity propagating along the neocortical sheet, e.g. propagation of alpha waves or cortical spreading depression.

NEW METHOD

Here, we developed a novel mathematical method (waveCSD) for CSD analysis of LFPs associated with a planar wave of neocortical neuronal activity propagating at a constant velocity towards a linear probe.

RESULTS

Results show that the algorithm is robust to the presence of noise in LFP data and uncertainties in knowledge of propagation velocity. Also, results show high level of accuracy of the method in a wide range of electrode resolutions. Using in vivo experimental recordings from the rat neocortex, we employed waveCSD to characterize transmembrane currents associated with cortical spreading depressions.

COMPARISON WITH EXISTING METHOD(S)

Simulation results indicate that waveCSD has a significantly higher reconstruction accuracy compared to the widely-used inverse CSD method (iCSD), and the regularized kernel CSD method (kCSD), in the analysis of CSDs originating from propagating neuronal activity.

CONCLUSIONS

The waveCSD method provides a theoretical platform for estimation of transmembrane currents from their LFPs in experimental paradigms involving wave propagation.

Laminar population analysis: estimating firing rates and evoked synaptic activity from multielectrode recordings in rat barrel cortex

We present a new method, laminar population analysis (LPA), for analysis of laminar-electrode (linear multielectrode) data, where physiological constraints are explicitly incorporated in the mathematical model: the high-frequency band [multiunit activity (MUA)] is modeled as a sum over contributions from firing activity of multiple cortical populations, whereas the low-frequency band [local field potential (LFP)] is assumed to reflect the dendritic currents caused by synaptic inputs evoked by this firing. The method is applied to stimulus-averaged laminar-electrode data from barrel cortex of anesthetized rat after single whisker flicks. Two sample data sets, distinguished by stimulus paradigm, type of applied anesthesia, and electrical boundary conditions, are studied in detail. These data sets are well accounted for by a model with four cortical populations: one supragranular, one granular, and two infragranular populations. Population current source densities (CSDs; the CSD signatures after firing in a particular population) provided by LPA are further used to estimate the synaptic connection pattern between the various populations using a new LFP template-fitting technique, where LFP population templates are found by the electrostatic forward solution based on results from compartmental modeling of morphologically reconstructed neurons. Our analysis confirms previous experimental findings regarding the synaptic connections from neurons in the granular layer onto neurons in the supragranular layers and provides predictions about other synaptic connections. Furthermore, the time dependence of the stimulus-evoked population firing activity is predicted, and the temporal ordering of response onset is found to be compatible with earlier findings.

Epileptiform activity preferentially arises outside tumor invasion zone in glioma xenotransplants

Seizures occur commonly with brain tumors. The underlying mechanisms are not understood. We analyzed network and cellular excitability changes in tumor-invaded and sham neocortical tissue in vitro using a rat glioblastoma model. Rat C6 glioma cells were transplanted into rat neocortex, yielding diffusely invading gliomas resembling human glioblastomas. We hypothesized that network excitability would increase in regions neighboring the tumor, and that initiation of epileptic discharges might be correlated to a higher density of intrinsically bursting neurones. Voltage-sensitive dye imaging revealed epileptic activity to be initiated in paratumoral zones (1-2 mm from main tumor mass), in contrast to control tissue, where epileptic foci appeared randomly throughout the neocortex. Neuronal firing patterns revealed significantly more intrinsically bursting neurones within these initiation zones than in regions directly adjacent to the tumor or in control tissue. We conclude that gliomas are associated with a higher density of intrinsically bursting neurones, and that these may preferentially initiate epileptiform events.

Negative myoclonus induced by cortical electrical stimulation in epileptic patients

Negative myoclonus (NM) is a motor disorder characterized by a sudden and abrupt interruption of muscular activity. The EMG correlate of NM is a brief (<500 ms) silent period (SP) not preceded by any enhancement of EMG activity (i.e. myoclonus). This study investigated the role of premotor cortex (PMC), primary motor cortex (MI), primary somatosensory area (SI) and supplementary motor area (SMA) in the pathophysiology of cortical NM by means of intracerebral low frequency (1 Hz) electrical stimulation. In three drug-resistant epileptic patients undergoing presurgical evaluation, we delivered single electric pulses (stimulus duration: 3 ms; stimulus intensity ranging from 0.4 to 3 mA) to PMC (2 patients), MI (1 patient), SI and SMA through stereo-EEG electrodes; surface EMG was collected from both deltoids. The results showed that (i) the stimulation of PMC or MI could evoke a motor evoked potential (MEP) either at rest or during contraction, in this latter case followed by an SP; however, in two patients, at the lowest stimulus intensities (0.4 mA), 50% of stimuli could induce a pure SP, i.e. not preceded by an MEP; raising the intensity of stimulation (0.6 mA), the SPs showed an antecedent MEP in >80% of stimuli; (ii) the stimulation of SI at low stimulus intensities (from 0.4 to 0.8 mA) induced in two patients only SPs, never associated with an antecedent MEP, whereas in the third subject the SPs could be inconstantly preceded by an MEP; by incrementing the stimulus intensity (up to 3 mA), in all three patients the SPs tended to be preceded, although not constantly, by an MEP; stimulus intensity affected SP duration (i.e. the higher the intensity, the longer the SP), without influencing the latency of onset of the SPs; (iii) the stimulation of SMA induced only pure SPs, at all stimulus intensities up to 3 mA; as for SI, increment of stimulus intensity was paralleled by an increase in SP duration, without influencing the onset latency of SPs. We conclude that single electric pulse stimulation of PMC, MI, SI and SMA through stereo-EEG electrodes can induce pure SPs, not preceded by an MEP, which clinically appear as NM, suggesting therefore that these cortical areas may be involved in the genesis of this motor phenomenon. However, it must be pointed out that SMA stimulation induced only pure SPs, regardless of the stimulus intensity, whereas occurrence of pure SPs following stimulation of PMC, MI, and SI depended mainly on the intensity of stimulation.

Horizontal spread of activity in neocortical inhibitory networks

In the presence of 4-aminopyridine (4-AP) and excitatory amino acid receptor blockers, GABAergic networks in the neocortex give rise to large spontaneous GABA-mediated depolarizations. We used voltage-sensitive dye techniques to explore the network properties of depolarizing GABA responses. Voltage-sensitive dye signals demonstrated that the superficial layers support the propagation of depolarizing GABA responses, with only minimal signals detected in deeper cortical layers. GABA responses propagated at a speed of 2.7 +/- 0.2 mm/s, a rate intermediate to fast synaptic transmission and spreading depression. Changes in the extracellular potassium concentration altered the propagation speed of the depolarizing GABA response. Taken together, these data support a role for both direct synaptic action of GABA at GABA(A) receptors and nonsynaptic mechanisms in the generation and propagation of depolarizing GABA responses.

Interictal spikes in focal epileptogenesis

Interictal electroencephalography (EEG) potentials in focal epilepsies are sustained by synchronous paroxysmal membrane depolarization generated by assemblies of hyperexcitable neurons. It is currently believed that interictal spiking sets a condition that preludes to the onset of an ictal discharge. Such an assumption is based on little experimental evidence. Human pre-surgical studies and recordings in chronic and acute models of focal epilepsy showed that: (i) interictal spikes (IS) and ictal discharges are generated by different populations of neuron through different cellular and network mechanisms; (ii) the cortical region that generates IS (irritative area) does not coincide with the ictal-onset area; (iii) IS frequency does not increase before a seizure and is enhanced just after an ictal event; (iv) spike suppression is found to herald ictal discharges; and (v) enhancement of interictal spiking suppresses ictal events. Several experimental evidences indicate that the highly synchronous cellular discharge associated with an IS is generated by a multitude of mechanisms involving synaptic and non-synaptic communication between neurons. The synchronized neuronal discharge associated with a single IS induces and is followed by a profound and prolonged refractory period sustained by inhibitory potentials and by activity-dependent changes in the ionic composition of the extracellular space. Post-spike depression may be responsible for pacing interictal spiking periodicity commonly observed in both animal models and human focal epilepsies. It is proposed that the strong after-inhibition produced by IS protects against the occurrence of ictal discharges by maintaining a low level of excitation in a general condition of hyperexcitability determined by the primary epileptogenic dysfunction.

Current-source-density profiles associated with sharp waves in human epileptic neocortical tissue

In human neocortical slices obtained during epilepsy surgery, sharp waves have been described to appear spontaneously, the shape of which met all criteria of epileptiform field potentials. In the present investigation, the current sinks and sources underlying these potentials were analysed. The cortical tissue used in the present study was a small portion of the tissue blocks excised for treatment of pharmacoresistant focal epilepsy. The tissue came from the temporal (n = 26), frontal (n = 1) and parietal (n = 1) lobes. Slices of 500 microm thickness were cut in the frontal plane perpendicular to the pial surface. Field potentials were recorded using a linear array of eight wire electrodes (diameter: 33 microm) with interelectrode distances of 300 microm. To scan a slice for sharp field potentials, this array was placed perpendicular to the pial surface at the midsection of each preparation, and consecutively at the respective midsections of the resulting halves of the slice. Each of these locations was termed a recording line. Depending on the appearance of spontaneous potentials, recording lines and slices were classified as either spontaneous or non-spontaneous. With both spontaneous and zero Mg(2+)-induced interictal discharges, in spontaneous slices, current sinks were preferentially located in layers II and III. In non-spontaneous slices, current sinks associated with interictal potentials could be found throughout all cortical laminae. With zero Mg(2+)-induced ictal activity, in spontaneous slices, the initial sinks were preferentially located in cortical laminae II and IIIa, and were shifted to lower ones after additional application of bicuculline. In non-spontaneous slices, no ictal-type discharges could be induced with omission of Mg2+ from the superfusate. Only addition of bicuculline elicited ictal-type activity, and sinks associated with this were preferentially located in layers II and IIIa. The results suggest that the supragranular layers, especially layer II, change qualitatively in functional organization in slices showing spontaneous discharges. We think that this special feature represents the function of the upper layers and can be blocked by bicuculline. This interpretation is supported by the observation that ictal discharges normally started in the upper layers in spontaneous and non-spontaneous slices, except for spontaneous slices with bicuculline, where the zone initiating discharges was translocated to deeper layers.

The functional organization of the interictal spike complex in benign rolandic epilepsy

PURPOSE

We studied the functional organization of the interictal epileptic spike complex in patients with benign rolandic epilepsy of childhood (BREC).

METHODS

We recorded interictal epileptiform spikes and somatosensory evoked potentials after median nerve stimulation, providing a biologic marker for the location of the central sulcus in 12 patients with BREC. We used multiple dipole modeling to assess the number, the three-dimensional intracerebral location, and the time activity of the underlying neuronal sources.

RESULTS

Although the interictal spike complex could be modeled by a single tangential dipolar source in seven patients (group 1), in the remaining five patients, two sources-a radial and a tangential dipole-were necessary adequately to explain the interictal spikes (group 2). The tangential source was located deeper than the radial source and was characterized by a frontal positivity and a centroparietal negativity with a phase reversal across the central sulcus, suggesting that the interictal spikes originated in the anterior wall of the central sulcus. The radial source showed a single electronegativity over the ipsilateral central region, which would be compatible with involvement of the top of either the pre- or postcentral gyrus. Both sources showed biphasic time patterns with an average latency difference of 30 ms.

CONCLUSIONS

Our results indicate that in some patients with typical BREC, the interictal epileptiform spike complex is generated by multiple, simultaneously active neuronal populations within the central region and that epileptiform activity is propagated between these two adjacent cortical areas.

Binaural vs. monaural auditory evoked potentials in rat neocortex

High spatial resolution epicortical recording techniques and numerical modeling were used to investigate laterality effects on the middle latency auditory evoked potential (MAEP) complex. Our data confirm previous reports that auditory stimulus laterality has a consistent effect on the amplitude, timing, and spatial distribution of the MAEP complex. The earliest temporal components (P1a, P1b and N1) show the greatest sensitivity, and are absent during ipsilateral stimulation. The later positive slow wave (P2) is present at the same amplitude during all stimulation conditions. Generation of the P2 appears to be independent of prior activation of areas 36 and 41 reflected in the early components, suggesting its generation by a more diffuse thalamocortical pathway, possibly from the medial division of the medial geniculate. Serial vs. parallel activation of rodent auditory cortex is discussed in the context of laterality-sensitive MAEP components.

Separation of different interictal discharge patterns in acute experimentally induced epileptic foci of the rat in vivo

Epileptic discharge patterns in an acute experimental model of epilepsy were analyzed. Epileptic foci were induced by epicortical application of penicillin on the rat motor cortex in vivo. Patterns with regular 1/s discharges, patterns with irregular discharges of about 0.5/s as well as compound patterns comprising discharges with intervals of about 300 ms could be differentiated by means of interval histograms and autocorrelation functions. These patterns occurred in an ordered sequence indicating that the different rhythms are activated by a progressive enlargement of the focus and duration of focal activity. The experiments suggest that different interictal discharge patterns can occur within the same brain regions and are not specific for a certain etiology; instead they seem to represent 'resonance' frequencies characteristic for the brain tissue which are disclosed under pathophysiological conditions.

Empirical comparison of the MEG and EEG: animal models of the direct cortical response and epileptiform activity in neocortex

This review directly addresses the appropriateness of the dipole model as a physical representation of neocortical sources produced by evoked and spontaneous epileptiform activity in neocortex. Three dimensional electrical measurements of cellular currents in rat sensory neocortex are compared to the extracranial magnetic fields these currents produce. Comparisons are performed for the direct cortical response (DCR) evoked by electrical stimulation of the cortical surface, and for evoked and spontaneous interictal and ictal discharge of the penicillin focus in the same animal preparation. Our data support the hypothesis that evoked and epileptiform magnetic fields result from intradendritic currents oriented perpendicular to the cortical surface. Furthermore, magnetic fields can be detected from epileptic foci smaller than 3 x 3 mm2. This work provides an empirical foundation for physical models with which to interpret noninvasive neuromagnetic recordings of epileptic discharge in human focal seizure disorders. The dipole approximation appears to be appropriate for the interpretation of magnetic field phenomena in neocortex.