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Facilitating the Integration of Modern Neuroscience into Noninvasive BCIs
Published in Chang S. Nam, Anton Nijholt, Fabien Lotte, Brain–Computer Interfaces Handbook, 2018
Mark Wronkiewicz, Eric Larson, Adrian K.C. Lee
EEG largely reflects the summed activity from large populations of pyramidal neurons (Hämäläinen et al. 1993), which are the largest and most common excitatory neurons in the cortex. Pyramidal neurons are generally oriented with the “trunk” of their large dendritic tree (called the apical dendrite) oriented normal to the cortical surface. Therefore, when pyramidal neurons are influenced by excitatory or inhibitory postsynaptic potentials (PSPs), charged ions flow within the neuron primarily along an axis aligned with the apical dendrite. This charge flow is often referred to as the “primary current.” Since charge cannot accumulate in the brain, “secondary” (or volume) current loops also flow extracellularly throughout the head to compensate for the primary current (Lopes da Silva & Van Rotterdam 2011). Therefore, an activation pattern resembling a source-sink configuration arises that mimics the characteristics of a current dipole—a fact that simplifies source imaging (Dale & Sereno 1993; Hämäläinen et al. 1993; Lopes da Silva & Van Rotterdam 2011) as discussed later. These PSP activations (lasting tens to hundreds of milliseconds) are much slower than action potentials (lasting ~1 ms), so the PSP activations of a population of neurons are more likely to temporally overlap (Lopes da Silva 2010). Combined with the aligned arrangement of these neurons, the field potentials of simultaneously active neurons add constructively in space and time to become detectible outside the head (Lopes da Silva & Van Rotterdam 2011).
Measurement of Electrical Potentials and Magnetic Fields from the Body Surface
Published in Robert B. Northrop, Non-Invasive Instrumentation and Measurement in Medical Diagnosis, 2017
Owing to its low-frequency nature, it is evident that fast action potentials on the axons of brain neurons contribute little to the EEG potential. The spiking axons run in various directions, and their very small, external potentials tend to average out in the volume conductors of the brain, the CSF, the meninges, and the skull and scalp. The generation of EEG potentials requires a neural source close to the inside surface of the skull that is coherent, that is, all the neurons must be aligned similarly and act together electrically. It turns out that the pyramidal cells in the center layers of the cerebral cortex are, in fact, the major source of the EEG potentials. Figure 4.38 illustrates schematically the various cells found in a radial slice through the cerebral cortex, including the pyramidal cells. Note that the apical dendritic branches of the pyramidal cells lie in the outermost layer of the cortex, next to the skull. These dendrites receive excitatory and/or inhibitory inputs from surrounding neurons and ascending axons. If the apical dendrites are receiving excitatory inputs, some positive ion current carried by an ion such as Na+ enters them, depolarizing the pyramidal cell toward firing. The inward, apical JNa is supplied by an extracellular current flowing outward from deeper layers in the cortex. This current flow is in response to the apical portions of the stimulated pyramidal cell going negative, while the deep portions are positive, creating an effective dipole on the cortex around the stimulated cell. If the apical dendrites of a pyramidal cell receive inhibitory inputs, there is a net outward flow of positive ions (or a net inward flow of negative ions such as JCl). Thus, inhibition of a pyramidal cell causes its apex to go positive, reversing the external current flow, and making the outer surface of the cortex positive around the inhibited cell. Many pyramidal cells in a region of cortex surface must be excited or inhibited together to create a local dipole large enough to be sensed through the skull by electrodes on the scalp.
Human Brain-Computer Interface
Published in Alexa Riehle, Eilon Vaadia, Motor Cortex in Voluntary Movements, 2004
Gert Pfurtscheller, Christa Neuper, Niels Birbaumer
The family of SCPs originates in the upper layers (layers I and II) of the cortex. However, slow potential changes, probably of the same origin, can be found in all parts of the nervous system. Whether the mechanisms of physiological generation are the same is still a matter of debate. The apical dendrites receive most of their input from intracortical fibers, callosal input from the other hemisphere, and input from the medial and reticular thalamus, the so-called nonspecific ascending activation system. Cholinergic inflow to the apical dendrites arrives primarily from structures in the basal forebrain and the basal ganglia.24 The amplitudes of slow cortical potentials are highly sensitive to the manipulation of cholinergic, monoaminergic, and glutamatergic transmission. SCPs tending toward the negative reflect slow EPSP and glial potentials and, therefore, indicate longer-lasting depolarization of the dendritic network. SCPs tending toward the positive are more difficult to analyze because they may result from a reduction of inflow to the apical dendrites, or, probably in rare occasions, from direct inhibitory activity at the level of layer I or II. Finally, SCPs tending toward the positive may indicate the inversion of the cortical dipole between the upper input layers and the lower output layers of the cortex.25 Therefore, the interpretation of slow potentials tending toward the positive requires a description of the experimental and physiological context of recording. Note also that in areas where the convexity of the cortex results in an inversion of the usual cortical dipole directions, such as in the orbitofrontal cortex, the inferior temporal cortex, part of area 17 of the occipital cortex, and the interhemispheric sulcus, inversion of the polarity of slow cortical potentials may be found. Therefore, polarity changes recorded at electrodes from the scalp alone can only be interpreted with great caution. Birbaumer et al.24 have described a frontal corticothalamic and basal ganglia network responsible for the attentional regulation of all types of preparatory SCPs. They have shown that slow cortical potentials are part of an excitatory and
The effect of experimentally-induced diabetes on rat hippocampus and the potential neuroprotective effect of Cerebrolysin combined with insulin. A histological and immunohistochemical study
Published in Egyptian Journal of Basic and Applied Sciences, 2023
Doaa El-Adli, Salwa A. Gawish, Amany AbdElFattah Mohamed AbdElFattah, Mona Fm. Soliman
The hippocampal formation consisted of hippocampus proprius, dentate gyrus (DG) and the subicular cortex (SC). The hippocampus proprius could be differentiated into CA1, CA2, CA3 and CA4 regions. The DG had a crest and upper and lower blades surrounding CA4 (Figure 1 (a and b)). The hippocampus proprius was formed of the following layers; the alveus, stratum oriens (st.or), stratum pyramidale (st.py), stratum radiatum (st.rd) and stratum lacunosum-moleculare (st.lm). The alveus was the innermost layer containing nerve fibers and neuroglial cells. St.or showed scattered cells within the nerve fibers. St.py consisted of 5–6 layers of pyramidal cells. St.rd showed a radial streaking pattern of fibers. Finally, St.lm showed horizontal fibers, neuroglial cells and blood vessels (Figure 1 (c)). Pyramidal cells of CA3 appeared as large sized, loosely packed triangular cells with vesicular nuclei and prominent nucleoli. Each cell showed an apical dendrite ramifying toward St.rd and basal dendrites (Figure 1 (d)). The DG consisted of molecular, granular and polymorphic layers. The polymorphic layer showed scattered polymorphic nuclei. The granule cell layer (GCL) contained 8–9 compactly arranged layers of granule cells with vesicular nuclei and prominent nucleoli. Spindle-shaped immature cells with oval darkly stained nuclei were seen in the subgranular zone (SGZ) (Figure 1 (e and f)).