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OECC , paper 13P Powered by Drupal. The resulting dipole contributes to the measured LFP depending on the arrangement of the generating cell with respect to the local population.
Therefore, dendritic processes on pyramidal cells in the cortex are thought to contribute strongly to the LFP, as potential differences from the local network do not cancel each other due to their geometrical arrangement Figure 4 B. Dipoles generated by the spiking activity itself are much faster, however, and therefore do not directly add to the LFP power.
While it was originally thought that excitatory postsynaptic potentials form the most important source of LFP generating dipoles Mitzdorf, , , more recently also other sources such as inhibitory synaptic input Hasenstaub et al. Figure 4. A Illustration of a pyramidal cell, where the dendritic tree is shown schematically on the left, the cell body and axon on the right. A synaptic potential creates a current sink on the dendritic tree and a current-source at the soma.
Adapted from Johnston and Wu B Pyramidal cells are aligned in a very stereotype fashion, with large dendritic arbours facing one direction and somata facing to the other. In this so-called open field arrangement, synchronized synaptic input creates strong dipoles, since currents flow from individual cells do not cancel each other. C Illustration of the excitatory-inhibitory cortical network involved in the generation of gamma-oscillations. Thalamic inputs activate populations of infragranular inf and supragranular and granular sup glutamergic cells, as well as GABAergic cells.
These three groups interact with each other. Inhibitory synapses are indicated by a square symbol , excitatory synapses by a circle symbol. Adapted from Logothetis The extracellular space acts like a volume conductor, which can, in principle, carry electrical signals over large distances and whose resistance depends on the geometry and layout of the neurons Johnston and Wu, ; Logothetis et al.
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The impedance of the cortical tissue has been found to be isotropic and independent of frequency Logothetis et al. While recent modelling work suggests that even in purely resistive extracellular medium frequency dependent attenuation can be caused by the dendritic morphology of the neurons Pettersen and Einevoll, , the specific shape of the LFP spectrum as shown in Figure 2 B likely also reflects underlying neural processes.
Oscillators contributing to the LFP at different frequencies are possibly generated by processes on varying scale and strength Buzsaki and Draguhn, ; Logothetis et al. Low frequency oscillations, on the one hand, are thought to be formed by global neuromodulatory inputs Steriade, giving rise to strong slow oscillators.
Gamma-rhythms, on the other hand, most likely originate locally within cortical microcircuits consisting of pyramidal cells and interneurons Fries et al. Experimentally, this has been shown to be the source of the gamma-oscillations in the hippocampus of rats Csicsvari et al. Similarly, we postulate that such circuits in cortex get activated by thalamic input, as surround stimulation alone was not sufficient to evoke gamma-oscillations Gieselmann and Thiele, Thus, while pyramidal neurons may be the strongest mediator of dipoles contributing to the LFP due to their size and geometry, interneurons play an important role in generating the dipoles underlying gamma-oscillations: They act as rhythm generators via rhythmic inhibition of pyramidal neurons and synchronized inhibitory synaptic potentials contribute significantly to the membrane oscillations on pyramidal neurons Bartos et al.
This is in good agreement with the idea that enhanced gamma activity during high contrast or surround stimulation reflects increased interneuron drive see above. For a more detailed discussion of interneuron networks and their role for cortical gamma-oscillations, see the excellent reviews by Bartos et al. Mazzoni et al. Interestingly, this simple model reproduced quantitatively accurate the dependence of LFP gamma-power on stimulus contrast as well as the dependence of information content in different LFP bands measured under naturalistic stimulation.
How exactly these active network processes on different scales interact with passive low-pass filtering properties due to dendritic morphology Pettersen and Einevoll, to form the signal measured as LFP is yet to be determined. If the gamma-band of the LFP originates in local cortical circuits as also revealed by reverse correlation with very localized stimuli Goense and Logothetis, , why is the spatial resolution apparently so much poorer compared to MU activity?
We postulate that this results from the interplay of two factors: First, strong oscillations — especially like those present under visual stimulation — can spread in the extracellular medium for considerable distances, as this acts as an isotropic frequency independent resistor Logothetis et al. This results in spatial blurring of the LFP signal.
Note, however, that this effect becomes only noticeable when large stimuli that excite extended networks of neurons are used see Discussion of Berens et al. Second, the spatial extent of microcircuits oscillating in phase and thus the size of the oscillating dipole is determined by the spread of thalamic axon fields, the size of the dendritic arbours of cortical neurons and intracortical horizontal connections Angelucci et al.
This will lead to contributions to the LFP signal by tissue at considerable distance from the electrode tip, as for instance thalamic axons in upper layer 4 fan out to cover an area of up to 1. In addition, the LFP also integrates subthreshold oscillations, which do not lead to spiking activity.
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