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Abstract
AbstractExtracellular voltage fluctuations (local field potentials, LFPs) reflecting neural mass action are ubiquitous across species and brain regions. Numerous studies have characterized the properties of LFP signals in the cortex to study sensory and motor computations as well as cognitive processes like attention, perception and memory. In addition, its extracranial counterpart – the electroencephalogram – is widely used in clinical applications. However, the link between LFP signals and the underlying activity of local populations of neurons is still largely elusive. For the LFP to aid our understanding of cortical computation, however, we need to know as precisely as possible what aspects of neural mass action it reflects. In this chapter, we examine recent advances and results regarding the origin, the feature selectivity and the spatial resolution of the local field potential and discuss its relationship to local spiking activity as well as the BOLD signal used in fMRI. We place particular focus on the gamma-band of the local field potential since it has long been implicated to play an important role in sensory processing. We conclude that in contrast to spikes, the local field potential does not measure the output of the computation performed by a cortical circuit, but are rather indicative of the synaptic and dendritic processes, as well as the dynamics of cortical computation.
Highlights
One important approach to gain insight into the computational and dynamical properties of neural ensembles is by recording the action potentials of a large number of neurons simultaneously (Csicsvari et al, 2003a; Buzsáki, 2004; Tolias et al, 2007)
We examine recent advances and results regarding the origin of the local field potential and discuss its relationship to local spiking activity as well as the more global BOLD mechanisms used in fMRI
While pyramidal neurons may be the strongest mediator of dipoles contributing to the local field potential (LFP) due to their size and geometry, interneurons play an important role in generating the dipoles underlying gammaoscillations: 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 (Whittington et al, 1995; Hasenstaub et al, 2005; Bartos et al, 2007; Fries et al, 2007)
Summary
An extracellular electrode placed in the brain measures the mean extracellular field potential, comprised of the aggregate electrical activity generated by various neural processes in a cell ensemble around the electrode tip (Figure 1A). While the work of Brunel and Wang studied the population spike count and not the LFP itself, Mazzoni et al used a model network consisting of sparsely and randomly connected leaky integrateand-fire neurons with one excitatory and one inhibitory population and simulated the generation of the LFP from excitatory and inhibitory synaptic currents (Mazzoni et al, 2008) Even such a simple model without the fine-structure of cortical microcircuits 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 in V1 (see below). In addition to such reduced models, detailed biophysical modeling will likely help to further untangle the importance of different contributions to the LFP (Makarov et al, 2010; Pettersen et al, 2008)
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