Local Field Potential in the Visual System

Biophysical Origin It is thought that the main contribution to the local field potential (LFP) derives from synchronous activation of neurons in the surrounding cortex. The LFP represents the sum over typically many thousands of local electrical fields that are generated within individual neurons. When a neuron is activated by the arrival of an excitatory postsynaptic potential on its dendrite, charged ions pass through its membrane which normally acts as an electrical insulator (Eccles 1951). An inflow of positive ions is termed current sink by convention, whereas an outflow of positive ions is termed current source. Since inflowing positive ions must be compensated for by an appropriate outflow of positive ions, there must be a current source corresponding to every sink. While current sinks reflect membrane depolarization and neuronal excitation, current sources can, for example, result from hyperpolarization or the action of ionic pumps of the neuron that restore the membrane potential to its equilibrium value. The spatial separation between sources and sinks during neural activation gives rise to an electrical dipole within each neuron, whose magnitude depends on the transmembrane ionic flow. The geometry of neural dendrites determines whether a measurable LFP signal is generated from the dipoles within individual neurons. If all neurons had symmetric “closed field” dendritic geometry or exhibited randomly oriented dendritic trees, then individual electrical dipoles would tend to cancel each other and no LFP signal could be measured. However, pyramidal neurons of the cortex exhibit an “open field geometry,” such that their apical dendrites are all aligned parallel to each other and perpendicular to the cortical layer structure. This causes an alignment of the electrical dipoles in these pyramidal cells when these are synchronously activated, which permits their summation and in turn generates a robust LFP signal. In addition to synaptic contributions described above, there are a number of additional factors that can influence the LFP. For example, intrinsic properties of the membrane may exhibit resonance such that depolarization or activation of the neuron results in a self-sustaining voltage oscillation (Silva et al. 1991). Other neuronal events have also been shown to contribute to the LFP, including direct electrical communication between neurons via gap junctions (Traub et al. 2004), interactions between neurons and glial cells (Poskanzer and Yuste 2011), and potentially also dendritic calcium spikes. Unlike the action potential, which has a clear functional role as transmitting information from