Decoupling Action Potential Bias from Cortical Local Field Potentials

Neurophysiologists have recently become interested in studying neuronal population activity through local field potential (LFP) recordings during experiments that also record the activity of single neurons. This experimental approach differs from early LFP studies because it uses high impendence electrodes that can also isolate single neuron activity. A possible complication for such studies is that the synaptic potentials and action potentials of the small subset of isolated neurons may contribute disproportionately to the LFP signal, biasing activity in the larger nearby neuronal population to appear synchronous and cotuned with these neurons. To address this problem, we used linear filtering techniques to remove features correlated with spike events from LFP recordings. This filtering procedure can be applied for well-isolated single units or multiunit activity. We illustrate the effects of this correction in simulation and on spike data recorded from primary auditory cortex. We find that local spiking activity can explain a significant portion of LFP power at most recording sites and demonstrate that removing the spike-correlated component can affect measurements of auditory tuning of the LFP. 1. Introduction The local field potential (LFP) is the integrated electrical activity of a large number of anatomically neighboring neurons, reflecting a combination of synchronous synaptic potentials [1] and action potentials [2]. The LFP is the object of growing interest in the neuroscience community because it may provide a valuable link between single neuron recordings and larger-scale neurophysiological signals such as EEG [3], fMRI [2], and ECoG [4]. These latter signals also offer a means to measure synchronous neural activity both within a single brain area [5, 6] and between brain areas [7, 8]. Historically, methods for studying LFP were developed with low impedance electrodes that integrated electrical potentials over a large brain volume ( 1？M , [1]). During the recent resurgence of interest in LFP, most studies have focused on data acquired with high impedance electrodes (1–5？M ) that can isolate the activity of single neurons. Single- or multiunit activity is typically extracted from the voltage trace by high-pass filtering ( 600？Hz, [6, 9]), and the LFP is extracted by low-pass filtering the same signal ( 300？Hz, [5, 6, 10]). Little is known about how the high impedance and specialized tip geometry of electrodes used for single unit recordings affect LFP signals in the lower frequency band. Thus, it is possible that the nature of the LFP signal