Local field potential (LFP) recording is a very useful electrophysiological method to study brain processes. However, this method is criticized for recording low frequency activity in a large area of extracellular space potentially contaminated by distal activity. Here, we theoretically and experimentally compare ground-referenced (RR) with differential recordings (DR). We analyze electrical activity in the rat cortex with these two methods. Compared with RR, DR reveals the importance of local phasic oscillatory activities and their coherence between cortical areas. Finally, we show that DR provides a more faithful assessment of functional connectivity caused by an increase in the signal to noise ratio, and of the delay in the propagation of information between two cortical structures. Introduction 1 LFP recording of cortical structures constitutes a powerful tool to detect functional 2 signatures of cognitive processes. However, several studies have suggested that 3 recording methods suffer of major caveats due to the recording of activity in distant 4 neural populations [1-4]. Thus, theta oscillations (6-10Hz) during active wake seem to 5 propagate from the hippocampus to the frontal cortical areas [5]. Despite these 6 important studies, LFP recording has revealed important features of cortical 7 organizations [6, 7]. For example, cortical slow wave oscillations of NREM sleep, which 8 constitute a prominent feature of this vigilance state, contribute moderately to 9 coherence between cortical areas [7]. In contrast, weak slow wave oscillations during 10 active wake contribute to a relatively high level of coherence between cortical 11 areas [6, 7]. LFPs are mainly generated by post-synaptic response to pre-synaptic 12 activity of neurons [8-11] and constitutes a natural integrator of action potentials 13coming from a given cortical region [12][13][14]. In its usual description, LFP recording 14 appears to be less local than multi-unit activity recordings. Indeed, the usual 15 PLOS 1/15 recording mode of LFP consists in implanting a single electrode in the investigated 16 cortical region and a second one in a supposed neutral site. This simple recording 17 configuration, called monopolar or referential recording (RR) mode, is well adapted to 18 evaluate a global brain state. Unlike single and multi-unit probe, the impedance of the 19 standard electrode used for LFP recording is usually low in order to record neural 20 activity of a larger area. However, this method may detect activities from distant 21 cortical areas located between the recording and the reference electrode [1, 13-19], a 22phenomenon called volume conduction. We propose here to compare monopolar or RR 23 mode to bipolar or differential recording (DR), which consists in setting a pair of 24 electrodes in the same cortical area and measuring the voltage difference between 25 them. The main historical reasons why RR is widely used [7,20] are: 1) its simplicity 26 because of the low number of wires that needs to be implanted (contributing to the 27 ...

Local field potential (LFP) recording is a very useful electrophysiological method to study brain processes. However, this method is criticized for recording low frequency activity in a large area of extracellular space potentially contaminated by distal activity. Here, we theoretically and experimentally compare ground-referenced (RR) with differential recordings (DR). We analyze electrical activity in the rat cortex with these two methods. Compared with RR, DR reveals the importance of local phasic oscillatory activities and their coherence between cortical areas. Finally, we show that DR provides a more faithful assessment of functional connectivity caused by an increase in the signal to noise ratio, and of the delay in the propagation of information between two cortical structures.