Carbon monofilament electrodes for unit recording and functional MRI in same subjects

&NA; Extracellular electrophysiology and functional MRI are complementary techniques that provide information about cellular and network‐level neural activity, respectively. However, electrodes for electrophysiology are typically made from metals, which cause significant susceptibility artifacts on MR images. Previous work has demonstrated that insulated carbon fiber bundle electrodes reduce the volume of magnetic susceptibility artifacts and can be used to record local field potentials (LFP), but the relatively large diameter of the probes make them unsuitable for multi‐ and single‐unit recordings. Although single carbon fiber electrodes have recently been used to record single‐unit activity, these probes require modifications in order to aid insertion and the use of these probes in fMRI has yet to be validated. Therefore, there is a need for a single‐carbon fiber electrode design that (1) minimizes the volume of the susceptibility artifact, (2) can record from a wide frequency band that includes LFP and multi‐ and single‐unit recording, and (3) is practical to insert without additional modifications. Here, we demonstrate that carbon‐fiber electrodes made from single carbon monofilaments (35 &mgr;m in diameter) meet all of these criteria. Carbon monofilament electrodes modified with the conductive polymer poly(3,4‐ethylenedioxythiophene) (PEDOT) have lower impedances and higher signal‐to‐noise ratio recordings than platinum‐iridium electrodes, a current gold standard for chronic single‐unit recording. Furthermore, these probes distort a significantly smaller volume of voxels compared to tungsten and platinum‐iridium electrodes in agarose phantom and in vivo MR images, leading to higher contrast‐to‐noise ratio in regions proximal to the electrode implantation site during fMRI. Collectively, this work establishes that carbon monofilaments are a practical choice for combined electrophysiology‐fMRI experiments.