Title : Ultra - small and customizable multichannel 1 electrodes for extracellular recordings

29 Increasing demand exists for smaller multichannel electrodes that enable simultaneous 30 recordings of many neurons in a non-invasive manner. We report a novel method for 31 manufacturing ultra-small carbon fiber electrodes with up to 7 closely spaced recording sites. 32 The electrodes were designed to minimize damage to neuronal circuitry and to be fully 33 customizable in 3D so that their dimensions can be optimally matched to those of the targeted 34 neuron population. 35 36 INTRODUCTION 37 Extracellular recording technique with microelectrodes has played a fundamental role in building 38 up the understanding on how the nervous system works. The electrophysiological experiments 39 are performed by inserting microelectrodes in the nervous tissue, which enables recording 40 extracellular voltage signals that are generated by action potentials or “spikes” (Robinson 1968). 41 However, it is often exceedingly difficult or even impossible to assign recorded spikes to specific 42 neurons if only a single electrode is used (Lewicki 1998). Therefore, electrodes with multiple 43 closely spaced recording sites are used to enable population recordings with single-unit 44 resolution (Buzsaki 2004). 45 Wire tetrodes are a highly successful multichannel electrode type (Jog et al. 2002; O'Keefe 46 and Recce 1993; Wilson and McNaughton 1993), which employs 4 recording sites with centre47 to-centre spacing between 10-50 μm. Because the spike amplitude recorded by each tip is 48 inversely proportional to the distance between the source and the recording sites, the four49 dimensional signal recorded by a tetrode can be used to identify the source with much higher 50 accuracy in comparison to recordings where only one recording site is employed (Gray et al. 51 1995). However, increasing the number of recording sites leads to increased electrode diameter 52 and thus more damage to neural circuitry is caused during the electrode insertion (Buzsaki 2004; 53