Relating Electrode Impedance and Recording Quality in the Rat Hippocampus

The viability of surgical brain implants depend on high quality electrode development and construction. In our lab, we construct tetrodes (four twisted electrodes) that allow us to isolate single unit activity in dense hippocampal cell layers. Gold-plating electrode tips lowers impedance, improving the yield of isolated single units, increasing signal-to-noise ratios, and minimizing noise overlap. However, excessive gold-plating increase the probability of wires shorting. Tetrodes with shorts effectively become tritrodes or stereotrodes, resulting in reduced spatial dimensions and decreased ability to sort the electrophysiological signal into distinguishable single unit clusters. In this study, we correlated the recording quality of isolated single units as a function of electrode impedance, in an effort to determine an optimal impedance level for our electrophysiological experiments. Our study found that after gold plating was used to reduce the impedance level below 400 kΩ, reducing the impedance past that point, below 400 kΩ had no effect on improving recording quality. For future tetrode construction, the gold plating process will be limited once the impedance level is below 400 kΩ to decrease the risk of shorting the tetrode