Does Impedance Matter When Recording Spikes With Polytrodes?

Abstract

Extracellular microelectrodes have been widely used to measure brain activity, yet there are still basic questions about the requirements for a good extracellular microelectrode. One common source of confusion is how much an electrode’s impedance affects the amplitude of extracellular spikes and background noise. Here we quantify the effect of an electrode’s impedance on data quality in extracellular recordings, which is crucial for both the detection of spikes and their assignment to the correct neurons. This study employs commercial polytrodes containing 32 electrodes (177 μm2) arranged in a dense array. This allowed us to directly compare, side-by-side, the same extracellular signals measured by modified low impedance (∼100 kΩ) microelectrodes with unmodified high impedance (∼1 MΩ) microelectrodes. We begin with an evaluation of existing protocols to lower the impedance of the electrodes. The poly (3,4-ethylenedioxythiophene)-polystyrene sulfonate (PEDOT-PSS) electrodeposition protocol is a simple, stable, and reliable method for decreasing the impedance of a microelectrode up to 10-fold. We next record in vivo using polytrodes that are modified in a ‘chess board’ pattern, such that the signal of one neuron is detected by multiple coated and non-coated electrodes. The performance of the coated and non-coated electrodes is then compared on measures of background noise and amplitude of the detected action potentials. If the proper recording system is used, then the impedance of a microelectrode within the range of standard polytrodes (∼0.1 to 2 MΩ) does not greatly affect data quality and spike sorting. This study should encourage neuroscientists to stop worrying about one more unknown.

Artifact-free and high-temporal-resolution in vivo opto-electrophysiology with microLED optoelectrodes

“…1). The impedance magnitude and phase of the recording electrodes were measured as 1.15 ± 0.07 MΩ and -68.33 ± 5.11°at 1 kHz (n = 54, mean ± SD), respectively, acceptable for high-quality in vivo extracellular recordings 32 .…”

Sputtered porous Pt for wafer-scale manufacture of low-impedance flexible microelectrodes

“…There has recently been debate as to what extent impedance reduction is useful to improve neural recordings 38,39 . In fact, in the case of intracellular recordings, raising the impedance from the standard 5 MΩ to several hundred MΩ still produced good intracellular measurements provided that one uses appropriate amplifying electronics 40 .…”

“…Thus, many publications suggest that it is not necessary to prioritize low impedance once it is significantly below the input impedance of the amplifier. Neto et al (2018) 38 One often undervalued parameter is the cost and ease of manufacturing. Though many varieties of experimental electrode have been published in the last decade, relatively few designs have proliferated and become commonly used.…”

Artifact-free, high-temporal-resolutionin vivoopto-electrophysiology with microLED optoelectrodes

“…We confirmed that the effect of substrate doping density on the electrical and optical characteristics of the fabricated LEDs is not as significant as the die-to-die variation in a wafer ( Supplementary Figure 1). The impedance magnitude and phase of the recording electrodes were measured as 1.15 ± 0.07 M and -68.33 ± 5.11 ° at 1 kHz (n = 54, mean ± SD), respectively, acceptable for high-quality in vivo extracellular recordings 30 .…”