Polytrodes: High-Density Silicon Electrode Arrays for Large-Scale Multiunit Recording

Abstract

We developed a variety of 54-channel high-density silicon electrode arrays (polytrodes) designed to record from large numbers of neurons spanning millimeters of brain. In cat visual cortex, it was possible to make simultaneous recordings from >100 well-isolated neurons. Using standard clustering methods, polytrodes provide a quality of single-unit isolation that surpasses that attainable with tetrodes. Guidelines for successful in vivo recording and precise electrode positioning are described. We also describe a high-bandwidth continuous data-acquisition system designed specifically for polytrodes and an automated impedance meter for testing polytrode site integrity. Despite having smaller interconnect pitches than earlier silicon-based electrodes of this type, these polytrodes have negligible channel crosstalk, comparable reliability, and low site impedances and are capable of making high-fidelity multiunit recordings with minimal tissue damage. The relatively benign nature of planar electrode arrays is evident both histologically and in experiments where the polytrode was repeatedly advanced and retracted hundreds of microns over periods of many hours. It was possible to maintain stable recordings from active neurons adjacent to the polytrode without change in their absolute positions, neurophysiological or receptive field properties.

State-of-the-art MEMS and microsystem tools for brain research

“…where I i is the current of one point source, σ is the conductivity of the extracellular space, and r i is the distance from the source to the electrode (a dipole assumption results in a more complex isopotential and an amplitude proportional to 1/r 2 , but arguably the ratio is neither a monopole or dipole [28][29][30][31] ). Given the close spacing of tetrodes, multiple spike signals can be used to triangulate and localize a specific cell in space 32 .…”

“…This technique spread throughout the MEMS community and formed the basis for many other novel electromechanical structures. The planar lithography approach has evolved over the years to include integrated interconnects 38 , active electronics 35,39,40 , cochlear implants 41,42 , polytrodes 31 , and three-dimensional arrays 29,[43][44][45] . An important simplification for defining and releasing fine neural probe structures has been the use of silicon-on-insulator (SOI) wafers and deep reactive ion etching (DRIE) 46,47 that many groups have adopted.…”

“…Another study extended this logic and used a dense 2D array of 54 channels on 1 shank to compare the performance of virtual tetrodes (grouping four adjacent electrodes) and polytrodes (more than four electrodes). The authors' data were limited to relatively large site pitches (43-65 μm) but showed that polytrodes outperformed even tetrodes in isolating multi-unit clusters into individual single units 31 . This same study also found as many as 24 single units in a virtual tetrode in the relatively sparse visual cortex.…”

“…Using MEMS-based arrays to improve software tools for neuroscientists also involves neuronal location information 9,24,29,31 and cell classification of neurons 4,83 , which are critical in brain mapping. The need for technologies to enable cell typing in brain research was highlighted as the number one priority by the U.S.…”

Intracranial neuronal ensemble recordings and analysis in epilepsy

“…The ability to record a single unit on more than one contact allows the reconstruction of the unit in space (Blanche, 2005;Dombovári et al, 2014). This arrangement limits the spatial coverage of neurons by concentrating the microelectrodes to a local region.…”