Reliability of signals from a chronically implanted, silicon-based electrode array in non-human primate primary motor cortex

Abstract

Multiple-electrode arrays are valuable both as a research tool and as a sensor for neuromotor prosthetic devices, which could potentially restore voluntary motion and functional independence to paralyzed humans. Long-term array reliability is an important requirement for these applications. Here, we demonstrate the reliability of a regular array of 100 microelectrodes to obtain neural recordings from primary motor cortex (MI) of monkeys for at least three months and up to 1.5 years. We implanted Bionic (Cyberkinetics, Inc., Foxboro, MA) silicon probe arrays in MI of three Macaque monkeys. Neural signals were recorded during performance of an eight-direction, push-button task. Recording reliability was evaluated for 18, 35, or 51 sessions distributed over 83, 179, and 569 days after implantation, respectively, using qualitative and quantitative measures. A four-point signal quality scale was defined based on the waveform amplitude relative to noise. A single observer applied this scale to score signal quality for each electrode. A mean of 120 (+/- 17.6 SD), 146 (+/- 7.3), and 119 (+/- 16.9) neural-like waveforms were observed from 65-85 electrodes across subjects for all recording sessions of which over 80% were of high quality. Quantitative measures demonstrated that waveforms had signal-to-noise ratio (SNR) up to 20 with maximum peak-to-peak amplitude of over 1200 microv with a mean SNR of 4.8 for signals ranked as high quality. Mean signal quality did not change over the duration of the evaluation period (slope 0.001, 0.0068 and 0.03; NS). By contrast, neural waveform shape varied between, but not within days in all animals, suggesting a shifting population of recorded neurons over time. Arm-movement related modulation was common and 66% of all recorded neurons were tuned to reach direction. The ability for the array to record neural signals from parietal cortex was also established. These results demonstrate that neural recordings that can provide movement related signals for neural prostheses, as well as for fundamental research applications, can be reliably obtained for long time periods using a monolithic microelectrode array in primate MI and potentially from other cortical areas as well.

Implanted intracortical electrodes as chronic neural interfaces to the central nervous system

“…First, long term viability is questionable. While in some cases, implants seem to last for years, in others they become unusable after just a few months [5,75,77,68]. Because of the widely varied results within studies, more thorough investigation is needed, with more tightly monitored controls, to determine long term viability and possible reasons for variation.…”

“…Studies [75] have shown that day to day fluctuations occur within the consistency of a signal waveform and patterns, as well as signal viability. Harris et al suggest that this is largely due to either human operator error or wrongful attribution of spikes to certain neurons [76].…”

“…It seems as though the fluctuations in signal stability seen in various studies, have inconclusive results due to the possible error rates of microarray attributions. Additionally, the measure for signal stability involves comparing similarity of waveforms for a neuron action potential [75]. It is possible that this measure is not reflective of signal stability, but rather the plasticity of the brain in that signal pathways grow and change over time.…”

“…Other studies of the Utah microarray with a parylene encapsulation (as opposed to the traditional polyimide encapsulation) suggested viability after more than 12 months of in vitro submersion in a phosphate buffer saline at room temperature [80,81]. Suner et al performed an extensive study on Utah electrode microarrays in monkeys yielding viabilities ranging from a minimum of 3 months to greater than 1.5 years [75]. All this data suggests the possibility for long term use, but a high level of variability in vivo, where some electrodes cease to be viable early on, yet others remain viable for years (as in the previously described patient in [5]).…”

Intracranial neuronal ensemble recordings and analysis in epilepsy

“…There are studies however, demonstrating long term biocompatibility of microelectrodes. Suner et al reported no evidence of SNR change and a poor relationship between impedance and SNR during long term microelectrode recordings (Suner et al, 2005). The carrier, or insulating agents encapsulating the wire electrodes can be important in this process.…”