title: Intra-day signal instabilities affect decoding performance in an intracortical neural interface system.
abstract: OBJECTIVE
  Motor neural interface systems (NIS) aim to convert neural signals into motor prosthetic or assistive device control, allowing people with paralysis to regain movement or control over their immediate environment. Effector or prosthetic control can degrade if the relationship between recorded neural signals and intended motor behavior changes. Therefore, characterizing both biological and technological sources of signal variability is important for a reliable NIS.
  
  
  METHODS
  To address the frequency and causes of neural signal variability in a spike-based NIS, we analyzed within-day fluctuations in spiking activity and action potential amplitude recorded with silicon microelectrode arrays implanted in the motor cortex of three people with tetraplegia (BrainGate pilot clinical trial, IDE).
  
  
  RESULTS
  84% of the recorded units showed a statistically significant change in apparent firing rate (3.8 ± 8.71 Hz or 49% of the mean rate) across several-minute epochs of tasks performed on a single session, and 74% of the units showed a significant change in spike amplitude (3.7 ± 6.5 µV or 5.5% of mean spike amplitude). 40% of the recording sessions showed a significant correlation in the occurrence of amplitude changes across electrodes, suggesting array micro-movement. Despite the relatively frequent amplitude changes, only 15% of the observed within-day rate changes originated from recording artifacts such as spike amplitude change or electrical noise, while 85% of the rate changes most likely emerged from physiological mechanisms. Computer simulations confirmed that systematic rate changes of individual neurons could produce a directional 'bias' in the decoded neural cursor movements. Instability in apparent neuronal spike rates indeed yielded a directional bias in 56% of all performance assessments in participant cursor control (n = 2 participants, 108 and 20 assessments over two years), resulting in suboptimal performance in these sessions.
  
  
  CONCLUSIONS
  We anticipate that signal acquisition and decoding methods that can adapt to the reported instabilities will further improve the performance of intracortically-based NISs.
authors.affiliation: School of Engineering, Brown University, Providence, RI, USA. janos_perge@brown.edu
authors.forename: János A
authors.initials: JA
authors.lastname: Perge
authors.forename: Mark L
authors.initials: ML
authors.lastname: Homer
authors.forename: Wasim Q
authors.initials: WQ
authors.lastname: Malik
authors.forename: Sydney
authors.initials: S
authors.lastname: Cash
authors.forename: Emad
authors.initials: E
authors.lastname: Eskandar
authors.forename: Gerhard
authors.initials: G
authors.lastname: Friehs
authors.forename: John P
authors.initials: JP
authors.lastname: Donoghue
authors.forename: Leigh R
authors.initials: LR
authors.lastname: Hochberg
mesh_terms: D000328:Adult; D000465:Algorithms; D062207:Brain-Computer Interfaces; D002940:Circadian Rhythm; D004569:Electroencephalography / Q000379:methods*; D019054:Evoked Potentials, Motor; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D009044:Motor Cortex / Q000503:physiopathology*; D009415:Nerve Net / Q000503:physiopathology*; D011782:Quadriplegia / Q000503:physiopathology*; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity