PMC

Computer simulation modeling the effect of neural signal changes on closed-loop neural cursor control.

A. A representative unit demonstrates large spike amplitude instability. The gray shaded area covers the amplitude range between the upper and lower boundaries of the window discriminator as determined manually by the experimenter. Spikes falling outside of these boundaries remained undetected during the online experiment. Inset: average spike waveforms during selected time periods indicated by elongated rectangles. B. Spike rates as determined by online (within window, black) and retrospectively discriminated spikes (resorted, gray). Apparent decline in the online firing rate results from failure of the smaller waveforms to satisfy the discriminator parameters.

A. Simulated closed-loop neural cursor trajectories in a radial-8 center out and back assessment with no perturbation. B and C: Rate change leads to directional bias, i.e. cursor drift in a constant direction, when using a decoder calibrated with neural data in A. D. Change in preferred direction leads to centrifugal distortion of cursor movements. E. Random insertion of spiking events results in jittery trajectories. F. Removing task related modulation of one or more units increases jitter of cursor movements. See text for details.

A. Rate changes show comparable magnitudes across 3 participants over the first two months of implantation and over 5 years in one participant. Inset: cumulative rate change. B. Corresponding magnitude of spike amplitude changes across data sets. C. All but the lowest isolation quality units show similar magnitude of rate change. Only units with statistically significant rate change are included. Insets show representative examples of spike waveforms with different isolation qualities. The gray area indicates the contribution of recording instability estimated by resorting a subset of the units (see text). SNR: signal-to-noise-ratio following .

The participant was asked to direct the neural cursor (paths shown as black dots) from the center of the screen (white or black dashed circle) to one of eight peripheral targets (gray discs), click and then return to the center. Black discs: click locations of successfully acquired targets. Cursor trajectories are relatively straight and mark out the eight principal target directions with 100% successful target acquisition rate B. Poor control with 15% correctly acquired targets. Cursor trajectories show frequent direction change and a strong tendency to move towards the bottom right corner of the screen

Small dots indicate the amplitude of individual spikes at the most negative deflection. Clusters of spikes correspond to experimental epochs (with different tasks and instructions) where larger circles and thick line indicate the mean spike amplitude within an epoch. Gaps between spike clusters indicate breaks between epochs, when data collection was paused. Inset: Due to their characteristic shapes and different amplitudes, spikes could be well discriminated into two classes. Over the duration of the experiment, spike amplitudes changed in both units, but with different dynamics. Alterations in electrode impedance, cross-talk, or other system parameters cannot explain these changes, as they would impact both units similarly. Thin gray lines: mean spike amplitude of units on other electrodes show no systematic amplitude change across the array. For clarity, only ten randomly selected amplitude traces are shown. Numbers indicate electrode and unit label.

A. In a series of radial-8 center-out-and-back assessments, performance decreased from 100% correctly acquired targets to zero percent, but after another forty minutes control recovered spontaneously and reached full performance. This decrease in performance was correlated with a change in one unit's mean firing rate (B). Top insets: cursor trajectories over the session. Arrow indicates bias direction. B. Mean firing rates of twenty-six units used in decoding. Unit 3 (thick black line) decreased its firing rate by 75% over the first ∼70min. The rate change of this unit correlated strongly with decreased performance (Pearson's cc.: 0.87, p≪0.01), and its preferred direction aligned with the axis of directional bias. The reason for these rate changes was unknown. Thicker lines: three units with significant rate change (p<0.01). Insets: direction tuning curves of the three units.