Interception and Interpretation JongMo Seo Technological evolution of

Interception and Interpretation Jong-Mo Seo Technological evolution of BCI • 1970 s: research algorithms to reconstruct movements from motor cortex neurons • 1980 s: Johns Hopkins researchers found a mathematical relationship between electrical responses of single motor-cortex neurons in rhesus macaque monkeys and the direction that monkeys moved their arms (based on a cosine function). • 1990 s: Several groups able to capture complex brain motor centre signals using recordings from neurons and use these to control external devices • The common thread throughout the research is the remarkable cortical plasticity of the brain, which often adapts to BCIs

Interception and Interpretation Jong-Mo Seo Practical Elements • Signal acquisition: the BCI system's recorded + digitised brain signal input. • Signal processing: conversion of raw information into device command – Feature extraction = the determination of a meaningful change in signal Feature translation = the conversion of that signal alteration to a device command. – Statistical analysis on the basis of the probability function that an electrophysiological event correlates with a given cognitive or motor task. • Device output: the overt command or control functions produced. – word processing, communication, wheel chair, prosthetic limb. – new output channel, therefore must have feedback to improve how they alter their electro physiological signal. • Operating protocol: the manner in which the system is turned on/off.

Interception and Interpretation Jong-Mo Seo Neurosurgical issues • • • Safety Durability/Reliability: scar, removal and re-implantation Consistency, Useful complexity (DOF) Suitability Speed and accuracy Efficacy: Technical vs. Practical

Interception and Interpretation Jong-Mo Seo EEG-based BCI • P 300 evoked potentials – brain's response to infrequent or significant stimuli from its response to routine stimuli. • Sensorimotor Cortex Rhythms – Movement or preparation for movement is typically accompanied by a decrease in µ (8– 12 Hz) and beta (18– 26 Hz) activity over sensorimotor cortex – People, including those with ALS or SCI have learned to control µ or beta amplitudes in the absence of movement or sensation • susceptible to external forces (i. e. , electrode movement) and contamination • less fidelity and spatial specificity and a limited frequency detection (<40 Hz), resulting in prolonged user training for higher levels of control. • Spatial and frequency limitations prohibits complexity of movements supported by EEG

Interception and Interpretation Jong-Mo Seo Single unit-based BCI • Best signal for BCI control has been achieved with multiple, single-unit action potentials recorded in parallel directly from cerebral cortex, in terms of accuracy, speed and DOF than single unit data. • Obtaining long-term stability of single unit recordings has proven difficult - Only provide a year of BCI control • Require insertion of a recording electrode into the brain parenchyma • Prone to scarring, implanted in eloquent regions of cortex

Interception and Interpretation Simultaneously recorded intracellular and extracellular signals Jong-Mo Seo

Interception and Interpretation Jong-Mo Seo Spatial dependence of spike waveform

Interception and Interpretation Chronic recording electronics Jong-Mo Seo

Interception and Interpretation Jong-Mo Seo How do you know you have a single unit? • Spike train autocorrelation • Always verify refractory period relative to long-time asymptote of autocorrelation function • Latency Variability Analysis

Interception and Interpretation Antidromic Spike Collision Jong-Mo Seo

Interception and Interpretation Jong-Mo Seo Voltage (A/D Levels) Multiple neural signals Voltage (A/D Levels) Time (sec) 3 msec

Interception and Interpretation Spike sorting Jong-Mo Seo Region from previous slide Raw Data Spike Detector Neuron #1 Spikes Neuron #2 Spikes Time (sec)

Interception and Interpretation Jong-Mo Seo The ‘graduate student’ algorithm Voltage (A/D Levels) Raw Data Threshold detector at 3 2 Time (sec) Interspike Interval Histogram Width (msec) Voltage (A/D Levels) Spike Height vs. Width Plot # of Intervals Candidate Waveforms Time (msec) Height (A/D Levels)

Interception and Interpretation Jong-Mo Seo General framework Locate Spikes Preprocess Waveforms Density Estimation Spike Classification Quality Measures

Interception and Interpretation Principal Component Analysis • Create “feature vector” for each spike • “Feature space” Jong-Mo Seo

Interception and Interpretation Jong-Mo Seo Post-Stimulus Time Histogram (PSTH) 40 µV 200 ms

Interception and Interpretation Semi-automatic Clustering Jong-Mo Seo

Interception and Interpretation Jong-Mo Seo Electrocorticogram-based system • Electrocorticogram (Eco. G) measures electrical activity of the brain taken from beneath the skull (subdural or epidural) • Gamma rhythms as well as µ and beta rhythms are prominent in ECo. G during movements • higher spatial resolution, better signal-to-noise ratio, wider frequency range, and lesser training requirements than scalprecorded EEG

Interception and Interpretation Jong-Mo Seo

Interception and Interpretation Jong-Mo Seo

Interception and Interpretation Electric current dipole fields Jong-Mo Seo

Interception and Interpretation Jong-Mo Seo Power spectrum handling & auto correlation function

Interception and Interpretation Jong-Mo Seo Artifact detection in EEG EOG EMG

Interception and Interpretation Jong-Mo Seo Stimulation mapping to locate cortical areas

Interception and Interpretation Jong-Mo Seo Electromyography • Indicator for muscle activation/deactivation • Electrode Categories – Inserted • Fine-wire (Intra-muscular) • Needle – Surface

Interception and Interpretation Jong-Mo Seo Fine wire electrode vs. surface electrode • Pros • Cons – – Extremely sensitive Record single muscle activity Access to deep musculature Little cross-talk concern – Extremely sensitive – Requires medical personnel, certification – Repositioning nearly impossible – Detection area may not be representative of entire muscle – Quick, easy to apply – No medical supervision, required certification – Minimal discomfort – Generally used only for superficial muscles – Cross-talk concerns – No standard electrode placement – May affect movement patterns of subject – Limitations with recording dynamic muscle activity

Interception and Interpretation Jong-Mo Seo Characteristics of EMG signal and noise • Signal – Amplitude range: 0– 10 m. V (+5 to -5) prior to amplification – Usable energy: Range of 0 - 500 Hz – Dominant energy: 50 – 150 Hz • Noise – Inherent noise in electronics equipment: 0 – thousands Hz • Cannot be eliminated • Reduced by using high quality components – Ambient noise: e. g. 60 Hz • Radio transmission, electrical wires, fluorescent lights • Essentially impossible to avoid • Amplitude: 1 – 3 x EMG signal – Motion artifact: 0 – 20 Hz • Electrode/skin interface, electrode cable • Reducible by proper circuitry and set-up – Inherent instability of signal • Amplitude is somewhat random in nature • Frequency range of 0 – 20 Hz is especially unstable > removal

Interception and Interpretation Jong-Mo Seo Maximizing quality of EMG signal • Signal-to-noise ratio – Highest amount of information from EMG signal as possible – Minimum amount of noise contamination • As minimal distortion of EMG signal as possible – No unnecessary filtering – No distortion of signal peaks – No notch filters recommended, e. g. 60 Hz • Differential amplification – Reduces electromagnetic radiation noise – Dual electrodes • Electrode stability – Time for chemical reaction to stabilize – Important factors: electrode movement, perspiration, humidity changes • Improved quality of electrodes – Less need for skin abrasion, hair removal

Interception and Interpretation Jong-Mo Seo

Interception and Interpretation Nerve conduction measurement • motor nerve conduction study, MNCS) • sensory nerve conduction study, SNCS) Jong-Mo Seo

Interception and Interpretation Jong-Mo Seo