Nineteen electrodes are placed on the scalp using the standard 10-20 system. Each one continuously streams simulated band power across delta, theta, alpha/mu, beta and gamma — the vertical bar above each electrode is its instantaneous amplitude, coloured by whichever band currently dominates there. Imagining a hand movement (without actually moving) produces event-related desynchronisation (ERD): mu/beta power drops over the motor cortex on the side of the brain opposite the imagined hand — C4 dips for left-hand imagery, C3 dips for right-hand imagery. That's the real signal Wolpaw-style sensorimotor-rhythm BCIs decode to move a cursor.
asymmetry = (C4_power − C3_power) / (C4_power + C3_power)
asymmetry > +threshold → "Right hand" command
asymmetry < −threshold → "Left hand" command
otherwise → no command (rest)
- Rest — baseline mu/alpha rhythm at rest, mild posterior alpha, no consistent asymmetry.
- Left hand MI — imagining a left-hand movement desynchronises the right motor cortex (C4), pushing the cursor left.
- Right hand MI — imagining a right-hand movement desynchronises the left motor cortex (C3), pushing the cursor right.
- Focus — sustained attention raises frontal beta power (Fz/F3/F4) without a strong C3/C4 asymmetry, so the classifier reports no motor command — this is what a real system must reject as a false positive.
- Signal noise — amplitude of random EEG/EMG/movement artifact mixed into every channel; push it high enough and the true ERD signal gets buried, exactly like a noisy real electrode.
- Electrode gain — amplifier sensitivity; too low and real signal is lost in the classifier's threshold, too high and noise dominates.
- Classifier threshold — how large the asymmetry must be before a command fires; raising it demands a cleaner, more confident signal and cuts false triggers at the cost of slower response.
Real BCIs run this same pipeline — acquisition, band-power feature extraction, a linear/asymmetry classifier, then a control command — to let people with paralysis move a cursor, a wheelchair or a robotic arm using sensorimotor imagery alone.