Project D: Eyes as a joystick

The eyeball is a battery. The front of the eye is a few millivolts positive relative to the back, so when you look left, the skin beside your left eye becomes slightly more positive and the skin beside your right eye slightly more negative. Two electrodesElectrode The conductor that turns ion currents in tissue into electron currents in a wire; its chemistry and contact quality set the noise you live with. Related: Silver/silver-chloride (Ag/AgCl)Silver/silver-chloride (Ag/AgCl) The standard electrode material for biopotentials, because its chemistry produces a stable, low-noise, low-drift interface with salty gel. , Electrode impedanceElectrode impedance How hard it is to push current through the skin-electrode contact, typically 5 to 50 kilohms with gel and megohms dry, higher at low frequencies. , Half-cell potentialHalf-cell potential The DC voltage that appears wherever a metal meets an electrolyte; two electrodes with different half-cell potentials produce an offset that can swamp the signal. . Glossary entry at the outer corners of the eyes pick up about 15 microvolts per degree of gaze. Amplify it, read it on an Arduino, and a dot on the screen follows your eyes. This is electrooculographyElectrooculography (EOG) Recording eye position from the standing voltage across the eyeball, which moves with the eyes; also the main source of blink artifacts in EEG. Related: Blink artifactBlink artifact The large, slow deflection an eye blink produces in frontal channels, often ten times the size of the brain signal., Electroencephalography (EEG)Electroencephalography (EEG) Recording the brain's electrical activity from electrodes on the scalp, a few tens of microvolts of blurred, summed cortical activity.. Glossary entry. It is also why blinks wreck EEGElectroencephalography (EEG) Recording the brain's electrical activity from electrodes on the scalp, a few tens of microvolts of blurred, summed cortical activity. Related: Electrocorticography (ECoG)Electrocorticography (ECoG) Recording from electrodes placed on the surface of the brain under the skull, usually during epilepsy surgery evaluation; cleaner and sharper than EEG. , Magnetoencephalography (MEG)Magnetoencephalography (MEG) Recording the tiny magnetic fields produced by cortical currents with superconducting sensors in a shielded room; less blurred by the skull than EEG, and far more expensive. , Volume conductionVolume conduction The spreading of currents from a source through the conductive tissues of the head, which is why one source shows up on many electrodes and why scalp EEG is blurry. , Microvolt (µV)Microvolt (µV) One millionth of a volt. Scalp EEG is tens of microvolts; a AA battery is 1.5 million of them. . Glossary entry, and it is the first stage of the speller you might build for someone with ALSAmyotrophic lateral sclerosis (ALS) A progressive disease of motor neurons that removes movement and eventually speech while leaving thought intact; the population many BCIs are built for. Related: Locked-in syndromeLocked-in syndrome Full awareness with almost no voluntary movement, sometimes only the eyes; communication depends entirely on assistive technology. , Augmentative and alternative communication (AAC)Augmentative and alternative communication (AAC) Any tool that helps someone who cannot speak communicate, from picture boards to eye-gaze spellers to BCIs. . Glossary entry in Phase 5.

Parts

The circuit

The Project B amplifier with the high-pass lowered as far as it will go. Replace the 1 µF and 330 kΩ high-pass with 10 µF and 1 MΩ, for a corner at about 0.016 Hz, which holds a step for a minute before it decays. Keep the gainGain How many times an amplifier multiplies its input; 1000× turns 50 microvolts into 50 millivolts, enough for an ADC to see. Related: Instrumentation amplifierInstrumentation amplifier A precision differential amplifier with very high input impedance and high CMRR, built from three op-amps and sold as a single chip such as the INA128 or AD8232. . Glossary entry at 2000: a 30° look is about 500 µV, which becomes 1 V at the output, a comfortable swing for the Arduino. Keep the 35 Hz low-pass. For a second, vertical channel, build a second copy of the amplifier or use the AD8232 breakout (its high-pass is too high for clean EOGElectrooculography (EOG) Recording eye position from the standing voltage across the eyeball, which moves with the eyes; also the main source of blink artifacts in EEG. Related: Blink artifactBlink artifact The large, slow deflection an eye blink produces in frontal channels, often ten times the size of the brain signal. , Electroencephalography (EEG)Electroencephalography (EEG) Recording the brain's electrical activity from electrodes on the scalp, a few tens of microvolts of blurred, summed cortical activity. . Glossary entry but it will show you the deflections).

Electrodes

Horizontal channel: one gold cup at the outer corner of each eye, about a centimetre from the eye on the temple bone. Vertical channel, if you have it: one above the eyebrow and one below the eye on the cheekbone, same eye. Reference and DRLDriven-right-leg circuit A third electrode fed with an inverted copy of the common-mode signal, so the body's hum is actively cancelled instead of merely rejected. Related: Common-mode signalCommon-mode signal A voltage that appears equally on both inputs of a differential amplifier, such as 60 Hz hum picked up by the whole body. , Ground electrodeGround electrode The electrode that ties the body to the amplifier's zero and, in a good design, is driven to cancel common-mode hum. . Glossary entry on the forehead between the eyebrows, or on an earlobe.

Software

Read the channel, subtract a baseline captured while looking at the centre of the screen, scale to pixels, move a dot. Drift is the whole problem: the electrodeElectrode The conductor that turns ion currents in tissue into electron currents in a wire; its chemistry and contact quality set the noise you live with. Related: Silver/silver-chloride (Ag/AgCl)Silver/silver-chloride (Ag/AgCl) The standard electrode material for biopotentials, because its chemistry produces a stable, low-noise, low-drift interface with salty gel. , Electrode impedanceElectrode impedance How hard it is to push current through the skin-electrode contact, typically 5 to 50 kilohms with gel and megohms dry, higher at low frequencies. , Half-cell potentialHalf-cell potential The DC voltage that appears wherever a metal meets an electrolyte; two electrodes with different half-cell potentials produce an offset that can swamp the signal. . Glossary entry offset wanders by hundreds of microvolts over minutes, which looks exactly like a slow gaze shift. Add a key that recentres the baseline, and later a rule that slowly pulls the baseline toward the current value whenever the eyes have been still for a while.

A minimal Python listener, with pyserial and pygame:

import serial, pygame
ser = serial.Serial('/dev/ttyUSB0', 115200)
pygame.init(); screen = pygame.display.set_mode((800, 600))
base = None; gain = 0.8
while True:
    line = ser.readline().decode(errors='ignore').strip()
    if not line: continue
    h = float(line.split(',')[0])
    if base is None: base = h
    for e in pygame.event.get():
        if e.type == pygame.KEYDOWN and e.key == pygame.K_SPACE: base = h  # recentre
    x = int(400 + gain * (h - base))
    screen.fill((16, 21, 18)); pygame.draw.circle(screen, (124, 255, 178), (x, 300), 12); pygame.display.flip()

Why this project matters

Eye-tracking is the most reliable channel a person with advanced ALSAmyotrophic lateral sclerosis (ALS) A progressive disease of motor neurons that removes movement and eventually speech while leaving thought intact; the population many BCIs are built for. Related: Locked-in syndromeLocked-in syndrome Full awareness with almost no voluntary movement, sometimes only the eyes; communication depends entirely on assistive technology. , Augmentative and alternative communication (AAC)Augmentative and alternative communication (AAC) Any tool that helps someone who cannot speak communicate, from picture boards to eye-gaze spellers to BCIs. . Glossary entry has, until the eyes fail too. Commercial eye-gaze systems are cameras and cost thousands of dollars. EOGElectrooculography (EOG) Recording eye position from the standing voltage across the eyeball, which moves with the eyes; also the main source of blink artifacts in EEG. Related: Blink artifactBlink artifact The large, slow deflection an eye blink produces in frontal channels, often ten times the size of the brain signal. , Electroencephalography (EEG)Electroencephalography (EEG) Recording the brain's electrical activity from electrodes on the scalp, a few tens of microvolts of blurred, summed cortical activity. . Glossary entry is a different sensing principle that works with the eyes closed, in the dark, and for a few dollars, and it is the foundation of the hybrid speller in Phase 5. The drift problem you just met is the entire reason that project is hard.