The artifact bestiary

Most of what a scalp 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 records is not brain. Blinks are ten times the size of alpha. A clenched jaw drowns everything above 20 Hz. The heart leaks in as a small spikeSpike Shorthand for an action potential as seen by an electrode, a brief blip about a millisecond wide. Related: Action potentialAction potential The brief, all-or-nothing voltage spike a neuron fires, about a millisecond long and about 100 millivolts tall across its membrane. , Spike sortingSpike sorting Deciding which spikes on an electrode came from which neuron, by clustering their waveform shapes. , Extracellular recordingExtracellular recording Recording with an electrode near neurons rather than inside one; you see small, fast blips from nearby cells rather than the full membrane swing. . Glossary entry once a second. Sweat makes the whole trace wander. Learn to recognize each one by eye and you will spend the rest of your career not fooling yourself. Every specimen below is on the simulator; press its button and watch.

The specimens

Blink. A smooth, rounded deflection, 100 to 300 µV, about 300 milliseconds wide, largest at the forehead (Fp1, Fp2) and shrinking toward the back, same sign in both frontal channels. It is the eyeball rotating upward under the closing lid, which moves the eye’s standing dipole. Nobody blinks less than about ten times a minute, so a recording without blinks is a recording with a bad frontal 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. At the source: ask people to blink freely between trials and hold still during them; never ask them not to blink, which produces a tense person who blinks anyway.

Eye movement. A step, not a bump, on the channels beside the eyes, opposite signs left and right for a horizontal look. Held gaze holds the step. Slow drifts of gaze look like slow drifts of baseline. At the source: a fixation point.

Jaw and face muscle. A burst of dense, spiky, high-frequency activity, mostly above 20 Hz, largest at the temples and back of the head where the temporalis and neck muscles are. Chewing, talking, swallowing, frowning, and the tense clench of someone trying too hard. It overlaps the beta and gamma bands and cannot be filtered away without losing them. At the source: relax the jaw, rest the head, and do not record a person who is speaking.

Heart. A small, sharp spikeSpike Shorthand for an action potential as seen by an electrode, a brief blip about a millisecond wide. Related: Action potentialAction potential The brief, all-or-nothing voltage spike a neuron fires, about a millisecond long and about 100 millivolts tall across its membrane. , Spike sortingSpike sorting Deciding which spikes on an electrode came from which neuron, by clustering their waveform shapes. , Extracellular recordingExtracellular recording Recording with an electrode near neurons rather than inside one; you see small, fast blips from nearby cells rather than the full membrane swing. . Glossary entry about once a second, regular, most visible near the ears and on the reference, tiny at the vertex. It is the ECGElectrocardiography (ECG) Recording the heart's electrical activity, about a millivolt on the chest; it leaks into EEG as a small regular spike. Related: ArtifactArtifact Anything in the recording that did not come from the brain: blinks, muscle, heartbeat, movement, sweat, mains hum, electrode pops. . Glossary entry’s QRS complex, a millivolt on the chest leaking to the head as a few microvolts. In an averaged ERPEvent-related potential (ERP) The small, stereotyped voltage waveform the brain produces in response to an event, invisible in a single trial and revealed by averaging many. Related: P300P300 A positive bump about 300 milliseconds after a rare or meaningful stimulus, the basis of the classic BCI speller. , N170N170 A negative deflection about 170 milliseconds after seeing a face, over the back of the head on the right. , Error-related negativity (ERN)Error-related negativity (ERN) A sharp negative wave about 50 to 100 milliseconds after you make a mistake, before you consciously notice. , Trial averagingTrial averaging Adding many time-locked epochs together so the consistent response grows and the random noise shrinks by the square root of the number of trials. . Glossary entry it can appear as a fake component if the heartbeat happens to line up with the stimulus. At the source: mostly you cannot; you learn to see it, and ICAIndependent component analysis (ICA) A method that unmixes multichannel data into statistically independent sources, so blinks and heartbeat can be identified and removed. Related: ArtifactArtifact Anything in the recording that did not come from the brain: blinks, muscle, heartbeat, movement, sweat, mains hum, electrode pops. . Glossary entry removes it in Phase 2.

Sweat and drift. A slow wander of the whole baseline over seconds to minutes, on one or all channels. Sweat changes the skin’s chemistry under 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 and shifts its offset; so does the paste slowly settling. At the source: a cool room, time for the 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 to settle, and a 0.5 Hz high-pass.

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 pop. A sudden step or spikeSpike Shorthand for an action potential as seen by an electrode, a brief blip about a millisecond wide. Related: Action potentialAction potential The brief, all-or-nothing voltage spike a neuron fires, about a millisecond long and about 100 millivolts tall across its membrane. , Spike sortingSpike sorting Deciding which spikes on an electrode came from which neuron, by clustering their waveform shapes. , Extracellular recordingExtracellular recording Recording with an electrode near neurons rather than inside one; you see small, fast blips from nearby cells rather than the full membrane swing. . Glossary entry on one channel, followed by a slow exponential recovery, with nothing on the neighbours. 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’s contact changed: a cup lifted, the paste cracked, a lead was tugged. At the source: fresh paste, tape, and leads that do not pull.

Hum. A steady sinusoid at 60 Hz (50 outside North America), on every channel, that changes size when you move a lamp cord. See why the hum for the ordered list.

Movement. Everything at once: a large low-frequency swing plus muscle plus a pop, as the head moves and the 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 shift. Recognizable because it is on all channels and ugly. At the source: a comfortable chair and a short recording.

Pulse. A slow rhythmic wobble at the heart rate, on one channel, when an 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 sits on an artery (the temples are the usual place). It is the vessel moving 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. Distinguish it from the heart spikeSpike Shorthand for an action potential as seen by an electrode, a brief blip about a millisecond wide. Related: Action potentialAction potential The brief, all-or-nothing voltage spike a neuron fires, about a millisecond long and about 100 millivolts tall across its membrane. , Spike sortingSpike sorting Deciding which spikes on an electrode came from which neuron, by clustering their waveform shapes. , Extracellular recordingExtracellular recording Recording with an electrode near neurons rather than inside one; you see small, fast blips from nearby cells rather than the full membrane swing. . Glossary entry by its shape: smooth, not sharp. At the source: move 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 a centimetre.

Reference contamination. Every channel shows the same artifactArtifact Anything in the recording that did not come from the brain: blinks, muscle, heartbeat, movement, sweat, mains hum, electrode pops. Related: Blink artifactBlink artifact The large, slow deflection an eye blink produces in frontal channels, often ten times the size of the brain signal. , Independent component analysis (ICA)Independent component analysis (ICA) A method that unmixes multichannel data into statistically independent sources, so blinks and heartbeat can be identified and removed. , Artifact subspace reconstruction (ASR)Artifact subspace reconstruction (ASR) An automatic method that detects short high-variance bursts and reconstructs them from clean data; common in real-time pipelines. . Glossary entry at the same time, whatever it is. That means it is on the reference electrodeReference electrode The electrode every other channel is measured against; there is no neutral spot on the head, so the choice shapes every trace. Related: MontageMontage The particular arrangement of which electrodes are subtracted from which, such as referential, bipolar, or average reference. , Common average referenceCommon average reference Re-referencing every channel to the mean of all channels, which approximates a neutral reference when electrodes cover the head well. . Glossary entry, which is subtracted from every channel. At the source: a reference somewhere quiet, and a linked-ears or average reference in Phase 2.

How to tell brain from not-brain

Three questions. Does it have the right shape? Brain rhythms wax and wane; artifactsArtifact Anything in the recording that did not come from the brain: blinks, muscle, heartbeat, movement, sweat, mains hum, electrode pops. Related: Blink artifactBlink artifact The large, slow deflection an eye blink produces in frontal channels, often ten times the size of the brain signal. , Independent component analysis (ICA)Independent component analysis (ICA) A method that unmixes multichannel data into statistically independent sources, so blinks and heartbeat can be identified and removed. , Artifact subspace reconstruction (ASR)Artifact subspace reconstruction (ASR) An automatic method that detects short high-variance bursts and reconstructs them from clean data; common in real-time pipelines. . Glossary entry are steps, spikesSpike Shorthand for an action potential as seen by an electrode, a brief blip about a millisecond wide. Related: Action potentialAction potential The brief, all-or-nothing voltage spike a neuron fires, about a millisecond long and about 100 millivolts tall across its membrane. , Spike sortingSpike sorting Deciding which spikes on an electrode came from which neuron, by clustering their waveform shapes. , Extracellular recordingExtracellular recording Recording with an electrode near neurons rather than inside one; you see small, fast blips from nearby cells rather than the full membrane swing. . Glossary entry, or steady tones. Does it have the right geography? Alpha is at the back, mu is at the top, blinks are at the front, muscle is at the edges, and an artifactArtifact Anything in the recording that did not come from the brain: blinks, muscle, heartbeat, movement, sweat, mains hum, electrode pops. Related: Blink artifactBlink artifact The large, slow deflection an eye blink produces in frontal channels, often ten times the size of the brain signal. , Independent component analysis (ICA)Independent component analysis (ICA) A method that unmixes multichannel data into statistically independent sources, so blinks and heartbeat can be identified and removed. , Artifact subspace reconstruction (ASR)Artifact subspace reconstruction (ASR) An automatic method that detects short high-variance bursts and reconstructs them from clean data; common in real-time pipelines. . Glossary entry on one channel alone is an 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. Does it happen at the right time? Anything locked to a blink, a heartbeat, a swallow, or the moment the subject moved is not the response you were looking for.