The artifact atlas

Why this matters

Most of what is large in a raw EEG trace is not brain. Blinks are ten times bigger than an ERP; a clenched jaw covers the whole gamma band; one loose electrode can dominate an average reference. Every cleaning method in Level 2 assumes you already know what these things look like, because the methods are only as good as your check on their output. This lesson is the vocabulary; the drill at the end is the test.

Concepts

Each artifact below is described on the same five axes — channels affected · morphology · duration · frequency content · polarity relative to the reference — because those are the five questions that separate one artifact from another and from brain activity. Polarity statements assume a referential montage with a reference that is far from the artifact source (mastoids, average); with a different reference the sign can flip, which is itself a useful check.

Blinks

• Channels: frontal-polar (Fp1, Fp2) largest, falling off steeply toward F3/F4 and Fz, essentially absent occipitally.
• Morphology: a smooth, rounded, monophasic deflection, roughly symmetric between the two sides.
• Duration: about 200–400 ms.
• Frequency content: low; most energy below a few Hz.
• Polarity: positive at Fp1/Fp2 with respect to a posterior or mastoid reference, because the eyeball is a dipole with the cornea positive and the eyelid sliding over it changes the potential seen from the forehead. Amplitude at Fp is typically well over 100 µV (TODO(confirm) typical range).
• Distinguish from: frontal slow waves (which are not confined to Fp and do not come in the stereotyped shape) and from vertical saccades (sharper onset).

Horizontal saccades

• Channels: lateral frontal (F7, F8; also Fp1/Fp2 and AF sites), with opposite polarity on the two sides.
• Morphology: a step — a rapid onset followed by a plateau that lasts until the next eye movement (a “square wave”).
• Duration: onset within tens of ms; plateau from hundreds of ms to seconds.
• Frequency content: very low (a step is mostly DC and low frequencies), with a brief broadband edge at the onset.
• Polarity: the side the eyes move toward goes positive (the cornea approaches that electrode), the other side goes negative; F7 and F8 are mirror images. Reading is a train of small saccades in one direction with a large return saccade.
• Distinguish from: blinks (same polarity on both sides, no plateau) and from lateral EMG (fast, spiky, no step).

EMG (muscle)

• Channels: wherever a muscle is under the electrode — temporal and lateral frontal (T7, T8, F7, F8, FT sites) for jaw clenching and the temporalis; occipital and posterior temporal for neck tension; frontal for the forehead; any site during swallowing or talking.
• Morphology: dense, spiky, irregular fast activity that thickens the trace; it starts and stops with the muscle contraction.
• Duration: as long as the contraction — a swallow (a second or two), a sustained clench (tens of seconds), a whole tense recording.
• Frequency content: broadband, from about 20 Hz to well above the EEG range, with no single peak; it raises the whole high-frequency floor of the spectrum.
• Polarity: none meaningful; it is high-frequency noise superimposed on the trace.
• Distinguish from: beta (lower amplitude, rhythmic, does not track jaw or neck tension) and from line noise (a single sharp spectral line, sinusoidal). The pitfall of reporting EMG as gamma is previewed below and returns in L7.4 and L7.5.

ECG and pulse

• ECG (electrical): small, sharp, regular spikes — the QRS complex — at the heart rate (roughly once per second), widespread across channels with a consistent shape, most visible when the reference is near the heart’s field (an ear or mastoid reference) and in low-amplitude segments. Frequency content: sharp transients, so broadband but brief.
• Pulse (mechanical): a slow, rounded, rhythmic wave at the heart rate confined to one electrode that sits over a scalp artery (often temporal); it is the electrode moving with the pulse, not an electrical signal from the heart. Frequency content: about 1 Hz and its harmonics.
• Distinguish from: brain rhythms by the metronome regularity and the match to heart rate; ECG from pulse by spike vs wave and by widespread vs single-channel.

Sweat and skin-potential drift

• Channels: one or a few electrodes, or all channels at once if the whole scalp is sweating.
• Morphology: slow, large, wandering baseline shifts; the trace tilts and drifts off the slot.
• Duration: seconds to minutes.
• Frequency content: below about 0.5 Hz.
• Polarity: either; sweat changes the electrode–skin interface potential.
• Distinguish from: slow eye movements (frontal and mirror-symmetric) and from delta activity (rhythmic, physiological amplitude). On a recording with no hardware high-pass — ds-eegbci was recorded with none — drift is plainly visible and must not be mistaken for a physiological slow wave.

Electrode pops

• Channels: a single channel.
• Morphology: an abrupt step or spike followed by an exponential return toward baseline, sometimes repeated as the contact flickers.
• Duration: onset in one or a few samples; recovery over tens of ms to seconds.
• Frequency content: the sharp edge is broadband; the recovery is slow.
• Polarity: either.
• Distinguish from: everything else by its confinement to one channel and its edge sharpness. Pops are also the canonical demonstration of filter ringing in L1.5, because a filter turns the sharp edge into oscillations on both sides.

Cable and motion artifacts

• Channels: several channels at once, often a group that shares a cable bundle, or all channels during head movement.
• Morphology: irregular, large, non-stereotyped deflections that coincide with movement; may include pops and drift.
• Duration: matches the movement.
• Frequency content: mostly low, with broadband edges.
• Distinguish from: physiological artifacts by their irregularity and their coincidence with events you can see (subject shifts, cable tugged, cap adjusted).

Line noise

• Channels: all channels if the ground or common-mode rejection is poor; single channels with high impedance otherwise.
• Morphology: a constant sinusoid at the mains frequency; at 10 s per page it appears as a uniform thickening or “fuzz” on the trace, not as individual cycles.
• Duration: continuous, though it can wax and wane as equipment switches on and off.
• Frequency content: a sharp line at 50 Hz or 60 Hz (by country) plus harmonics. In ds-eegbci (60 Hz mains, 160 Hz sampling) only the 60 Hz fundamental is below the 80 Hz Nyquist frequency, which L1.6 returns to.
• Distinguish from: EMG (broadband, irregular) by looking at the spectrum: a line, not a floor.

Electrode bridging

• Channels: two (or more) neighbouring electrodes.
• Morphology: the two traces are near-identical, including their noise; their difference is close to zero.
• Duration: the whole recording (a gel bridge between electrodes, usually from over-application of gel), or from the moment of the bridge.
• Frequency content: whatever the shared signal has; the tell is the identity, not the frequency.
• Distinguish from: genuinely correlated neighbours by looking at the difference channel or a correlation matrix. Bridged channels look fine in isolation, interpolation “works”, and the montage has silently lost a channel — the pitfall is previewed in L0.2 and treated in L2.2.

Flat and dead channels

• Channels: one channel (or the recording reference, which is flat by construction in some systems).
• Morphology: a straight line, or a line with only line noise on it.
• Duration: whole recording, or from the moment of disconnection.
• Distinguish from: a channel that is merely quiet by checking its variance against neighbours; a truly zero-variance channel is either dead, disconnected, or the reference.

Drowsiness-related slowing

Drowsiness is a state, not an artifact, but it lands in the atlas because it is the commonest reason a “resting” file is not what its label says. Look for the pattern from L0.4: alpha fragments and drops out, the background slows toward theta, slow roving eye movements appear frontally, and vertex sharp waves may appear at Cz. Channels: posterior for the alpha dropout, frontal for the eye movements, Cz for vertex waves. Duration: tens of seconds to minutes.

The same artifacts in a spectrum and a topography

The notebook reproduces each artifact type as a figure with its PSD and topography. The point of that exercise is that each artifact has a spectral and a spatial signature as well as a temporal one: blinks and saccades are frontal and low-frequency; EMG is lateral and broadband; line noise is a single line; pops and drift are single-channel and low-frequency. Level 2 will lean on these signatures when it classifies ICA components — a component’s topography, time course and spectrum are the same three views.

Explore

The item bank holds at least 60 labelled 5-s segments with 8 or more channels each. Items are drawn from ds-eegbci (no hardware filters, so drift, pops and line noise are real) and ds-lemon raw (which has a VEOG channel for blink and saccade items). Labels are label_source: algorithmic until the author reviews every item (§4.5); labels that Phase 0 data cannot supply (for example ECG and bridging) may be absent from the bank, and the drill says so.

Practice

The notebook reproduces each artifact type as a figure with its PSD and topography, so that the temporal, spectral and spatial signatures are side by side.

Exercises