Event-related potentials and oscillations

The P300P300 A positive bump about 300 milliseconds after a rare or meaningful stimulus, the basis of the classic BCI speller. Related: Event-related potential (ERP)Event-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. , Oddball paradigmOddball paradigm An experiment in which a rare target is mixed in among frequent standards; the rare one produces a P300. . Glossary entry speller worked by averaging: the same waveform, locked to each flash, grew while the noise shrank. The motor imageryMotor imagery Imagining a movement without making it, which changes rhythms over motor cortex in a way a classifier can sometimes detect. Related: Sensorimotor rhythm (mu)Sensorimotor rhythm (mu) An 8 to 13 Hz rhythm over motor cortex that weakens when you move, or imagine moving, the corresponding body part. , Event-related desynchronization (ERD)Event-related desynchronization (ERD) The drop in rhythmic power over a cortical area when it becomes active, such as mu weakening when you imagine moving your hand. , BCI inefficiencyBCI inefficiency The finding that around one in five people cannot control a motor-imagery BCI even after training; once called "BCI illiteracy," a label the field is retiring. . Glossary entry classifierClassifier An algorithm that assigns each trial a label, such as left hand or right hand, after being trained on labelled examples. Related: Linear discriminant analysis (LDA)Linear discriminant analysis (LDA) A simple, robust classifier that draws a straight boundary between classes; the workhorse of EEG decoding for two decades. , Support vector machine (SVM)Support vector machine (SVM) A classifier that finds the boundary with the widest margin between classes; strong on small datasets. , Cross-validationCross-validation Testing a model on data it never saw during training, repeatedly, so the reported accuracy means something. . Glossary entry worked by band power: a rhythm got weaker over one hemisphere. Those are the two languages of 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. An event-related potentialEvent-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 is a change in voltage locked in time to an event. An oscillation is a rhythm whose power or phase changes. They are analyzed differently, they can hide each other, and understanding both is the foundation of every decoderDecoder The algorithm that maps neural signals to intended actions, such as a classifier or a Kalman filter. Related: ClassifierClassifier An algorithm that assigns each trial a label, such as left hand or right hand, after being trained on labelled examples. , Kalman filterKalman filter An estimator that blends a prediction from a motion model with a noisy measurement, weighting each by how much it trusts it; the decoder behind most intracortical cursor control. , Brain-computer interface (BCI)Brain-computer interface (BCI) A system that turns measured brain activity into control of something outside the body, from a cursor to a wheelchair to a speech synthesizer. . Glossary entry in this phase.

Averaging keeps what is locked

Add fifty epochsEpoch A fixed-length slice of the recording locked to an event, such as one second starting 200 milliseconds before each stimulus. Related: 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. , Baseline correctionBaseline correction Subtracting the mean of the pre-stimulus interval from each epoch so trials line up. . Glossary entry, each aligned to its stimulus, and divide by fifty. Anything with the same shape and timing on every trial survives. Anything with random timing, or random sign, shrinks toward zero at a rate of one over the square root of the number of trials. The survivor is the 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. Its named bumps are called components: P1, N1, P2 (early sensory), N170N170 A negative deflection about 170 milliseconds after seeing a face, over the back of the head on the right. Related: Event-related potential (ERP)Event-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. . Glossary entry (faces), P300P300 A positive bump about 300 milliseconds after a rare or meaningful stimulus, the basis of the classic BCI speller. Related: Event-related potential (ERP)Event-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. , Oddball paradigmOddball paradigm An experiment in which a rare target is mixed in among frequent standards; the rare one produces a P300. . Glossary entry (rare targets), N400 (semantic surprise), the ERNError-related negativity (ERN) A sharp negative wave about 50 to 100 milliseconds after you make a mistake, before you consciously notice. Related: Event-related potential (ERP)Event-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. . Glossary entry (errors), each with a typical latency, polarity, and scalp location.

Averaging assumes the response is the same every time. It is not, quite; latency jitters from trial to trial, which smears the average and makes fast components look smaller. That is a limitation to know about, not a reason to stop averaging.

Oscillations change in power and phase

Alpha does not have a fixed shape locked to anything; it waxes and wanes on its own schedule. Averaging the raw signal across trials cancels it, because its phase differs on every trial. To see it, compute power (square the band-passed signal, or take the spectrogramSpectrogram A spectrum computed over sliding windows and plotted against time, showing how frequency content changes. Related: Wavelet transformWavelet transform A time-frequency analysis that uses short wiggles at high frequencies and long ones at low, matching the trade-off to the frequency. , SpectrumSpectrum A plot of how much of a signal lives at each frequency. . Glossary entry) on each trial first, then average the power. Now a rhythm that reliably gets weaker after a movement cue shows up as a dip in mu power, the event-related desynchronizationEvent-related desynchronization (ERD) The drop in rhythmic power over a cortical area when it becomes active, such as mu weakening when you imagine moving your hand. Related: Sensorimotor rhythm (mu)Sensorimotor rhythm (mu) An 8 to 13 Hz rhythm over motor cortex that weakens when you move, or imagine moving, the corresponding body part., Motor imageryMotor imagery Imagining a movement without making it, which changes rhythms over motor cortex in a way a classifier can sometimes detect.. Glossary entry, even though the raw average shows nothing.

So: average the signal to see ERPsEvent-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; average the power to see oscillatory changes. Doing the wrong one shows nothing and leads people to conclude there is nothing.

Time-frequency analysis

The spectrogramSpectrogram A spectrum computed over sliding windows and plotted against time, showing how frequency content changes. Related: Wavelet transformWavelet transform A time-frequency analysis that uses short wiggles at high frequencies and long ones at low, matching the trade-off to the frequency., SpectrumSpectrum A plot of how much of a signal lives at each frequency.. Glossary entry or the wavelet transformWavelet transform A time-frequency analysis that uses short wiggles at high frequencies and long ones at low, matching the trade-off to the frequency. Related: SpectrogramSpectrogram A spectrum computed over sliding windows and plotted against time, showing how frequency content changes.. Glossary entry, applied per trial and averaged, gives a picture with time along the bottom, frequency up the side, and colour for power relative to baseline. ERDEvent-related desynchronization (ERD) The drop in rhythmic power over a cortical area when it becomes active, such as mu weakening when you imagine moving your hand. Related: Sensorimotor rhythm (mu)Sensorimotor rhythm (mu) An 8 to 13 Hz rhythm over motor cortex that weakens when you move, or imagine moving, the corresponding body part. , Motor imageryMotor imagery Imagining a movement without making it, which changes rhythms over motor cortex in a way a classifier can sometimes detect. . Glossary entry appears as a blue patch in the mu band during movement. A rebound of beta after movement appears red. Gamma bursts during attention appear, weakly, and often turn out to be muscle. The trade the spectrumSpectrum A plot of how much of a signal lives at each frequency. Related: Fast Fourier transform (FFT)Fast Fourier transform (FFT) The algorithm that breaks a stretch of signal into the sinusoids that would add up to it, giving amplitude and phase at each frequency. , Power spectral density (PSD)Power spectral density (PSD) The spectrum expressed as power per hertz, the standard way to quantify how strong alpha is. . Glossary entry page described holds: short windows for time, long for frequency, and wavelets adjust the window to the frequency automatically.

Phase

Beyond power, oscillations have phase, and phase carries information. Inter-trial phase coherence measures whether a rhythm’s phase is consistent across trials at each time; a strong value right after a stimulus means the stimulus reset the rhythm. Phase-locking between two channels measures whether two regions oscillate in step, which is the most common (and most over-interpreted) measure of “connectivity.” And an 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 can itself be thought of as a phase reset of ongoing rhythms; the debate over whether that is what ERPsEvent-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 are has run for twenty years.

Where decoders live

P300P300 A positive bump about 300 milliseconds after a rare or meaningful stimulus, the basis of the classic BCI speller. Related: Event-related potential (ERP)Event-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. , Oddball paradigmOddball paradigm An experiment in which a rare target is mixed in among frequent standards; the rare one produces a P300. . Glossary entry spellers decode an 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: the classifierClassifier An algorithm that assigns each trial a label, such as left hand or right hand, after being trained on labelled examples. Related: Linear discriminant analysis (LDA)Linear discriminant analysis (LDA) A simple, robust classifier that draws a straight boundary between classes; the workhorse of EEG decoding for two decades. , Support vector machine (SVM)Support vector machine (SVM) A classifier that finds the boundary with the widest margin between classes; strong on small datasets. , Cross-validationCross-validation Testing a model on data it never saw during training, repeatedly, so the reported accuracy means something. . Glossary entry looks at the average-like shape of each epochEpoch A fixed-length slice of the recording locked to an event, such as one second starting 200 milliseconds before each stimulus. Related: 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. , Baseline correctionBaseline correction Subtracting the mean of the pre-stimulus interval from each epoch so trials line up. . Glossary entry. Motor imageryMotor imagery Imagining a movement without making it, which changes rhythms over motor cortex in a way a classifier can sometimes detect. Related: Sensorimotor rhythm (mu)Sensorimotor rhythm (mu) An 8 to 13 Hz rhythm over motor cortex that weakens when you move, or imagine moving, the corresponding body part. , Event-related desynchronization (ERD)Event-related desynchronization (ERD) The drop in rhythmic power over a cortical area when it becomes active, such as mu weakening when you imagine moving your hand. , BCI inefficiencyBCI inefficiency The finding that around one in five people cannot control a motor-imagery BCI even after training; once called "BCI illiteracy," a label the field is retiring. . Glossary entry decodes oscillations: the classifierClassifier An algorithm that assigns each trial a label, such as left hand or right hand, after being trained on labelled examples. Related: Linear discriminant analysis (LDA)Linear discriminant analysis (LDA) A simple, robust classifier that draws a straight boundary between classes; the workhorse of EEG decoding for two decades. , Support vector machine (SVM)Support vector machine (SVM) A classifier that finds the boundary with the widest margin between classes; strong on small datasets. , Cross-validationCross-validation Testing a model on data it never saw during training, repeatedly, so the reported accuracy means something. . Glossary entry looks at band power over channels. SSVEPSteady-state visual evoked potential (SSVEP) When you stare at something flickering at a fixed rate, visual cortex oscillates at that rate, and the frequency can be read straight out of the EEG spectrum. Related: Canonical correlation analysis (CCA)Canonical correlation analysis (CCA) Finding the combination of channels that correlates best with reference sinusoids at each candidate frequency; the standard SSVEP decoder. , Brain-computer interface (BCI)Brain-computer interface (BCI) A system that turns measured brain activity into control of something outside the body, from a cursor to a wheelchair to a speech synthesizer. . Glossary entry decodes a driven oscillation: the classifierClassifier An algorithm that assigns each trial a label, such as left hand or right hand, after being trained on labelled examples. Related: Linear discriminant analysis (LDA)Linear discriminant analysis (LDA) A simple, robust classifier that draws a straight boundary between classes; the workhorse of EEG decoding for two decades. , Support vector machine (SVM)Support vector machine (SVM) A classifier that finds the boundary with the widest margin between classes; strong on small datasets. , Cross-validationCross-validation Testing a model on data it never saw during training, repeatedly, so the reported accuracy means something. . Glossary entry looks for a frequency. Knowing which language your paradigm speaks tells you what features to extract and what preprocessing will destroy the signal. High-pass at 1 Hz kills slow ERPsEvent-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; averaging raw signal kills induced responses.