Where oscillations come from
Pacemaker cells, excitation-inhibition loops, thalamocortical circuits, and why aligned pyramidal neurons make a field the scalp can see. The mechanisms behind alpha, spindles, slow waves, and gamma.
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Your Brian2Brian2 A Python simulator for spiking neural networks in which you write the equations directly. Related: Leaky integrate-and-fire neuronLeaky integrate-and-fire neuron The simplest useful neuron model: a leaky capacitor that fires and resets when it reaches a threshold. . Glossary entry network of four thousand neurons, none of which oscillated alone, fell into a rhythm when you tuned the inhibition. That is the general answer to where oscillations come from: from the interaction of excitation and inhibition with a delay, which makes a population fire, silence itself, recover, and fire again. The specific rhythms 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 each have a specific circuit. Let’s take them in order of frequency, and then ask why any of it reaches the scalp.
Two ways to make a rhythm
A pacemaker. Some neurons oscillate alone, because they carry channels whose interplay produces a limit cycle: a slow inward current that depolarizes, a slow outward current that repolarizes, repeat. Thalamic relay neurons have a low-threshold calcium current and a hyperpolarization-activated current that together make them burst rhythmically when they are hyperpolarized and quiet. Sinoatrial cells in the heart are the same idea.
A loop with a delay. A population of excitatory cells drives a population of inhibitory cells, which shut the excitatory cells down, which removes the drive to the inhibitory cells, which lets the excitatory cells recover. The period is set by the synaptic and membrane time constants. This is the mechanism of gamma (fast, local, inhibitory interneurons with fast synapsesSynapse The junction where one neuron signals to another, usually by releasing a chemical that opens channels on the next cell. Related: Synaptic plasticitySynaptic plasticity The strengthening or weakening of synapses with use, the physical basis of learning. , NeurotransmitterNeurotransmitter A chemical released at a synapse that changes the next neuron's membrane potential. . Glossary entry) and, with slower elements and long-range loops, of alpha and spindles.
Slow waves, under 1 Hz
In deep sleep the whole cortex alternates between an up state, where neurons fire, and a down state, where they are silent, roughly once a second. The mechanism is cortical: during the up state, activity-dependent potassium currents and synaptic depression build up until the network collapses into silence; during silence they recover until a few neurons’ spontaneous firing reignites the network. Slow wavesSlow wave A large, slow (under 1 Hz) oscillation of deep sleep during which most cortical neurons fall silent and then fire together. Related: Delta wavesDelta waves Slow, large waves below about 4 Hz that dominate deep sleep. , HypnogramHypnogram The staircase plot of sleep stage against time across a night, scored in 30-second epochs from EEG, EOG, and EMG. . Glossary entry are the largest signal in 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 because the whole cortex does this nearly in synchrony. Your sleep band was detecting them.
Spindles, 11 to 16 Hz
The thalamus’s reticular nucleus, a shell of inhibitory neurons, inhibits the relay neurons, which respond with a rebound burst (the calcium current), which excites the reticular cells, and round again at spindle frequency. The bursts are relayed to cortex, which produces the waxing-and-waning spindle. It gates sensory input during sleep and is tied to memory consolidation.
Alpha, 8 to 12 Hz
The classic account is thalamocortical: relay neurons in the visual thalamus, when not driven by input, fall into a rhythm near 10 Hz through pacemaker properties and their loop with the reticular nucleus, and visual cortex follows. The newer account adds a cortical generator: layer 5 pyramidal neurons in visual cortex can pace at alpha on their own. Both are probably true. The functional story (idling, or active inhibition) is in the Phase 1 explainer.
Beta and gamma, 15 to 80 Hz
Gamma is the loop with a delay at its fastest: pyramidal cells drive fast-spiking inhibitory interneurons, which inhibit them back within a few milliseconds. The period is set by the inhibitory synapseSynapse The junction where one neuron signals to another, usually by releasing a chemical that opens channels on the next cell. Related: Synaptic plasticitySynaptic plasticity The strengthening or weakening of synapses with use, the physical basis of learning. , NeurotransmitterNeurotransmitter A chemical released at a synapse that changes the next neuron's membrane potential. . Glossary entry’s time constant, around 10 to 25 ms, giving 40 to 100 Hz. Gamma is local (a few millimetres), tied to active processing, and weak on the scalp. Beta over motor cortex is a longer-range rhythm associated with holding a state, and its brief bursts, rather than its average power, appear to be the meaningful unit.
Millions of neurons in visual cortex each produce a tiny current when they fire. Under what condition does the sum reach the scalp as a measurable voltage?
When they fire together and their currents point the same way. Currents that are out of phase cancel; currents that point in random directions cancel. Pyramidal neurons have long dendrites all oriented perpendicular to the cortical surface, so their synaptic currents are aligned like tiny parallel batteries. When they are also synchronized, as in a rhythm, the sum is a dipole layer big enough to reach the scalp. Interneurons, whose dendrites point every way, contribute almost nothing to 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 however active they are.
Why the scalp sees any of it
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 is not 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. 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 are too brief and too asynchronous; they cancel. 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 is mostly the slower synaptic currents in the dendrites of pyramidal neurons, which last tens of milliseconds and can synchronize. A patch of cortex a few square centimetres, with its pyramidal cells receiving synchronized input, forms a current dipoleCurrent dipole The simplest model of a patch of active cortex: a tiny current source and sink close together, with a strength and a direction. Related: 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. , Forward problemForward problem Given a source in the head, compute the voltages on the scalp; it has a unique answer. . Glossary entry layer whose field passes through the skull (blurred, as the forward model showed) to the scalp. Alpha is visible because visual cortex is large, superficial, and synchronized. Gamma is barely visible because it is local and fast. Deep structures like the hippocampus are essentially invisible however rhythmic, because they are far away and their geometry is not a neat open layer.
The local field potentialLocal field potential (LFP) The slow, summed electrical activity of many neurons near an electrode, below a few hundred hertz, as opposed to individual spikes. Related: 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., 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 recorded by an intracortical 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 is the same synaptic current summed over a millimetre instead of centimetres, which is why LFPLocal field potential (LFP) The slow, summed electrical activity of many neurons near an electrode, below a few hundred hertz, as opposed to individual spikes. Related: 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. , 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 has more gamma and sharper features than 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 why intracortical BCIsBrain-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. Related: DecoderDecoder The algorithm that maps neural signals to intended actions, such as a classifier or a Kalman filter. , Closed loopClosed loop A system in which the output feeds back to influence the input, such as a user adjusting their brain activity while watching the cursor it controls. . Glossary entry sometimes decode from LFPLocal field potential (LFP) The slow, summed electrical activity of many neurons near an electrode, below a few hundred hertz, as opposed to individual spikes. Related: 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. , 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 when the 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 are gone.
Deep dive Cross-frequency coupling 2 min
Slow rhythms often modulate fast ones: gamma bursts ride on the peaks of theta in the hippocampus; spindles ride on the up-states of slow wavesSlow wave A large, slow (under 1 Hz) oscillation of deep sleep during which most cortical neurons fall silent and then fire together. Related: Delta wavesDelta waves Slow, large waves below about 4 Hz that dominate deep sleep. , HypnogramHypnogram The staircase plot of sleep stage against time across a night, scored in 30-second epochs from EEG, EOG, and EMG. . Glossary entry. This nesting is thought to organize when different computations happen and when synapsesSynapse The junction where one neuron signals to another, usually by releasing a chemical that opens channels on the next cell. Related: Synaptic plasticitySynaptic plasticity The strengthening or weakening of synapses with use, the physical basis of learning. , NeurotransmitterNeurotransmitter A chemical released at a synapse that changes the next neuron's membrane potential. . Glossary entry can change, and it is the mechanism by which closed-loop slow-wave stimulation could affect memory: enlarge the slow waveSlow wave A large, slow (under 1 Hz) oscillation of deep sleep during which most cortical neurons fall silent and then fire together. Related: Delta wavesDelta waves Slow, large waves below about 4 Hz that dominate deep sleep. , HypnogramHypnogram The staircase plot of sleep stage against time across a night, scored in 30-second epochs from EEG, EOG, and EMG. . Glossary entry, and you change when the spindles and the hippocampal replay they carry occur.
Deep dive Pathological rhythms 3 min
Seizures are runaway synchrony: too much excitation, too little inhibition, and the whole network locks into a rhythm that spreads. Parkinson’s disease produces excessive beta synchrony in the basal ganglia, and deep brain stimulation may work in part by disrupting it, which is why adaptive DBSDeep brain stimulation (DBS) Electrodes implanted deep in the brain and pulsed continuously, the standard treatment for advanced Parkinson's disease and essential tremor. Related: Charge densityCharge density The charge injected per pulse divided by electrode area; keeping it below safe limits prevents electrode damage and tissue injury. . Glossary entry that triggers on beta bursts is in trials. Absence epilepsyEpilepsy A tendency to recurrent seizures; its monitoring units are where seizure-detection algorithms get validated and where ECoG data comes from. Related: Epilepsy monitoring unit (EMU)Epilepsy monitoring unit (EMU) The hospital ward where patients are recorded on video-EEG for days to capture seizures before surgery. , Seizure detectionSeizure detection Automatically flagging seizures in EEG, a real clinical need and a good first machine-learning project on the TUH corpus. . Glossary entry is spindle circuitry gone wrong. Understanding the normal generator is how each of these was worked out.
Explain what this page was about to your roommate in three sentences. No jargon they would not know.
We make rhythms two ways, through cells that burst on their own when held quiet and through groups where excitation wakes inhibition that then shuts the group down until it recovers. We see spindles at 11 to 16 hertz from the thalamus looping with its inhibitory shell, alpha near 8 to 12 hertz from thalamus and cortex, and gamma near 40 to 100 hertz set by inhibitory timing of about 10 to 25 milliseconds. We pick these up on the scalp because lined up surface cells add their slow input currents into one large layer, while brief spikes and crooked cells cancel out.
Yours does not have to match. Check that the three ideas are there.