Brain waves · Grey Matter

Brain waves are the rhythmic fluctuations of voltage that large groups of neurons produce when they are active together, and they are the first thing any recording of the brain is sorted by: the frequency band of a signal says much about the state of the tissue that made it.

Brain waves. Brain waves are the rhythmic fluctuations of voltage that large groups of neurons produce when they are active together, and they are the first thing any recording of the brain is sorted by: the frequency band of a signal says much about the state of the tissue that made it.

A rhythm appears when many neurons receive their inputs in step. Their synaptic currents add up instead of cancelling, and the sum rises and falls at the pace of the cycle. The pace is set by circuits: loops between cortex and thalamus, and networks of fast inhibitory interneurons that open and close short windows for firing. Recordings are divided into conventional bands, whose edges vary by a hertz or so between textbooks:

Delta, below about 4 Hz, dominates deep sleep. Theta, about 4 to 7 Hz, is prominent in drowsiness and, in the hippocampus, during exploration and memory tasks.

Alpha, about 8 to 12 Hz, is strongest over the back of the head with the eyes closed and at rest, and drops when the eyes open or attention engages; it is the rhythm Hans Berger first recorded in 1929.

Beta, about 13 to 30 Hz, is linked to an active, alert cortex and to holding a motor state. Gamma, above about 30 Hz, comes from fast interneuron-paced inhibition and accompanies local processing; it is weak at the scalp and clearest in recordings near the cortex.

For technology the bands are features. A decoder reads power in a band over a region (alpha falling over motor cortex when a movement is imagined, for example) long before it reads anything about single neurons.

Amplitude measures synchrony more than activity. A desynchronised, busy cortex produces a small, irregular trace; a large wave means many cells moving together, which is why sleep and seizures are so visible.

Frequency and amplitude trade off. Slow rhythms recruit large territories and are large at the scalp; fast rhythms are local and small, and the skull filters them further.

A brain wave is a measure of how many neurons agree on the timing.

That makes rhythms the easiest thing to read from outside and the hardest to turn into what any one neuron is doing.

Questions: How do gap junctions between interneurons help the cortex oscillate? Fast-spiking interneurons of the same type are linked by gap junctions made of connexin 36, so when some of them depolarise, current leaks into their neighbours and pulls them toward firing at the same moment. Synchronised interneurons deliver their inhibition to many pyramidal cells together, opening and closing a common window for firing, which is the core of gamma rhythms. The coupling passes slow voltage changes better than brief spikes, so it aligns the cells' rhythm more than it transmits individual spikes. Mice lacking connexin 36 still oscillate, but with weaker gamma, which shows the junctions help without being the only mechanism. How could two brain areas use a shared rhythm to talk to each other? In an oscillating group of neurons, inhibition opens and closes in each cycle, so the cells are most excitable during a short window of every cycle. If a sending area's spikes arrive at a receiving area during its excitable window, they have a strong effect; if they arrive during its inhibited phase, they are largely ignored. The communication-through-coherence hypothesis of Fries proposes that areas select whom to listen to by aligning the phase of their rhythms, gamma for feedforward signals and alpha-beta for feedback in the visual system. Too much synchrony would defeat the purpose, since a network in which everything fires together carries little information. How does the thalamus help produce the slow waves and spindles of sleep? As arousal falls, the brainstem's neuromodulators withdraw and thalamic relay cells hyperpolarise, which switches them from faithful relaying to rhythmic bursts. The thalamic reticular nucleus, inhibiting the relay cells in rhythm, generates the sleep spindles (bursts of about 11 to 15 Hz), and the loops between thalamus and cortex, together with the cortex's own slow oscillation, produce the large slow waves of deep sleep. In this mode the thalamus transmits little from the senses, which is part of why the sleeping brain is cut off from the world. What does it mean when an EEG shows strong alpha or strong delta? An EEG is usually read by splitting it into bands: delta below about 4 Hz, theta about 4 to 7 Hz, alpha about 8 to 12 Hz, beta about 13 to 30 Hz and gamma above about 30 Hz, with edges that vary slightly between textbooks. Strong alpha over the back of the head is the normal pattern of an awake person at rest with the eyes closed, and it shrinks when the eyes open. Strong delta is normal in deep sleep and abnormal in an awake adult, where it can point to a damaged or depressed patch of cortex. A band's power says how many neurons are oscillating together at that pace, so it describes the state of the tissue rather than any specific thought. How does inhibition set the pace of fast brain rhythms? When a group of fast-spiking interneurons fires, the GABA-A inhibition it delivers silences the pyramidal cells around it for a few milliseconds. As that inhibition wears off, the pyramidal cells that are being driven fire together in the short window that opens, excite the interneurons again, and the cycle repeats. The decay time of GABA-A inhibition sets the period, which is why such loops oscillate in the gamma range, roughly 30 to 90 Hz. Because these rhythms come from the inhibitory side of the circuit, gamma is often read as a sign of how well inhibition is working.