Twenty things a human brain does, placed by how big they are
and how long they take. Both axes are powers of ten, because no other scale
fits an ion channel and a lifetime on one picture. Six of the boxes carry a
real recording or measurement of real people; click any of them.
The five rhythms
Delta, theta, alpha, beta and gamma
are not five different recordings. They are one recording, from one channel
at the back of one person's head, separated by filtering. All five traces
above share a single vertical scale, so the size difference between them is
real rather than a drawing choice.
Closing the eyes multiplies alpha's share of the total power by
4.2. That is Hans Berger's 1929 result, and you can still get it out
of a free file in about a second of arithmetic.
Do not read the eyes-open delta as sleep. Delta
takes nearly half the power in the eyes-open trace, and almost all of that
is eye movement and blinking rather than cortical delta. Low frequency
power on a scalp electrode is the least trustworthy thing on this page, and
the filter cannot tell an eyeball from a brain.
Where each rhythm sits
They do not all come from the same
place. Alpha is the back of the head, and dramatically so: with the eyes
closed it is 72% of the power at PO8, over the occipital lobe, and
15% at T10 beside the ear. That is the visual cortex idling, and it
is why the eyes-closed trace on this page came from Oz.
Two of these five maps are mostly artefact, and it
matters. Delta peaks at AF8, right above the eye, because that is where
a blink is loudest, not because the front of the head is asleep. Beta and
gamma peak at T8 and T10, over the temporalis muscle, because clenching a
jaw looks like fast activity to an electrode. A filter cannot tell muscle
from brain. Alpha's occipital peak is the one pattern here that is clean,
which is a fair picture of how much of EEG analysis is spent on this
problem.
This is a scalp map, not a source map. Skull and
scalp blur everything, so a hot spot means the signal is strongest there on
the head, not that the cortex directly beneath it is generating it.
Recovering the generators is a whole inverse problem, and it is not one this
page has solved, which is why these are drawn as heads rather than painted
onto the cortical surface a page away.
Solving
Band
Eyes
Watch it
Surface
View
quietstrong
On the cortex, not on the scalp
The head maps above are scalp maps. This is the same recording pushed
through the other side of the problem: a three-shell model of brain, skull
and scalp says how a current at each of 20,484 points on the cortex
would look at each of the 64 electrodes, and the inverse runs that
backwards to ask which cortical currents best explain what was measured.
Alpha lands where a century of neurology says it should. With the eyes
closed, 70 per cent of its strongest sources fall in visual cortex,
concentrated in the lateral occipital cortex, the lingual gyrus and the
cuneus, and it is 19.4 times stronger than with the eyes open. The
build script refuses to write the file unless all of that holds.
What you are looking at. This is the cortex
inflated: the real thing is folded so tightly that two thirds of it
is buried inside sulci and invisible from outside, so it is blown up like a
balloon until the folds flatten out and you can see all of it at once. It
stops looking like a brain, which is the price. Switch to folded for
the real shape and watch most of the signal disappear into the creases. The
two hemispheres are also pushed apart in the inflated view, because
inflating them makes each one expand through where the other used to be.
Press play and it moves. The colours above are
average power over fifty seconds, which is a still photograph of something
that is not still. Playing shows the actual band-filtered signal, frame by
frame, three seconds of it at a thirtieth of a second per frame. It runs at
an eighth of real speed, because ten cycles a second on a screen is a
flicker rather than a rhythm. Blue and orange are the two directions the
current swings, on a square root scale because cortical current is very
unevenly spread and a linear one leaves everything but the few strongest
sources sitting at the dark centre. No value ever crosses zero, which is
the one thing that would be a lie about an oscillation. Both eye states are
drawn on one scale so the collapse
when the eyes open is visible rather than normalised away: eyes open reach
28 per cent of the eyes closed amplitude.
A better surface is not better data. This is drawn
on the full fsaverage cortex, 163,842 vertices per hemisphere, but
the inverse was solved on 10,242. Each vertex takes the value of the source
nearest to it, measured on the sphere so that distance means distance along
the cortical sheet rather than through the skull. Nothing is smoothed and
no value is invented: the patches you see are exactly the size of the
source space's own resolution. Switch to folded to see it on the
real anatomy, where most of the cortex is hidden inside sulci, which is why
the inflated view exists at all.
An inverse solution is not a measurement. Infinitely
many arrangements of current inside a head produce exactly the same
voltages on its surface, so the problem has no unique answer and what you
get back is what your assumptions asked for. This one assumes current is
spread smoothly over the cortical sheet. That is a standard and published
kind of estimate, and it is still an estimate. The head is fsaverage, an
average of many brains, not the head that was recorded.
Watch what happens to gamma. It localises to the
superior temporal cortex, 54 per cent of its strongest sources, with more
in the middle and inferior temporal cortex. That is not a brain rhythm. It
is the temporalis muscle, and the inverse solution has faithfully placed a
clenched jaw on the nearest available cortex. Solving the inverse problem
does not remove an artefact. It gives it an anatomical address, which is
more convincing and no more true.
Delta moved, and that is the noise model working. On
the scalp, delta peaked at AF8 above the eye. Here it does not, because the
covariance that whitens the data was computed from the eyes-open recording,
which is full of blinks, so the solution treats eye movement as noise and
discounts it. What survives is not frontal. That is a choice this page made
and it is visible in the result.
What the picture says
Almost everything sits in a band running from bottom left to top right:
the bigger a thing is, the longer it takes. That band is not drawn by hand.
It is the least squares line through the boxes themselves, so the shape of
the cloud is what puts it there.
One process sits a long way off it. A nerve impulse crosses most of a body
in about the time a single synapse takes to answer. Everything else that
reaches across a metre takes years. That is what myelin buys, and it is why
the most common cell in the cubic millimetre on this site's first page is
not a neuron at all but the oligodendrocyte, the cell that makes it:
20,139 of 49,379.
Where the numbers come from
Every box is tagged. Measured here means a script in this repository
produced the number from data in this repository. Cited means it is a
published range nobody here measured, and the citation is on the box.
Nothing is asserted without one or the other.
The spatial anchors come from H01, the cubic millimetre of human temporal
cortex on the first page: dendritic trees run 228 to 1,147
micrometres, cell bodies sit a median 10.7 micrometres apart, the tissue
packs 31,240 cell bodies into every cubic millimetre, and the cortex is
2,626 micrometres deep.
The temporal ones could not come from there. H01 is one instant, fixed in
resin, and the tractography is an average of
shapes. Neither has a time axis at all. So the recordings are other
people's: a whole cell current clamp from a human cortical neuron, scalp
EEG from someone opening and closing their eyes, and depth electrodes
sitting in a living human hippocampus. All three are openly published under
CC0 or ODC-BY, and the numbers printed beside them are measured off the
traces by scripts/fetch_signals.py, not copied from a paper.
A unit error that got caught. The first version of
that script reported the action potential rising at three hundred million
volts per second, because the derivative was taken against the wrong time
spacing. A real one rises at a few hundred. The script now asserts that the
spike is tens of millivolts tall, under a millisecond wide, and rising at
hundreds of volts per second, and refuses to write the file otherwise. The
corrected figures agree to within about one per cent with an independent
extraction of the same sweep.
The clipped channel. The hippocampal recording has
two channels and one of them is pinned against the amplifier rail: it never
goes positive at all. The script picks the channel that is not clipped and
prints the state of both, rather than plotting a flat ceiling as though it
were physiology.
Two clocks, not one
The slow end of this map has a trap in it, and the first version of this
page fell into it. Losing a synapse and the decline of synaptic density are
not the same process on different scales. They are two clocks.
The removal is fast and largely nocturnal. Synapses strengthened during
the day are broadly scaled back across a night's sleep, some are removed
outright, and microglia do much of their pruning then, which is also when
the day's memories are consolidated. That is the overnight box, five
to eight hours wide.
The decline is what is left over after twenty years of those nights. A
child's cortex builds far more synapses than it keeps, and the count comes
down from the first years into the early twenties. That is the two years
to twenty box, and it is a net trend rather than an event.
Myelination had the same fault. Ensheathing one axon takes weeks;
finishing the job across the brain takes into the fourth decade, and both
now sit in that box rather than one standing in for the other.
The one slow thing we can actually measure
Six boxes carry real data and five of them happen in under a second. The
sixth takes sixty years. Myelin water fraction was measured in 45 people
aged 18 to 79, and it does not do the same thing everywhere: parietal white
matter loses myelin steadily across adult life, at a correlation with age of
−0.89, while the corticospinal tract holds on to it at
−0.09, which on 45 people is no reliable change at all.
That is the same corticospinal tract drawn on
the tract page running from the brainstem to the
motor cortex, and the same insulation that puts the nerve impulse so far off
the diagonal. The tract that most needs to stay fast is the one that stays
myelinated.
What is missing
The right hand side of this map is still thinner than the left, and
honestly so. Openly downloadable numbers get scarce as the time axis grows.
Synaptic density across a human lifespan, the classic result that a child
builds far more synapses than it keeps, does not exist anywhere as a
downloadable table: the papers predate supplementary data and the modern
imaging work is behind paywalls. Those boxes are cited rather than measured,
and they will stay that way until something real turns up.