New theory explains how beta waves arise in the brain
Beta rhythms, or waves of brain activity with an approximately 20 Hz
frequency, accompany vital fundamental behaviors such as attention,
sensation and motion and are associated with some disorders such as
Parkinson’s disease. Scientists have debated how the spontaneous waves
emerge, and they have not yet determined whether the waves are just a
byproduct of activity, or play a causal role in brain functions. Now in a
new paper led by Brown University neuroscientists, they have a specific
new mechanistic explanation of beta waves to consider.
The new theory, presented in the Proceedings of the National Academy
of Sciences, is the product of several lines of evidence: external
brainwave readings from human subjects, sophisticated computational
simulations and detailed electrical recordings from two mammalian model
“A first step to understanding beta’s causal role in behavior or
pathology, and how to manipulate it for optimal function, is to
understand where it comes from at the cellular and circuit level,” said
corresponding author Stephanie Jones, research associate professor of
neuroscience at Brown University. “Our study combined several techniques
to address this question and proposed a novel mechanism for spontaneous
neocortical beta. This discovery suggests several possible mechanisms
through which beta may impact function.”
The team started by using external magnetoencephalography (MEG)
sensors to observe beta waves in the human somatosensory cortex, which
processes sense of touch, and the inferior frontal cortex, which is
associated with higher cognition.
They closely analyzed the beta waves, finding they lasted at most a
mere 150 milliseconds and had a characteristic wave shape, featuring a
large, steep valley in the middle of the wave.
The question from there was what neural activity in the cortex could
produce such waves. The team attempted to recreate the waves using a
computer model of a cortical circuitry, made up of a multilayered
cortical column that contained multiple cell types across different
layers. Importantly, the model was designed to include a cell type
called pyramidal neurons, whose activity is thought to dominate the
human MEG recordings.
They found that they could closely replicate the shape of the beta
waves in the model by delivering two kinds of excitatory synaptic
stimulation to distinct layers in the cortical columns of cells: one
that was weak and broad in duration to the lower layers, contacting
spiny dendrites on the pyramidal neurons close to the cell body; and
another that was stronger and briefer, lasting 50 milliseconds (i.e.,
one beta period), to the upper layers, contacting dendrites farther away
from the cell body. The strong distal drive created the valley in the
waveform that determined the beta frequency.
Meanwhile they tried to model other hypotheses about how beta waves emerge, but found those unsuccessful.
With a model of what to look for, the team then tested it by looking
for a real biological correlate of it in two animal models. The team
analyzed measurements in the cortex of mice and rhesus macaques and
found direct confirmation that this kind of stimulation and response
occurred across the cortical layers in the animal models.
“The ultimate test of the model predictions is to record the
electrical signals inside the brain,” Jones said. “These recordings
supported our model predictions.”
Beta in the brain
Neither the computer models nor the measurements traced the source of
the excitatory synaptic stimulations that drive the pyramidal neurons
to produce the beta waves, but Jones and her co-authors posit that they
likely come from the thalamus, deeper in the brain. Projections from the
thalamus happen to be in exactly the right places needed to deliver
signals to the right positions on the dendrites of pyramidal neurons in
the cortex. The thalamus is also known to send out bursts of activity
that last 50 milliseconds, as predicted by their theory.
With a new biophysical theory of how the waves emerge, the
researchers hope the field can now investigate whether beta rhythms
affect or merely reflect behavior and disease. Jones’s team in
collaboration with Professor of Neuroscience Christopher Moore at Brown
is now testing predictions from the theory that beta may decrease
sensory or motor information processing functions in the brain. New
hypotheses are that the inputs that create beta may also stimulate
inhibitory neurons in the top layers of the cortex, or that they may may
saturate the activity of the pyramidal neurons, thereby reducing their
ability to process information; or that the thalamic bursts that give
rise to beta occupy the thalamus to the point where it doesn’t pass
information along to the cortex.
Figuring this out could lead to new therapies based on manipulating beta, Jones said.
“An active and growing field of neuroscience research is trying to
manipulate brain rhythms for optimal function with stimulation
techniques,” she said. “We hope that our novel finding on the neural
origin of beta will help guide research to manipulate beta, and possibly
other rhythms, for improved function in sensorimotor pathologies.”
It had been exceedingly windy, and even after the storm had passed the sea still seemed to be quite angry, battering the rocks and the beach with considerable vigour. But at least it had stopped raining for a while, so Algy tucked himself in flat against the rock, holding on tightly with both wings, and spent a happy hour or two just watching the waves pounding on the shore.