Neural networks that look and act like computer circuits are only part of the picture
From: Salk Institute
April 22, 2022 -- For
years, the brain has been thought of as a biological computer that processes
information through traditional circuits, whereby data zips straight from one
cell to another. While that model is still accurate, a new study led by Salk
Professor Thomas Albright and Staff Scientist Sergei Gepshtein shows that
there's also a second, very different way that the brain parses information:
through the interactions of waves of neural activity. The findings, published
in Science Advances on April 22, 2022, help researchers better
understand how the brain processes information.
"We now have a new
understanding of how the computational machinery of the brain is working,"
says Albright, the Conrad T. Prebys Chair in Vision Research and director of
Salk's Vision Center Laboratory. "The model helps explain how the brain's
underlying state can change, affecting people's attention, focus, or ability to
process information."
Researchers have long
known that waves of electrical activity exist in the brain, both during sleep and
wakefulness. But the underlying theories as to how the brain processes
information -- particularly sensory information, like the sight of a light or
the sound of a bell -- have revolved around information being detected by
specialized brain cells and then shuttled from one neuron to the next like a
relay.
This traditional model
of the brain, however, couldn't explain how a single sensory cell can react so
differently to the same thing under different conditions. A cell, for instance,
might become activated in response to a quick flash of light when an animal is
particularly alert, but will remain inactive in response to the same light if
the animal's attention is focused on something else.
Gepshtein likens the
new understanding to wave-particle duality in physics and chemistry -- the idea
that light and matter have properties of both particles and waves. In some
situations, light behaves as if it is a particle (also known as a photon). In
other situations, it behaves as if it is a wave. Particles are confined to a
specific location, and waves are distributed across many locations. Both views
of light are needed to explain its complex behavior.
"The traditional
view of brain function describes brain activity as an interaction of neurons.
Since every neuron is confined to a specific location, this view is akin to the
description of light as a particle," says Gepshtein, director of Salk's
Collaboratory for Adaptive Sensory Technologies. "We've found that in some
situations, brain activity is better described as interaction of waves, which
is similar to the description of light as a wave. Both views are needed for
understanding the brain."
Some sensory cell
properties observed in the past were not easy to explain given the
"particle" approach to the brain. In the new study, the team observed
the activity of 139 neurons in an animal model to better understand how the
cells coordinated their response to visual information. In collaboration with
physicist Sergey Savel'ev of Loughborough University, they created a mathematical
framework to interpret the activity of neurons and to predict new phenomena.
The best way to explain
how the neurons were behaving, they discovered, was through interaction of
microscopic waves of activity rather than interaction of individual neurons.
Rather than a flash of light activating specialized sensory cells, the
researchers showed how it creates distributed patterns: waves of activity
across many neighboring cells, with alternating peaks and troughs of activation
-- like ocean waves.
When these waves are
being simultaneously generated in different places in the brain, they
inevitably crash into one another. If two peaks of activity meet, they generate
an even higher activity, while if a trough of low activity meets a peak, it
might cancel it out. This process is called wave interference.
"When you're out
in the world, there are many, many inputs and so all these different waves are
generated," says Albright. "The net response of the brain to the
world around you has to do with how all these waves interact."
To test their
mathematical model of how neural waves occur in the brain, the team designed an
accompanying visual experiment. Two people were asked to detect a thin faint
line ("probe") located on a screen and flanked by other light patterns.
How well the people performed this task, the researchers found, depended on
where the probe was. The ability to detect the probe was elevated at some
locations and depressed at other locations, forming a spatial wave predicted by
the model.
"Your ability to
see this probe at every location will depend on how neural waves superimpose at
that location," says Gepshtein, who is also a member of Salk's Center for
the Neurobiology of Vision. "And we've now proposed how the brain mediates
that."
The discovery of how
neural waves interact is much more far-reaching than explaining this optical
illusion. The researchers hypothesize that the same kinds of waves are being
generated -- and interacting with each other -- in every part of the brain's
cortex, not just the part responsible for the analysis of visual information.
That means waves generated by the brain itself, by subtle cues in the
environment or internal moods, can change the waves generated by sensory
inputs.
This may explain how
the brain's response to something can shift from day to day, the researchers
say.
Additional co-authors
of the paper include Ambarish Pawar of Salk and Sunwoo Kwon of the University
of California, Berkeley.
The work was supported
in part by the Salk Institute's Sloan-Swartz Center for Theoretical
Neurobiology, the Kavli Institute for Brain and Mind, the Conrad T. Prebys
Foundation, the National Institutes of Health (R01-EY018613, R01-EY029117) and
the Engineering and Physical Sciences Research Council (EP/S032843/1).
https://www.sciencedaily.com/releases/2022/04/220422161527.htm
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