Anesthetic drastically diverts the travels of brain waves

Date:
April 27, 2022
Source:
Picower Institute at MIT
Summary:
Under propofol general anesthesia very slow frequency traveling
waves transform and dominate, redirecting and disrupting the higher
frequency traveling waves associated with conscious function.



FULL STORY ========================================================================== Under propofol general anesthesia very slow frequency traveling waves
transform and dominate, redirecting and disrupting the higher frequency traveling waves associated with conscious function.


========================================================================== Imagine the conscious brain as a sea roiling with the collisions and
dispersals of waves of different sizes and shapes, swirling around and
flowing across in many different directions. Now imagine that an ocean
liner lumbers through, flattening everything that trails behind with its powerful, parting wake. A new study finds that unconsciousness induced
by the commonly used drug propofol has something like that metaphorical
effect on higher frequency brain waves, appearing to sweep them aside and,
as an apparent consequence, sweeping consciousness away as well.

Put more prosaically, the study in the Journal of Cognitive Neuroscience
by MIT scientists at The Picower Institute for Learning and Memory shows
that propofol substantially alters how different frequencies of brain
waves travel across the brain's surface, or cortex. Whereas conscious
brains exhibit a mixture of waves of various frequencies rotating or
traveling straight in various directions, brains under propofol anesthesia became dominated by powerful, very low frequency "delta" waves that
roll straight outward in opposite directions instead of slowly rotating
around central points as they do during consciousness. Higher frequency
"beta" waves, meanwhile, became fewer and more erratically structured, traveling only in directions not dominated by the surging delta waves.

Traveling waves are hypothesized to perform many important functions as
they coordinate the activity of brain cells over the areas of the brain
they cover.

These include reading information out from memory and holding it there
while it waits to be used in cognition. They may also aid in perception
and act as a means of time keeping in the brain. The findings therefore illustrate how profoundly anesthesia alters the state of the state of the
brain as it induces and maintains unconsciousness, said senior author
Earl K. Miller, Picower Professor of Neuroscience in MIT's Department
of Brain and Cognitive Sciences.

"The rhythms that we associate with higher cognition are drastically
altered by propofol," Miller said. "The beta traveling waves seen during wakefulness are pushed aside, redirected by delta traveling waves
that have been altered and made more powerful by the anesthetic. The
deltas come through like a bull in a china shop." Co-senior author
Emery N. Brown said the findings illustrate that there are many ways
anesthetic drugs can act on the brain.



==========================================================================
"The traveling waves generated by propofol help us appreciate that
there are many dynamical phenomena that anesthetics create that can
contribute to altered arousal states such as unconsciousness," said Brown,
an anesthesiologist at Massachusetts General Hospital and Edward Hood
Taplin Professor of Computational Neuroscience and Health Sciences and Technology at MIT. "It is unreasonable therefore, to think that there
is a single mechanism of action for all anesthetics." Lead author
Sayak Bhattacharya, a postdoctoral Picower Fellow in Miller's lab,
led the study by re-analyzing a data set recorded from two animals
as they underwent propofol anesthesia, stayed in that state for a
while, and then were brought back to consciousness. That's important, Bhattacharya said, because while traveling waves have been observed in
subjects under anesthesia many times, this study is among the first to
track them in subjects all the way through the process of losing and
regaining consciousness.

"No study has observed how traveling waves change directly from awake to anesthesia and then back to awake in the same experiment," he said. "We continuously monitored how these waves behaved when the animals were in
the awake state, and then how they changed when loss-of-consciousness
occurred, and then again when recovery-of-consciousness occurred --
all in the same animals, in a continuous experimental session. This
allowed us to explore in real-time how neural pathways that produce
the waves were altered." The lab's first analysis of the data set,
published in 2021, documented a profound shift toward delta waves but
hadn't measured the waves' traveling nature.

In the new analysis, Bhattacharya and co-authors found significant
shifts not only by frequency, but also wave direction, speed, structural organization and planar vs. rotational form.



========================================================================== "Slow-frequency delta (~1 Hz) waves increased while higher-frequency
(8-30 Hz) waves decreased," the authors wrote. "The slow-delta waves sped
up and became more spatially organized. They became more planar (and
less rotating) and increased mirror-image waves traveling in opposite directions. Whatever directions slow-delta waves flowed in after loss
of consciousness, they dominated. Higher-frequency waves decreased and
lost structure after loss of consciousness, despite showing increased
[wave] power, and flowed preferentially in directions where slow-delta
waves were less frequent." After the animals regained consciousness,
their wave patterns all returned to where they were before propofol administration. The clear association between these two regimes
(unfettered beta before or after anesthesia vs. delta dominance during anesthesia) and the state of consciousness strongly suggests a connection, Bhattacharya said.

"We hypothesize that the drastic breakdown of beta traveling waves
and their redirection could contribute to loss of consciousness under
propofol anesthesia," he said.

Notably, the study's findings suggest an important way in which
anesthesia differs from sleep (it's a common misconception that the
two states of unconsciousness are similar). In normal sleep delta waves
travel in a rotating pattern that may provide the timing that induces the "spike timing dependent plasticity" needed for consolidating memories of experiences accumulated during the day. Under propofol, however, delta
waves become planar rather than rotating, disrupting this memory-aiding mechanism and depriving the brain of a key function of sleep.

In addition to Bhattacharya, Miller and Brown the paper's other authors
are Jacob Donoghue, Meredith Mahnke and Scott Brincat.

The JPB Foundation, the National Institute of General Medical Sciences
and the Office of Naval Research funded the research.


========================================================================== Story Source: Materials provided by Picower_Institute_at_MIT. Note:
Content may be edited for style and length.


========================================================================== Journal Reference:
1. Sayak Bhattacharya, Jacob A. Donoghue, Meredith Mahnke, Scott
L. Brincat,
Emery N. Brown, Earl K. Miller. Propofol Anesthesia Alters Cortical
Traveling Waves. Journal of Cognitive Neuroscience, 2022; 1 DOI:
10.1162/ jocn_a_01856 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/04/220427100602.htm

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