Paying attention to the neurons behind our alertness

Date:
August 20, 2020
Source:
Okinawa Institute of Science and Technology (OIST) Graduate
University
Summary:
The neurons of layer 6 - the deepest layer of the cortex - were
examined by researchers to uncover how they react to sensory
stimulation in different behavioral states.



FULL STORY ==========================================================================
It's probably quite easy to relate to. You're in a lecture or meeting,
avidly taking notes and engaged in the topic. You're alert and aware
of what's being discussed, keen to absorb as much as you can. And then, suddenly, your thoughts drift away. You stop hearing what's being said
or seeing what's being shown.

You become drowsy, your eyelids start to droop, and your mind switches
off.


========================================================================== Although this scenario is familiar to many people, the reason behind why
our brains do this is not properly understood. This is mainly because
many different brain structures regulate our attention span and one
of them is in layer 6 -- the deepest layer in the cortex. The cortex,
or cerebral cortex, is the outermost part of the brain in humans and
other mammals. It is composed of functionally specialized and highly interconnected sensory, motor and association areas. Abstract thinking,
an internal model of the outside world, and conscious perception all
arise from this part of the brain.

"We know layer 6 of the cortex regulates how we are connected to the
external world in a behavior state-related manner. It's involved in why
we are fully aware in some situations and why our attention suddenly
goes when we become drowsy," explained Dr. Sigita Augustinaite, from
the Optical Neuroimaging Unit at the Okinawa Institute of Science and Technology Graduate University (OIST).

"But it's a very difficult area of the brain to study." For this
research, which was published in Current Biology, Dr. Augustinaite
worked with Professor Bernd Kuhn, who leads the Unit. They used a
powerful microscopy technique -- two-photon imaging -- to examine the
neuronal activity of visual cortex layer 6 in mice. This was the first
time that layer 6 neurons had been studied with subjects that were awake
and behaving.

Neurons are specialized cells, which transmit information to other nerve
cells and muscles around the body. Within the brain, they form networks
and communicate with each other via electrochemical signals. Given
their importance, finding out more about their activity is vital for understanding how our brains work.

In each experiment, the mouse was placed in a fixed yet comfortable
position and periodically shown different kinds of visual stimuli. It
had the freedom to run, sit, groom itself and even sleep, all while hours
of recordings were being taken from deep within its brain with cellular resolution. To determine its alertness, the researchers also recorded
the electrical activity -- or electroencephalogram -- in the brain of
the mouse, as well as its running speed and pupil size. The result was
the ability to observe the activity of hundreds of individual neurons
at once in response to different sensory conditions whilst the mouse
was in different behavioral states.

The researchers found three distinct populations of neurons that reacted differently to the visual stimulation. Around one-third of the neurons
were classed as visual stimulus activated (VSA) neurons. These were
inactive when there were no visual stimuli but were activated when the
mouse was shown a stimulus. Other neurons had the opposite reaction. They
were spontaneously active without a visual stimulus but, once a stimulus
was shown, the activity dropped away. These were classed as visual
stimulus suppressed (VSS) neurons.

There was also a population of quiet neurons. These were neurons that
didn't show any activity during this experiment. But, as Dr. Augustinaite explained, they might simply have shown no activity because the right
kind of stimuli wasn't being presented to activate them. "There's a
possibility that if the right stimuli were shown, then these so-called
quiet neurons would have started acting as VSA neurons." The majority
of VSA and VSS neurons were most active when the mouse was most alert,
and their activity gradually decreased as the mouse became sleepier.

However, this was not always the case. Some neurons, especially among
the VSS population, were most active during low alertness -- when
the mouse was drowsy or asleep. And interestingly, while many neurons
did not discriminate between locomotion and stationary states, others
did. Some neurons were only active during locomotion, whilst others
were only active during stationary periods. In fact, if there's one
thing this research has shown, it's that there's no constant. Rather,
the neurons in layer 6 are diverse and dynamic.

"This is how we think layer 6 regulates what's happening,"
Dr. Augustinaite said. "The activity of these different populations
of neurons complements each other." For example, if there's a flow of
visual information, VSA neurons send information outside of the cortex
to other parts of the brain. But if it's dark, these sub-cortical areas
still need to know what's happening in the cortex. So, the VSS neurons
take over. This activity of layer 6 neurons regulates the information
flow from the sensory organs to the cortex and is therefore one of the
key factors that makes one pay attention or lose focus.

There's still a lot of research to be done in layer 6 to determine exactly
how this circuit contributes to our attention spans and to different
disorders, such as attention deficit, autism or schizophrenia, but
Dr. Augustinaite emphasized that if we want to understand what's happening
in our mind, then we must work with awake and behaving subjects. "This
is the first-time layer 6 was investigated with this method."

========================================================================== Story Source: Materials provided by Okinawa_Institute_of_Science_and_Technology_(OIST)
Graduate_University. Original written by Lucy Dickie. Note: Content may
be edited for style and length.


========================================================================== Journal Reference:
1. Sigita Augustinaite, Bernd Kuhn. Complementary Ca2 Activity
of Sensory
Activated and Suppressed Layer 6 Corticothalamic Neurons
Reflects Behavioral State. Current Biology, 2020; DOI:
10.1016/j.cub.2020.07.069 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/08/200820143847.htm

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