Nav: Home

Is this brain cell your 'mind's eye'?

September 30, 2019

No-one knows what connects awareness - the state of consciousness - with its contents, i.e. thoughts and experiences. Now researchers propose an elegant solution: a literal, structural connection.

'Content circuits' within the cortex are plugged into 'switchboard circuits' that allocate awareness, says the theory, via cortical cells called L5p neurons.

Writing in Frontiers in Systems Neuroscience, the group offers evidence - and caveats. Their challenge to experimentalists: if consciousness requires L5p neurons, all brain activity without them must be unconscious.

State vs. contents of conscious

Most neuroscientists chasing the neural mechanisms of consciousness focus on its contents, measuring changes in the brain when it thinks about a particular thing - a smell, a memory, an emotion. Quite separately, others study how the brain behaves during different conscious states, like alert wakefulness, dreaming, deep sleep or anesthesia.

Most agree the two are indivisible: you can't think or feel or experience anything without being aware, nor be 'aware' of nothing. But because of the divided approach, "nobody knows how and why the contents and state of consciousness are so tightly coupled," says Dr. Jaan Aru, neuroscientist at Humboldt University, Berlin, and lead author of the new theory.

Separate circuits

The divide created between state and contents of consciousness is anatomical.

Our conscious state is thought to depend on the activity of so-called 'thalamo-cortical' circuits. These are connections between neurons in the cortex, and neurons in the thalamus - a thumb-sized relay center in the middle of the brain that controls information inflow from the senses (except smell). Thalamocortical circuits are thought to be the target of general anesthesia, and damage to these neurons due to tumors or stroke often results in coma.

In contrast, functional brain imaging studies locate the contents of consciousness mostly within the cortex, in 'cortico-cortical' circuits.

The missing link?

Aru and colleagues believe that L5p neurons are uniquely placed to bridge the divide.

"Thalamo-cortical and cortico-cortical circuits intersect via L5p neurons," explains Aru. "Studies tracing these cells under the microscope suggest they participate in both circuits, by exchanging connections with both thalamus and cortex."

Functional brain studies suggest these cells may indeed couple the state and contents of consciousness. Cellular-level brain imaging in mice shows that L5p neurons respond to a sensory stimulus (air puff to the leg); that this response increases when the animal is awake; and that it is strongest by far when the animal reacts to the stimulus (moves its leg).

"We can't tell what the mouse is thinking," concedes Aru. "But if we assume that it reacts only when it is conscious of the stimulus, then this study demonstrates the interaction between the state [wakefulness] and contents [sensory experience] of consciousness in L5p neurons."

The assumption is consistent with a similar mouse study. This one went further, showing that directly activating the stimulus-responsive L5p neurons (e.g. with drugs) makes the animal react to a weaker sensory stimulus - and sometimes without any stimulus.

"It's as if the mouse experiences an illusory stimulus; as if L5p stimulation creates consciousness," Aru adds.

Testing the theory

The theory is a first iteration that needs refinement, stresses Aru.

"Our goal here is to convince others that future work on the mechanisms of consciousness should specifically target L5p neurons."

Nevertheless, this general arrangement could account for some well-known quirks of consciousness.

For example, the processing delay of this long relay - from cortico-cortical circuit to thalamo-cortical and back again via L5p neurons - could explain why rapid changes of stimuli often escape conscious perception. (Think subliminal messages spliced into video.)

One feature of this phenomenon is 'backward masking': when two images are presented briefly in rapid succession (50-100 ms), only the second image is consciously perceived. In this case, posits Aru, "by the time the stimulus completes the L5p-thalamus-L5p relay, the second image has taken over early cortical representation and steals the limelight lit by the first image."

The theory could also help explain why we usually have little conscious insight into some brain processes, like planning movement or even syntax.

"All brain activity that does not (sufficiently) involve L5p neurons remains unconscious," predicts Aru.

Therein lies the key to testing this exciting theory.
-end-
Please link to the original research article in your reporting: https://www.frontiersin.org/articles/10.3389/fnsys.2019.00043/full

Corresponding author: Dr. Jaan Aru, jaan.aru@gmail.com; @jaaanaru

Frontiers is an award-winning Open Science platform and leading open-access scholarly publisher. Our mission is to make high-quality, peer-reviewed research articles rapidly and freely available to everybody in the world, thereby accelerating scientific and technological innovation, societal progress and economic growth. Frontiers received the 2014 ALPSP Gold Award for Innovation in Publishing. For more information, visit http://www.frontiersin.org and follow @Frontiersin on Twitter.

Frontiers

Related Neurons Articles:

The first 3D map of the heart's neurons
An interdisciplinary research team establishes a new technological pipeline to build a 3D map of the neurons in the heart, revealing foundational insight into their role in heart attacks and other cardiac conditions.
Mapping the neurons of the rat heart in 3D
A team of researchers has developed a virtual 3D heart, digitally showcasing the heart's unique network of neurons for the first time.
How to put neurons into cages
Football-shaped microscale cages have been created using special laser technologies.
A molecule that directs neurons
A research team coordinated by the University of Trento studied a mass of brain cells, the habenula, linked to disorders like autism, schizophrenia and depression.
Shaping the social networks of neurons
Identification of a protein complex that attracts or repels nerve cells during development.
With these neurons, extinguishing fear is its own reward
The same neurons responsible for encoding reward also form new memories to suppress fearful ones, according to new research by scientists at The Picower Institute for Learning and Memory at MIT.
How do we get so many different types of neurons in our brain?
SMU (Southern Methodist University) researchers have discovered another layer of complexity in gene expression, which could help explain how we're able to have so many billions of neurons in our brain.
These neurons affect how much you do, or don't, want to eat
University of Arizona researchers have identified a network of neurons that coordinate with other brain regions to influence eating behaviors.
Mood neurons mature during adolescence
Researchers have discovered a mysterious group of neurons in the amygdala -- a key center for emotional processing in the brain -- that stay in an immature, prenatal developmental state throughout childhood.
Connecting neurons in the brain
Leuven researchers uncover new mechanisms of brain development that determine when, where and how strongly distinct brain cells interconnect.
More Neurons News and Neurons Current Events

Trending Science News

Current Coronavirus (COVID-19) News

Top Science Podcasts

We have hand picked the top science podcasts of 2020.
Now Playing: TED Radio Hour

Listen Again: The Biology Of Sex
Original broadcast date: May 8, 2020. Many of us were taught biological sex is a question of female or male, XX or XY ... but it's far more complicated. This hour, TED speakers explore what determines our sex. Guests on the show include artist Emily Quinn, journalist Molly Webster, neuroscientist Lisa Mosconi, and structural biologist Karissa Sanbonmatsu.
Now Playing: Science for the People

#569 Facing Fear
What do you fear? I mean really fear? Well, ok, maybe right now that's tough. We're living in a new age and definition of fear. But what do we do about it? Eva Holland has faced her fears, including trauma and phobia. She lived to tell the tale and write a book: "Nerve: Adventures in the Science of Fear".
Now Playing: Radiolab

The Wubi Effect
When we think of China today, we think of a technological superpower. From Huweai and 5G to TikTok and viral social media, China is stride for stride with the United States in the world of computing. However, China's technological renaissance almost didn't happen. And for one very basic reason: The Chinese language, with its 70,000 plus characters, couldn't fit on a keyboard.  Today, we tell the story of Professor Wang Yongmin, a hard headed computer programmer who solved this puzzle and laid the foundation for the China we know today. This episode was reported and produced by Simon Adler with reporting assistance from Yang Yang. Special thanks to Martin Howard. You can view his renowned collection of typewriters at: antiquetypewriters.com Support Radiolab today at Radiolab.org/donate.