Mountains and minds
Can geological processes inform us about consciousness?
While visiting Eureka Dunes I had a eureka moment. I am meant to study consciousness, and to help others do the same. My first question to be considered is, can we learn anything about consciousness from our understanding of the geological processes that created Eureka Dunes?
I will answer this question in two parts. For each part I will describe a geological process, then draw an analogy to a brain process. Lastly, I will try to dig deeper and see if these comparisons lead to new theoretical or experimental ideas to be pursued.
Mountains and minds: Cycling between potential and kinetic energy
One of the most interesting things to me about geological processes is that they are cyclical. In a previous post, “Where does sand come from?,” I focused on coastal sand. Mountains and water engage in a dance as they cycle through different forms, with water taking the lead.
Rain to river to ocean to cloud to rain.
Mountain to rock to sand to rock to mountain.
Desert sand is different. It is land locked. Compartmentalized. The sand that makes up Eureka Dunes is not going anywhere for a long time.
Eureka Valley, home to Eureka Dunes, is a flat basin surrounded on all sides by mountain ranges. You can see evidence of water run off from the ranges onto the valley floor (see picture below or satellite view here). With water comes eroding rocks. Those rocks are further eroded by wind.
Some rocks become small enough that the strong winds ripping through the valley can lift and carry them to the south end, where they are deposited. So much sand has been deposited in Eureka Valley that it has created one of the largest dunes in North America, up to 680 feet high.
This is called an Aeolian process, in homage to the Greek god Aeolus, keeper of the winds. Aeolian processes contribute to shaping the land more in deserts.
“Wind. Wind strong enough to lift dust and clay thousands of feet; wind powerful enough to move rocks. It is wind that is the signature of the desert, if not its prime mover.” Mary Hill, “California Landscape: Origin and Evolution,” University of California Press (1984).
The prime mover is still water, though it comes rarely. The Sierra Nevada mountains to the west rob the lands of rainfall. Though rain and snow storms can frequently be seen along the ridge of that great range, they rarely reach beyond.
“Most of the deserts of North America have been robbed of water by mountains that intercept rain before it can reach the thirsty deserts. In this manner, lands of eastern California and Nevada are left dry because they lie in the rain shadow of the high Sierra Nevada.” Mary Hill.
Even when water does come, it serves to move more rocks down to the valley floor, not to transport them to oceans. Some of the lightest particles of silt can be carried away by wind, but the rest remains. The basin becomes laden with sand and salts. Only a great flood, such as might be brought about by large scale climate change, could wash the sand and salt out of the basin.
Consciousness is also cyclical in nature. Waking consciousness is very different from sleeping consciousness. Most of the scientific literature equates non-REM sleep with being unconscious. This conscious or not delineation is being revisited and refined, but I think we can all agree that we are most conscious when awake, and that we need sleep to restore consciousness to its fullest potential.
Are the processes happening in the brain during sleep comparable in any way to geological cycles? In geology, water is the prime mover, pushing mountain waste (rocks) out to the ocean so that it may be recycled. In the brain, it turns out, water is also a prime mover.
In 2013 Maiken Nedergaard published an article that blew my mind: “Sleep Drives Metabolite Clearance from the Adult Brain.” In it Lulu Xie and Hongyi Kang et al. show how cerebrospinal fluid (CSF) infiltrates the mouse brain, churns through the spaces between cells, then flushes back out. CSF does not move through brain tissue through diffusion, a slow, tortuous path, but is pushed through rapidly by convective exchange of CSF and interstitial fluid.
Most amazingly, they found that this process only occurs when the mouse is asleep or anesthetized. Upon waking, the faucet is closed. CSF no longer infiltrates.
See for yourself:
“Convection is basically like a river. It is fluid flow driven by a pressure gradient or gravity, for example. In convection, it does not matter whether you are a small peptide, like beta amyloid, that’s about 4 kilo dalton, versus a larger waste product, as tau, that is 10 times larger or 40 kilo dalton, because these proteins would flow with the same velocity as the fluid flow.” Maiken Nedergaard, in her presentation for The Physiological Society, 2018.
The authors hypothesized that this sleepy time brain bath is important for flushing out protein waste products. In the same 2013 paper, they showed that Alzheimer’s Disease related protein amyloid beta was cleared more rapidly during sleep. Since then the Nedergaard lab and others have done numerous experiments that strengthen and extend their hypothesis.
This system in the brain works similarly to the lymphatic system in the body. Since glial cells, specifically the astrocytic endfeet that surround 98.4% of brain vasculature, play a big role in opening and closing the faucet through the aquaporin 4 water channels they express, Nedergaard has named this the glymphatic system.
Prior to this discovery it was believed that the brain could only remove dead cells and protein debris through cellular degradation processes, like autophagy and ubiquitination, because there are no lymphatic vessels in the brain to remove waste. Cellular degradation is another process for removing brain waste, akin to the ceaseless mechanical processes that break down rock. Even then, some waste gets stuck.
That is where the glymphatic system comes in. Like the sand stuck in Eureka Valley, this is the flood that resets the cycle.
Cycling between potential and kinetic energy
Geological cycles are dependent upon the transfer of energy. A rock atop a mountain has a great deal of potential energy, both gravitational and chemical. After wind and rain, rivers and oceans have exerted their forces to mechanically and chemically break down the rock, its potential energy is spent.
In the case of sand reaching the ocean, the tiny rocks that make up sand are eventually recycled to form new rock. In the case of sand reaching Eureka Valley, the tiny rocks are stuck much longer, but eventually the flood will come. That sand, too, will be recycled. In both cases, once a full cycle is complete, the potential energy of the rock is restored.
I argue that the processes that generate consciousness are also dependent on cycling between potential and kinetic energy. Nedergaard has convinced me that while our consciousness is at rest, the glymphatic system flushes out our brain. She has largely focused her attention on the mechanisms that enable this to happen, and on clearance of proteins related to neurodegenerative disorders. But many waste products build up in the brain.
I believe that the glymphatic system restores the brain to a state of maximal potential energy. I think this potential energy is slowly exhausted throughout our waking hours as metabolites are used and neurons and other cell types signal to each other, dumping ions and proteins into the extracellular space — as mountains turn to sand.
Interestingly, the Nedergaard lab has more recently found that lactate is removed by glymphatic clearance. Lactate is the metabolic waste product of glucose, the primary energy source of the brain. What else might the glymphatic system remove? Neuromodulators. Neurotransmitters. Ions. Intercellular signaling molecules stuck in the extracellular matrix. There are many possibilities waiting to be investigated.
I further argue that if we want to understand how the raw materials that produce consciousness are used and recycled we need to study what is happening in the spaces between neurons — the extracellular space, i.e. the basins of the brain.
It is an exciting time to be interested in the diffusion and convection of different molecules through the extracellular space of the brain. Labs around the globe are just now converging on the technologies required to study these questions in live animals and human beings.
Some questions that will be interesting to pursue and model might be:
- What raw materials store the potential energy necessary for the production of consciousness?
- What are the primary forms of kinetic energy that generate consciousness?
- How does the degradation of identified raw materials contribute to that energy?
- How is potential energy restored in the system?
Singing sand and consciousness: resonant frequencies, constructive interference, and emergent properties
Ok, now things are going to get weird.
Though stuck in the valley, the sand that makes up Eureka Dunes is in constant flux. Eureka Dunes is one of 40 identified “booming” dunes. In the heat of summer, the top sheet of dry sand sometimes falls down the slip face of the dune. The avalanche produces an audible sound that is sustained for several minutes, long after the avalanche has ended. The booming is a musical sound, with a dominant frequency being heard alongside higher-order harmonics, comparable to the vibration of a plucked cello string. The most common dominant frequency is 85 Hz (+/- 4 Hz), either the musical note E or F.
“Where the music is heard in circumstances which admit of no mechanical or artificial causation, the wind is capable by itself of playing upon the chords, and producing the vibration that is necessary for the manufacture of the sound (Curzon 1923).” From “Booming Sand Dunes,” by Melany L. Hunt and Nathalie M. Vriend.
Melany Hunt asserts that the booming sound is created because the dunes are not totally uniform. They have a compacted, damp interior and a loose, dry surface layer. An impact on the loose sand creates an acoustic pulse that initiates a wave. Acoustic waves are trapped between the air and the damp interior layer, which can be thought of as a waveguide that promotes amplification of the wave. The booming sound can only be emitted when enough energy is supplied, and an avalanche does the trick. Avalanching sand converts gravitational energy into kinetic energy, further amplifying the acoustic waves through constructive interference. If and only if conditions are right such that the threshold for booming frequency is reached, some of that energy can escape into the air as an emitted sound.
Hunt has taken things beyond description and has developed a mathematical model of this system. See “The waveguide theory for booming sand dunes,” and “Linear and nonlinear wave propagation in booming sand dunes.”
The emission of an audible note from sand dunes can be thought of as an emergent property, because it depends on actions of the system as a whole. A single grain of sand can not produce sound, but the collective vibrational energy of sand can.
Quite a few neuroscientists and philosophers studying consciousness believe that consciousness is an emergent property. But what property is consciousness? At what scale can it be detected? What type of energy drives it, and what type of energy conversion takes place to produce consciousness? The answers are unknown.
One popular idea is that the synchronization of brain oscillations between brain areas produces consciousness. Of particular interest is synchronization of distinct thalamocortical networks. Thalamocortical networks are bi-directional connections between neurons in the thalamus and cortex, with distinct neural networks for processing of distinct information.
Brain oscillations (AKA brain waves) can be seen when looking at the activity of large populations (thousands or millions) of neurons at once. Populations of neurons tend to oscillate between active (on average, probability of firing action potential is high) and not (on average, probability of firing action potential is low), and that oscillation is rhythmic. Brain oscillations occur at resonant frequencies, and a lot of research has gone into trying to understand the functional significance of each of five frequency bands.
In thalamocortical networks the high frequency gamma band (typically 40 Hz, but ranging from 25–100 Hz) has been of interest, since thalamocortical gamma oscillations synchronize during difficult cognitive tasks. It is believed that constructive interference between synchronous waves amplifies the signal.
For many years after their discovery, brain oscillations were thought to be functionally insignificant noise, the ongoing background activity of the brain. It has since been found that a certain threshold of oscillatory activity must be reached in order for incoming stimuli to be perceived, though that is debated. Everything about this field is debated. Check out the review article, “Neural oscillations and the initiation of voluntary movement,” to get a sense of it.
That said, if what I have presented here is right, the oscillations can be thought of as the avalanche, required to generate enough energy to produce the emergent property of consciousness.
Brain as sand dune
The brain is a highly complex structure. Efforts are underway to create a census of specialized cell types in the brain as a starting point to understand how they work in concert (see the Allen Brain Atlas). There are a lot of them. Even within cell types, there are a lot of individual differences between cells. As a molecular and cellular neuroscientist, this is how I have always thought of the brain. It is fascinating and important, but…what if we threw out most of that information and just thought of the brain as a sand dune?
Now comes the armchair philosophizing. The following is not to be taken as truth, but as the presentation of a new framework for thinking about consciousness whose merits are up for debate.
Does the brain have a simple internal structure that could explain consciousness, like the damp sand with dry sand on top? Damp sand is more compact. The dry sand on top would be farther apart and more fluid, more free to vibrate. Is the brain similar?
I will start by considering the fundamental structure of an individual neuron. Neurons have a cell body the extends a branched process called a dendrite and an unbranched process called an axon. Dendrites are kind of like filter feeders in the ocean. They are nets that can “catch” the various neurochemical signals floating by in the extracellular space. Axons are more like an extension cord. They are essential for the transmission of signals over long distances. Axons that are transmitting signals to the same destination group together into nerve bundles, also called axon tracts, that all terminate (“plug in”) at the same region of the brain or location in the periphery. They adhere to each other, like a good IT person wrapping velcro around groups of wires to keep things tidy.
I can’t decide if this is the dumbest question ever or of critical importance — Does the transmission of an action potential down the length of the axon cause a physical vibration?
Action potentials do propagate along the length of an axon like a wave. The influx of ions changes the osmolarity of the interior of the cell. Perhaps that causes extracellular fluid to rush in, causing the axon volume to swell slightly. It isn’t such a crazy idea. However, most axons are coated with myelin, and only exchange ions with the extracellular space at one micrometer gaps called the nodes of Ranvier. Even so, myelin is kind of like a lipid and protein rich water balloon. If the axon was to swell or vibrate at the nodes, wouldn’t that be amplified by such a structure?
Perhaps the energy created by that tiny bit of movement can be amplified further when axons within a nerve bundle are all firing together. Is there a capacitive displacement sensor sensitive enough to detect the tiny vibrations that might be happening? How might we track other types of energy that might be generated during the concerted activity of axons within a nerve bundle?
Could axon tracts be the cello string, and consciousness be the audible note?
Taking a more macroscopic view, there is a newish type of human brain imaging called diffusion tensor imaging that specifically images the structure of axon tracts. Looking at these images while thinking of energy traveling through wet and dry sand, I notice a few things (you can take a look for yourself, here).
First, most of the major tracts in the interior of the brain do not travel in a straight line. They curve. Perhaps this is simply because they are surrounding the curved structures of the gray matter. Or perhaps that yields some benefit in nerve transmission.
The bilateral symmetry of the brain is remarkable. Some of the curving tracts almost make a circle, or at least an egg shape, when looking at axon tracts in both hemispheres. The thalamic radiations are particularly stunning. It is tempting to imagine the synchronous activity of the layered, circular axon tracts as an electromagnetic coil, but I don’t think that is the case.
Second, the tracts in the interior of the brain tend to be dense, closely packed, thick bundles. In contrast, images of the thin tracts extending outward into the cortex in all directions are reminiscent of a koosh ball.
Like a sand dune, axon tracts form a densely packed interior with a relatively spacious outer layer. It makes sense to me that these thin tracts, distributed in almost a grid throughout the cortex, would have different physical properties, and might transfer energy to the surrounding tissue differently.
And that, my friends, is all the energy my own brain has to think about this today. Please take it from here. The hope in presenting a comparison between the brain and a sand dune is to inspire new ways of thinking about how the brain works. Please do pick this post apart and tell me all the things I don’t know.