The Evolution of Attention
Consciousness can be understood when knowing the Evolution of Attention from the first multicellular lifeforms to modern mammals. When did our ancestors first experience consciousness?
This is an excerpt from the book series Philosophy for Heroes: Act. You can preorder the e-book here.
If we were able to pinpoint the moment in time when this happened (and the corresponding evolutionary changes in the brain), we could deduce the underlying mechanism of consciousness. In this context, it is best to first limit our examination to attention, which refers to the ability to select between competing or even contradicting sense perceptions. For example, you hear something on your left, see something moving on your right, you are hungry, and tired; which sense perception gets your attention first?
Attention Attention is the brain’s process of limiting alternative thought patterns, then increasing the most dominant thought pattern’s strength. It is like a simple majority rule: the most successful thought pattern gets all the resources while other thought patterns are suppressed. While we can jump back and forth between different thoughts, we cannot have two dominant thought patterns at the same time.
The brain parts involved that help us make this decision underwent half a billion years of evolution and can be traced back to simple multi-cellular organisms (see [Kaas, 2017, p. 547-554] and https://www.wanderingsolace.com/a-brief-history-of-brain-evolution.html):
|Millions of years ago||Species||Brain part|
|750||sponges||no nervous system|
|580||hydras||basic nerve net|
|520||fish||optic tectum / superior colliculus (controlling eye movement)|
|520||fish||thalamus (integration of information)|
|520||fish||basal ganglia (resolving conflicting thoughts)|
|520||fish||amygdala (evaluation of different decisions)|
|520||fish||(predecessor of the) hippocampus (spatiotemporal memory)|
|450||sharks||cerebellum (movement programs)|
|300||reptilians||wulst / neocortex (high level planning)|
|Figure 5.1:||Timeline of mammal ancestry with a list of animal species that branched off the evolutionary tree and which brain part first appeared at that time.|
Given this long history, the brain was never designed as a thinking machine as such. It is a collection of different functions, layered like an onion, with many systems having overlapping responsibilities. If a neural pathway helped one of your ancestors to avoid danger, it survived, even if that meant that the architecture became somewhat chaotic (“complex”). Nature does not care about simplicity in this regard, it cares only about what works and what does not. Sometimes, the organization of a certain function into clearly distinct brain parts had an evolutionary advantage (for example, separating the neocortex from the rest of the brain). In other cases, the most efficient way of building a brain was to put one function directly beside the other (for example, the different brain regions in the necortex).
Yet, for our understanding of consciousness, it is sufficient to look at the brain as a system of separate parts interacting with each other. We just have to keep in mind that brain parts usually blend into neighboring brain parts, and that there are a lot more interactions and connections between brain parts than listed here.
5.1.1 Nerve Nets in Hydras
Looking at our evolutionary tree of animal ancestors in regard to brain development, sponges were the first to branch off (more than 750 million years ago). They are sea animals that are mostly immobile and simply filter oxygen and nutrients from the ocean water. Although they have a primitive way of pushing water out of their body when it is toxic or otherwise polluted, they lack any form of nervous system as we understand it. Their cells communicate directly with each other with calcium signalling. Each cell contains a concentration of calcium that can be released if it receives calcium from neighboring cells. This way, it creates a calcium wave propagating throughout the organism. You can imagine it like having many square containers grouped together and filled up to the brink with water. When you take one container and pour it into its neighboring containers, all containers will overflow.
|Figure 5.2:||An image of a hydra, an animal that has a basic nerve net to catch prey with its tentacles (image source: Shutterstock).|
The first animals with some resemblance of a brain were hydras (see Figure 5.1.1). They branched off our evolutionary tree more than 540 million years ago. They are small (around 10 millimeters in length) animals that usually attach themselves to the surface of an object in their environment (e.g., a rock) and can slowly move over it or detach themselves and float in the water. They have a basic nervous system that allows them to use their tentacles to attack prey. If another animal (mostly tiny planktonic crustaceans like Daphnia or Cyclops (up to 5 millimeters in length) touches a tentacle, the nerve cells activate the tentacle to take it into the hydra’s mouth. There is no central nervous system that organizes this activation. Instead, nerve cells are spread throughout the body of the hydra in a nerve net. This enables the hydra to respond to its environment, but not to detect where this original stimulus came from. Any signal leads to the same reaction (see Figure 5.3).
|Figure 5.3:||In a nerve net, any signal leads to a single reaction (for example, a touch leads to a contraction of all muscles at the same time).|
Nerve net A nerve net is nervous system without a central organization. This means that any signal (that exceeds a certain signal strength) that the nerve net receives through its senses causes a singular reaction. Hydras have this kind of nervous system to contract their body when prey touches their tentacles.
If the human body had a nerve net instead of a nervous system and brain, we would not be able to figure out where we were touched, only that we felt something and as a result, had to come up with a general response to this touch. Such a general response is comparable to our hormonal system: for example, the adrenaline released in a situation of danger does not cause specific actions but prepares the whole body for a possible injury or energy exertion. Another example would be the regulation of body temperature which, again, is a general response to certain conditions instead of a specific movement. The opposite is, for example, the reflex of suddenly jerking back your hand after touching a hot stove: a specific signal leads to a specific reaction.
5.1.2 The Olfactory System in Lancelets
The olfactory system’s roots probably evolved very early, as detecting molecules is closely connected to a lifeform’s search for nutrients. While not directly related with us (they diverged from our ancestors around 700 million years ago), we share some of our olfactory-related genes with lancelets (see Figure 5.4). They can be seen as predecessors of fish with similar organs but in more primitive form. For example, their gill-slits are used for feeding but not for respiration. Likewise, their circulatory system transports nutrients but no oxygen. While they have no centralized olfactory system (we humans do), their olfactory receptors are studded along their flanks to detect possible sources of nutrition in their aquatic environment.
|Figure 5.4:||An image of a lancelet. Lancelets can be seen as predecessors of fish with similar but more primitive organs (image source: Shutterstock).|
Molecules connect to the olfactory system over the peripheral olfactory system. In aquatic animals this happens directly via the contact with the water. In land animals with lungs this happens by having the airborne molecules dissolve into a mucus on top of the olfactory receptor cells. If the molecule binds with the receptor cell, a nerve signal is created and transmitted to the brain.
5.1.3 Classification of Signals in Arthropods
A basic form of attention appeared at the time the arthropods (insects, spiders, crabs, etc.) split off the evolutionary tree around 550 million years ago. With this new form of attention, instead of treating all sensory input as equal, the information is pre-processed and can thus be amplified and classified. For example, imagine noticing something suddenly moving in the grass—it immediately draws your attention. Once you see it emerging from the grass, you classify it as a particular concept, for example a snake.
By comparing several images on your retina for changes, your visual system can make out which moving part belongs to which previously seen part. For example, if your visual system identifies a dog and then the same dog in subsequent images, you perceive any changes in those images as movements of the dog. If you closed your eyes every second, you would perceive the dog “jumping” from place to place. You would have to use your short-term memory as a workaround and remember where the dog was earlier to decide whether or not he had moved. Figure 5.5 shows an example that represents the filter function XOR. When passing two images through such a XOR filter, it would highlight changes between both images and thus detect movement.
|Figure 5.5:||In this nerve system, the dashed lines inhibit the corresponding reaction. If signal 1 and signal 2 appear at the same time, no action is taken. If only signal 1 appears, action A is taken. If only signal 2 appears, action B is taken.|
At its core, this classification is filtering of information we do not need and amplifying and abstracting the remaining information. Most multi-layered nervous systems (including our own) support this kind of filtering. It is now heavily used in artificial intelligence research to, for example, classify images that contain particular objects or faces. While it also applies to other senses like sense of touch or hearing, for the explanation of pre-processing we will focus here on the visual sense because it is easiest to explain.
Visual pre-processing is done partly by the retina of our eyes, detecting edges and changes, and compressing the data-stream toward the rest of the visual system. To understand what is happening, imagine looking at a picture of a tree in front of a white background. How do you determine there is a tree? Do you “scan” the picture line by line from top left to bottom right? What does the brain do with the raw sense data coming in from the eyes?
When drawing a picture, we start out by creating the contours of the objects with a pencil and then filling them with color. When looking at a picture, we go the opposite way: first, we perceive the raw image, then our visual system extracts the edges of objects (see Figure 5.6).
|Figure 5.6:||Edge detection applied to a photograph, by John McLoone, 2010.|
Beyond just being a one-way street of information (classifying the image data to abstract information), classification systems can also help you to direct attention. When we see things that are new or unusual, our brain allocates resources to finding out what they are. This could be just by turning our head, refocusing our eyes, looking at things from a different perspective, going closer, or asking others about the new or unusual things.
5.1.4 The Optic Tectum in Fish
Controlling eye movements made it necessary for fish (they split from the evolutionary tree around 520 million years ago) to develop central processing, namely the optic tectum (in mammals, this organ is called the superior colliculus). It helps fish (and us) to track moving objects and is responsible for blinking and pupillary and head-turning reflexes. In the brain, it sits right behind the optic chiasm where both the nerves from the left and right eye cross (see Figure 5.7). If you are not actively (consciously!) focused on something else, and an object moves outside your inner field of sight, this part of the brain is responsible for bringing it to your attention, after which you might move your head, and re-focus your eyes to get a better picture of the possible threat. Imagine you did not have this reflex and you had to consciously move your head and eyes to see anything moving in your environment. The risk of injury (say, from an oncoming tiger) would be much higher because of your long reaction times.
Optic tectum The optic tectum or superior colliculus (in mammals) helps the eyes to track objects, and controls blinking, pupillary, and head-turning reflexes.
|Figure 5.7:||The basic visual projection pathway from our eyes to our neocortex. Both eyes send the left side of their perceived visual field to the right hemisphere, and the right side of their perceived visual field to the left hemisphere.|
Interestingly, if something or someone demonstrates the same capabilities that the optic tectum has (tracking a moving object with its head and eyes) that is enough for us to automatically attribute some sort of “consciousness” to it. A example is the robot “Pepper” (see Figure 5.8) which (who?) is programmed to recognize faces and move its (her?) head.
|Figure 5.8:||The robot “Pepper” (image source: Shutterstock).|
5.1.5 The Thalamus in Fish
Fish also developed the thalamus, an organ to combine and pre-process different sources of sensory information into a coherent whole before relaying it to other parts of the brain. One example is what we have discussed in Chapter 5.3.3 about how the brain integrates the information from the red, green, and blue cone cells in the retina into colors. While further processing takes place in the neocortex, the first part responsible for this color encoding is the lateral geniculate nucleus (or LGN). Another example is integration of visual and auditory signals. If you hear a sound slightly to your left, the auditory system communicates this information to, among other parts of the brain, the visual system (through the LGN, McAlonan et al. ). It is also involved in a number of image pre-processing tasks. For example, it provides a three-dimensional model of the environment by combining the information from the left and right eyes.
Thalamus The thalamus integrates different sensory information and relays the information to other brain parts. For example, it combines sense data from the retina’s cones into colors, or calculates the three-dimensional information from the two-dimensional images from both eyes.
As an example, imagine seeing a barking dog. It is thanks to our thalamus that we recognize that the barking sound and the image of the dog belong to the same entity (or at least that we can turn our visual attention to the barking dog). Without the thalamus, we would recognize them as two separate perceptions. For an example of when this fails us, imagine hearing a plane far above you: not taking into account the speed of sound, the distance between you and the plane, and the high speed of the airplane, your thalamus fails to combine the information correctly. You end up looking where you (falsely) expect the plane to be (where it had been a few moments earlier) instead of where it actually is. The neocortex could help out here of course, too: you could sit down and do mathematical calculations on paper to determine the actual position, considering the distance to the plane and the speed of sound.
Given its central role when creating a coherent and whole experience of the world, we will revisit the thalamus several times in later sections of this book.
5.1.6 The Basal Ganglia in Fish
The basal ganglia is thought to originate from the need to arbitrate between different courses of motor neuron activation. It identifies the “best” among several possibly contradicting courses of actions. Rules determine what “best” in the context means. With the analogy of the neural committee, the motor program with the most votes from the committee gets selected to be executed.
The basal ganglia is also involved in coordinating entire motor programs. Imagine an orchestra without a conductor: sure, the musicians could play their correct parts, but at different speeds and starting at different points in time. The basal ganglia is like a conductor of an orchestra, synchronizing the different motor programs.
Basal ganglia The basal ganglia is a part of the brain that arbitrates decisions by the neural committees. Also, it coordinates the sequence of entire motor programs. In both cases, it is not a decider but merely provides rules and structure like a referee.
5.1.7 The Amygdala in Fish
Yet another brain part, the amygdala, first appeared in fish. Arbitrating and catching our attention with the basal ganglia is one thing, how we prioritize the signal is another. While we can make decisions based on the strength of the signal—turning our head to the loudest noise seems to be a good strategy—we also need to put the signal into context. For example, instead of always running away from a tiger, we might consider whether or not to take the risk and first pick some berries and only then run away—especially if we are very hungry. This demonstrates how the amygdala uses information from a number of sources to prioritize different courses of action.
Amygdala The amygdala is the brain’s value center. Instead of relying on mere signal strength, the basal ganglia relies on the amygdala to evaluate different signals depending on the context. It is also connected to the hippothalamus providing a bridge to the hormonal system.
The amygdala is also connected to our hippothalamus, providing a bridge between the brain and the hormonal system. As such it can provide a fight, flight, or fawn response:
- Attack the predator (“fight” response),
- Run away from the predator (“flight” response), or
- Freeze (“fawn” response).
The “fawn” response works against predators because many predators’ instincts depend on motion. If their prey is not moving, their hunting instinct is not activated. For example, a frog would starve in a box full of dead flies. Beyond helping with the execution of the immediate response (for example, releasing adrenaline), it can also serve as an early basic form of memory. The hormonal changes caused by, for example, the flight response remain long after a predator has vanished from an animal’s view and will cause it to be more careful or head home to a safe place.
The information the amygdala is using is limited to immediately available sensory data, though. It activates emotional reactions based on mapping the input from the thalamus to emotional behavior. For example, the sight of fresh berries might evoke a positive emotional response while the voice of a rival might evoke a fight, flight, or fawn response. This response can overrule any rational evaluation of the situation because it is quicker and possibly stronger than signals coming from the neocortex.
What makes this mechanism so powerful is that—while it requires very little processing power—it can cover a wide arrange of sensations. As opposed to complex mappings and interactions going on in the neocortex, all the amygdala is doing is mapping a signal to a limited array of emotions. Imagine walking through an art museum and all the amygdala is saying about each image would be “good” or “bad” while your neocortex writes an entire analysis of the structure and coloring of each image.
5.1.8 The Hippocampus in Fish
A predecessor of our hippocampus also developed around the time of the first fishes. The actual hippocampus is unique to mammals but there are theories that similar structures evolved from a common ancestor of reptiles and mammals around 320 million years ago. Its main task is to create a mental map of your environment so you remember where possible food and water sources are located. It also helps you to navigate, remember paths you have taken, relate spatially to other people, animals or objects [Danjo et al., 2018], and recognize places for orientation. A good example for the use of the hippocampus is squirrels burying nuts as food stashes for the winter. Our current understanding is that this map is not a literal map but instead makes heavy use of points of orientation. While many people can construct a mental image of a map, we prefer to orient ourselves by seeing something we know and then putting our goal in relation to the landmark. For example, when describing to another person the path to a location, we might say “Walk down the street until you get to the large tower, then turn right.”
Hippocampus The brain’s hippocampus provides us with a mental map for navigation. It also has a temporary component tracking, for example, which areas in our environment we have already foraged. The hippocampus together with the olfactory system (sense of smell) make up the allocortex.
Allocortex The allocortex is part of the cerebral cortex (the neocortex is the other part) and consists of the olfactory system and the hippocampus.
While earlier animals could drift and react to sensory inputs (evading predators and approaching food), once the food was out of sight, it was also out of mind. The hippocampus allowed those animals to forage more efficiently by avoiding areas that they had already foraged and exploring areas where they had not foraged. This requires mapping the environment based on odors and sights, as well as prioritizing those according to the time they should be visited [Murray et al., 2018]. The hippocampus evolved to handle tasks in serial order with the right timing and in the right context, as well as into the gateway for long-term memory of the neocortex. Its function becomes most visible during dreams, when experiences are played back that were retained during the day for long-term memory backup.
5.1.9 The Cerebellum in Sharks
More than 450 million years ago, sharks with a cerebellum emerged. This organ coordinates complex, time-critical behavior. You can imagine that when hunting, the shark might have to outmaneuver its prey and then bite at the right moment. Similarly, its prey had to come up with movement strategies to navigate through the water to evade predators, locking predator and prey into an evolutionary race. To understand the role of the cerebellum in our daily lives, imagine that you had to execute all actions consciously, like when you first learned to ride a bicycle or walk: the programs to coordinate all your motor neurons were not yet transferred to your cerebellum (“learned”). Thus, they were not yet optimized and consequently, they were very slow. Until that optimization happened, walking or riding a bike had not become “second nature.” You had to take “baby steps” and focus on one step or motion at a time before making the next one.
In more general terms, the cerebellum is used for anything that needs to be replayed. Beyond pure movement programs, current research points to an additional role of the cerebellum in language fluency, social behavior, and the reward system [Carta et al., 2019]. Given that both the cerebellum and the basal ganglia are involved in coordinating motor programs, it is no surprise that they also form an integrated network to exchange information [Bostan and Strick, 2018].
Cerebellum The cerebellum is the brain part that helps with coordination of complex and time-critical behavior. Its main focus is any task that can be replayed, for example, language or specific motor programs.
Very few people are known to live without a cerebellum. In such a case, it seems that the rest of the brain can compensate to a degree, but showing late speech development, slurred and slowed speech, and a reduced control of pitch and loudness. Given that there is a direct connection from the inner ear (which provides information about one’s balance in the three-dimensional space) to the cerebellum, it is also of no surprise that in such cases, we see late walking development, reduced gait speed, unsteady gait, and a reduced ability to stand in darkness or when the eyes are closed [Yu et al., 2014].
5.1.10 The Neocortex in Mammals
The challenge with the hippocampus’ mental map is that the environment can change quickly. Food stashes or the location of fruit trees or hills might be relatively permanent, but what about the location of predators or possible prey? Without the ability to track objects and predict those objects’ possible locations, if you see a tiger vanishing behind a tree, the tiger would also vanish from your mind. Likewise, a piece of bread would vanish from your mind simply because you put it into the toaster—and it would miraculously reappear as toast.
Object permanence The ability of object permanence allows us to track predicted positions or movements of objects even after they have vanished from our field of view. By running a simplified simulation of the world, we are aware that a tiger that has jumped behind a tree is still there.
Without this ability to track objects when they are no longer visible (object permanence), you could rely only on the more primitive form of emotionally-driven behavior provided by the amygdala. You would still be in a state of shock from seeing the tiger in the first place, and you might even connect a general negative feeling with the area where you have seen the tiger. But after the tiger vanished behind the tree, this emotional state would be the only thing on which you could base your actions.
These were the reasons that, more than 300 million years ago, the neocortex emerged, layered over the collicular control of attention (above the previously mentioned optic tectum or superior colliculus). Following the Permian-Triassic mass extinction event 252 million years ago (extinguishing 70% of land biodiversity), both the dinosaurs and mammals emerged. Given that dinosaurs still dominated the planet, mammals had to move into a niche and become nocturnal animals. According to the nocturnal bottleneck hypothesis, traits of growing fur, managing body temperature, and well-developed senses of smell, hearing, and touch helped our ancestors to stay active at night while evading predators during the day. Creating a map based on smell takes more effort than if you use your eyes. Smells are generally temporary and tell you (or at least our mammal ancestors) about the past. But being able to tell when a predator, prey, potential partner, or competitor passed by can be essential in survival and procreation. To process the information, our ancestors’ brains, especially the neocortex, grew significantly [Rowe et al., 2011, cf.].
Nocturnal bottleneck hypothesis The nocturnal bottleneck hypothesis states that many mammalian traits like growing fur, managing body temperature, and well-developed senses of smell, hearing, and touch were adaptions to moving into a niche to become nocturnal animals. This adaption was necessary to evade the dominant dinosaurs who roamed the Earth during daytime.
The neocortex’ tasks are long-term planning, learning, and coming up with creative solutions using evolutionary algorithms and neuronal committees. For example, you might want to calculate a path to food sources, considering all the other places you want to visit (drinking from the lake, checking out your burrow, and so on). It also allows you to make predictions about the future by tracking the position and state (e.g., berry bush is blooming) of objects or even the mental state of other people. In your neocortex’ world, the previously vanished tiger still exists, despite you no longer seeing it. Other functions of the neocortex are sensory perception, cognition, directed motor commands, spatial reasoning, communication, and planning.
In addition, the neocortex allows direct self-generated (meaning that there needs to be no external stimulus) control of your skeletal muscles. Life without the neocortex could be imagined as being in the gym, and the only way you could do any exercise would be if someone scared you each time to make you jump. Likewise, your eyes would be controlled by reflexes and would move only as a reaction to another moving object within your field of vision—sufficient to duck, but not enough to actually make decisions that are not just reactions to your immediate environment or your inner state.
Neocortex The neocortex is the newest part of the mammalian brain and consists of the cerebral hemispheres. Its main tasks are focus, language, long-term planning, and modelling of the world. It can generate strategies that involve detours if goal-directed behavior is not successful (for example, going around a fence instead of trying to get through it).
Cerebral hemispheres The cerebral hemispheres consist of the occipital lobes, the temporal lobes, the parietal lobes, and the frontal lobes. The two hemispheres are joined by the corpus callosum.
Lobe A lobe is a clear anatomical division or extension of an organ.
Not all animals with a neocortex have all the abilities listed above. For example, besides primates, so far it has only been shown that dogs, cats, and a few species of birds have the ability to track objects when they are no longer visible (object permanence). Surprisingly, many predator animals do not have this ability. When the prey suddenly vanishes underwater or behind a tree, they are confused and visibly unsure where it went. Their basic emotional system still works and they are still in hunting mode. But without an idea of the location of their prey, they might give up their hunt quickly.
Beyond model-building and motor control, the neocortex also deals with classification of sense data. The visual field is analyzed to create a mental representation of objects and their location. Instead of the raw data, this mental representation of the world is then used by the rest of the brain in, for example, coordination with motor control, language (reading this book), or face recognition. Similarly, other senses (hearing, touch, smell, etc.) are processed, and their signals are categorized and prioritized.
Last but not least, the neocortex also allows you to focus. Generating an internal signal, it can override external sense inputs or even signals from other brain parts. For example, some people are afraid of speaking in front of an audience. Their neocortex can still negotiate with other parts of the brain (especially the amygdala) to risk such a (in their eyes) dangerous decision, weighing the pros and cons. Similarly, soldiers usually undergo some sort of training to overrule their fear of facing an adversary and staying on task in high-stress situations. While getting angry (mobilizing strength) solved many of the problems in pre-historic times (for example, when being stuck, moving a rock, or fighting an opponent), it is now seen as a maladjusted behavior. Bashing your fists against your defective car or computer will (usually!) not solve the issue.
As all skeletal muscles are under control of the neocortex, it can even override automatic functions like breathing. The neocortex can even overrule the amygdala so that a hormonal reaction does not lead to a physical response. For example, you might be angry at a person but the social situation does not allow you to act on that emotion without significant social repercussions. That is the neocortex overruling the amygdala in favor of long-term goals.
|Figure 5.9:||My lungs: *breathes automatically* Me: *thinks about it* My lungs: *well, now I’m not doing it* (image source: Shutterstock).|
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