By Siha Hoque
Since the development of medical sciences, our understanding of the human brain and its various functions has been constantly evolving. One of the first proposals for the purpose of the brain was by Aristotle in 335 BC, who claimed that it served to ventilate blood in the heart, which would otherwise overheat. Around two thousand years later, we recognise the brain as the seat of intelligence, memory, personality and emotion.
Various scientists have often likened the human brain, an organ composed of 86 billion neurons and 100 trillion synapses, and 100 neurotransmitters transferring signals between them, to a computer. In fact, it remains true that many parallels exist between the two systems, that both utilise electrical signals, receive input, process it, and produce output. It could be even said that the tangible tissues that form the brain are ‘hardware’, and that our cognition is ‘software’.
In spite of the above, the human brain also outperforms any machine existent today, and this can be summarised as due to an unattainable level of efficiency; the organ operates on under 20 watts, and in order to complete 1% of brain activity, a computer would require millions. The brain’s neurons and synapses all communicate by electrical and chemical signalling between cells simultaneously in parallel processing, whereas the best modern computer would require over a hundred times the space to do the same; and as well as that, humans can learn an entirely new concept with only one experience, whereas current artificial systems require at least thousands of data points to acquire the same accuracy.
See above: El Capitan, currently the world's most powerful supercomputer.
It is in fact difficult to conceptualise the myriad of processes that occur, unnoticed by most, on a daily basis. The average school day is approximately seven hours long, which is 25200 seconds long. Let’s say within 100 of these seconds, you were to sit down in a noisy classroom, at a table, with a worksheet in front of you. As you begin to read and answer the questions, your teacher gives you instructions. Within these, and every following 100 seconds that pass, your brain completes 100 quintillion operations. These can be categorised into constant processes and mechanisms which allow you to perceive, interact with and learn from the world around you.
Brain Anatomy (Simplified)
See above: A simplified diagram showing where in the brain grey and white matter is present.
Overall, the organ is composed of two main tissues, white matter (formed from nerve fibres) and grey matter (formed from neuronal cell bodies). The grey matter is typically found on the outside of the organ, and on certain internal structures, such as of the limbic system.
It is first important to understand the brain’s complex anatomy, which can first be simplified into three main segments.
The cerebellum is at the back of the skull, in control of fine motor skills, balance and posture. Below this is the brainstem, connected to the spinal cord and controlling subconscious functions like breathing and heartbeat. Above these two regions is the cerebrum, the largest part of the brain, divided into two hemispheres that are connected by a structure known as the corpus callosum. It is responsible for thought, learning, speech and conscious movement. The cerebrum can be divided into four distinct lobes, the frontal lobe, parietal lobe, temporal lobe and occipital lobe, each with their own specific purpose.
Within this however, are also several separate structures, namely the thalamus, hypothalamus, hippocampus and the amygdala. These are often grouped as the limbic system - controlling emotions and memory. The outside of the brain is known as the cerebral cortex responsible for high level processing, as well as the integration of sensory information and further dictating our movement.
However, it must be said that much of the brain functions as many-to-one, a principle referred to be neuroscientists as degeneracy, where distinct parts of the brain, or different neurons, can together produce the same outcome or serve the same function as another set.
Default Mode Network and Task Positive Network
In your school day, your brain may be entering and exiting many states. This may begin with Default Mode Network. DMN occurs during wakeful periods without active stimulation, or goal-based work. It involves regions including the prefrontal cortex and the medial temporal lobe - areas of the brain specialising in self-referential thought, high-level thinking, and memory retrieval. DMN is characterised by self-reflection, the recall of memories, planning for the future, daydreaming and social cognition.
When you begin to focus, perhaps when called during registration, the brain switches into the Task Positive Network (TPN) used for goal-orientated, focused tasks. This activates a different area of the prefrontal cortex (the dorsolateral region) responsible for high-level cognitive focus and planning, as well as the inferior parietal lobe which manages your sensory input and motor response. The change between the two states is dictated by the salience network, which identifies and filters the most relevant stimuli you are being exposed to.
See above: fMRI scan showing neural activity during DMN.
Neural Oscillations
Neural oscillations are more commonly known as brainwaves, and these refer to repeating, rhythmic patterns of electrical signalling through the central nervous system. These can be measured by electroencephalography (EEG) which is where electrodes woven into a cap are placed on the head to take measurements of the electrode potentials within. Brainwaves are categorised by frequency bands.
See above: An EEG cap.
Delta brainwaves have the lowest frequency, 0.5-4.0Hz, and occur only during deep sleep, the low frequency removing metabolic stress from the brain. These oscillations trigger a surge in hormones responsible for cell regeneration, hence growth and repair, and are highly important in maintaining good immune strength.
Theta (4-8Hz) and alpha (8-12Hz) brainwaves are observed during moments of physical relaxation, often alongside daydreaming or meditation, only with slightly more controlled attention present to reach alpha brainwaves. Motor control and more active thinking, like when using a calculator, commonly result in beta brainwaves, with a frequency of 12-30Hz. The highest frequency oscillations above 30Hz are known as gamma waves, and these are observed during advanced information processing, where multiple regions of the brain operate simultaneously at a high level.
DMN usually occupies the lower frequencies of the alpha band, however, returning to the school day, if you were to begin to read a question, the oscillations would transition into beta waves as you switch to TPN. DMN is disengaged, and several areas become involved in understanding the words of the question: the Dorsal Attention Network in the cerebral cortex engages to bring visual attention to words, whilst the ventral visual cortex in the occipital lobe, left temporal lobe and Broca’s Area in the left frontal lobe are activated to recognise these words as meaningful language.
Processing Sensory Input
The brain combines information from the five senses of sight, smell, sound, taste and feel to produce an overarching perception of the environment through specialised regions known as multimodal association areas, such as in the Parietal Lobe. Prior to this, most of the sensory data must first enter the thalamus (excluding smells, which are sent by olfactory receptors in the nose through a more efficient route to the amygdala in the temporal lobe, where they are processed quicker) The thalamus sends these signals to the appropriate locations in the cerebral cortex for interpretation - the visual, auditory, insular and somatosensory cortices. This is a constantly occurring process during wakeful hours, and so the brain has adapted to apply filtering, known as sensory gating, to remove irrelevant background sensory input - such as the sound of people playing a sport outside - and prioritise other stimuli, like further instruction from a teacher.
There are three continuously monitored senses beyond the five stated above, and they are: interoception - subconscious monitoring of temperature, heartbeat and breathing; proprioception - muscular positioning and movement, and vestibular - balance and orientation in space.
Fixational eye movements are occurring constantly when focusing on a single point or object. These are small, involuntary, tremor-like movements of the eyes which are required for continuous vision. If the eyes were totally still, the light receptors in the retina would stop responding to the stationary image, resulting in it fading. Despite all this, the image produced by the eyes is not camera-perfect, as our brains are perpetually filling in gaps in sight caused by blind spots, blurry peripheral zones, or brief obstructions. It does this through a mechanism known as predictive coding, which is the recognition of patterns and utilisation of surrounding cues to effectively generate textures, colours and shapes. Studies have shown that when a gap is encountered, the brain identifies borders of the object, before filling it in with colour, known as edge completion and surface filling. It is because of these processes that you can be aware of what the person next to you is wearing even if they are on the very edge of your periphery.
See above: Diagram of Semicircular canals.
Proprioception is the knowledge of where your body parts are in space, and the pressure they are exerting on their surroundings. This sense comes from the stimulation of proprioreceptors in all muscles and joints, which send electrical signals to the somatosensory cortex, where they are interpreted. This sense allows you to pick up a pen and uncap it, even whilst looking ahead at a board. Utilising sensors located in the ear, the vestibular sense is what gives you balance. It consists of semicircular canals - three fluid-filled tubes - detecting rotational movement, such as nodding, and otolith organs, which detect linear movement and gravity through the movement of calcium crystals within them. Then, as you move the pen to write, the cerebellum creates a comparison between what it expects to feel and the actual feedback from the senses. If they do not align - for example, you feel like you are about to drop the pen, the brain sends a reflex to correct your grip.
Cognitive and Emotional Processing:
Actually learning new information, on a microscopic level, occurs through neuroplasticity - which is defined by Dr Micheal Merzenich, a pioneering neuroscientist on brain plasticity research, as “The brain’s capacity to change itself physically and functionally throughout life”. When acquiring unfamiliar data or skills, neurons can form new connections, also known as synapses, to map this new information. Through repetition of these skills, and when sleeping, these neural pathways become further solidified. A hundred seconds in the school day would require a large amount of cognitive processing and the key structures involved in this are the prefrontal cortex, amygdala and hippocampus.
See above: Microscope image of a synapse. (Image from Okinawa Institute of Science and Technology)
The prefrontal cortex (PFC) is in the frontal lobe, located behind the eyes, and it manages all high-level thought through the executive functions of planning, decision making, working memory and the impulse control, overarchingly facilitating goal-based behaviour. When you are learning, the PFC acts as a command centre, one of its key roles being filtering out distractions. Chemically, this is enabled by the inhibitory neurotransmitter Gamma-Aminobutyric Acid - GABA - being released to suppress the activity of neurons responding to unrelated stimuli, such as background conversation in a classroom. GABA does this by causing individual neuronal cells to become too negatively charged to send an electrical signal. Another neurotransmitter called Norepinephrine intensifies the desired signals, one of the ways in which it does this is by increasing the likelihood of the carrying neurons firing, by raising the positivity of their charge. The PFC also manages your working memory, allowing you to ‘hold’ task-relevant data in mind, in order to bridge the gap between event and response. What is different here is that usually, after a stimulus is removed, neurons stop firing, yet, in the working memory, they continue to fire even after the removal of the initial stimulus. In terms of decision making, perhaps for selecting a strategy for solving a problem, the PFC evaluates the expected rewards and therefore enables an individual to choose what will give the best outcome. When you learn a new concept for the first time, the PFC is highly active, yet after strengthening the ability through repetition, the PFC has to monitor performance less as other regions of the brain take charge.
As stated earlier, the amygdala is part of the limbic system. It is a cluster of nuclei found in the temporal lobe, and it provides emotional significance to cognitive processes. In early humans it was a crucial survival system, used for perceiving danger and threats, particularly through the interpretation of others’ facial expressions. In your brain at school, it consolidates long-term memories, especially of emotionally intense moments, by releasing a cascade of neurotransmitters and hormones which effectively ‘tag’ an experience as special, and then strengthening the connection between the neurons mapping this. Some of the excitatory chemicals in this cascade are glutamate and calcium - glutamate opening receptors in neurons, allowing an influx of calcium to enter, and in turn triggering a surge in the production of proteins that can stabilise a memory.
Also a part of the limbic system, the hippocampus, responsible for the transfer of memory from short-term to long-term, as well as spatial navigation, resides near to the amygdala, deep within the temporal lobe. Chemically, when learning, experiences are converted to long-lasting memory by a mechanism known as Long-Term Potentiation, a process that physically strengthens the connections between neurons. The creation of a memory begins with initial signalling. Glutamate binds to receptors on the neuron when new information is first encountered, and if repeated or strong, some of these receptors open, which therefore allows an influx of calcium to enter, which in turn stimulates the creation of more receptor proteins. This makes the synapse of the neurone permanently more sensitive to future signals. As glutamate builds the memory, the neurotransmitter dopamine determines how relevant it is - if the memory created was surprising or rewarding in some way, dopamine is released in the hippocampus, and allows LTP to occur over a broader time window.
These three structures link both cognition and emotional processing. They are not, as once thought, distinct functions, but in fact co-dependent. Emotional associations act as a catalyst for learning new information, particularly in directing attention, and as observed with the limbic system, this is highly important for the encoding of memory.
Automatic Life Functions - The Subconscious
It is also important to appreciate the fact that there are many aspects the brain controls in 100 seconds without any conscious input, in order to keep our organs functioning in homeostasis. These automatic functions are controlled by the brain's oldest structures, including the brainstem, which is composed of the midbrain, pons and medulla oblongata.
See above: Microscope image of the tissue of the medulla oblangata.
Heart rate is controlled by the medulla oblangata, which receives sensory input from chemical and pressure receptors in the blood vessels, and in response adjusts the heart rate and blood pressure through the nervous system. There are two opposing branches of the nervous system, the sympathetic nervous system - responsible for alert or energising responses, including increasing heart rate, and the parasympathetic nervous system, responsible for resting, and decreasing heart rate.
Directly above the brainstem, managing body temperature and hunger, is the hypothalamus. Here, clusters of neurons function as detectors and integrators, and can activate the body’s natural cooling system, sweating; or heating system, shivering, should it detect a change from the optimal range. Central thermoreceptors sensitive to blood temperature, and peripheral thermoreceptors on the skin detecting outer temperature, are used to supply input.
See above: Chemical structure of the hormone ghrelin.
The management of appetite however, is more complex. A small cluster of neurons known as the Arcuate Nucleus, or ARC, is responsible for the integration and interpretation of signals from the stomach and intestines, which travel across the gut-brain axis, a bidirectional communication network between both zones of the body. When your energy lowers, the hunger hormone, ghrelin, produced in the stomach, activates certain neurons in the hypothalamus, which in turn activate peptides that stimulate your appetite. This is why for certain weight-loss surgeries, specifically gastric sleeve surgery, the part of the stomach producing the majority of ghrelin is removed. If, prior to the 100 seconds we are focusing on, you were hungry and ate breakfast, mechanoreceptors (receptors in the stomach tissue detecting stretch) would have continuously signalled to the brain across the gut-brain axis, in order to trigger the mixing of the food, and also to give you the sense of being full, to prevent over-eating. However, if you did not eat, the lack of glucose would stimulate the production of the stress hormone cortisol, impairing your focus by disrupting the activity of the PFC. This is why breakfast is considered to be the most important meal.
In conclusion, despite the human brain being an organ which weighs only a mere kilogram, it has been evolving over seven millions years to reach this point in the present day, and its sheer complexity means it is near impossible to map in its entirety. In one hundred seconds of your average school day, it can complete an inconceivable number of processes, and possesses the neuroplasticity to learn a great deal of new skills with ease and accuracy.
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