ELearning/Foundations/PhysicalBrain
Before we discuss the physical basis of learning, we would do well to review the basic structure and functioning of the brain. This is a very abbreviated view and you are encouraged to explore further if this description interests or confuses you. We move from the macro to the micro.
Energy requirements
Human brains are expensive - metabolically speaking. It takes lot of energy to run our master organ. While making up about 2% of body weight, it consumes 20% of its energy. About two-thirds of the brain's energy budget is used to help nerve cells "fire" or send signals. The remaining third is used for "housekeeping," or cell-health maintenance (Swaminathan, 2008). Energy travels to the brain via blood vessels in the form of glucose, which is transported across the blood-brain barrier and used to produce adenosine triphosphate (ATP), the main currency of chemical energy within cells. Unlike how strenuous physical activity drains the body's energy much more intensely than sitting in a chair, high demand cognitive tasks (taking the SATs for example) increase energy consumption a tiny amount compared with the brain's gluttonous baseline intake (Jabr, 2012). Mental exhaustion, then, is much more a psychological phenomenon than a physical one.
Anatomy
There are about 100 trillion (100,000,000,000,000) connections between the 86 billion neurons in the brain. The average neuron makes about 1,000 synaptic connections with other neurons. Many of these connections occur with neighboring cells, but just as many occur with distance cells throughout the brain. It is a reality, then, that the brain is a complex, dense network of computational nodes communicating flexibly and along multiple pathways at nanosecond speeds. This interaction gives rise to our thoughts, feelings, and who we are. Truly amazing.
There are many ways in which to divide the brain. Here we begin with a simple three-part model.
The hindbrain is the oldest part of the brain, from both evolutionary and growth perspectives. It includes the cerebellum, the pons and the medulla oblongata, which function collectively to support vital bodily processes. The medulla is joined to the spinal cord and controls unconscious body functions such as breathing, swallowing, blood circulation and muscle tone. Located above the medulla is the pons which serves as a bridge to connect the brainstem and the cerebellum. The pons receives information from visual areas to control eye and body movements and also plays a role in controlling patterns of sleep and arousal. Information is relayed from the pons to the cerebellum to control the coordination of muscular movements and maintain equilibrium. The cerebellum is always involved in learning that involves movement.
The midbrain sits between the forebrain and the hindbrain and forms a major part of the brainstem, which connects the spinal cord and the forebrain. It includes the tectum, cerebral penduncles, and the substantia nigra. These structures form connections between the cerebral cortex and the brainstem and spinal cord to control sensorymotor processes such as movement. Interestingly, the midbrain is the sole source of dopamine for the brain.
The forebrain is the largest part of the brain, most of which is made up of the cerebrum, or cerebral cortex. Other structures include components of the limbic system and basal nuclei (also called the basal ganglia), plus the corpus callosum connecting the hemispheres.
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| 2. External cortices of the brain | 3. Prefrontal cortices | 4. Limbic system components |
Cerebral cortex
The cortex consists of six layers, each with different neuronal shapes, sizes and density as well as different organizations of nerve fibers. Each cortical hemisphere (Figure 2) is divided into lobes and subcortices: frontal or prefrontal (conscious thought), motor, somatosensory, parietal (integration of senses), occipital (sight), temporal (sound and smell), cingulate (also called the limbic cortex and cingulate gyrus (a gyrus is a rounded fold of the cortex, as see on the exposed brain), considered part of the limbic system), and the deeply buried insular cortex.
The sensory cortices contain both primary sensory areas, that receive lightly processed information from the sense organs, and accompanying association areas that integrate and recognize patterns such as objects, music, food, and odors. Interestingly, the association areas operate on a longer timescale (seconds) than sensory areas (milliseconds), thought to be due to the transient nature of our senses and the higher-level processing accomplished by the association areas (Runyan et al., 2017). Association areas, in turn, contribute to more complex cognitive and behavioral functions in the frontal cortex. It's interesting to note that 30% of the brain's neurons are devoted to the visual system, 8% for touch, 3% for hearing, and under 1% for taste and smell (Zhang, 2018).
The frontal lobes are further divided into premotor and prefrontal (abstract thought and analysis) cortices. The prefrontal cortex is again divided into multiple subcortices as we see in Figure 3. The cerebral cortex controls perception, memory, and all higher cognitive functions, including the ability to concentrate, reason and think in abstract form, with the subdivisions taking on their specific functions. The human cortex has historically been viewed as especially large in comparison to other animals, including the great apes. This size was thought to be the cause of our superior intelligence. Research (Barton & Venditti, 2012), however, demonstrates that the source of superior human intelligence lies in the distributed neural networks in human brains, and not on the size of its cerebrum. Given that intelligence depends on one's ability to integrate information, the networked brain makes perfect sense.
Situated beneath the cortex are the limbic system and the basal nuclei. Subcortical structures of the limbic system and basal nuclei consist of multiple neuronal clusters, or knots of neurons referred to as nuclei, that together perform the multiple functions of the structure.
Limbic system
The limbic system (Figure 4) includes, in both hemispheres, the hippocampus, fornix, dentate gyrus, amygdala, thalamus, hypothalamus, pituitary gland, mammillary body and olfactory bulb. Also known as the 'emotional brain', the limbic system is important in the formation of memories and in controlling emotions, decisions, motivation and learning.
The hippocampus is deeply involved in learning and memory. Among its functions are episodic memory (sequence of events), generalization, spatial navigation and memory, behavioral inhibition, as well as transference of short-term memory to long-term memory, recognition and recall. Partially wrapped with the hippocampus, the dentate gyrus is especially sensitive to pattern differences, allowing us to differentiate objects and ideas. It is also one of two structures that produces new neurons throughout our lives. The mammillary body is also involved in memory recall, and the fornix carries signals from the hippocampus to the mammillary body and thalamus.
The thalamus is considered the primary "relay station" between the senses and the forebrain, acting as an attentional screen determining the specific signals that get passed to different regions of the cortex, limbic system, and basal nuclei. It is also involved with the regulation of voluntary body movement, consciousness and sleep, arousal, and alertness. New research indicates that the thalamus is also the master "timekeeper" for actions requiring timing and coordination (Wang et al., 2017). The mechanism it uses is to increase or decrease neuron firing rates that send signals to the dorsomedial PFC and caudate nucleus (part of the basal nuclei) to match the requirements of the task. The hypothalamus regulates visceral functions, such as temperature, reproductive functions, eating, sleeping and the display of emotion. The amygdala is responsible for the integration of emotions, emotional behavior, and motivation. It receives input from all the senses as well as visceral inputs. It is essential to emotional and social learning, and thought to determine emotional intelligence. The amygdala is also involved in the expression and regulation of aggression and sexual behavior.
Basal Nuclei
Many sources include the basal nuclei (Figure 5) as part of the limbic system, but just as many do not. Located on both sides of the thalamus, the basal nuclei consist of the caudate nucleus and putamen (together called the ventral striatum), and the globus pallidus. Embodying a "go no-go system" the basal nuclei are involved in virtually all decision making. They receive information from and signals back to the cortex to regulate skeletal movement (initiating and terminating action, including timing and control of specific muscle movements) and other higher motor functions, reinforcing adaptive and suppressing maladaptive behaviors. Recent evidence indicates the same effect on cognition (Wessel et al., 2016). This go no-go system can be seen in action when we begin entering an elevator and suddenly realize another person is in front of us trying to leave the elevator, and also when we are reading and the phone rings. The cognitive cost of this action is that we forget what we've been reading immediately before the ringing phone (i.e., the contents of working memory are essentially erased). This new finding suggests a neural basis of distraction. The striatum is involved in most types of learning, especially behavioral conditioning and procedural learning. Habits and expertise, with their largely automatic and intuitive operation, are dependent upon the basal nuclei.
While this description suggests very distinct functions, in reality the structures form circuits of neural processing that meld into a wide array of emotional, cognitive, and physical responses.
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| 6. Gray and white matter | 7. The axon bundles of white matter |
As we see in Figure 6, the entire cortex, as well as other parts of the nervous system, is made of "gray" matter and "white" matter. Gray matter consists of neural cell bodies, dendrites, unmyelinated (unprotected) axons for local communication, glial cells, and blood vessels. White matter is composed of bundles of myelinated axons (Figure 7) that connect various gray matter regions to each other and carry nerve impulses between neurons . White matter only contains the axons of the nerve cells, and not the cell bodies. They are the "transmission lines" of brain networks. Research indicates that white matter integrity abnormalities (e.g., reduced myelination, reduced axon fiber coherence) are strongly associated with major depression (Shen et al., 2017; Jung et al., 2010). Scientists hypothesize that this results in a lack of frontal cortical control over the limbic system and other structures involved in emotion processing.
Maturation generally proceeds from the back to the front of the brain, with the prefrontal cortex maturing the latest, during early adulthood.
Hemispheric differences
While research as debunked the myth of left-brain (logical), right-brain (intuitive) thinking, there are real differences behind the myth. Referred to as functional lateralization, specialization of some functions in the let and right hemispheres has been know to scientists for some time and is associated with improved cognitive, fine motor, and verbal and nonverbal communication ability (Gotts et al., 2013). The great majority of research has centered on the cerebral cortex, but lateralization within the limbic system and basal nuclei has also been found.
- Broca's (speech production) and Werneke's (language comprehension) areas are located predominantly in the temporal lobe of the left hemisphere (95% for right-handers and 70% for left-handers).
- The left hemisphere is more active in language and fine motor coordination, while the right hemisphere is more active in visuospatial and attentional processing.
- The left hemisphere shows a preference to interact exclusively with itself, while the right hemisphere interacts in a more integrated way with both hemispheres.
- The right hemisphere determines handedness.
- Language lateralization (favoring the left hemisphere) is most pronounced in right-handed males.
- The left hippocampus maintains bidirectional communication with other portions of the default mode network, indicating shared influence. The right hippocampus, on the other hand, maintains a one-direction communication, receiving signals but not sending them.
Individual differences
While the general structures of the brain are quite consistent among humans, there are individual differences that impact personality, learning, and other individual characteristics. We cite these to reinforce the idea that individual differences do indeed impact learning and that individualized teaching strategies may be necessary to accommodate them. Identified relationships:
- The brains of women are significantly more active in many more areas of the brain than men, especially in the prefrontal cortex, involved with focus and impulse control, and the limbic system, the emotional area of the brain involved with mood and anxiety. The visual and coordination centers of the brain were more active in men (Amen et al., 2017).
- There is greater variety in the size of men's brains than of women's. This could help explain why some psychiatric disorders such as ADHD and autism are more prevalent in boys (Wierenga et al., 2017).
- Reduced thickness of the anterior cingulate and dorsolateral cortices is predictive of sensation seeking and substance abuse (Holmes et al., 2016).
- Smaller volume (generally thickness and density) in the parietal cortex is associated with higher aversion to risk and increased sensitivity to physical and emotional pain (Datal, et al., 2014; Emerson, et al., 2014).
- Amygdala volume (size) correlates positively with both the number and complexity of individuals' social networks, as well as "reading" others' faces.
- Persons higher in "emotional empathy" (feeling emotion along with the other person) have higher-density insular cortices, while those higher in "cognitive empathy" (simply knowing what the other person is feeling and what they might be thinking) have higher-density in the midcingulate cortex (Eres et al., 2015).
- The right inferior gyrus (part of the ventral cortex) in "critical thinkers" is more highly activated when evaluating information than the less-critical. This region is associated with cognitive inhibition, in which the individual is able to stop or override certain mental processes (stop unwanted thoughts, weed out irrelevant information) while attending to new or contrary information (Lindeman et al., 2013).
- Different ways of experiencing the past are associated with distinct brain connectivity patterns that may be inherent to the individual and suggest a life-long 'memory trait'. Some people have richly detailed recollection of past experiences (episodic memory), while others tend to remember just the facts without details (semantic memory) (Sheldon, et al., 2016).
- With age, the hemisphere communicate bilaterally as a means of compensating for other negative aspects of aging, thus maintaining normal function. "Good roads make for efficient travel, and the brain is no different. By taking advantage of available pathways, aging brains may find an alternate route to complete the neural computations necessary for functioning" (Davis et al., 2017).
Developmental differences
Brain regions develop at roughly the same pace during normal brain maturation. An approach to measuring this coordinated development, called structural covariance, is being used to study neurodevelopmental disorders such as autism, ADHD, and conduct (antisocial) disorders. These findings support the hypothesis that these and other developmental disorders have neurodevelopmental origins.
Premature birth increases the chance of developing cognitive, motor and behavioural difficulties, problems that persist throughout school years (Allotey et al., 2017). "These results show children born prematurely have a higher risk of a number of developmental problems, including lower IQ and ADHD. It is important for the effect of a preterm birth on the neurodevelopment of children to be included in counselling for parents who expect or have had a preterm delivery. Likewise, any decision on the timing of delivery should take into consideration the long term effects of prematurity."
Gender The brains of men and women respond differently to prosocial and selfish behavior (Soutschek et al., 2017). The ventral striatum is responsible for the assessment of reward and is active whenever a decision is made. The striatum is more strongly activated in female brains during prosocial decisions (e.g., sharing found money) than selfish decisions (keeping the money for oneself). The opposite pattern is seen in male brains (higher activation during selfish decisons). Although the difference appears at the neurotransmitter level, the researchers speculate that cultural expectations account for the difference.
Autism The pace of development between cortical thickness in the inferior temporal and occipital lobes (called the fusiform gyrus) and amygdala volume are significantly out of sync in autistic persons when compared to normal subjects. The fusiform gyrus is key to social cognition and emotional awareness (Dziobeck, et al., 2010).
ADHD Attention deficit/hyperactivity disorder carries a number of neurological abnormalities. Children and teens with ADHD lag behind others of the same age in how quickly their brains develop (Jacobson et al., 2018) and form connections within, and between, key brain networks (Sripada, et al., 2014). Compared to matched (age, gender, etc.) normal controls, ADHD is marked by significant reductions in gray matter volume in the right insular cortex and right orbital prefrontal cortex, significantly decreased connectivity between the insular cortex and the right hippocampus, bilateral olfactory cortex, and caudate nucleus, and significantly increased activity between the insular cortex and the left middle temporal gyrus (Li et al., 2015). Further, Jacobson et al. (2018) found that the degree of volume deficit was correlated with the severity of symptoms.
Conduct disorders are separated into childhood-onset (CO-CD) and adolescence-onset (AO-CD). Examining the growth (in thickness) of the different cortical lobes (prefrontal, temporal, etc.), Fairchild et al. (2016) found an unusually high degree of uniform cortical lobe growth for CO-CD individuals, and an unusually low degree of uniformity of growth for AO-CD individuals. Growth of the different lobes for normal controls was somewhere in the middle.
Poverty is associated with tremendous differences in size, shape and functioning of children's brains, including less cortical volume, thickness and surface area, smaller hippocampus, and reduced plasticity (Noble, 2017). In general, children from disadvantaged homes tend to perform more poorly on tasks that test their language and memory skills, and the ability to exert self-control and avoid distraction. A study by Evans (2016) tracked 341 individuals over a 15-year period, beginning at age nine. Children in the study living in poverty had more psychological distress as adults, including more antisocial conduct like aggression and bullying and more helplessness behavior, than kids from middle-income backgrounds. Poor kids also had more chronic physiological stress and more deficits in short-term spatial memory.
Imagine how you might approach the same learning task for three different learners: one is risk-aversive, another highly sensitive to physical and psychic pain, and a third has difficulty focusing for more than a few minutes.
Blood supply
Although only 2% of the body's weight, the brain receives 20% of its blood supply. About two-thirds of this energy supports neuron firing with the remaining third committed to basic metabolism and maintenance functions (Swaminathan, 2008). The entire blood supply to the brain depends on two sets of branches from the dorsal aorta, the internal carotid arteries and the vertebral arteries. "Nerves and blood vessels lead intimately entwined lives. They grow up together, following similar cues as they spread throughout the body. Blood vessels supply nerves with oxygen and nutrients, while nerves control blood vessel dilation and heart rate. Studies have shown that when neurons work hard, blood flow increases to keep them nourished." (Dutchen, 2014). This intimate relationship is the basis of fMRI, which measures blood flow and not actual neuron activity.
The physiological demands served by the blood supply are particularly significant because neurons are more sensitive to oxygen deprivation than other kinds of cells with lower rates of metabolism. In addition, the brain is at risk from circulating toxins, and is specifically protected in this respect by the blood-brain barrier. As a result of the high metabolic rate of neurons, brain tissue deprived of oxygen and glucose is likely to sustain transient or permanent damage. Brief loss of blood supply (referred to as ischemia) can cause cellular changes, which, if not quickly reversed, can lead to cell death. Sustained loss of blood supply leads much more directly to death and degeneration of the deprived cells.
Very recent research shows that sensory stimulation during infancy results in richer, more complex neurovascular networks. The opposite is also true, with sensory deprivation resulting in shorter vessels and less branching. "The finding that neural activity is necessary and sufficient to trigger alterations of vascular networks reveals an important feature of neurovascular interactions" (Lacoste, 2014). This finding not only has implications for learning capacity, but also reveals the possibility for using sensory stimulation to generate new blood vessels in damaged areas of the brain. The research is described as "preliminary".
Neural networks and clusters
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| 8. The brain's neural pathways | 9. Concentrations of neural activity and their connecting pathways | 10. A functional cluster of neurons (dentate gyrus) within a mouse hippocampus |
The brain carries out its functions (including thinking, talking, listening) via neural networks and sub-networks, which connect the different structures within the brain so they can work seamlessly together to produce perception, thought, and action. These multi-part circuits respond together and regulate one another. They also vary in their connectivity according to the task at hand, increasing traffic here and silencing connections there. Physically, this means network member neurons are more densely connected to each other than to neurons of other networks. Consider how sidewalks provide preferred pathways between campus buildings. If the sidewalks don't provide the most convenient access, pedestrians create new pathways. And so it is with the brain. Mapping the networks and member neurons is referred to as the connectome, or the nervous system's wiring diagram.
Our previous understanding of these circuits likened the brain to a bowl of spaghetti with individual neurons reaching out to others in a more or less random fashion. However, imaging reveals a more organized pattern somewhat akin to the wiring in an office building where bundles of wires form the backbone of the system and smaller bundles radiate outward to specialized functions (Figure 8 above). As we move outward from the backbone, networks become smaller, as we see in Figure 9 above. Networks within larger networks operating at different oscillation bands (see brainwaves below).
Locally, neurons cluster to form anatomical structures, or nuclei (within the brain), with specialized functions. Figure 10 above is that of dentate gyrus neurons within a mouse hippocampus, which contributes to the formation of new episodic memories (Liu et al., 2012). Functional neural clusters don't only exist within the brain's structure, but also within sense organs and their neural pathways to the brain (referred to as ganglia). The eye and optic nerve separate incoming images into elements perceived by the brain. The olfactory bulb converts smells into signals for the brain to process. The ears are especially interesting in that the right ear is more geared toward speech and the left ear is attuned more to music. Children who have right-ear hearing impairment have more trouble in school than those with left-ear loss (vos Savant, 2014).
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| A 3-D view of the connectome |
Individual neurons are constituents of multiple networks, and function in a manner consistent with one circuit for a particular function, and in another manner with a different circuit serving a different function (Figure 11 below). The neuron fires rhythmically in tune with one pathway for one function, and at another rate for a different function. Although a single neuron or group of neurons may represent a single phenomenon, such as angle of visual orientation, it participates in different memories associated with the particular visual angle. Similarly, a particular melody may trigger memories of multiple songs. This phenomenon helps explain the benefit of associating new material with that already learned, and the mixing of memories we can all attest to.
Neuronal participation in multiple circuits is demonstrated in research by Huth et. al (2012) in which they mapped neural activity as subjects viewed over 1,000 objects and actions. Two examples are seen below (Figures 12 and 13), with red representing high activity and blue low activity. Of special interest is the fact that similar concepts cluster in the same brain areas while very different concepts cluster further apart. This structure allows the brain to forego maintaining separate cells for each of the vast array of semantic concepts, a physical impossibility.
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| 11. Single neurons take part in multiple circuits | 12. Neural activity while viewing a photo of a planet | 13. Neural activity while viewing the word argue |
A new video from Nature describes the brain's semantic map, or "brain dictionary". Also, see Huth et al., 2016.
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| The brain dictionary |
Three learning networks
We previously discussed how the brain can be divided in numerous ways for different purposes. Here we take a look from a learning perspective (Figure 14). Three types of brain networks are especially important to learning: recognition, strategic and affective networks, all part of the forebrain (plus the cerebellum). These networks are named for their function and not their anatomical configuration, using brain imaging technologies while subjects performed different types of tasks and thinking.
Recognition networks are specialized to sense and assign meaning to signals from within the body and the external environment. These networks assign meaning based on the patterns we perceive, allowing us to identify and understand information, ideas and concepts. Based in the back of the brain, recognition networks enable us to identify and interpret patterns of sound, light, taste, smell and touch. They enable us to recognize voices, faces and text, as well as more complex patterns like dance, justice and the structure of the atom.
Based in the frontal cortex and the cerebellum, strategic networks specialize in generating and overseeing mental and motor strategies. They enable us to plan, execute and monitor actions, tactics and strategies. These networks orchestrate virtually everything we do, from sweeping the floor, to composing an essay to executing a flight to the moon. They operate at the conscious level, as when we’re following travel directions, and subconsciously as our minds and bodies automatically carry out many aspects of walking or driving.
Affective networks, based in the subcortical and cortical portions of the limbic system, are specialized at evaluating recognition and strategic patterns and assigning them personal significance. Unpleasant stimuli are processed first via the more archaic subcortical region (recognizing danger) while pleasant stimuli are first processes through the cortical region (Paradiso el al., 1999). What we see is determined in large part by our internal state – a melting pot of emotions, needs, desires and memories. Whether we like it or not, everything we perceive and do is filtered through an affective screen made of, among others, our state of energy or fatigue, familiarity with and interest in aspects of the environment, our mood and personality.
At any instance, the many facets of the environment are competing for our attention. These demands require not only that we recognize them and formulate strategies, but that we also evaluate their significance and importance to ourselves. Think about it. Only affect answers the “why” of our lives.
Other important networks
Three important core networks are receiving considerable attention in explaining human behavior (McGrew, 2011):
The salience network is a controller or network switcher. Think of the salience network as the air traffic controller of the brain. Its job is to scan all information bombarding us from the outside world and from within our own brains. This controller decides which information is most urgent, task relevant, and which should receive priority, sending signals to areas of the brain for processing. This controlling network must suppress either the default or executive networks depending on the task at hand. It must suppress one, and activate the other. The salience network is centered in the anterior insular cortex and dorsal anterior cingulate, with connections to the thalamus, amygdala, orbitofrontal PFC, olfactory cortex, and the superior temporal cortex.
The default mode network (DMN) manages what your brain does when not engaged in specific tasks. It is the busy or active part of your brain when you are mentally passive. This network comprises an integrated system for autobiographical, self-monitoring and social cognitive functions. As such, it is especially important to our self-concept. The default mode network is also responsible for the spontaneous mind wandering and internal self-talk we engage in when not working on a specific task or, when carrying out a task that is so automatized (e.g., driving a car) that our mind starts to wander and generate spontaneous thoughts. The DMN is centered on the ventromedial PFC with connections with the orbitofrontal PFC, hypothalamus, amygdala, and a portion of the midbrain.
The central-executive network (CEN) produces higher-order cognition and attentional control. In other words, when you must engage your conscious brain to work on a problem, place information in your working memory as you think, focus your attention on a task or problem, etc., you are “thinking” and focusing your attention. The CEN is concentrated in the dorsolateral PFC and posterior parietal cortex.
There appears to be a direct, inverse relationship between the default mode network and the central executive network in that as one activates, the other largely deactivates (Chapman et al., 2017). The function of this relationship remains unsettled, with some experiments demonstrating high activity in the DMN and low activity in the CEN during creative tasks while others indicate the opposite.
The mirror neuron system is involved in understanding others' actions and their intentions behind them, and it underlies observational learning. There are two main networks of mirror neurons: one residing in the parietal lobe and the premotor cortex plus the posterior ventrolateral PFC (parietofrontal mirror system). This network is active when we carry out goal-directed behaviors and also when we observe the goal-directed behavior of others. In a sense, our brains are internally mimicing others' behavior in rehearsal for copying their actions. The other network, the limbic mirror system residing in the insula and posterior medial PFC, is devoted to the recognition of affective (emotional) behavior. This network is heavily involved in social reasoning and behavior.
Individual differences
The Human Connectome Project, launched in 2010 by several nations, is beginning to reveal new and interesting facts about the brain's wiring patterns that impact or reflect the human condition. Smith et al., (2015) compared regional brain connectivity (active communication) of 461 individuals in a resting state with multiple demographic and psychometric measures. Their major finding was that those with increased brain connectivity had higher life success factors like education and income levels, life satisfaction, and overall physical fitness. Hilger et al. (2017) found that higher interconnectivity between brain regions, especially when the anterior insula and the anterior cingulate cortex are more integrated with the larger network, to be associated with higher intelligence. On the other hand, they also identified brain regions (e.g., the junction area between temporal and parietal cortex) that are more strongly 'de-coupled' from the rest of the network in more intelligent people. This may result in better protection against distracting and irrelevant inputs.
Connectivity patterns between brain regions have been implicated in a variety of psychological and cognitive disorders. "In psychiatric disorders, functional connectivity, which is measured by temporal correlations between some brain regions, is too much increased or decreased compared to healthy control. It has been suggested that these abnormal connections cause the decrement of cognitive function" (Yamashita et al, 2017). The group has developed a technique called "connectivity neurofeedback training" to restore normal functioning. See ELearning/Brain enhancement for more on the technique.
Testing for creativity and connectivity, Durante and Dunson (2016) found high creativity to be significantly correlated with connectivity between the two hemispheres, especially in the frontal cortex. Beaty et al. (2017) found that highly creative people show three networks working in sync during creative tasks: the default mode, salience, and executive networks. Unlike normal people where these networks generally work in opposition to each other, the networks are activated simultaneously in highly creative people.
Neurons and neuroglia
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| 15. A prototypical neuron (nerve, nerve cell) | 16. Astrocyte, one type of neuroglia (glia, glial cell) on a background of neural axons and dendrites | 17. Neuron axon branches show electrical activity (voltage) |
At its most fundamental level, the brain consists of specialized cells communicating with each other and maintaining brain health. Globally, there is an approximate 1 to 1 ratio (1:1) of neurons and glial cells in the brain, with the ratio varying significantly between regions (von Bartheld et al., 2016; Jabr, 2012). On one extreme, cortical white matter contains 15 glia for every neuron (15:1). The limbic system and basal nuclei contain about 11 glia for every one neuron (11:1). Cortical grey matter contains about 1.5 glia to each neuron (1.5:1). At the other extreme, the cerebellum contains about one glia for every four neurons (1:4). Both neurons and glial cells come in a variety of types. "There are hundreds, if not thousands, of types of brain cells that have different functions and behaviors" (Luo et al., 2017).
Neurons
The basic neuronal structure is seen in Figure 15. Remember that this is an illustration and neurons come in hundreds of configurations, all with the same basic parts.
Dendrites are the signal receiving protrusions branching from the cell body, and are the site of memory formation (Seibt et al., 2017). These protrusions grow and deconstruct as learning occurs, governed by specific proteins (Gao et al., 2017). Dendrite receptors, located on tiny spines protruding from the dendrite branches, come in different "families" even when they receive the same neurotransmitter. Each receptor type performs different functions for the neuron. Glutamate receptors, for example, include NMDA receptors, AMPA receptors, and kainate receptors, which perform different cellular functions like opening ion channels into the cell body, helping to build action potential. AMPA receptors are important to memory in that as these receptors fall into disuse, they fade away and with them specific memory traces (Migues et al., 2016). Further, some spines are responsive to different nuanced signals from the senses, such as particular angles in our field of vision (Wilson et al., 2016). Some respond only to vertical edges, others respond to only horizontal edges, while others respond to other angles. This constitutes a form of signal (information) processing that contributes to the perception of our world.
The central cell body is the largest part of a neuron and contains the neuron's nucleus, containing DNA, and other cell structures such as mitochondria and organelles. The nucleus encodes messenger RNA, which is transported to specific cites and builds proteins necessary for cell functions, including learning and memory. The cell body also integrates the multiple signals coming from other neurons, generates electrical impulses and propagates them down the axon, and maintains cellular health.
The axon carries nerve impulses away from the cell body. A neuron typically has one axon with multiple branches (see Figure 17) at the end that connect it with other neurons or with muscle or gland cells. Some axons may be quite long, reaching, for example, from the spinal cord down to a toe. Axons that extend to other brain and body regions are enclosed in a myelin sheath, which increases the speed of impulse transmission; some large axons may transmit impulses at speeds up to 300 feet (90 meters) per second. These axons make up the brain's "white matter". Short axons within grey matter that communicate locally are not generally myleanated.
Neuronal subtypes
Scientists have classified neurons into four main groups based on differences in shape (Jabr, 2012). Multipolar neurons are the most common neuron in the vertebrate nervous system and their structure most closely matches that of the model neuron: a cell body from which emerges a single long axon as well as a crown of many shorter branching dendrites. Unipolar neurons feature a single primary projection that functions as both axon and dendrites. Bipolar neurons usually inhabit sensory organs like the eye and nose. Their dendrites ferry signals from those organs to the cell body and their axons send signals from the cell body to the brain and spinal cord. Pseudo-unipolar neurons, a variant of bipolar neurons that sense pressure, touch and pain, have no true dendrites. Instead, a single axon emerges from the cell body and heads in two opposite directions, one end heading for the skin, joints and muscle and the other end travelling to the spinal cord.
Researchers also categorize neurons by function. Sensory neurons collect information from sensory organs—from the eyes, nose, tongue and skin, for example. Motor neurons carry signals from the brain and spinal cord to muscles. Interneurons (also called relay neurons) connect sensory and motor neurons of the peripheral nervous system with the central nervous system: the long axons of projection interneuons link distant brain regions to create and regulate networks; the shorter axons of local interneurons form smaller circuits between neighboring cells (Muñoz et al., 2017). A single type, parvalbumin-expressing neurons (PV cells), in the medial prefrontal cortex are essential to goal-driven attention (Kim et al., 2016) and can be activated using optogenetics. There are place cells within the hippocampus that record and track our physical location. There are mirror neurons that fire when we observe others' actions and also when we perform the same action.
These broad categories do not capture the true diversity of the nervous system. Not even close. In the cerebellum alone, there are nine subtypes of neurons (Jabr, 2012), and 21 in the frontal cortex (Luo et al., 2017). Neurolex.org includes a list of 310 neural subcategories.
In the past, it was thought that neurons were solely responsible for "brain work" while glial cells supported them by holding them in place, supplying them with nutrients and oxygen, insulating one neuron from another, and destroying and removing the carcasses of dead neurons (clean up). We now know that this picture is incomplete, and therefore inaccurate.
Neuroglia
While the support function of neuroglia have proven true, recent discoveries tell us that glia are also intimately involved in brain work. For example, glial cells are responsible for modulating the shape of nerve endings (Singhvi et al., 2016; Sakry, et al., 2014), affecting cognitive processing, sensory perception, and learning. "The glia release a specific protein fragment that influences neuronal cross-talk, most likely by binding to the synaptic contacts that neurons use for communication. Disruption of this information flow from the glia results in changes in the neural network, for example during learning processes." Tso et. al. (2017) have implicated astrocytes, a specialized glial cell, as the primary drivers of our internal clocks (circadian rhythms). Glia cells also play a pivotal role in the initial organization of the infant brain, coaxing neurons into specific pathways so that proper brain assembly can ensue (Rapti et al., 2017).
Glia subtypes
Glial cells maintain the brain's environment, regulate synapses and neurotransmitters, respond to injuries, and in certain cases can even become neurons (Yuhas, 2012). Five types of glia researchers have discovered so far:
Astrocytes (Figure 18, green) are the most common type of glial cell in the brain, and secrete GPC4 proteins into synapses affecting both axon terminals and dendrite receptors, enabling neuron-to-neuron communication (Farhy-Tselnicker, et al., 2017). They also take up neurotransmitters, cleaning up after neuronal activity. They wrap themselves around capillaries, forming the blood-brain barrier, and provide nutrition to nerve cells. During maturation of the chaotic young brain, astrocytes deactivate many synapses to create a more orderly network. The network remains relatively stable throughout adulthood, but astrocytes begin anew with their trimming function in elderly brains, especially in the hypothalamus and cerebellum, effectively reducing the number of synapses in the brain. This action is thought to cause or contribute to decreased metabolism and problems of physical coordination (Boisvert et al., 2018). Finally, astrocytes communicate with each other via calcium waves, similar to electrical/chemical brain waves in neurons. One function of this communication is the regulation of blood flow to the brain, and additional functions are postulated (Bazargani & Atwell, 2016).
Oligodendrocytes (blue) wrap the tips of their tentacles around axons in a fatty white coating called myelin (myelin sheath). Schwann cells in the peripheral nervous system function as both astrocytes and oligodendrocytes. Microglia (deep red) are the brain's rapid response team. Since the immune system's molecular machines can't cross the blood-brain barrier, microglia defend the brain from invaders. Finally, Ependymal cells (pink) produce cerebrospinal fluid and form the boundary of the brain's ventricles (cavities containing the fluid) and the central canal of the spinal cord.
Neuronal communication
Communication between nerve cells occurs at the synapse, a tiny gap between the end of one neuron and the top of another. Neurons communicate in one of two ways: The vast majority of neurons communicate chemically using neurotransmitters. A much smaller number communicate via electrical impulses. Neurons propagate their messages down the axon body and through the terminal branches via electrical pulses toward synapses with other cells.
In chemical synapses (Figure 19 below), this electrical pulse triggers the release of neurotransmitters from vesicles, essentially small sacs of neurotransmitter. The neurotransmitter receiving dendrites are populated with tiny mushroom-shaped buttons called dendritic spines. These spines play a major role in the strength and duration of neuronal communication. For example, in some instances, there are a large number of spines congregated together to receive incoming neurotransmitter while in other instances the number of spines is quite low (Kusters & Storm, 2014). Variation in the shape of spines also plays an important role, with some shapes creating more permanent memories and others temporary memories. We can also note that spine shapes can and do change in response to increased or decreased memory permanence. Chemical synapses communicate in one direction only.
In electrical synapses (Figure 20), neurons communicate via gap junctions. The gap junction contains a number of ion channels, allowing the flow of electricity between neurons in both directions. In this case, the gap between neurons is much smaller than chemical synapses. The most thoroughly studied electrical synapses occur between excitatory projection neurons of the inferior olivary nucleus (part of the medulla oblongata) and between inhibitory interneurons of the neocortex, hippocampus, and thalamus. These synapses require a special gap junction protein for robust coupling. Electrical synapses allow for quicker signal transmission, bidirectional communication, and they are thought to synchronize neural activity; a regulatory agent for other neurons.
Evidence (Qui et al., 2015; Fröhlich & McCormick, 2010) suggests there is a third form of transmission between neurons, with low amplitude electrical fields, starting with a single cell or group of cells, propagating wave transmissions through populations of cells, thus communicating without synaptic transmission. The electrical fields are thought to guide neocortical network activity within the brain, and wound healing in the body.
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| 19. Chemical Synapse (credit Univ of Tokyo) | 20. Electrical Synapse (credit Univ of Tokyo) | 21. Hundreds of signals acting on a single neuron |
Excitation and inhibition
Functionally, there is an important distinction between neurons important to learning: excitation and inhibition. Excititory neurons carry the information flow, promoting neuron firing in other cells both locally and long-range with other regions. Inhibitory nerves communicate locally to regulate the activity of excititory nerves. This action is considered to be the basis for single neurons participating in multiple pathways, with inhibitory nerves closing off some pathways and opening up others. Selective attention and the ability to screen out distractions, separating the "signal from the noise" is one manifestation of inhibitory nerves in action (Yang et al., 2016). During periods of focused learning, inhibitory neurons fire about half as often as normal (Kuhlman, 2013). About 80% of neurons are excititory and 20% inhibitory.
Signals from other neurons are received via the dendrites and travel down the axon to the terminal buttons, communicating with the dendrites of additional neurons. Key to this activity is action potential. Signals from other neurons do not necessarily result in the cell "firing". Figure 21 above shows the multitude of signals impinging on a single neuron, with excitatory signals in green and inhibitory signals in red (click Figure 21 to view full size). Each incoming signal adds to or decreases the action potential of the cell. Signal strength between neurons varies over two orders of magnitude, with a few strong connections among large numbers of weak ones (Cossell et al., 2015). Most of this difference comes from the size of the synapse, with larger ones including more vesicles of neurotransmitter released and uptook. Once potential passes a critical level, the cell fires and sends its message along to other neurons. Why do neurons share such a large number of weak connections? Cossell speculates that they may have significance for learning. "If neurons need to change their behavior, weak connections are already in place to be strengthened, perhaps ensuring rapid plasticity (thus learning) in the brain." As a result, the brain can quickly adapt to changes in the environment."
Uncoordinated signals of individual brain cells can be noisy, imprecise, and even contradictory - which means our brains cannot rely solely on the activity of single neurons to make accurate decisions about the world. Instead, during active cognition, the brain combines the activity of thousands to millions of neurons to ensure a more accurate message, which makes effective communication among large populations of neurons a central feature of the brain (Smith, G. et al., 2015).
Brainwaves
Activity in the brain is not constantly high or low, but rather organized in waves that come and go. Brain waves (technically called rhythmic neuronal oscillations) constitute patterns of electrical pulses from masses of neurons communicating with each other. In other words, groups of neurons fire together in wave-like patterns (fire-rest-fire-rest) that propagate through neural networks to carry information from one part of the brain to others. Your thoughts are literally carried on the waves of neuronal oscillations. Research tells us that brainwaves play an integral role in learning – they are the carriers of learning, and any other process involving communication between brain structures (Brincat & Mille, 2015). More on that in a minute. First, let's look at the phenomenon itself.
As we see in Figure 22, brain waves vary along two dimensions – frequency (Hertz: distance between peaks) and strength (amplitude: height of peaks and valleys). The higher the frequency, the lower the amplitude. This continuum is divided into “bands” and labeled as we see below (cps is short for cycles per second). Speedy gamma waves form a tight frenetic pattern while lazy delta waves are big and loopy. It happens that these patterns are closely associated with our state of mind, and also how we learn. Note that, while one wave pattern may dominate at any one time (e.g., "theta state"), depending on the current state of mind, all five patterns are generally present in the brain at all times.
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| 22. Brainwaves are patterns of neuron groups firing at different strengths and frequencies (Hertz and amplitude) |
Wave patterns
Pseudo science has endowed particular wave patterns with magic-like properties and make recommendations based on them: “The Alpha-Theta border, from 7 to 8Hz, is the optimal range for visualization, mind programming and using the creative power of your mind.” While we know that electrical and magnetic stimulation, chemicals, biofeedback, the very recent neurofeedback, and meditation do change brainwave patterns, we have not attained that level of precision. Here, we use information from Cetin (2010) and Hermann (1997).
Gamma wave patterns are associated with the synchronous cooperation of various cell networks of the brain, associated with high-level perception and sense making – attuned to the present and also integrating the present with relevant memories. Recent research suggests it is associated with bursts of insight and high-level information processing, and the efficient formation of memories in the hippocampus. Debaters, professional athletes, theorists, programmers and others involved in intense cognitive and psychomotor effort would be in gamma. Interestingly, food-seeking behavior in humans and animals is also mediated by gamma wave patterns (Carus-Cadavieco et al., 2017). It's also known that activity in this frequency range is markedly reduced in Alzheimer patients.
Beta wave-like activity is characteristic of an engaged mind. A person in active conversation would be in beta. A person making a speech, or a teacher, or a talk show host would all be in beta when they are engaged in their work.
Alpha waves represent non-arousal, relaxation, and lack of focus. Consider sitting on a bench and looking out at the world with nothing particular in mind. The alpha state, then, is characteristic of mind-wandering, day dreaming, and lapses of attention. One can easily imagine how anyone listening to a lecture or driving on long stretches of isolated highways would oscillate between beta and alpha states.
Theta waves are important to the formation of memories, specifically emotional and contextual memories. Theta oscillations in the medial temporal lobe occur more often when someone is navigating an unfamiliar environment, and that the more quickly a person moves, the more theta oscillations take place—presumably to process incoming information faster. In an unexpected finding, theta oscillations were most prominent in a blind person who relied on a cane to move (Aghajan et al., 2017).
When dominant, theta waves constitute the border between sleep and wakefulness; a state of deep relaxation and a disengaged intellect. Meditation, biofeedback, hypnosis, and yoga all give rise to a theta state. Recent research suggests that the brain is reviewing and consolidating these memories during REM (rapid eye movement), or dream sleep when theta waves dominate among the hippocampus, amygdala, and the neocortex (Boyce et al., 2016).
Delta waves indicate deep dreamless sleep. Falling asleep, our brain waves calm, descending into slower and slower patterns, falling to less than one cycle per second. Humans dream in approximate 90 minute cycles, which grow longer through the night. Dreaming brings an increase in theta waves which again fall off into the delta band. Sleep spindles, the primary mechanism for transferring short-term memories, and perhaps other types of memories, emanating from the hippocampus to the prefrontal cortex, occur during dreamless sleep. For example, we know that deep sleep is critical to visual learning (Durkin, et. al., 2017).
The emerging picture, then, is of different types of memories being transferred to long-term memory during REM (theta band) and deep dreamless (delta band) sleep. In both cases, the hippocampus directs the cortex to replay and interpret important short-term memories and thus strengthen their presence in long-term memory, referred to as memory traces.
As mentioned above, neural networks operate at differing oscillations (bands) as we see below (Chennu et al., 2014). Subjects wore EEG caps while in relaxed eyes-open wakefulness, thus we see activity in the three lower bands only. Colors represent different networks. Thus, we see a multitude of networks functioning simultaneously, carrying on the many forms of brain work.
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| 23. Alpha wave networks | 24. Theta wave networks | 25. Delta wave networks |
Brainwaves and brain work
Although the above descriptions of brainwaves are generally true, they fail to describe the utility of brainwaves in the more dynamic flow of daily life. Michalareas et al. (2016) have uncovered how bottom-up and top-down communication occurs between lower (i.e., sensory) and higher (i.e., cognitive) centers of the brain. Bottom-up communication occurs when lower brain areas, like when information enters through the eyes and flows from lower to higher visual areas, enlist higher areas to adopt their wave patterns in the gamma range and consciously perceive the visual field. Higher areas use past experience to organize and interpret the information, assign priority and direct the attention of lower areas, using alpha and beta bands, between 10 and 20 Hertz waves. We see, then, how the brain uses brain waves - rhythmic neuronal oscillations - to accomplish its work.
Sense-making. Measuring brainwaves while subjects were involved in a variety of tasks, Colgin (2010) found the CA3 portion of the hippocampus (Figure 26), with its ability to recall past experience, and the medial temporal lobe, which deals with information about the present, communicating in the gamma band with a third region called CA1, also part of the hippocampus. Even more revealing, the temporal lobe communicated in the higher frequency portion of the gamma band while CA3 communicated in the lower frequency portion of the gamma band. CA1 rapidly and sporadically switched back and forth between the differing frequencies, apparently receiving signals from the two areas in turn and integrating their input.
Memory. Memory involves many structures within the brain, and their interconnectivity is key to both memory encoding and retrieval. Solomon et al. (2017) found that both encoding and retrieval are mediated with slow theta wave (3-8 Hz) network connectivity, thus synchronizing activity, essentially constituting parallel processing. On the other hand, fast gamma (30-100 Hz) network activity desynschronizes brain structures, creating asynchronous activity.
Learning. Explicit and implicit learning are accompanied by different patterns of brainwave activity (Loonis et al., 2017; Chafee & Crowe, 2017). During explicit learning tasks, correct choices are followed by an extended increase in alpha-beta waves (oscillating at 10-30 hertz) whereas incorrect choices lead to a brief increase in delta-theta waves (3-7 hertz). Incorrect choices also lead to neural spikes, indicative of high amplitude (high strength) neuronal firing. Further, alpha-beta waves decrease as learning progresses. Loonis speculates that alpha-beta waves may be indicative of the learner building a mental model of the task, which fades as the model is completed. In contrast, implicit learning increases delta-theta wave patterns only with correct responses, which fade as learning is consolidated. This pattern may reflect neural “rewiring” that encodes the implicit skill during learning. During explicit learning, wave activity is more pronounced in the hippocampus, prefrontal cortex, and medial temporal lobes. Pronounced activity is seen in the amygdala, striatum, motor cortex, and cerebellum during implicit learning.
Decision-making. Daniels et al. (2017) have uncovered the decision making process at the neuronal level (collective computation), recording neuron firing within the posterior prefrontal cortex of a macaque monkey. In very similar fashion to the recording below, the monkey is confronted with a decision. During initial processing, neurons fire in seemingly random fashion and, as the monkey approaches a decision, the neurons commence to firing at the same rate; from a multitude of firing patterns toward a synchronized gamma wave pattern. There is a slow(er) aggregation phase followed by a fast propagation phase. Daniels interprets this two-stage process as the central executive polling many sensory neurons for maximum input. "At first the 'neural voice' is heterogeneous and collective, but as the decision network gets close to the decision point, the 'neural voice' becomes homogeneous..." Further, they found that a lack of homogeneity during the propagation phase resulted in timid and poorly executed action following the decision point.
Pisauro et al. (2017) have identified a decision network in humans for perceptual-, value-, and reward-based decisions. The posterior medial frontal cortex maintains coupling with the ventromedial PFC and striatum (brain areas known to encode the subjective value of the decision alternatives) during the same process of evidence accumulation found by Daniels et al. above.
The following video (Figure 27) records the firing within a population of neurons of a mouse hippocampus as it searches for and finds food. The cells were modified so they would fluoresce when firing. Jennings et al. (2015) records neurons as they fire while the mouse searches for food (random firing; aggregation phase) until it comes upon and decides to eat the food (strong uniform pattern; propagation phase).
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| 27. Active hippocampal neurons during a mouse's search for food. |
Also inherent in decision making is the action that follows. Especially in the realm of visual-motor activity, such as walking down the street or playing basketball, the motor cortex prepares itself for multiple possibilities during the aggregation phase before a decision is made. In this way, the brain is prepared to execute motor commands whatever the decision. "The brain is continuously translating visual targets into possible actions that can be performed on those targets" (Gallivan et al., 2017).
Neurotransmitters
Neurotransmitters are the brain chemicals transferred across the synapse between neurons as they communicate. They can be broadly distributed throughout the central and peripheral nervous systems, constrained to specialized pathways, or anywhere in between. Most neurons make two or more neurotransmitters, which are released at different frequencies. More than fifty have been identified. We mention only the most common.
Excitatory neurotransmitters
Acetycholine was the first neurotransmitter to be identified, and is the most abundant in the nervous system. Its primary function is to excite the body's muscle structures.
Catecholamines dopamine, epinephrine, and norepinepherine are involved in reward-pleasure and learning. Dopamine is associated with alertness and energy, attention, and is the principle neurotransmitter involved in addiction due to its dominant role in the brain's reward and pleasure centers. Dopamine insufficiency is also associated with ADHD and schizophrenia.
Glutamate is considered to be the major mediator of excitatory signals in the central nervous system and is involved in most aspects of normal brain function including cognition, memory and learning. It not only mediates information processing, but also regulates brain development, cellular survival, differentiation and elimination as well as formation and elimination of synapses. It is located almost exclusively within cell bodies (99.99%) and remains inactive until expelled. Glial cells as well as neurons possess glutamate receptors, which uptake the substance rather than release it back into the synapse for reuptake. Glutamate is both essential and highly toxic at the same time, and very low levels must be maintained within the extracellular fluid.
Serotonin is broadly distributed in the brain and body and is involved in emotions, impulse control, sleep, dreaming, hunger, and arousal. Depletion of serotonin is known to be associated with depression, obsessions, and compulsions. Many biological clues suggest that serotonin, as well as other small molecules released from blood platelets when they are activated during clotting, play an integral role in cardiovascular disease. In addition, studies have shown that people who have suffered through major depression are more than four times more likely to have a heart attack.
Inhibitory neurotransmitters
GABA, gamma-aminobutyric acid, is the major inhibitory neurotransmitter in the central nervous center. It is involved in regulating anxiety and motor learning and may be related to eating and sleep disorders. GABA plays an outsized role in the infant brain, establishing the proper balance between excitatory and inhibitory synapses, laying the foundation for circuit formation (Oh et al., 2016). Reduced presence of GABA in adults manifests as poor attention and memory. "Higher brain functions depend on well-balanced neural activity within the underlying brain regions (McGarrity et al,, 2016)."
Endorphins, Enkephalins and Substance P. Endorphins and enkephalins are the body's natural opiates inducing feelings of wellbeing and reducing pain perception. They also depress physical functions like breathing and heartbeat. Substance P is the primary mediator of pain signals.
Purines adenine, guanine, hypoxanthine, and xanthine exist in both the central and peripheral nervous systems, and are implicated in learning and memory, locomotor and feeding behaviors, and sleep. Adenine and guanine also play crucial roles in DNA and RNA functions. A derivative of adenine, adenosine, is known to reduce the production of glutimate in the auditory thalamus (part of the thalamus), leading to reduced auditory learning ability in adults.
Endocannabinoids promote homeostasis throughout the body and are also involved in learning and memory, feeding, fear, and anxiety. They are believed to be synthesized by the body 'on-demand' rather than made and stored for later use. A recently uncovered function of endocannabinoids (CB2) is to increase and decrease the excitation (firing) threshold of neurons in the hippocampus, thus serving a regulatory function for hippocampus signaling to other parts of the brain (Stempel et al., 2016).
Genetics
On the frontier of brain research, the new field of genomics has revealed that the genes within our neurons constitute a second level of organization within the brain. Genomics examines the entire set of genetic information and activity contained within our cells. It reveals that the brain's genes are considerably more involved in regulating memory and behavior than ever imagined. Almost every cell in the body contains a complete set of DNA, each with about 25,000 genes. Genes are specific sequences of bases (DNA: A, T, G, C) that encode instructions for making proteins. We see in Figure 23 that genes are made of base pairs that together contain instructions for making proteins. These instructions vary in size from a couple hundred pairs to over two million.
A full 84% are active in the brain at any moment (Lein & Hawrylycz, 2014), with distinctive collections of genes at work in each major brain structure (and the rest of the body). The cortex, with its hemispheres and sub-cortices, is remarkably homogeneous in its genetic makeup. And, although there is individual variety, patterns of gene activity are quite consistent among humans.
Most genes do their work (are expressed) by being copied by short strands of transcription RNA, which in turn build specific proteins using amino acids. Genes, then, are essentially recipes for building proteins. Proteins, each consisting of between 1,000 and 10,000 molecules, have a large range of functions within the cell, including acting as antibodies to protect the cell from foreign invaders, enzymes that direct chemical reactions, messengers to transmit signals that coordinate biological processes, structural components of cell bodies, and the transport and storage of small molecules within the cell. With 42 million protein molecules within each cell, they are truly the workhorses of cell function.
An example of how genes impact learning and memory is the CREB gene, which encodes a protein that regulates the expression of other genes necessary for memory. CREB not only orchestrates memory formation, but also directs memory formation to brain cells containing the CREB protein (Silva, 2017). High concentrations of CREB are present in the young, which decreases with age. The CREB protein is key to linkages between memories; see Physical basis of memory for additional information.
Genes carry inherited behavior patterns, regulate the growth and number of dendrites (Molumby et al., 2016), determine which cells neurons communicate with and how (Paul et al., 2017), and influence learning abilities and disabilities. For example, genes synthesize new proteins to form memories (Rizzo et al., 2016), within dendrites of the neuron. Even though every cell in our body contains the same DNA, there is a surprising level of individuation within nerve cells that make each unique in structure and function (Erwin et al., 2016). This helps explain why identical twins can be so different.
Within the brain, repeated neural activity changes behavioral genes over time and these genes can eventually drive neural activity. Influence becomes a two-way street. Social anxiety, for example, may develop because of a series of embarrassing incidents. The initial response is driven by neural activity, but over time, long-term behavioral patterns emerge as a result of changes in gene expression, thus creating new and modified proteins that drive neural activity. We see, then, that personality is not a matter of nature or nurture, it is a matter of nature AND nurture. Thus, genetic changes within the brain's neurons reflect the accumulated experiences of the individual, forming a coherent perception of the world and behavioral patterns of the individual. Further, since genes are also the basic unit of heredity, the behavioral patterns can be passed on to offspring as much as physical characteristics.
The timing when different genes are first expressed appears to follow a strict pattern across the lifespan, a "genetic lifespan calendar" (Skene et al., 2017), taking place within the transcript RNA that copies DNA instructions to build proteins. The biggest changes occur during young adulthood, peaking around age 26, with most changes completed by middle age. This calendar is somewhat delayed in women, suggesting that the female brain ages more slowly than men.
Summary
The brain processes information in a hierarchy. As you are reading this page, the signal coming in through your eyes enters your brain through the thalamus, which temporally organizes it. That information then goes on to the primary visual cortex at the back of the brain, where populations of neurons respond to very specific basic properties. For instance, one set of neurons might fire up because the text on your screen is black and another set might activate because there are vertical lines. This population will then trigger a secondary set of neurons in the visual association cortex that respond to more complex shapes like circles, and so on until you have a complete picture of the physical properties of the text.. From there, signals move to the left temporal lobe, which comprehends the meaning of the words, and on to working memory in the prefrontal cortex, which comprehends the ideas and concepts being communicated. As the ideas are processed, other regions light up in response to perceived informational and emotional importance. If you react emotionally based on the information, the limbic system sends signals to the striatum, which initiates physical movement. If the information is new and makes an impression, it is temporally stored in the hippocampus until it is consolidated into long-term memory located throughout the cortex, based on the memory's components (faces, words, emotions).
Our look at the physical brain does not do justice to its elegance and complexity. We recommend you further your understanding by visiting websites on the subject. One place to begin is the Brainfacts.org website.
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