The Anatomy and Function of the Brain in Memory and Creativity
This page describes, in a simplified way, the anatomy and function of the brain in memory and creativity. Sensory perception will be described on a page soon to be constructed.
Structure of the neuron
The nervous system consists of nerve cells, also called neurons, and a large number of different "support" cells (=glial cells) which provide both physical support and supplies of essential chemicals.
The general structure of a neuron can be seen in the figure below.
Messages from other neurons come to this neuron through branches, called dendrites, which extend from its cell body. The axons of the other neurons are attached to these dendrites through structures called synapses. Once the message from another neuron is transmitted to this neuron through its dendrites, it sends a translation of this message to other neurons through its own axon.
The kind of translation will depend on the role of the neuron. For example, the intensity or strength of the message that is sent on can be modified. Some neurons always react to a message by sending an amplified message to the next neuron, while other neurons may always send an inhibitory message to the next neuron. Still other neurons may respond by amplification or inhibition depending on the neuron from which they got the message, or depending on whether they have been sending messages recently or have been inactive. Thus a neuron that has been repeatedly bombarded with messages may become inhibited itself, and not send on a message, or send on a message to inhibit the next neurons' response. This is what happens when a neuron becomes habituated.
Structure of the central nervous systemThe central nervous system consists of the cerebrum (what we commonly think of as the brain), the cerebellum, the brain stem and the spinal cord.
Memory and the central nervous system
The overall structure of the cerebrum consists of an outer cortex (grey matter), consisting of layers of interconnected nerve cells (= neurons), an inner layer of white matter, which is made of the cables (=axons) that connect these neurons to the other parts of the nervous system, and, deep inside the brain, collections of interconnected neurons called nuclei.
Structures in the cerebrum that are involved in memory include the prefrontal cortex, which is involved in working memory, a form of memory which lasts for seconds, the hippocampus, which is involved in short-term memory (hours to days or longer), and the cerebral cortex itself, which is involved in long term memory.
The prefrontal cortex is located on the sides of the brain above your eyes and a little to the rear. The prefrontal cortex is responsible for holding on to memory for a very short time -on the order of seconds.
Whether you are moving through space, or reading this sentence, you first store the images or words in working memory -- this is the memory you use when you need to remember something for a few seconds at most (about 10 seconds, in fact). It is also the memory you use to "keep in mind" something you are thinking about, and the one you use to make lists of things to remember, to make comparisons between two things in front of you, etc. It has a limited capacity - it can only keep "on-line" only 7≠2 chunks of information at a time. By chunks, I mean for example, a telephone number (which is 7 digits long). However, if the number beginns with an exchange you know well, say 253-, that exchange constitutes a chunk, so you can remember more digits or other items. Because of the limited capacity and time frame for retaining an item in working memory, you can "lose track" of something that is stored in working memory only, if you get distracted from your task.
Many neuroscientists have ascribed a critical role to working memory in the feeling we have of our sense of self and of continuity in our flow of consciousness. Because working memory contains whatever one is aware of at each very moment, its content is constantly changing, but at the same time there is overlap in the arrival times of these very short term memories in the brain, so you can have a sense of continuity from one situation to the next. Furthermore messages concerning the "self" are constantly arriving at the pre-frontal cortex. While we may move in space, or the environment around us changes, there are always constants about the self which change very slowly if at all, and which are represented in working memory. This self probably exists as a chunk in working memory, thus providing a sense of unity as well as continuity.
There is also a role of the prefrontal cortex in keeping goals in mind, both short-term and long-term goals -- have you ever asked, "Now what was I looking for?" -- that kind of question comes when your inner dialogue or outer experience (with its potentially more important goals) has displaced the goal of, say, finding a book, or your glasses, or the snack you had left behind when the phone rang.
The hippocampus, a group of cells in the lower inner side of the temporal lobe, is responsible for short-term memory, and takes over, in terms of duration of memory, from the prefrontal cortex. The hippocampus can "remember" things for minutes, hours, days, and weeks, and it has a very much greater storage capacity than the pre-frontal cortex. You use your hippocampus when you remember where you parked your car this morning, or what you learned by studying yesterday.
For memories to be very long-term, they must be consolidated through storage in the cerebral cortex. The hippocampus serves as a distribution center for these memories. This distribution process is ongoing, once a memory enters the hippocampus, and will depend on the nature of the memory, your active desire to retain it, whether the activity is repeated with variations (such as remembering where you put your car -- you may remember where you put it this morning, because you have to use it tonight, but you don't need to remember today's location next week, because you will have moved the car in the interim), to mention a few factors.
Functioning of the hippocampus can be severely affected by stress. Both the stress hormone cortisol, produced by the pituitary gland, and the hormone which cause the pituitary to release cortisol, called corticotropic hormone releasing factor (CRH), produced by the hypothalamus, interfere with hippocampal function. In this way, the brain inhibits storage of memories which otherwise would be too painful. Of course people differ in the extent to which stress triggers release of these hormones, and in the type of stress which may trigger the hormone release. Thus two people who experience the same trauma may have widely differing memories of the trauma, both shortly after the trauma and in the long run.
The amygdalaA critical component of memory is its emotional content. Emotion does not simply imply happiness, or sadness, or anger, but also involvement, salience, motivation - importance, in other words. The amygdala is located close to the hippocampus in the temporal lobe, and provides the emotional color to experience and memories. Without this emotional color, we would not have interest in our surroundings, and would not know how to respond to our environment.
I have often advised students to study at least a couple of times in the room where they are going to have quizzes or finals. Why? Because the cerebral cortex, which is involved in long-term memories, can grasp onto those memories best when you are in the same situation as when the memory trace was first constructed.
What do I mean by a memory trace? When memories are shifted into long-term memory, the memory is not distributed in a single spot in the brain, complete with address to reach it. Rather it is distributed in bits and pieces, which are reassembled as needed to provide the memory. The trigger for the reassembly -- in a way the entry point -- is often a similar experience. So if you study in given surroundings, you can look around you and (unconsciously) activate the "entry points" for connecting together all the neurons which, when activated in the proper order, reconsitute what we have as the conscious memory.
A classic example of such an "entry point" is the madeleine that Proust ate, which triggered all the memories that poured forth in his series of books "Remembrance of Things Past."
The cerebellum, located beneath the cerebrum, and attached to it via the brain stem, is the part of the nervous system that is responsible for procedural memory - that is, the memory of how to do things -- the movements of the mouth and throat in speech, riding a bicycle, and so forth. It is also responsible (together with nuclei in the midbrain and brainstem) for coordination, that is the proper sequencing of movements and the correction of error when you do things.
You are not consciously aware of the activity of the cerebellum. For example, when you type, or play a musical piece from memory, you do not have to be conscious of each and every finger movement, because the cerebellum has learned the activity. When you first begin to learn to type or play an instrument, you are conscious of ordering each move to be carried out - a very sl o o o o w process. Once you have practice, the cerebellum takes over the sequencing and the checking, and the activity becomes automatic, and very rapid, because the processing time is greatly shortened.
The frontal cortex is responsible for planning -- organizing thought, and envisioning the consequences of actions in the future. It is closely linked to the prefrontal cortex, and guides the choices of items to be placed in working memory.
Planning is critical to behavior because without this organizing principal, we would behave in response to the emotion or the external trigger of the moment, without regard to the consequences.
The right frontal cortex tends to create plans in terms of the negative consequences of potential actions, especially negative consequences to life and limb. We should note again here that consciousness of the body-as-entity is lodged in the right hemisphere, so it is not surprising that the right frontal cortex should concern itself with safety of life and limb. The left frontal cortex tends to create plans in terms of the rewards, but it also concerns itself with social behavior, both success and failure. Thus a person who has had a stroke affecting the right frontal cortex, will behave in an overly optimistic way and perform in a socially correct fashion, while the person with a stroke on the left will tend to look at the more gloomy side of things, but may not care about the social consequences of his or her actions.
People who are depressive tend to have low activity in the left frontal cortex. At the same time their evaluation of situations tends to correspond more closely to objective reality, which often makes it difficult to argue logically with a person who sees things in terms of gloom and doom. In light of the role of the left brain in social behavior as well as in optimism, a person who is depressed will not be concerned with social niceties or grooming.
It has been suggested that the reason women are more likely than men to become depressed, and to be able to talk about it (remember, speech is primarily located in the left hemisphere), lies in the fact that the connections between the two hemispheres are more abundant in women than in men. In fact it is possible that men become depressed just as often as women, but because verbal access to the depressive state is not as easy, men may not "know" they are depressed, but may simply behave in ways that reflect the depression and their depressive/realistic view of the world.
The reticular system, or reticular formation, is a collection of interconnected cells which are linked together all the way from the end of the spinal cord to the brian stem, and from there they are connected to the thalamus. While all of the reticular system is responsible for arousal of the nervous system around it, the area of the reticular system located in the brain stem is responsible for the level of arousal we call consciousness.
Signals coming in from the sensory systems have a relay in the reticular system, so that if we are not alert, a string enough signal will serve to arouse us. The reticular system then sends a signal to the thalamus, which is responsible for maintaining consciousness.
The thalamus is a paired collection of neurons, grouped into serveral nuclei, which is situated centrally deep in the cerebrum. One member of the pair lies on each side of the brain's midline. The thalamus may be thought of as the "nerve center" of the brain, because most messages to and from the cerebral cortex pass through the thalamus. The only sensory modality which does not use the thalamus as a way station to the cortex is the sense of smell. Nerves carrying all the other sensory information synapse with their next neurons in line at the thalamus.
The reticular system, which, as noted above is responsible for arousal, sends its "wake-up" message to the thalamus, which then produces a steady electrical pulse with a frequency of 8-12 seconds. This pulse spreads to the entire cortex, providing the alpha waves seen on the electroencephalograph (EEG) when taken with eyes closed. This pulsation also occurs when we have our eyes open, but cannot be seen due on the EEG to the high level of "noise" produced by the visual system.
It is thought that this pulse provides the underlying electrical activity that "binds" the cortex with the underlying central nervous system in a steady background of neurons blinking on and off. We do not completely understand how this binding occurs. My (perhaps overly simplistic) conception is based on two important properties of neurons. The first is that they can, and -- when left to their own devices -- do, fire (that is, send an electrical discharge down their axons) at random. Thus there must be some system to curb (though not completely eliminate, for reasons which I will explain later) this random firing.
One system is to increase the threshold for firing by hyperpolarizing the neuron's membrane -- a membrane which is hyperpolarized takes a longer time for its membrane potential to drift down to the point where it can fire spontaneously. This is the approach used by inhibitory neurons -- the ones that inhibit the firing of their neighbors.
This is not, however, an approach that could be used successfully by the entire brain. If the entire brain were hyperpolarized, then random firing may be curbed, but genuine messages would have a very difficult time getting through -- only the very strongest signals would have a chance. So another approach is needed.
This second approach is based on the fact that, after firing, neurons cannot fire again for a short while, called the refractory period. By imposing a rhythmical on-off signal on the brain, in other words by making a large number of the brain's neurons fire off in a rhythmical manner, the thalamus imposes a relatively simultaneous refractory period, so the neurons are ready to fire together at the end of the refractory period. In this way, when a message comes into the brain from the periphery, it will encounter neurons all over the cortex which are waiting to be fired, so the message can be spread over all of the relevant parts of the cortex. Thus an incoming signal can activate all the parts of the cortex that should respond to it. It is this organized widespread activation which may well be what enables us to be conscious -> certainly we are unconscious without this thalamic firing. It is also likely that it is this organized widespread activation which allows the brain to carry out unconscious processing when we are awake, and to report this processing to consciousness.
This rhythmical activation during waking should be contrasted with two other types of activation - that which occurs in epilepsy and and that which occurs in sleep.
Epilepsy is the result of the spontaneous localized firing of a neuron or neurons, which then spreads indiscriminantly to neighboring neurons, and in the case of grand mal seizures, to the entire brain. Because the neurons which are firing are not in the thalamus, but elsewhere in the brain, they override the thalamic signal; because the firing is indiscriminant, the pattern of firing is not organized for conscious thought, and the person will be unconscious, although he or she may behave in a stereotyped manner, which may seem to be conscious.
Sleep can be divided into four basic stages, which are associated with different types of EEG's and different levels of arousal. As we fall asleep, we first move through all four sleep stages, accompanied by a progresssively larger amplitude of brain waves, with a slower rhythm. After the first 80-90 minute long episode of deep sleep, we experience alternate episodes of rapid-eye-movement (REM) sleep and the other stages of sleep.
A purpose for sleep in memory, and an neural network explanation for imagination and creativity
Yaneer Bar-Yam has argued cogently that the reason for these stages of sleep lies in the need of the brain to organize and prune memory traces. As you can probably see already, the brain is a highly subdivided collection of networks. When we are awake, these networks of neurons can fire in appropriate sequences to create thought and behavior.
These networks of neurons are "tied" together through a process called imprinting -- the more often a group of neurons fire in a particular pattern, the easier it becomes to trigger this sequence. "Memory" can really be thought of as the result of this imprinting. In fact, what we do when we learn is to cause specific neurons to tend to fire in a certain order. The more we practice, be it a memory or an activity, the more likely the group of neurons corresponding to that memory or activity are to fire in the appropriate pattern, and the easier it is to recall the memory -- you can think of the process of developing a memory as developing a roadmap in your brain.
At the same time, when a pattern of firing is imprinted on a group of neurons, the chances that any of these neurons can fire as part of another pattern is diminished. All the experiences you have in a day that become translated into networks of co-firing neurons in the the hippocampus or cerebellum have the potential to become permanent co-firing networks. However, the cortext has only a limited capacity to organize these networks before there becomes excessive overlap among the neurons used for one memory's network and those used for another memory's network. As a result, there is a practical limit to the amount of memories that can usefully be held among groups of neurons.
According to Bar-Yam, the purpose of sleep is to allow neurons to disconnect from networks, somewhat at random. By disconnect I mean that, in a grouping of co-firing neurons, neuron B no longer fires when neuron A fires. With progressive disconnection of neurons, and reassignment of these other neurons to other groups, the system can contain continuously updated information from the environment. What determines a network's demise is the strength of the connections among the neurons in the network. The more likely the whole ensemble of neurons in a network is to fire together, the less likely the network is to fall apart. Therefore, if we think about a memory repeatedly, it will beome more and more consolidated in long term memory, and less and less easy to dislodge.
It is the role of sleep to take apart and put together these networks. By taking the cortex and other parts of the nervous system "off-line," that is by not "binding" it together, pruning of network connections can occur without interference of ongoing sensory input. We experience this type of pruning semi-consciously in REM sleep -- that is we can sometimes report what we have dreamt about. The dream of REM sleep is the result of this process of memory trace activation, which results in pruning of neurons from networks and reconnecting others into networks. This pruning and reconnecting of conscious non-sleep experiences gives dreams a quality of quasi-reality. This pruning may also lead to rearrangement of networks, and the formation of new connections between networks, which can give the solution of problems encountered during the conscious waking hours.
We should note here that, while sleep is critical for reinforcing memories that are associated with a strongly interconnected network, and diminishing the strength of others, it is not the only condition where connection and disconnection among neurons occurs. When we are awake, and developing memories of our experiences, and working to solve problems, we are creating networks, some of which may have only a fleeting existence, while some may survive the nightly pruning. The stronger the interconnections created during the day, the greater the likelihood that some aspect of a future experience will trigger firing of one or another of the neurons in the network, and therefore the greater the likelihood that you will remember the previous experience, ad/or will repeat that experience. If the experience is playing a piece of music, or performing a special kind of jump, then this imprinting may be something to celebrate, but if the experience is something that is negative, it may lead to the formation of an obsession, which by dint of repetition may become more and more imprinted, and therefore stronger.
Single-mindedness is a particularly apt term for an obsession, because when obsessed, more and more neurons become linked together in the representation of the obsession, to the point that almost any signal that starts its way through the brain will encounter neurons that are part of the obsession's pattern, so that almost anything will trigger obsessive thoughts. In fact, brain imaging of persons with obsessive-compulsive disorder (OCD) shows that their brain activity is excessively coordinated, and that recovery from OCD is associated with decreased coordination.
A critical point for this course: Imagination and creativity -- or rather the propensity to be imaginative and creative -- probably lies in the "looseness" of the networks formed in the creative person's cortex. Creativity and imagination can be thought of as the process of making associations between two ideas, such that suddenly one idea triggers the other. At the neuronal level this association between ideas is the result of one network suddenly triggering another, to which it may at first be only loosely connected: in other words, some neurons from network A (and their attendant memories/thoughts) may combine with some neurons of network B to form a new network C. The moment of the transition from A + B to C is the 'aha' moment.
By contrast, the propensity to become imprinted to the point where individual neurons in a network can only participate in one particular network, and where connections of individual neurons with other networks are severed, leads to a lack of creativity and imagination, at least in the domain of thought and behavior associated with the over-imprinted networks.
Have you ever worked on a problem, and come back again and again to an unsatisfactory solution? The more you come back to that solution, the harder it is to think of another way of looking at the problem -- in other words the unsatisfactory solution has been deeply imprinted, and you can't jump out of the rut. Have you then said: "I'll sleep on it"? In fact given the role of sleep described above, this is just the right thing to do!
Copyright V. Utermohlen, MD
Last updated January 29th, 200 1