Memory

As shown in Figure 03a, the ability to modify our behaviour in response to life's experiences is shared by all animals including the bacteria E. coli. Such feat requires the brain's willingness to learn. Learning results in the formation of memories and in humans this process reaches its most sophisticated form, allowing us creatively to associate different reflections on the past, to generate new ideas, and most importantly to acquire language as a medium of expression and communication. Memory requires the brain to be physically altered by experience and it is this remarkable property that makes thought, consciousness, and language possible. The basic mechanism of memory formation is highly conservative over

Figure 24a Memory Classification [view large image]

billion years of biological evolution. The difference in humans is that we have a lot more of the stuffs. There are about 100 trillion synaptic connections in our brain.

There are many ways to classify the memory. The concept of explicit and implicit memory refers to whether or not the recollection is produced consciously and intentionally. While the scheme of declarative and nondeclarative memory depend on the retrieval that can be declared verbally or not. Associative memory is triggered by clues; nonassociative memory can be habitual or sensitive. There are also short term and long term memory. One of the classification schemes is shown in Figure 24a. Table 06 is an attempt to put them all together. In the table, the declarative, and the procedural memory are explicit with the rest of nondeclarative memories being implicit. Only the working memory belongs to the category of short term memory fading away in hours, while the others are long term, and available for retrieval in years. Figure 24b shows the components,

Figure 24b Types of Memory [view large image]

locations, and pathways for many types of memory.

Table 06 Types of Memory

• Working Memory -Most of our memories are fleeting because few of the many experiences we have in the course of an average day are remembered for very long, nor do they need to be. Transient memories are absolutely essential to the process of understanding the meaning of events as they occur in the present. This type of very short-term memory for things being experienced now is known as working memory; it allows you to comprehend what you are reading or to figure out the meaning of what has just been said to your in a conversation. Working memory can be thought of as a low capacity information reservoir that is always full, sensations flowing into it continuously at about the same rate that they are forgotten. Some of the information held in short-term storage may be important enough to be remembered for a long time and must therefore be transferred to a more stable form of storage, which is represented by far more robust

  alterations in the brain's chemical and physical make-up in the form of synaptic connections. It is not necessarily for an important experience to trigger the formation of long-term memories, other factors such emotion, practice, and rehearsal also facilitate the transformation. Experiments show that in all cases the most important underlying distinction between the short- and long-term memory formation is that the latter requires a dialogue between synapses and genes and the former does not.

  Figure 25 Working Memory [view large image]

  The working memory itself is located in the prefrontal cortex. As experimental techniques became refined, it has become clear that there is no rigid dividing line between a memory and a thought. A model of working memory has been developed to combine perceptions, memories and concepts together, and consists of three parts:
  
   
  • Phonological loop - Memory in this area (see Figure 25) enables us to remember sequences of approximately seven digits, letter, or words. The language areas of the brain are mainly in the left hemisphere, around and above the ear. The language loop start with hearing words in the auditory cortex and/or reading words in the visual cortex. Perception of language results from the convergence of auditory and visual information in Wernicke's area. Expression of language is controlled by Broca's area; while the angular gyrus is concerned with meaning.
  • Visual-spatial scratch pad - It is like a sort of inner eye, which receives and codes data into visual or spatial images. For example, it comes into play when we need to remember where we were on a page when we start reading a book again. Functional imaging suggests that this complex structure represents the "what" and "where" in short-term memory (see Figure 25).
  • Central executive - This most important yet least well understood component of the working memory model, is postulated to be responsible for the selection, initiation, and termination of processing routines (e.g., encoding, storing, retrieving). It is believed that this component coordinates information from a number of sources, directs the ability to focus and switch attention, organizes incoming material and the retrieval of old memories and combines information arriving via the other two temporary storage systems. It performs various tasks such as reasoning or doing mental arithmetic - rather like the RAM (Radom Access Memory) of a computer.
  
  
   

  Nondeclarative memory - Nondeclarative memory includes skill learning, implicit learning, priming, simple classical conditioning, and habituation. These forms of learning are similar in that it is experience which changes the neural makeup, and the conscious access to past episodes is not essential for the formation of these memories. Implicit memory is not flexible and does not allow for the recombination of learned information. Nondeclarative memory does not require the hippocampus or related structures. Instead, the implicit learning of skills and habits depends on the neostriatum (basal ganglia and its connections to the frontal lobes). The conditioning simple skeletal muscle reactions depends on the cerebellum. The amygdala is

  Figure 26 Nondeclarative Memory [view large image]

  essential for emotional conditioning. Nondeclarative memory can be classified to five main groups:

  • Procedural memory - It is the repository of such skills as handwriting or driving. These skills are essential part of our memory store, but it is difficult to describe the "know-how" in words. In this sense the memory is said to be implicit or non-declarative (Figure 26); you just cannot explain how to ride a bicycle. The skills may be difficult to acquire, but once learnt they are never forgotten, even without occasional practice. Thus it seems that the know-ledge or information required for the execution of very complex motor routines or procedures is somehow laid down in a robust permanent memory store. The parts of the brain involved in the acquisition of complex motor skills are the cerebellum and putamen (see Figure 24). Deeply ingrained habits are stored in the caudate nucleus.
  • Classical conditioning - Along with motor skills, conditioning is part of non-declarative memory. The desire for food at a particular time of day - regardless of whether hungry or not - is one example of such conditioning. A classical example is to associate the ring of a bell to food when feeding a dog. After repeating the training many times, the dog shows salivation at the ring of the bell even without food (see Figure 26).
  • Fear memory - Recent study in delivering shocks to mice suggests that fear memory does not occur immediately after a painful event; rather, it takes time for the memory to become part of our consciousness. The initial event activates NMDA receptors - molecules on cells that receive messages and then produce specific physiological effect in the cell - which are normally quiet but triggered when the brain receives a shock. Over time, the receptors leave their imprint on brain cells. A phobia is an excessive or unreasonable fear of an object, place or situation. Examples include fears of specific things such as insect, snake, mouse, and flying. It seems that people can learn to suppress a fright reaction by repeatedly confronting, in a safe manner, the fear-triggering memory or stimulus. It is found that for specific phobias, up to 90% of people can be cured through such exposure therapy.
  • Nonassociative memory - Nonassociative memory includes two forms of learning called habituation and sensitization. Habituation is defined as a decreased in response to a repeated stimulus such as a certain odor. On the other hand, sensitization is an increased responsiveness such as more sensitive in touching a cut in the skin. Nonassociative learning involves reflex pathways in the spinal cord and elsewhere.
  • Remote memory - The memory of events that occurred in the distant past is referred to as remote memory. The underlying anatomy of remote memory is poorly understood, in part because testing this type of memory must be personalized to a patient�s autobiographical past. What is known is that, like semantic memory, remote memory eventually becomes independent of the hippocampus. One memory model shows a linear representation of how experience is processed as memory: Stimulus Sensory Registration Attention Short Term Memory Consolidation - Retrieval Long Term Memory Remote Memory. At the stage of sensory registration, there is a matching/assigning of the pattern to a meaning. Short-term memory is temporary and is limited in space. If short-term memory is not repeated, the information is lost fairly quickly. Long term memory is consolidated and stored throughout the nervous system. Remote memories represent the foundation memories upon which more recent memories are built. Since early acquired information is the foundation for new memories and may be linked to many more new memories, such memory is less subject to change and/or loss. Similar to the short-term memory, the remote memories are not usually affected by aging.
  
  
   

  Declarative memory - Declarative memory covers the memory of facts such as events and names, which do not need to be repeated for them to sink in. Those experiences destined to be laid down as long-term memories are shunted down to the hippocampus where they are held in storage for 2 - 3 years. During this time the hippocampus replays the experiences back up to the cortex, and each rehearsal etches it deeper into the cortex. Eventually the memories are so firmly established in the cortex that the hippocampus is no longer needed for their retrieval. Much of the hippocampal replay is thought to happen during sleep. Dreams consist partly of a rerun of things that have happened during the day, fired up to the cortex by the hippocampus. The visual areas generate rerun of daily sightings (Figure27).

  Figure 27 Long-term Memory [view large image]

  Semantic, and episodic memory are the subclasses of declarative memory:

  Episodic memory - It is about an event in one's life and everything about it, including emotional reactions. Remembering an episode, e.g., the attack on Pearl Harbour (Figure 28), is to create a memory for a unique event that only happened once and there is no opportunity for learning the event by rehearsal. Episodic memories are not very reliable, they are highly personal, selective, idiosyncratic and varying over time, but they may also be richly complex and movie-like in character. They constitute the stories we tell ourselves about our past, they are the things we would write about in our autobiography. Episodic memories can be recalled deliberately or are triggered by evocative sensory stimuli - particularly by the sense of

  Figure 28 Declarative Memory [view large image]

  smell. Episodic memory involves the use of the hippocampus for forming memories and the cortex for storage (see diagram D, Figure 24).

  • Semantic memory - Semantic memory is the knowledge of facts - numbers, addresses, language and concepts - which the brain files in categories and which seems to involve the left temporal lobe. Retrieval is then carried out by the frontal lobes (see diagram E, Figure 24). We assume all of the facts that constitute our knowledge of things must be stored in an organized fashion to be useful. Though this has not been demonstrated, it seems likely that the brain stores our semantic memories as modules that have some logical links to one another; they are grouped by category for instance. On retrieval, the brain knows where to find the memory according to the address of that particular category. Semantic memory is essential to the understanding of how things work and thus to an under-standing of the world we live in. It is a body of knowledge that helps us to regulate our behaviour according to and dependent on reliable factual memories. Navigational skills, for example, depend on our ability to deploy a complex store of semantic memory, including detailed spatial memories and representations of the world.

mechanisms and principles involved in its formation of short- and long-term memories are conserved throughout the animal kingdom, including in humans. Aplysia exhibits a behaviour of protective reflex in which the sea slug withdraws its gill into the safety of the mantel cavity in response to a mild touch stimulus to another part of the body called the siphon (Figure 29a). If the stimulus is repeated a number of times, the gill withdrawal reflex becomes weaker until finally the animal ignores the touch stimulus. The waning of sensitivity to repeated stimulation is known as habituation and is a very simple form of learning found in all animals, including humans. Another type of learning is sensitization, when we are exposed to an unexpected or strongly unpleasant stimulus. Generally the sensitizing effect of

Figure 29a Memory in Aplysia [view large image]

a single alarming stimulus is short-lived, lasting perhaps for just a few minutes. But if the alarming stimulus is repeated a number of times our senses may be heightened for days and now such sensitization becomes a form of long-term memory.

		It is reported in 2007 that the seat of memory has been pinpointed in mouse. By monitoring 260 neurons in the hippocampus (Figure 29b), researchers have discovered that different experience is recorded in different area called "clique", which can be categorized from very general to very specific. Furthermore, such brain activities can be translated into binary codes (Figure 29c). Supposedly, we can read the mind from such codes and tell what it is
Figure 29b Seat of Memory [view large image]	Figure 29c Memory Code [view large image]	thinking by the process of backward translation. The followings are steps to uncover the memory code:

1. Recorded Experiences - The mice are exposed to three startling experiences - a puff of air on the back (to mimic an owl attack from the sky), a fall in a container (the "elevator" drop), and shaking in a cage (the "earthquake") - while a recorded plotted firing from a large set of CA1 neurons. Each row in the plot captures firing of a single cell over time.
2. Patterns of Mental Activities - The points in the 520 dimensional phase space (corresponding to the activities of 260 neurons before and after an event) are projected into a 3 dimensional phase space. Different mental activity falls into different area in such plot starting from "rest". Temporal analysis revealed that the activity patterns associated with those startling experiences recurred spontaneously at intervals ranging from seconds to minutes after the actual event, but with smaller amplitudes than the original response. Such patterns provide evidence that the information traveling through the hippocampal system was inscribed into the brain's memory circuits. The replay corresponds to a recollection of the experience after the fact.
3. Coding Cliques - It is discovered that neuron ensembles active during an event contain subsets -termed neural cliques. The cells in a clique all show very similar firing patterns and are not part of the other cliques.
4. Organization of Memories - Further analyses showed that each clique encodes a different aspect of an experience, ranging from the general to the specific. It can be visualized as a hierarchical organization with the most general clique at bottom, and the very specific on top. Any given pyramid can be a component of a more general polyhedron representing all events of a given category, such as "all startling events".
5. Translated into Binary Code - The clique activity is represented as a string of binary code with 1 as being active and 0 signifies inactivity. Thus the earthquake binary code is 11001 corresponding to: "starting event", "disturbing motion", "air puff", "drop", and "shake". While the elevator drop binary code is 11010 for the same sequence.

Higher Functions

The frontal lobes are where ideas are created; plans constructed; thoughts joined with their associations to form new memories; and fleeting perceptions held in mind until they are dispatched to long-term memory or to oblivion. This brain region is the home of consciousness, where the products of the brain's subterranean assembly lines emerge for scrutiny. Self-awareness arises here, and emotions are transformed in this place from physical survival systems to subjective feelings. The area of the frontal lobe most closely associated with the generation of consciousness is in the prefrontal cortex. Figure 30 shows four areas, which endow human with fucntions not available in other animal:

Figure 30 Higher Functions [view large image]

1. Orbito-frontal cortex - This area inhibits inappropriate action, freeing us from the tyranny of our urges and allowing us to defer immediate reward in favour of long-term advantage.
2. Dorsolateral prefrontal cortex - Things are held "in mind" here, and manipulated to form plans and concepts. This area also seems to choose to do one thing rather than another.
3. Ventromedial cortex - This is where emotions are experienced and meaning bestowed on our perceptions.
4. Anterior cingulate cortex - It helps focus attention and "tune in" to own thoughts.

The answer was still elusive at the end of Francis Crick's life, when he was struggling in vain trying to understand the role of claustrum in consciousness. Subjectivity poses the more formidable scientific challenge. Each of us experiences a world of private and unique sensations that another person can only appreciate indirectly. If the senses ultimately produce experiences that are completely and personally subjective, then we cannot arrive at a general definition of consciousness because there would be an infinite number of them. This is the "hard problem" of consciousness. According to some researchers, science cannot take on consciousness without a significant change in methodology, a change that would enable scientists to identify and analyze the elements of subjective experience.

Figure 31 Consciousness [view large image]

Others argue that we only need an underlying theory. Just like the Newtonian mechanics, one theory is sufficient to describe the multitude of orbits and trajectories.

The nature of free will is another issue that can be tackled by the new biology of mind. Free will is the ability to act or make choices as a free and autonomous being and not solely as a result of compulsion or predestination. According to Freud's discovery of psychic determinism - the fact that much of our cognitive and affective life is unconscious - there is not much left for freedom of action. Experiment on the correlation between electrical activity of the brain and movement (lifting a finger for example), reveals that the electrical activity precedes the movement by 200 milliseconds. It is proposed that the process of initiating a voluntary action occurs in an unconscious part of the brain, but that just before the action is taken, consciousness is recruited to approve or veto the action. In the 200 milliseconds before a finger is lifted, consciousness determines whether it moves or not. Thus, our conscious mind may not have free will, but it can freely modify inappropriate behavior (Figure 32). This is the reason for the laws in our society to hold all of us accountable for our own action. It is suggested that we

Figure 32 Free Will [view large image]

should update our idea of free will to mean self-control over our behaviour.

Altered States

automatically (dreaming, for example, is thought to be common to all mammals). Others are attained through learned techniques such as meditation. Some are induced by drugs. Other still - the vision and trance states - are highly controversial, and many people doubt their existence. To understand altered states one must assess subjective accounts of what it is like to "be in" these states, along with objective research that tries to identify their physiological basis and effects. Figure 33 shows the brain scan for some of the altered states listed below.

Figure 33 Altered States [view large image]

Dreaming - Vivid visual dreams light up the visual cortex; nightmares trigger activity in the amygdala and the hippocampus flares up from time to time to replay recent events. The areas, which seem to be most commonly active are the pathways carrying alerting signals from the brainstem and the auditory cortex; supplementary motor area and visual association areas - all of which produce the "virtual reality" effect of dreaming. Activity is decreased in the dorsolateral prefrontal cortex, the area of waking thought and reality testing (Figure 33). Studies have shown that dreaming sleep occurs in a wide range of animal species. Figure 34 shows a dreaming cat. When its pons is surgically removed to permit movement during REM

Figure 34 Dreaming Cat [view large image]

sleep, the very nice cat becomes a vicious tiger when it is dreaming and throws itself at imaginary prey.

Daydream - Many surveys suggest that ordinary men and women, who are neither disturbed nor neurotic, spend a large part of each day in some sort of fantasy, reverie or daydream. This kind of quick fantasy rarely has a structured narrative. It is the moment when we stop paying attention to what we are seeing and hearing and switch into an inner theatre of the imagination where we can play at wish fulfillment (Figure 35). But there are other fantasies qualitatively different from these "wouldn't it be nice if ..." stories. These are sustained fantasies, which often seem to have been crafted, worked and reworked to meet some more profound psychological need. When one daydreams, normal inhibitions are bypassed. The

Figure 35 Daydream [view large image]

evidence of the rather macabre biographies of serial killers shows that they had frequently recurring violent fantasies before they turned to murder.

Meditation - One function of consciousness is to knit together our sense of self-identity. But many religious traditions believe that enlightenment can be achieved only by breaking the shackles of self and attaining "purer" states of consciousness through meditation (Figure 36). As well as its psychological benefits, the meditative state has marked physiological effects - these phenomena are measurable and reliably repeatable, and thus are a suitable object of scientific study. Such studies have revealed some remarkable effects: meditation can lower a subject's metabolic rate, decreasing blood pressure, pulse rate and muscle tension. One study shows that the subject could reduce his oxygen intake to one-third of the normal resting state. Scans of people in a self-induced state of "passive attention" have

Figure 36 Meditation [view large image]

been shown to "turn off" areas of the brain normally associated with seeking stimuli, including the parietal, anterior and premotor cortexes (Figure 33).

Hypnosis - Modern studies show that the brain waves of hypnotized subjects are much like those of the waking state. When subjects are hypnotized, they can speak, walk and carry out instructions. Yet there are some noticeable changes from normal consciousness: attention becomes very selective, with the subject ignoring everything but the hypnotist's voice; the subject rarely initiates thought or activity, but waits for suggestions from the hypnotist; and fantastic ideas or situations are more readily accepted as reality. It is almost as if the willing, relaxed subject relinquishes control over part of his or her consciousness to the hypnotist. The classic method of hypnotism is to put a subject into a relaxed frame of mind and ask him or her to concentrate on an object, such as a swinging pocket watch (Figure 37). Brain scans (Figure 33) show increased activity during hypnosis in the motor and sensory

Figure 37 Hypnosis [view large image]

areas suggesting heightened mental imagery. Increased blood flow in the right anterior cingulate cortex suggests that attention is focused on internal events. The brain activity seen in this state is quite different from that seen in normal waking or sleeping.

Sexual fantasy - This is the ability to use our imaginations erotically. It is found that people spend a surprising amount of time thinking about sex. There are vast cultural differences in what different societies consider acceptable material for sexual fantasy and fetishes. The Victorians considered fetish to be shocking and dangerous, the true dark side of sexuality; while the Freudian view treats fetishism as the result of linking unresolved childhood drives to object that seems "safe" such as the high-heeled shoe. Many therapists now consider that it is perfectly normal to have sexual fantasies, and some even believe that they can be used to achieve a more fulfilling sex life. Research into sexual fantasies is complicated and must rely on what patients report to their therapists, but some studies have found links with childhood events - either sexual violence or a strict, repressed upbringing. There is an obvious distinction between fantasy and action - a fantasy does not harm others. However, some people who have

Figure 38 Fantasy [view large image]

fantasies that involves inflicting pain to themselves or others (such as to the cat in Figure 38) claim that they feel compelled to act them out. Sometimes people with less extreme fantasies also choose to turn them into realities such as in the form of cross dressing.

Addiction - Drug addiction is caused by a similar train of events to hunger. However, unlike most types of food, addictive drugs cause changes in the receptors to which they bind, making them less sensitive. This creates tolerance and addiction. Most addictive drugs work by altering levels of neurotransmitters in the brain's reward circuitry centered on the limbic areas. Other brain areas are also involved and each type of drug works in a slightly different way to produce its characteristic effects. Opiates are drugs derived from the dried resin of the opium poppy (Figure 39), or synthetic versions of these chemicals, such as heroin, codeine and morphine. All

Figure 39 Drug Addiction [view large image]

have been used medicinally at some time for their pain-killing properties. They are used illegally for similar reasons: heroin gives the user a "high", reducing anxiety and producing a sense of temporary well-being.

experiences. There have been thousands of reports of near-death experiences, many noting the same types of sensations. Subjects feel as though they have left their bodies. Some people report travelling down a tunnel toward a bright light (Figure 42), where benevolent presences wait. Scientists have been unable to explain them conclusively. Some physiologists have suggested that hypoxia, or low oxygen levels in the brain, might cause a consistent pattern of hallucination in all sufferers. Other scientists argue

Figure 42 Vision [view large image]

that the experience stems from an acute bout of "REM intrusion" into the partially awakening state (in time of extreme stress) similar to narcolepsy - a neurological disorder characterised by uncontrollable bouts of sleep that can cause elaborate hallucinations and, sometimes, out-of-body experiences.

	An aplisa is like a squishy snail. In rain, in snow, in sleet, in hail. When it is angry, it shoots out ink. The ink is purple, it's not pink. An aplisa cannot live on land. It doesn't have feet so it can't stand. It has a very funny mouth. And in winter it goes to the south.
[view large image]

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