Memory

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Memory – the record of information and experiences, including sensory and behavioral, which is encoded, stored, and retrieved by various neural networks within the brain[1] – is critical for day-to-day functioning. Storing encoded information in memory is a matter of synaptic remodeling, and the strength of memories formed is modulated by emotion.[2]  Types of memory, and the underlying neural network, can be categorized by the duration of encoded information (e.g., sensory, short-term, intermediate-term, or long-term),[3] [4] by the type of information encoded, [1] or how the information is expressed (e.g., explicitly or implicitly).[4] Working memory as a cognitive function is realized when the object of attention is a subset of information recalled from memory.[5]

Memory and Synaptic Remodeling

It is well understood that dendrites are involved in the transmission of signals between neurons, serving particularly in a receptive capacity. However, a sizable and growing body of research exists which also indicates dendrites are neural learning centers.[6]

Dendritic Spines – the Centers of Learning and Memory

The ability of a neuron to learn – or more specifically, to quickly recognize patterns, even after only a single learning event and/or in the presence of background noise – is mediated by small projections from the dendrites called spines.[7] It is on the head of these spines that dendrites receive electrochemical signals from the transmitting cell.[8]

As late as the mid-1990s, the prevailing view was that dendritic spines were relatively stable structures. However, recent advances in neural imaging have evidenced spines as motile structures that are capable of changing size and shape – and possibly in their functional activity – on a minute-to-minute basis.[9]

New spines are formed, and existing spines are strengthened as a result of long-term potentiation (LTP).[8] LTP is a critical part of the production of new dendritic spines, a process known as spinogenesis.[7] From a CFDT perspective, spinogenesis is far more important than neurogenesis – the formation of new neurons. In general, neurogenesis is an activity limited to fetal development and specific anatomical regions (e.g., the hippocampus). Conversely, spinogenesis continues throughout the lifetime, is interactive in near-real-time with events, stimuli reception, and behaviors, and is the primary mediator of complexities yielding higher-order cognitive functions.

More is not always Better

Mature dendritic spines are the primary neurotransmitter receptors in mammalian brains. Spinogenesis produces more dendritic spines and strengthens existing spines. It would seem counterintuitive, then, that more spines do not necessarily result in better cognitive functioning.

In the past, beneficial spine pruning was believed to only take place during development or aging, as may be seen with neural pruning.[9] However, spine pruning has been observed during REM sleep[10] and is now known to be a desirable mechanism for the synaptic refinement.[11] The general pattern of spinogenesis and synaptic refinement is (1) rapid spinogenesis – which occurs in near-real-time with stimulation, (2) synaptogenesis – the establishment of a connection from an axon bouton to the newly formed spine – a process which usually takes one to two days, and (3) spinal pruning through which inappropriate, redundant, and weak spines and synapses are eliminated.[10] Such pruning is essential for efficient cognition and learning. Spinal pruning is highly dependent on sensory experience.[8]

Axon Terminals and Boutons

The observant reader may wonder whether the presynaptic partner to spines – boutons which form from axon terminals – might also actively participate in learning and memory formation in a manner similar to dendritic spines. Indeed, terminals have been observed to arborize like dendrites.

Moreover, terminal boutons – the specialized end of an axon that forms the presynaptic part of synapses – can develop rapidly in a manner similar to spinogenesis.[12] However, computational modeling consistent with empirical observations strongly suggests that the development of boutons is primarily a matter of neuron electrical homeostasis. In other words, it is an autonomous process, not an activity-driven process. Axon arborization and bouton formation keep the electrical activity of a neuron from becoming too high or too low.

Memory and Emotion

In general, chronic stress – such as may be observed by the expression of despair, withdrawal, and/or hopelessness – has been shown to cause dendritic atrophy and spine loss.[7] [13] Acute stress, on the other hand (i.e., significant stress-induced over a relatively short period of time, such as an hour), results in increased spine density and pruning of weak or inappropriate spines. In other words, short bursts of acute stress stimulate neural remodeling and reallocation of cognitive resources.

Acute stress can be associated with a range of emotions; stress should not be blanket-understood as a negative experience. For example, watching a competitive sporting activity is generally a pleasurable, fun activity, particularly if the game is “interesting.” But when your team is behind by two points and time is running out, the “fun stress” can be intense enough raise your blood pressure, make your heart race, or – in rare cases – even cause a stroke.

Emotions enhance memory representation through several macroscopic processes.[14] Emotions can capture attention, thus enabling the individual to sustain focus on emotionally salient stimuli. Emotions may also recruit more resources for the processing of stimuli, thus creating stronger encoding and more efficient storage.

Conversely, emotional loading can interfere with memory formation. The most likely cause of interference in this respect is attention being drawn away from the relevant stimuli – that which is to be remembered – and drawn toward the transient experience of the emotion. Moreover, since cognitive resources are divided between differing tasks, there will be fewer recruitable resources for stimuli processing.

A natural inquiry may arise, then, whether there is an emotional state that enhances memory performance and, if so, can that state be activity driven. The short answer, unfortunately, is no. Both positive and negative emotions can accentuate or interfere with encoding, storing, and retrieving information.

There is, however, another aspect of emotions. As mediated by the dopaminergic system, emotional experiences determine the individual’s motivational processes associated with a particular set of memories.[15] In other words, the experience of pleasure or reward with a specific activity does not only concern the release of endorphins (pleasure neurotransmitters). Instead, experiencing pleasure from an activity – the goal of which, in terms of neuropsychology, is to repeat said activity as often and as frequently as possible – has three stages:

  • The first stage in receiving a rewarding pleasure response is that the activity needs to produce a hedonic impact. That is, you need to like doing it. If, for example, you strongly dislike the taste of chocolate, you will not become a “chocoholic.”

  • The second stage is to develop a wanting for the activity.

  • The third state in developing a reward pleasure response is learning that the activity is both desirable and has a hedonistic impact, and how that activity can be willfully repeated.

For effective CFDT, the therapist can introduce a significant impact on all three stages of reward development by merely making the activity fun. What constitutes “fun” will differ from client to client, of course. Nonetheless, if the adaptive, spinogenetic, activities introduced in therapy are to produce long-lasting behaviors, the client needs to develop a reward response for engaging in them. The challenge for the effectual therapist will be in combining fun with properly modulated acute stress.

Spinogenesis, Synaptic Remodeling, and Therapeutic Delivery

The neuromolecularbiology of spinogenesis and synaptic remodeling has its theoretical appeal to the interested therapist, but how does it impact the delivery and dosing of therapy? Several general points should be kept in mind when formulating a therapeutic session that contributes to producing long-lasting, far-reaching transfer effects:

  • Spines are the basic units of memory and learning, and they form in near-real-time in relation to sensory experience. Therefore,

    • The first third or half of a session, thus, should be filled with numerous broad-based activities through which the client is required to perform short bursts of high-intensity exercises. The therapist encourages the client toward cognitive sprinting as a warm-up.

    • The last half or third of the session should be filled with fewer, targeted activities that are designed to improve the client’s deficit cognitive functions. The therapist directs the client toward sustained cognitive jogging.

    • In between, the therapist can transition from driving the client’s cognitive sprints to sustaining the client’s cognitive jog.

  • LTP is vital for the development of stable, mature spines. Therefore, while therapists are encouraged to engage in a broad range of activities to maximize spinogenesis – and, ultimately, far-reaching transferable effects – therapists should also construct successive sessions to exercise and develop the same or similar processes and functions over several weeks.

  • Synaptogenesis takes up to 48 hours to complete, whereas spinogenesis happens in near-real-time. Hence,

    • Client scheduling should be arranged, whenever possible, to allow at least a day between sessions. Greater frequency (i.e., daily sessions) will not, in most cases, provide additional beneficial outcomes or stronger neural remodeling or reallocation of cognitive resources.

    • If sessions must be scheduled on successive days, therapists should plan sessions to target different cognitive functions. For example, if a client is scheduled Monday, Tuesday, and Wednesday, sessions should be prepared to address either the attentional or the memory function on Mondays and Wednesdays, depending on which of the two functions show a more significant current deficit. The other function would then be addressed on Tuesdays.

  • Spinogenesis and synaptic remodeling are accentuated by acute stress, but associating rewards with a set of activities promotes habitualizing the behavior. Thus, to drive the development of long-lasting effects, therapy sessions need to be simultaneously stressful and rewarding. Perhaps the easiest way to combine these otherwise contradictory objectives is to develop a friendly competitive relationship in which the client either competes with him/herself (self-improvement), competes with the therapist, and/or competes with other clients. Doing so also has the benefit of developing goal-oriented executive functioning.

Categorizing Memory by Type

Categorization by Duration

A common scheme for classifying memory types is by the duration of memories. Duration lengths vary from the time scale of hundreds of milliseconds to an individual’s lifetime.

Sensory

Within current research models, each of our five senses is associated with multiple cognitive processing components: sensory perception, or the physical ability to perceive physical information about the environment; working and long-term memory capabilities; and what may be called sensory memory. Sensory memory is automatically processed,[4] and is a fundamental aspect of human sensory systems. It provides a trace of sensory information that remains available to various neural networks and processes for no more than approximately 100 ms after the removal of sensory perception. Although its duration is short, its saliency is equivalent to direct sensory input into working memory if it draws attention’s focus.[16] Visual sensory memory is known as “iconic memory.” Auditory sensory memory is known as “echoic memory.” And tactile sensory memory is known as “haptic memory.”[2]

Short-Term

Short-term memory has a storage span on the order of a few seconds and a capacity of around ten items.[4] Neurologically, it is a consequence of the dual processes of habituation and sensitization. These processes arise from short-term potentiation (STP) and short-term depression (STD) of neurotransmitter signals in secondary, collateral pathways. STP and STD yield short-term plasticity.[17] These neural activities are distinctly different from those involved in working memory. Thus, despite the sometimes synonymous usage within the research literature between short-term and working memory, the two are neurologically distinct.

Functionally, the difference between short-term memory and working memory may be observed as the former allows for recall of information without the need for conscious or effortful refreshing. Working memory, on the other hand, is more complicated. It entails attentional control both for storage and manipulation of information.[14]

Intermediate-Term

Some researchers posit the possibility of an intermediate-term memory. Items stored in intermediate-term memory do not need to be maintained by attentional control and are available for recall for up to 8 seconds. This memory system engages when an item is no longer within the attentional focus of working memory and disengages when the item is encoded for long-term memory.[3] In practice, a significant overlay may exist between the concepts of short-term and intermediate-term memory.

Long-Term

Items in long-term memory are encoded for time scales greater than eight seconds, even as long as the individual’s lifetime. Theoretically, LTM has unlimited capacity, but it is subject to neural remodeling, which can remove the dendritic spines within which the information is encoded.

Whereas information stored in short-term / intermediate-term and working memory is recalled consciously, long-term storage may be recalled either consciously (as in declarative / explicit memories) or unconsciously (as in non-declarative / implicit memories).[2] Encoding and storage of LTM involves mainly unconscious or preconscious processes. However, items may be intentionally encoded when spotlighted by the attentional system.[18]

Attentional spotlighting and associative binding seems to be related theta band coupling of gamma oscillations, allowing for the integration of new perceptual information as encoded memory traces. [18] [19] Other research, however, indicates that beta band power is predictive of LTM encoding.[20]

Categorization by Information Encoded &/or Expressed

Declarative / Explicit Memory

Declarative memory – sometimes called explicit memory – refers to stored information that can be consciously evoked. Declarative memory is processed through numerous neurologic areas including sensory cortexes, the medial temporal lobe (MTL) memory system, and the frontal cortex. [18] 

The MLT, in addition to playing a pivotal role in declarative memory, is also involved in emotional processing. The experience of significant adverse events (which may result in PTSD or CPTSD diagnoses), can produce structural and functional changes in the MLT.[21] The changes produce declarative memory impairments in traumatized individuals, with a bias for emotionally negative stimuli and trauma-relevant information.[22]

There are two basic types of declarative / explicit memories:[2]

  • Episodic Memory – refers to the ability to learn, store, and retrieve information related to personal experiences. Typically, these memories will include details about the event as well as information about the time and place.

  • Semantic Memory – refer to the creation of supramodal representations of abstract, schematic, or conceptual information. The stored representations allow for efficiently manipulating such information, thereby allowing highly creative thinking.

Non-Declarative / Implicit Memory

Non-declarative – sometimes called implied memory – encompasses all unconscious memories as well as certain abilities or skills:[2]

  • Procedural Memory – refers to that part of memory, which is involved in recalling motor and executive skills needed to do one or more tasks. Learning is Initially required for any task or skill acquisition. However, once learned, the task or skill can be automated and performed without conscious attention to the task or skill itself.

  • Associative Memory – is the storage and retrieval of information through association with other information.

  • Non-Associative Memory – requiring the processes of habituation and sensitization – involves to newly acquired information or learned behavior gained through repeated exposure to an isolated stimulus.

  • Priming – is not a memory, per se, but refers to an effect whereby exposure to certain stimuli focuses attention on memories to be drawn out at a later time.

Working Memory is a Unique Type of Memory

Within the CFDT framework, working memory is considered a primary function distinct from memory, as described above. While it may be considered a special type of memory system wherein the stored information can also be manipulated and thereby included by some researchers along with other forms of memory, there are important distinctions that make it unique.

To wit, memory proper is a result of short or long term neural remodeling through the processes of spinogenesis and synaptogenesis, and driven by STP, STD, LTP, and LTD. Information is thus learned and stored through neurochemical encoding of dendritic spines, and remain until such a time as the associated spine or spines undergo pruning.

Conversely, items stored in working memory are encoded as gamma frequency bursts held within a coupled theta frequency. Information stored in working memory, therefore, only remains as long as it remains the object of the attentional system. [13] [20] [23]

The unique nature of working memory as coupled transient neural oscillations gives rise to the observed “magical number” of seven items +/- two as the typical, limited capacity of working memory. Each items to be transitively stored is represented by one gamma burst, accounting for “one space” within the theta band carrier cycle. Overall, only seven +/- two such gamma bursts – dependent upon the nature of the information to be stored – fit into each theta-band carrier wave. [19]

Citations

  1. ^ 1.0 1.1 Eichenbaum, Howard. (2008). “Memory.” Scholarpedia. vol. 3, no. 3, article 1747. Article Link.
  2. ^ 2.0 2.1 2.2 2.3 2.4 Camina, Eduardo and Francisco Güell. (June 30, 2017). “The Neuroanatomical, Neurophysiological, and Psychological Basis of Memory: Current models and their origins.” Frontiers in Pharmacology. vol. 8, article 438. Article Link.
  3. ^ 3.0 3.1 Kamiński, Jan. (May 17, 2017). “Intermediate-Term Memory as a Bridge between Working and Long-Term Memory.” The Journal of Neuroscience. vol. 37, no. 20, pp. 5045-5047. Article Link.
  4. ^ 4.0 4.1 4.2 4.3 Takeda, Masaki. (June 30, 2018). “Brain Mechanisms of Visual Long-Term Memory Retrieval in Primates.” Neuroscience Research. vol. 142, pp. 7-15. Article Link.
  5. ^ Cowan, Nelson. (2000). “Processing Limits of Selective Attention and Working Memory.” Interpreting. vol. 5, no. 2, pp. 117-146. Article Link .
  6. ^ Wu, Xundong, et. al. (May 3, 2019). “How Dendrites Affect Online Recognition Memories.” PLoS: Computational Biology. vol. 15, no. 5, article e1006892. Article Link.
  7. ^ 7.0 7.1 7.2 Qiao, Hui, et. al. (2016). “Dendritic Spines in Depression: What We Learned from Animal Models.” Neural Plasticity. vol. 2016, article 8056370, pp. 1-26. Article Link.
  8. ^ 8.0 8.1 8.2 Yang, Yang, et. al. (April 2, 2019). “Changes of Synaptic Structures associated with Learning, Memory and Diseases.” Brain Science Advances. vol. 4, no. 2, pp. 99-117. Article Link.
  9. ^ 9.0 9.1 Segal, Menahem. (December 15, 2012). “History of Neuroscience: Dendritic Spines and Memory.” IBRO History of Neuroscience. Article Link.
  10. ^ 10.0 10.1 Stein, Ivar S. and Karen Zito. (November 26, 2014). “Dendritic Spine Elimination: Molecular Mechanisms and Implications.” The Neuroscientist. vol. 25, no. 1. Article Link.
  11. ^ Zhang, Zhong-weil et. al. (January 15, 2013). “Essential Role of Postsynaptic NMDA Receptors in Developmental Refinement of Excitatory Synapses.” PNAS. vol. 110, no. 3, pp. 1095–1100. Article Link.
  12. ^ Butz, Markus, and Arjen van Ooyen. (October 10, 2013. “A Simple Rule for Dendritic Spine and Axonal Bouton Formation can Account for Cortical Reorganization after Focal Retinal Lesions.” PLoS: Computational Biology. vol. 9, no. 10, article e1003259. Article Link.
  13. ^ 13.0 13.1 Marchetti, Giorgio. (August 14, 2014). “Attention and Working Memory: Two basic mechanisms for constructing temporal experiences.” Frontiers in Psychology. vol. 5, article 880. Article Link .
  14. ^ 14.0 14.1 Garrison, Katie E. and Brandon J. Schmeichel. (February 13, 2018). “Effects of Emotional Content on Working Memory Capacity.” Cognition and Emotion. vol. 33, no. 2, pp. 370-377. Article Link.
  15. ^ Berridge, Kent C. and Morten L. Kingelbach. (January 31, 2013). “Neuroscience of Affect: Brain mechanisms of pleasure and displeasure.” Current Opinions in Neurobiology. vol. 23, no. 3, pp. 294-303. Article Link.
  16. ^ Barton, Brian and Alyssa A. Brewer. (June 12, 2019). “Attention and Working Memory in Human Auditory Cortex.” Human Auditory System, ed. Prof. Stavros Hatzopoulos, Dr. Andrea Ciorba and Associate Prof. Piotr H. Skarzynski. Chapter Link .
  17. ^ Poon, Chi-Sang and Daniel L Young. (August 8, 2006). “Nonassociative Learning as Gated Neural Integrator and Differentiator in Stimulus-Response Pathways.” Behavioral and Brain Functions. vol. 2, no. 29. Article Link.
  18. ^ 18.0 18.1 18.2 Köster, Moritz, et. al. (December 6, 2018). “Memory Entrainment by Visually Evoked Theta-Gamma Coupling.” Neuroimage. Vol.118, pp. 181-187. Article Link.
  19. ^ 19.0 19.1 Hülsemann, Mareike Johanna. (September 2016). “The Role of Phase-Amplitude Coupling in the Relationship between Acute Stress and Executive Functioning.” Doctoral Thesis, Trier University. Thesis Link.
  20. ^ 20.0 20.1 Daume, Jonathan, et. al. (January 11, 2017). “Phase-Amplitude Coupling and Long-Range Phase Synchronization Reveal Frontotemporal Interactions during Visual Working Memory.” Journal of Neuroscience. vol. 37, no. 2, pp. 313-322. Article Link.
  21. ^ Thomaes, Kathleen, et. al. (2009). “Increased Activation of the Left Hippocampus Region in Complex PTSD During Encoding and Recognition of Emotional Words: A pilot study.” Psychiatry Research: Neuroimaging. vol. 171, pp. 44-53. Article Link.
  22. ^ Thomaes, Kathleen, et. al. (February 2013). “Increased Anterior Cingulate Cortex and Hippocampus Activation in Complex PTSD During Encoding of Negative Words.” Social Cognitive and Affective Neuroscience. vol. 8, no. 2, pp. 190-200. Article Link.
  23. ^ Alekseichuk, Ivan, et. al. (June 20, 2016). “Spatial Working Memory in Humans Depends on Theta and High Gamma Synchronization in the Prefrontal Cortex.” Current Biology. vol. 26, no. 12, pp. 1513-1521. Article Link.
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