“We are who we are because of what we learn and what we remember.”
Memory: the power of “retaining” and “recalling” past experience. (NCIt) A mental function by which we are able to retain and retrieve information about events that have happened in the past. (Cardwell, 149) Learning is the process whereby we acquire new knowledge about the world, and memory is the process whereby we retain that knowledge over time. Our brain creates, stores, and revises memories. We depend on memory for thinking, learning, decision making, and interacting with other people. (Kandel4, 107)
In 1894, Ramon y Cajal suggested that memory is stored not in “neurons” but in the growth of new connections between them. Eric Kandel proved this in the 1960’s. (SAM Oct/Nov 2007, 36) William James concluded in 1890 that memory must have at least two different processes: a short-term process and a long-term process. (Kandel, 210) Each form of memory storage requires specific changes in the brain. Short-term memory results from strengthening existing synaptic connections, making them function better, whereas long-term memory results from the growth of new synapses. Put another way, long-term memory leads to anatomical changes in the brain, whereas short-term memory does not. (Kandel4, 114) The brain has 100 trillion connections between neurons and each connection has the potential to be part of a memory. The links between individual neurons, which (combine) them into a single memory, are formed through a process called “LTP" – "long-term potentiation.” (Carter, 175-176) When changes in the network become lasting and robust, the information becomes firmly in long-term (memory). The memory of something and the "perception" of the same something share the same network. For example, viewing an image of an object activates the same network as viewing the actual object. (Goldberg, 110) Also referred to as ‘memory system.’
“The psychologist Adrianus de Groot showed chess masters and weaker players a chessboard on which the pieces were configured as they might be in the middle of an actual game. Even though they had seen the board for only a few seconds, chess masters were able to reconstruct it almost perfectly, but players below the master level had a lot of trouble. De Groot then showed a second board … that was not representative of a real game. The chess masters’ ability to reconstruct the random board was just as bad as that of the less skilled players.”
Long-term Memory: memory that lasts days and weeks. (Kandel5, 4) A set of processes that retain information over days, months, and years. Includes (memories for) sensory-motor skills (“implicit memory”) and memories for autobiographical details and facts (“explicit memory”). (Koch, 339) An example would be, remembering what you did on your last birthday. (Discover Aug 2007, 56) Memory for patterns. Describing the shared properties of whole classes of things. For example, knowing that tomatoes are red. “Generic memories” are accessed much more frequently than singular memories. (Goldberg, 132) To convert short-term memory to long-term memory involves “repetition.” (Kandel5, 4) Without experience and without the essential organizing tool of “language,” infants lack the capacity to embed their memories in a web of meaning that will make them accessible later in life. (Foer, 84) Both implicit and explicit memory can be stored in the long term for days, weeks, or even longer. Each form of memory storage requires specific changes in the brain. (Kandel4, 114) Also referred to as ‘generic memory.’ Editor's note - neuroscientists distinguish between two durations of memory, long-term and short-term, but often talk of "working memory." Working memory is classified here as a form of short-term memory.) The neurobiology of long-term memory differs fundamentally from short-term memory storage in requiring in its initial steps the "synthesis" of new “proteins.” (Kandel5, 4) Requires anatomical changes. Requires the creation of new "synapses." (SAM, Oct/Nov 2007, 35) Not only do the changes in “synaptic strength” last longer but, more amazingly, the actual number of synapses in the “circuit” changes. (Kandel, 213) Repeated “sensitization training” (practice) causes neurons to grow new terminals. (Kandel, 215) Enhancing long-term memory requires repeated training interspersed with periods of rest. (Kandel, 191) In 2004, Eric Kandel reported a protein bearing a close resemblance to a “prion” that plays a key role in the formulation of long-term memories. (Discover, Jan. 2005, 53) Long-term memory requires repeated training and the formation of new synaptic connections. In the interim, while a memory is being consolidated, it is sensitive to disruption. Recent studies have revealed that a similar sensitivity to disruption occurs when a memory is retrieved from long-term storage; that is memories become unstable for a short period of time after they have been retrieved. (For example), when a person recalls a memory that evokes the “fear response,” the memory is destabilized for several hours. If during that time the storage processes in the brain are perturbed, either behaviorally or with a drug, the memory often does not go back into storage properly. Instead, it is erased or made inaccessible. (Kandel4, 188) Although long-term memories are formed in the hippocampus and the “prefrontal cortex,” they are stored elsewhere, in knowledge-specific “cortical” regions. (Memory and Mind, John Gabrieli) Relatively invulnerable to decay. Can withstand the assault of “neurological” decline, even “dementia.” (Goldberg, 108) One fMRI study contrasted brain activity when subjects recorded experiences that would later be recalled vividly to those that were soon forgotten, and found that in children, increased activity was limited to the “medial temporal lobe” (which includes the hippocampus and surrounding structures), but in adults, the prefrontal cortex was also engaged by memorable events. (Memory and Mind, John Gabrieli)
Long-Term Depression (LTD): a chemical process which weakens and disconnects neurons from each other. (Doidge, 117) A persistent activity-dependent decrease in synaptic efficacy between neurons. It typically occurs following repeated low-frequency “afferent” “stimulation,” but it can be induced by other methods. Long-term depression appears to play a role in memory. (MeSH)
Long-term Habituation: stage of “habituation” in which the number of “presynaptic” connections among “sensory neurons” and “motor neurons” decreases. In working with “aplysia” in long-term habituation, the number of presynaptic terminals drops from 1300 to 850, and the number of active terminals diminishes from 500 to about 100 --an almost complete shutdown of "synaptic transmission." (Kandel, 213-214)
Long-Term Potentiation (LTP): a chemical process which occurs at the neuronal level. It strengthens the connections between neurons. (Doidge, 117) The links between individual neurons, which bind them into a single memory, are formed through (this process). (Carter, 176) A family of slightly different mechanisms, each of which increases the strength of the “synapse" in response to different ("firing rates") and patterns of stimulation. (Kandel, 283) A long-lasting increase in synaptic strength between two neurons. (Hockenbury, 248) A long-lasting increase in transmission at synapses in the hippocampus. (LeDoux, 139) The 'NMDA receptor' responds only to extraordinarily rapid trains of stimuli and is required for long-term potentiation. When a ”postsynaptic neuron" is stimulated repeatedly, the ‘AMPA receptor’ generates a powerful “synaptic potential” that “depolarizes” the cell “membrane.” This depolarization causes an “ion channel” in the NMDA receptor to open, allowing "calcium" to flow into the cell. The flow of calcium “ions” into the postsynaptic cell acts as a “second messenger” (much as “cyclic AMP” does), triggering long-term potentiation. Thus the NMDA receptor can translate the electrical signal of the synaptic potential into a biochemical signal. These biochemical reactions are important because they trigger molecular signals that can be broadcast throughout the cell and thus contribute to long-lasting synaptic modifications. Specifically, calcium “activates” a “kinase” that increases synaptic strength for about an hour. The calcium influx and the activation of this kinase lead to the strengthening of the synaptic connections by causing additional AMPA receptors to be assembled and inserted into the membrane of the postsynaptic cell. (Kandel, 284) In the five years beginning in 1975, there were only twelve (research) publications on LTP. Between 1990 and 1994, more than one thousand LTP papers were published, and the number almost doubled in the next five years. (LeDoux, 140)
Long-Term Sensitization: process by which sensory neurons grow new connections that persist as long as the memory is retained. In working with “aplysia” in long-term sensitization, the number of “synaptic terminals” more than doubles (from 1300 to 2700) and the proportion of “active terminals” increases from 40% to 60%. In addition there is an outgrowth from the (postsynaptic) motor neuron to receive some of the new connections. (Kandel, 213-214) Also referred to as ‘long-term facilitation.’
Memory Pathways: the medial "temporal" and medial "diencephalic" memory areas are interconnected both with each other and with widespread regions of “cortex” by a variety of “pathways” crucial for memory "consolidation" and retrieval. (Blumenfeld, 829)
Memory Trace: the brain changes associated with a particular stored memory. (Hockenbury, 242) The biological representation of an event in memory. This is thought to be biochemical in nature. (Cardwell, 149) Also referred to as an ‘engram.’
Mneme: the capacity for retaining after effects of experience or stimulation. (Oxford)
Re-Entry: the process of ongoing “reactivation” of critical neural networks in the “neocortex” involved in “memory” formation and recall. If the loops involve ‘distant brain regions’ the process is called ‘reverberation.’ If the loops are local to where the synaptic changes are taking place, the process is called “long-term potentiation” or “LTP.” (Goldberg, 112)
Short-term Memory: memory that lasts at most hours. Every form of (declarative and procedural) memory has this stage. (Kandel5, 4) A catch-all term for the temporary storage of information over tens of seconds. Compared to long-term memory, (short-term) memory is more (easily altered) and has only a very limited capacity. (Koch, 196) Some consider (short-term memory) interchangeable with “working memory.” (Hockenbury) Both implicit and explicit memory can be stored in the short term for minutes. (Kandel4, 114) Also referred to as ‘immediate memory.’ The neurobiology of short-term memory does not require “protein synthesis." (Kandel5, 5) Short-term memory is created by strengthening existing synapses. (SAM Oct/Nov 2007, 35) Short-term memory produces a change in the function of the synapse, strengthening or weakening preexisting connections. (Kandel, 215) Learning gives rise to short-term memory by producing transient changes in the strength of existing synaptic connections between neurons. Those short-term changes are mediated by proteins and other molecules already present at the synapse. Cyclic AMP and “protein kinase A” enhance the release of “glutamate” from the terminals of neurons, and this enhanced release is a key element in short-term memory formation. (Kandel, 240)
Iconic Memory: a form of high-capacity, rapidly decaying (within a second or so) visual memory. It exists in other sensory modalities as well. (Koch, 331) Also referred to as ‘fleeting memory.’
Working Memory: orchestrated processing and storage. A mental workspace that holds and manipulates information ... which is distinct from long-term memory. (Logie, 1) Memory that does not last. For example, remembering a phone number just long enough to dial it. (Discover August 2007, 56) Characterized by a small storage capacity, "semantic" representation, and short duration. (Koch, 197) (Words or a phrase that) take longer to say, take more space in working memory. This matters because anything that occupies working memory reduces the ability to think. (Kahneman, 29) A form of short-term memory. (Kandel, 111) Removing the prefrontal cortex of monkeys does not result in a generalized deficit in short-term memory but rather in a deficit in the functions described as working memory. (Kandel, 354)