Neurobiology
of Memory

Memories Develop in 4 Stages

Declarative Memories are formed in 4 stages differentiated by how long they last, the type of neurobiological changes encoding them, and the part of the brain that accesses these memories.

  • Duration: 18 seconds

  • Storage: neurochemical changes

  • Access: prefrontal cortex

  • Maintained by conscious repetition

  • Promoted to Short-term Memory by meaningful associations

  • 7 +/- 2 items

  • Duration: 1 to 3 hours

  • Storage: rearrangement of existing proteins

  • Access: hippocampus

  • Requires meaningful associations 

  • Promoted to Intermediate Memory by active recall 3 times within 30 minutes

  • Duration: 3 to 5 days

  • Storage: synthesis of new proteins

  • Access: hippocampus

  • Strengthened by repetition

  • Promoted to Long-term Memory by delayed active recall via cortico-cortical connections just prior to forgetting

  • Duration: up to 3 years

  • Storage: synthesis of new proteins

  • Access: hippocampus & cortico-cortical connections

  • Strengthened by spaced active recall

  • Promoted to Permanent Memory by applications in varied contexts

 
Working Memory

Working Memories can be maintained by the prefrontal cortex through continuous repetition.  The synaptic changes that occur during Working Memory are transient biochemical changes -- without any structural modifications to synapses.

Unless Working Memories are promoted to short-term memory through meaningful associations, they will be forgotten in about 18 seconds.

An example of working memory would be a young man who asks a girl for her phone number.  If he doesn't have paper and pen, he might repeat her number over and over to prevent forgetting it until he can write it down.

 

 
Short-term Memory

Short-term Memories last 1 to 3 hours.  They are encoded with synapses strengthened (Long-term Potentiation or LTP) by temporarily inserting existing receptors are into dendritic spines. 

 

Short-term Memories are fragile and can be lost due to physical trauma, electric shock (ECS), drugs, cold, and hypoxia.  Unless Short-term Memories are reinforced through active recall, the extra receptors migrate away from the synapse (Long-term Depression or LTD) as shown below.

 

Don't confuse "long-term" used in the terms LTP and LTD with Long-term Memory.  LTP and LTD are "long-term" compared with brief neurochemical synaptic changes.  Entirely new dendritic spines have been observed to form in only 30 minutes to 1 hour when individual neurons are stimulated, as shown in the images below:

Synapses enter "record mode" enabling changes to potentiation via new spines and receptors when both the upstream and downstream neurons fire at the same time.  This is consistent with the requirement for meaningful associations in order to promote Working Memories to Short-term Memories.

 

Unless Short-term Memories make the quantum jump to Intermediate Memory, they will be forgotten in a few hours.  Making the quantum jump to Intermediate Memory requires at least 3 active recalls within 30 minutes as explained below.

 
Intermediate Memory

Both Short-term and Intermediate Memories are accessed via the hippocampus, shaded here in blue. 

 

The hippocampus acts like an "index" for newly acquired memories enabling these memories to be easily recalled. 

 

The hippocampus also serves as a gateway determining which information from Working Memory is considered meaningful and important enough to be remembered.  

The amygdala assists the hippocampus in its gateway role.  The amygdala is the brain's center for fear and other strong emotions. When something is life-threatening or bizarre, the amygdala enhances the ability of the hippocampus to recall these events that may be important for survival.

Intermediate Memories last 3 to 5 days.  The quantum jump from 3 hours for Short-term memory to 3 days for Intermediate Memory results from the synthesis of new synaptic proteins!

When associations for Short-term Memories are actively recalled at least
3 times within 30 minutes, a quantum increase in memory duration occurs, known as Late Long-term Potentiation, or L-LTP.

Eric Kandel proposed that the mechanism behind the jump to L-LTP of Intermediate Memory was a build-up of sufficient protein kinase-A (PKA) to trigger protein synthesis in the nucleus.

Intermediate Memories can be maintained through frequent repetition.  While this will get you an "A" on Friday's test, repetition or "cramming" prevents the formation of Long-term Memories!  Newly acquired Intermediate Memories are accessed via the hippocampus.  Since hippocampal connections are constantly changing, once repetition stops, Intermediate Memories will rapidly fade within days!

 

 
Long-term Memory

Creating a new Long-term Memory requires delaying recall of a new idea until just prior to forgetting it!  Delayed, active recall (not mere repetition) will force the neural pathways that initiate the recall to shift away from the hippocampus towards longer lasting connections directly from one part of the cortex to another, known as cortico-cortical pathways.


Brainfire helps students quickly learn new material using special algorithms that rapidly promote Working Memories for new ideas to Short-term, then Intermediate, and Long-term Memories.  Students get an "A" on the weekly test, and more importantly, Brainfire maintains memories of key concepts long after the test has been taken!

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