Behavioral Neuroscience XIII
How do we Remember?
Memory
How many memories do you have? If a memory is defined as any skill, association, or experience your brain houses, you can easily see how the number of memories you have explodes into the millions.
How does our brain store so many memories? Well, this question is still an active area of research. So far, we have developed made significant strides in two areas: storage/structure of memories, and basic memory making.
Memory Structure & Storage
There are several difference kinds of memories. Short-term or working memory (STM), which is what you are thinking/learning/remembering in the moment and only lasts about 30 seconds. This is what you are consciously aware of in the moment and working memory collects and processes all the inputs for your senses to paint one picture. This combining of information is called encoding and helps bridge the gap between transduction and conscious thought. Because of its relation to conscious thought and your decision making, the prefrontal cortex is heavily involved.
Everything that you can remember that did not occur in the last 30 seconds is called Long Term Memory (LTM). Consolidation is the process of converting STM to LTM. LTM is built on a hierarchy, and each memory type follow different rules and are stored in different places throughout the brain. Here is the layout:
Declarative Memory are memories you can consciously recall like past events and facts. It is flexibly accessed and you can access similar declarative memories through association.
- Episodic Memory are memories of past events and experiences. They are stored in the cortex and the right, frontal, and temporal lobes. Consolidation of events involves all senses and emotions, and this is the reason that a certain smell or emotion can trigger a memory. Different attributes of an experience are processed in different parts of the brain.
- Semantic Memory are definitions of things. They are stored in the cortex and temporal lobe.
Procedural Memory are things that you can show, but can’t really explain, like skills and habits.
- Learned skills are stored in the Basal Ganglia, motor cortex, and cerebellum
- Repetition priming (being predisposed to guess an answer based on recently similar situations) is stored in the bilateral temporal lobe and the left frontal lobe
- Conditioning (being predisposed to act a certain way from a stimulus and reward) is stored in the cerebellar circuit (simple conditioning) and the hippocampus (complex conditioning)
Retrieval is the process of bringing of stored information in LTM and bringing it to STM and conscious awareness. When we retrieve a memory from LTM to STM we have to re-store it afterwards, which can lead to distorting the memory. This can be a positive, such as re-consolidating PTSD-related memories more neutrally, or a negative, when a witness mis-remembers information based on what others think happened.
Memory Making
While the above section explains where memories are stored, we still haven’t answered the fundamental question: how is a memory actually made? This is still a mystery but we have gained some insights into this.
Neural Plasticity is the idea that neurons can rewire themselves based on a recurring stimuli (or lack thereof). Neural plasticity in LTM is called Long-term Potentiation (LTP). When there is a recurring stimuli, there is a large influx of Ca++ through NMDA. The NMDA receptors activate CAMKII, a protein that locks in an activated state and promotes AMPA receptors to open while increasing their conductance (effectiveness). As these neurons continually fire together, their connection gets stronger, and the memory is encoded.
The opposite of LTP is Long-term Depression (LTD), in which a lack of Ca++ deactivates NMDA and instead activates phosphates, degrading the connection between neurons.
This above process for memory making has only been observed for episodic memory formation—other forms of memory creation are still being researched. Even with very little information, we do know that the above process of memory formation can be improved through exercise, environmental enrichment, and memory tasks.
Recap:
- Memories are stored throughout the brain in a hierarchical structure, separated into facts and processes
- Memories are formed by a series of neurons firing together long enough to all activate at the same time at a later date.