Here we attempt to answer what is memory, how is it stored and retrieved.
Memory is a process by which information is:
- Encoded
- Stored
- Retrieved The brain has different types of memories, and certain brain regions are specialized for this task.
Ebbinghaus Curves
Other experiments destroy parts of the cortex and correlate this with recall.
Types of memory
TODO see Kendal67-1 figure.
Sensory memory
- iconic memory (remembering images) 150-500 milliseconds
- Echoic memory (recognizing some sounds) usually retained for 1 to 2 seconds. This memory is filtered by consciousness/attention to be passed to short term working memory. The register capacity of this memory is considered to be quite large.
Short-term memory
it has an explicit storage of about 7 +- 2 items (so very small). Depending on attention level, it is retained for 2 to 18 seconds. It seems the representation here is often vocal.
Long-term memory
There is a division between declarative and non-declarative memory ones that can be recalled consciously or not. Conscious meaning you can recall it and transfer it to someone else. Often explicit motor experiences become implicit memories.
Priming is for example auto completion of visual stimuli.
Cell assembly theory
This theory has been proposed by Hebb to explain memory creation. After learning, partial information leads to full activation of the learned pattern -> engram.
Long term memory
Declarative memories
The forms of declarative memories divide into two main kinds:
- Episodic memory: memories of specific events (time and place of some events, or life experiences), they are usually very rich and full of details.
- Semantic memory: general knowledge (e.g. bananas are yellow), they are not associated with time and space. These are distilled from episodic memories somehow.
- The replays of episodic memories often happen in REM state (which is so linked to memory consolidation).
- This semantic memory is distributed in the brain, they are not localized.
Case of Henry Molaison
Henry Molaison was an amnesic patient that had his hippocampus removed in 1953. He could not form new episodic memories, but his semantic memory was intact. At the time it had no clue what the medial-temporal lobes were doing, he had seizures that started from that part of the brain, so the surgeons removed that part of his brain.
- IQ was normal
- Short term memory was normal
- Sensory and motor functions were normal
- He did a mirror-drawing task (he was able to learn the task).
- Semantic memory was ok (he had common sense, know basic facts about the world).
- Priming was ok (BERT like masking problems are ok).
But he could not be able to recall new people, neither episodic memories up to 2 years before surgery (older memories where intact).
The Hippocampus
In humans the cortex became larger but the Hippocampus remained similar, and it has been pushed back (into the posterior part). This means it is an ancient structure.
Bryan A. Strange et al. 2014
In different animals
Lateral PFC (attention areas) is also useful for memory recall to happen.
Long-term potentiation of the Schaffer Collateral
We want to know when the schaffer collateral is activated (remember from CA3 to CA1, after the pattern completion). They go to fiber bundles that are easily stimulated electrically (then we record the neurons in the CA1 region). The presynapse releases glutamate that binds to AMPA receptors that activate and bring potassium in that bring the potential of the post-synapse up. Glutamate also binds with NMDA receptors, now do nothing because magnesium is blocking the path.. But if the post-synaptic neuron fires, then the magnesium ion is removed (pushed away by the high potential) and the NMDA receptor can be activated, which means the neuron can learn, they fired together. NMDA lets calcium in, which creates a cascade of processes that bring more AMPA ion gates in the post-synapse.
CA1 have NMDA receptors that are important for this connection (if you add blockers then they are not stimulated anymore, these are important for brain plasticity, ketamines modulate NMDA receptors for example).
Morris Water Maze Experiment
In this experiment, the rat is put in a pool of water and has to find a platform to get out of the water (not pleasurable fro mices to stay in water). The rat is trained to find the platform (random walk before experience, then they are more directed), and then the platform is removed. The rat is then put back in the pool and the time to find the platform is measured. They mutated a gene to remove NMDAR, and they observed you are much slower in any learning in the rats. (It took more time for the rats to find the platform). They also did the opposite, overexpressing NMDAR made them learning faster.
- Checked the electrophysiology activations of transgenic and wild type
- And did behavioural analysis of mice performance in this task.
This experiment aims to measure spatial memory.
Place Cells
CA1 neurons are place specific, they fire if they recognize to be in some specific position. Then they used microdrives to record the firing of the neurons in the CA1 region of the hippocampus. The neurons fire in multiple places, not single place. They used information from multiple neurons to reconstruct information of the paths taken by the rat. They also saw that there was a large overlapping of place fields for cells that co-fire temporally. (cell ensemble code hypothesis). They saw that cells with small overlapping place fields, do not fire together (anti-correlation). This has been seen in many species (also in humans, monkeys, bats).
Grid Cells
They wanted to know the input of the place cells. Grid cells are found in the medial entorhinal cortex, they fire in a grid-like pattern (invariant of animal species, animal’s posture, landmarks etc), they are used to create a map of the environment. They are also found in humans. They also find some shifted grid patterns, rotated and similars. Grids that have different spacing in dorsal or ventral MEC part of the brain (ventral is coarser compared to dorsal), and they also fire at different periodicity, and change in different animals (flying or not, more complex for flying animals, or climbing animals). The grid looks like a Fourier transformation in space, not time domain (wavelet or fourier decomposition). A good reference is work by Yoram Burak (Accurate Path Integration in Continuous Attractor Network Models).
Even in the absence of external sensory cues, foraging rodents maintain an estimate of their position, allowing them to return home in a roughly straight line. This computation is known as dead reckoning or path integration. A discovery made three years ago in rats focused attention on the dorsolateral medial entorhinal cortex (dMEC) as a location in the rat’s brain where this computation might be performed. In this area, so-called grid cells fire whenever the rat is on any vertex of a triangular grid that tiles the plane. Here we propose a model that could generate grid-cell-like responses in a neural network. The inputs to the model network convey information about the rat’s velocity and heading, consistent with known inputs projecting into the dMEC. The network effectively integrates these inputs to produce a response that depends on the rat’s absolute position. We show that such a neural network can integrate position accurately and can reproduce grid-cell-like responses similar to those observed experimentally. ~Yoram Burak
In the thalamus we can observe some head-direction cells.
Allocentric Information
The brain represents some allocentric information which is not egocentric information, meaning it is not dependent of the point of view. The memories are created with allocentric information to create place cells.
Associative memories
Activating part of the network activates the other parts too!
But we have problems with memory interference.
Associativity at the Synapse level
Remember that often a single synapse is not sufficient to activate the synapse of the next neuron. You need cooperativity of a group of neurons or a strong synapse (Associativity). and some times the association is specific meaning some synapses are stronger than others even if the activation level is similar.
Multiple sensory streams help creating strong connections. (Hippocampus is difficult to excite, it needs often to be multi-modal to activate).