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The Future of Neuroscience: Memory Retrieval

By: Sai Srihaas Potu

The mechanism of memory remains one of the great unsolved problems of neuroscience. Grappling with the question more than a hundred years ago, the German zoologist Richard Semon formulated the concept of the engram. Neuroscientists now have the knowledge and tools to tackle this problem, however, there are still many unanswered questions about the different mechanisms of memory and whether or not memory retrieval is possible.


Memories are stored as chemical changes at the connection points between neurons in the brain. There are more than 100 trillion synapses in the human brain, and each memory is stored as slight changes to thousands or millions of synapses. In this sense, memories have a distributed representation: each memory involves thousands of synapses, and each synapse is involved in possibly thousands of memories.


Our memory has three basic functions: encoding, storing and retrieving information. Encoding is the act of getting information into our memory system through automatic or effortful processing. Storage is retention of the information, and retrieval is the act of getting information out of storage and into conscious awareness through recall, recognition, and relearning.

Once a memory is created, it must be stored. Many experts think there are three ways we store memories: first in the sensory stage; then in short-term memory; and ultimately, for some memories, in long-term memory. Because there is no need for us to maintain everything in our brain, the different stages of human memory function as a sort of filter that helps to protect us from the flood of information that we're confronted with daily.


If a memory survives until at least the next day, then its storage has already resulted in a possibly permanent change to thousands of synapses in the brain. The question is for how long will that memory be retrievable? And is it still there even if it is not retrievable?


The memory for past events is among the least understood processes in neuroscience. It is only known for certain to exist in humans, and the experiments required to study its neural mechanisms are problematic for logistical and ethical reasons.

A recent study looked at the activity of individual neurons in the episodic memory region, the hippocampus, of human patients undergoing surgery for epilepsy. Episodic memories undergo qualitative changes with time, but little is known about how different aspects of memory are affected. Different types of information in a memory, such as perceptual detail, and central themes, may be lost forever. In patients with medial temporal lobe damage, memory for perceptual details is severely impaired, which is why memory retrieval is a key aspect of current neuroscience research.

In the study, the researchers asked participants to learn the locations of words on a circle. They were then tested by being shown the word and asked to move a marker to its location on the circle. To discover whether precision fades over time, they tested groups of participants at different time intervals ranging from 10 minutes to four days after they had learned the information.

The researchers found that while participants who were tested later had forgotten a significant proportion of word-location associations, the ones they did remember were recalled with the same precision as the groups tested earlier. The researchers also looked at whether allowing participants to extract a pattern by clustering words on the same theme in one location would aid memory. When a pattern was present, they found that participants remembered more word-location associations which shows that accessibility went up, but the precision with which things were remembered went down.


This experiment shows that learning processes still happen even when a person cannot actively remember an experience. With diseases like Alzheimer’s, it is not the memory that is lost but rather the recalling of memory that is hindered. In these cases, the cells that are responsible for recalling information are disrupted. The findings of this research give hope to the future treatment of Alzheimer’s and other memory impairing diseases. Hopefully, it will be possible to artificially reactivate the ability to recall forgotten information, and counteract the damaging effects of Alzheimer’s disease and other similar diseases. As you see, even after all this past research, our brain is still full of surprises. And the journey to discover it has only just begun.


References:

1. Addante RJ, Ranganath C, Yonelinas AP. Examining ERP correlates of recognition

memory: evidence of accurate source recognition without recollection. Neuroimage. 2012.

2. Addis DR, Wong AT, Schacter DL. Remembering the past and imagining the future: Common and distinct neural substrates during event construction and elaboration. Neuropsychologia. 2007.

3. Addis DR, Pan L, Vu MA, Laiser N, Schacter DL. Constructive episodic simulation of the future and the past: Distinct subsystems of a core brain network mediate imagining and remembering. Neuropsychologia. 2009.

4. Aggleton JP, Brown MW. 1999. Episodic memory, amnesia, and the hippocampal–anterior thalamic axis. Behavioral and Brain Science. 1999.

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