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The Science Of Health: How Does The Brain Store Memories?

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Memories: Welcome back to “The Science Of Health”, ABP Live’s weekly health column. Last week, we discussed how the brain perceives and understands time, and how the organ’s inner clockwork drives one’s behaviour. This week, we discuss how the brain stores memories, and the significance of a memory’s location in the brain. Memories form an integral part of our lives, especially the happy ones, because they make us cherish the good times. People usually don’t want to remember the sad events from their past because these memories make them feel uncomfortable.

The region in the brain where a memory is stored can determine how useful it will be for future situations, a new study has found. 

The study describing the theory was recently published in the journal Nature Neuroscience. 

Memories are initially stored in the hippocampus, but are later transferred to the neocortex. The transfer of memories from the hippocampus to the neocortex is called systems consolidation. Memories reside in the neocortex for a long time. 

However, there are some exceptions. Not all memories undergo systems consolidation, which means that certain memories live in the hippocampus forever, and never move to the neocortex. 

No researcher had mathematically figured out what determines whether a memory resides in the hippocampus or moves to the neocortex, until now. Researchers from the Howard Hughes Medical Institute’s Janelia Research Campus have attempted to resolve this challenge by proposing a new mathematical neural network theory.

According to this theory, memories consolidate in the neocortex only if those memories improve generalisation. This refers to the act of inferring something as a general concept or occurrence based on common properties of specific instances. 

Reliable and predictable components of memories lead to generalisations, which can be applied in other situations. For instance, seeing or thinking of mountains makes one visualise snow-capped landscapes and valleys. 

This must not be confused with episodic memories, which are detailed recollections of the past that have unique features. For instance, an episodic memory will make us remember exact instances from an old trip to the mountains. 

The study explains that memories are not copied from one area of the brain to another in this form of consolidation, but a new form of memory is consolidated in the neocortex. This form of memory is a generalisation of previous memories. 

The amount of a memory that can be generalised determines whether the memory remains in the hippocampus, or is consolidated in the neocortex. 

Next, the researchers aim to conduct experiments to see how much of a memory can be consolidated.

Humans often show varied recollections of the same event. When multiple people believe that their distorted memories are accurate recollections, the phenomenon is referred to as the Mandela Effect. 

While it is believed that only long-term memories could be distorted, new research has shown that humans can also misremember events within a few seconds, leading to illusions. 

This happens because expectations often shape one’s perception of an event, whether it occurred a few seconds ago or a long time back. Sometimes, shaping long-term memories to fit one’s expectations can generate false memories. 

Similarly, short-term memory for perceptions formed just one or two seconds ago may not accurately represent what the person saw and perceived at the time of perception. In other words, people often incorrectly remember what they saw or perceived just a few seconds back. 

The study describing these findings was published April 5, 2023 in the journal PLOS One

The study was led by Marte Often from the University of Amsterdam in The Netherlands.

 

As part of the study, participants were made to perform four experiments, which focused on a subset of false memories, also called “memory illusions”. 

In the first experiment, the participants were made to undergo a training procedure to see if they were fit to continue with other experiments. They were made to perform basic visual memory tasks. 

Out of 45 participants, 40 were found to have normal or corrected-to-normal vision (the ability to see normally with glasses). These 40 participants were presented with circles of six or eight letters. In the circles, one or two letters were mirror-image forms, or pseudo-letters. 

After this, they were shown a second set of letters, but were asked to ignore it. 

Then, the participants were shown a target shape, and presented with a list of options. They were asked to choose from these options the location at which the target shape was present in the circle shown earlier. Also, they had to rate themselves, on a scale of 1 to 4, on how confident they were about the fact that they remembered the location of the target shape correctly. 

After this, the participants were made to perform three similar experiments. 

The four experiments showed that some participants reliably reported what was there, which means that their perceptual inference accurately reflected the input they were shown. Meanwhile, other participants erroneously but with high confidence reported what they expected the location of the target shape to be, which means that their memory report was strongly influenced by expectations. 

The experiments show that expectations can reshape perceptual representations over short time scales. This leads to short-term memory illusions. These refer to the inaccurate recollections of events that occurred a few seconds ago. 

According to the study, short-term memory illusions appeared when the participants saw a memory display which contained real and pseudo-letters or mirrored letters. 

A substantial increase in high confidence memory errors was observed within seconds after the memory display disappeared. This means that after the memory display was removed, a large number of participants wrongly recollected the location of a target letter, but were highly confident that their memory was accurate. 

According to the study authors, the increase in errors over time indicates that the high confidence errors do not purely result from an incorrect perception of the memory display, but due to mistakes in their short-term memory.

The authors concluded that prior expectations can shape memory traces, both short-term and long-term.

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