Memory works in strange ways. We remember some places, events and things, but not others. Why do we particularly remember the high points of experience? Scientists say they have an explanation. According to a new research from the University of California, Davis, Center for Neuroscience, our brains prioritize rewarding memories over others, and reinforce them by replaying them when we are at rest. Using functional MRI, UC Davis neuroscientists were able to identify a signal in the hippocampus associated with a rewarding memory.
The study was conducted by Professor Charan Ranganath, a neuroscientist at the University of California, Davis and Matthias Gruber, a postdoctoral researcher at UC-Davis, reportsScienceDaily.
Professor Charan Ranganath explained:
"Rewards help you remember things, because you want future rewards. The brain prioritizes memories that are going to be useful for future decisions."
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How Memory Works?
Memory is the process through which information is encoded, stored, and retrieved. We cannot work in the present or ponder about the future, if we don’t have memory of the past. Without memory we would not be able to remember what we did yesterday, what we have done today or what we plan to do tomorrow. And of course, we could not learn anything without memory. Memory processes vast amounts of information in our brains through different forms such as sounds, images or meaning.
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Let’s take a look at how memory works. As mentioned earlier – there are 3 phases of memory:
1. Encoding. When information travels through our sensory input into our memory system, it needs to be altered into a form that the system can handle, so that it can be stored. The process is more like changing our money into a different currency when we travel to a foreign country. For example, a word which is seen (in a book) may be stored if it is changed (encoded) into a sound or a meaning (i.e. semantic processing). There are 3 main methods in which information can be encoded – Visual (picture), Acoustic (sound) and Semantic (meaning).
2. Memory storage. There has been a significant amount of research regarding the differences between Short-Term Memory (STM) and Long-Term Memory (LTM). Most adults can store between 5 and 9 items in their short-term memory. Cognitive psychologist George A. Miller in his 1956 experiment put this idea forward and he called it the magic number 7. Information can only be stored for a brief duration in STM (0-30 seconds), but LTM can last a lifetime.
3. Memory Retrieval. If we can’t remember something, it may be because we are unable to retrieve it. When we are asked to retrieve something from memory, the differences between STM and LTM become very clear. STM is stored and retrieved sequentially. For example, if a group of participants are given a list of words to remember, and then asked to recall the fourth word on the list, participants go through the list in the order they heard it in order to retrieve the information. LTM is stored and retrieved by association. This is why you can remember what you went upstairs for if you go back to the room where you first thought about it. Organizing information can help aid retrieval. We can take look at an example: A doctor is prescribing various pills to a patient which should be taken at various times of the day. If the doctor gives these instructions in the order which they must be carried out throughout the day (i.e. in sequence of time), this will help the patient remember them.
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Ranganath/Gruber study used functional magnetic resonance (fMRI) imaging to scan the brains of participants as they answered yes/no questions regarding a short series of objects (eg: "do these objects weigh more than a basketball?"), including questions regarding their weight in comparison to each other. Each series was shown on a background image to give them context and the subjects were offered rewards, either large (in dollars) or small (in cents) for giving correct answers.
After the tests, the subjects were scanned during a resting period. Afterwards, there was a surprise memory test for all of the objects shown during the initial experiment.
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Although participants were not expecting the memory test outside the scanner, they were better at remembering objects that were associated with a high reward. Matthias Gruber, first author of the study explained:
"Also, when an object was associated with high reward, people remembered better the particular background scene that was on the screen during scanning.”
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Moreover, the brain activity observed during the fMRI scans during rest was able to predict memory performance - the brain scans of subjects at rest after the initial test was the same pattern of activity that was observed during the high-reward task. This implies that the subjects were apparently replaying the rewarding memories, strengthening connections and helping to fix the memory in place.
Better retention of these events was observed in people who showed more replay of high-reward memories.
According to some experts, these interactions were likelyrelated to release of dopamine, a neurotransmitter that is released in the brain when we expect rewards. Conditions such as Parkinson's disease or ageing often involve memory defects because they are linked to reduced dopamine.
The findings show how memory could be biased toward the high points of experience. Prof. Ranganath concluded:
"It speaks to a memory process that is normally hidden from us. Are you remembering what you really need to know? It could depend on what your brain does while you are at rest."
The study was published in the journal Neuron.
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