Neuroscientists Identify Brain Circuit Necessary For Memory Formation
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When we visit a friend or go to the beach, our brain stores a short-term memory of the experience in a part of the brain called the hippocampus. Those memories are later consolidated that is, transferred to another part of the brain for longer-term storage.
A new MIT study of the neural circuits that underlie this process reveals, for the first time, that memories are actually formed simultaneously in the hippocampus and the long-term storage location in the brains cortex. However, the long-term memories remain silent for about two weeks before reaching a mature state.
This and other findings in this paper provide a comprehensive circuit mechanism for consolidation of memory, says Susumu Tonegawa, the Picower Professor of Biology and Neuroscience, the director of the RIKEN-MIT Center for Neural Circuit Genetics at the Picower Institute for Learning and Memory, and the studys senior author.
The findings, which appear in Science on April 6, may force some revision of the dominant models of how memory consolidation occurs, the researchers say.
What Are The Stages Of Information Processing
Now that you know even more about the stages of memory and understanding how your brain categorizes things, you can begin to understand the actual process of bringing that information from the hippocampus to the surface of your brain, where you are actively remembering an event.
The first stage of information processing is known as the attending stage, and this is where you are actively participating in the event that you will remember. You are physically experiencing the senses, the events, and everything that is happening. By focusing on the event at hand, you are preparing your brain to create a memory for it, and this begins the first stage of information processing.
While you are participating in the event that is going to be remembered, you move on to the encoding stage. This stage happens while you are experiencing the event, and it can be equated to just taking notes on the event. When you place enough importance on something to pay attention to it, your brain will start encoding it into your memory. If you dont pay attention to an event, your brain will not bother encoding it, even if you are attending the event, which is why teachers put such an emphasis on paying attention in class.
The Locus Of The Long
Overall, the results described to this point would seem to demonstrate conclusively that the cerebellum is necessary for learning, retention, and expression of classical conditioning of the eyeblink and other discrete responses. The next and more critical issue concerns the locus of the memory traces. Evidence summarized below would seem to demonstrate conclusively that the long-term memory traces for this type of learning are formed and stored in the cerebellum.
We and our associates have developed a new approach to the problem of localizing memory traces in the brain, namely the use of methods of reversible inactivation, together with recording of neuronal activity. Reversible inactivation methods ,per se, have existed for some time and have been used very effectively to produce temporary lesions . What we have done is to apply this method systematically to the major structures and pathways in the cerebellarbrain stem circuit we have identified as the essential circuit for classical conditioning of discrete responses , during performance and during acquisition of the CR .
Inactivation of the magnocellular red nucleus is indicated in Fig.b. Inactivation by low doses of muscimol for 6 days of training or cooling for 5 days completely prevented the expression of the CR. Yet animals showed asymptotic learned performance of the CR from the beginning of postinactivation training .
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Where Do Memories Form And How Do We Know
Without memories, wed be lost. Theyre the threads that hold our lives together, connecting who we were to who we are.
But weve only recently pieced together the extraordinary brain science behind them a story that takes in amnesiacs, mind palaces and ghostly carnivals.
One of our first analogies for understanding memory comes from Ancient Greece, where Plato likened memories to etchings on a wax tablet, and his favourite student, Aristotle, continued to use this in his own writings.
Forgetfulness, said Aristotle, occurred in childhood because the wax was too soft, and in the elderly because it was too hard.
For him, memories were not located in the brain, but throughout the body. He thought that the brain was present merely to cool the hot heart the seat of our soul.
The School of Athens
A tendency to favour the heart over the brain continued for centuries in part because of the Churchs ban on dissecting the human brain. In fact, it wasnt until the 17th Century that people began to realise that the brain had any capacity for thought at all.
It was German philosopher Hermann Ebbinghaus who pioneered the first scientific study of memory in the late 19th Century. He cared less for where memories lay in the brain, and more for how memory works.
He also classified three types of memory in psychology: sensory memory, short-term memory and long-term memory labels that are still used today.
Putative Mechanisms Of Memory Storage In The Cerebellum
Classic theories of the cerebellum as a learning machine proposed that conjoint activation of Purkinje neurons by parallel fibers and climbing fibers would lead to alterations in synaptic efficacy of the parallel fiber synapses. Ito discovered that such conjoint activation led to a long-lasting depression of parallel fiber synaptic efficacy on Purkinje neuron dendrites, the process of cerebellar long-term depression . He and his associates developed considerable evidence that such a process plays a key role in adaptation of the vestibulo-ocular reflex .
In eyeblink conditioning, many of the Purkinje neurons that exhibit learning-related changes show decreases in simple spike responses in the CS period , consistent with a mechanism of LTD . Current evidence suggests that glutamate activation of AMPA and metabotropic receptors on Purkinje neuron dendrites together with increased intracellular calcium yields the persisting decrease in AMPA receptor function at parallel fiber synapses on Purkinje neuron dendrites that produces LTD .
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Storeroom As An Analogy
We can understand how the human brain stores information by using a simple analogy. Our brain is like a storeroom when we consider memory storage. Like in a store you want to keep the important things on the shelf because in that way they will become easily accessible. Similarly, our brain keeps important things on the surface levels of memory storage. You pay attention to the things that you like, that is why subjects of your interest are easier for you to remember as compared to the things which you do not like.
That is why some people believe that it helps to organize your mind by yourself, by trying to remember only the things which are important and matter to you. In this way, the desired information will always be available on the shelf, just like the sugar you want for your tea.
Learning Recalling And Thinking
The brain regulates an array of functions necessary to survival: the action of our five senses, the continuous monitoring of the spatial surround, contraction and relaxation of the digestive muscles, the rhythms of breathing and a regular heartbeat. As the vital functions maintain their steady course without our conscious exertion, we are accustomed to consider the brain as preeminently the organ of thought. The brain houses our mind and our memories, and we rely on its information-processing capacities when we set out to learn something new.
But where in the brain can we locate memory or thought itself? offered some clues about the ways scientific investigationfrom the molecular level to studies of the alert, behaving animalhas begun to define in physical terms an abstract quality such as “attention.” Similar techniques and approaches are being applied to other mental functions, too, even those as seemingly intangible as learning, remembering, or thinking about the outside world.
Learning and memory, which for many years were considered central problems in psychology, the social sciences, and philosophy, have recently assumed greater importance in the area of neurobiology, itself a confluence of several lines of investigation.
Most available evidence suggests that the functions of memory are carried out by the hippocampus and other related structures in the temporal lobe.
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What Is Information Processing And How Does It Work
Now that you know a little bit more about what memories are and what two of the most important types of memories are, you now need to understand a little bit about information processing. Information processing, to put it simply, is the way the brain processes and works with information. This includes retrieving memories from the hippocampus or the prefrontal lobe. There are two things to remember about information processing. First, there are three stages of memorizing something. Secondly, there are four steps of the actual information processing stage.
The three stages of memory are the motions your brain goes through to bring a memory to the surface of your mind. Now that you know about the different types of memories, you can begin to put this together. First things first, theres a good chance that a sensory memory will be your trigger. Whether it is a sight, sound, feeling, or smell that you experience, this will likely be the first part, or the trigger, to remembering something. Once this happens, the short-term memory plays a role. While it only holds information for a certain amount of time, it is useful when there is already a pre-established connection in the long-term memory department. When something is memorized enough, it will become a physical part of the hippocampus, becoming a long-term memory. There is no limit to how many long-term memories that you can have.
How Memories Are Formed Stored And Recalled
Since the 1940s scientists have surmised that memories are held within groups of neurons, or nerve cells, called cell assemblies. Those interconnected cells fire as a group in response to a specific stimulus, whether it’s your friend’s face or the smell of freshly baked bread. The more the neurons fire together, the more the cells’ interconnections strengthen. That way, when a future stimulus triggers the cells, it’s more likely that the whole assembly fires. The nerves’ collective activity transcribes what we experience as a memory. Scientists are still working through the details of how it works.
For a short-term memory to become a long-term memory, it must be strengthened for long-term storage, a process called memory consolidation. Consolidation is thought to take place by several processes. One, called long-term potentiation, consists of individual nerves modifying themselves to grow and talk to their neighboring nerves differently. That remodeling alters the nerves’ connections in the long term, which stabilizes the memory. All animals that have long-term memories use this same basic cellular machinery; scientists worked out the details of long-term potentiation . However, not all long-term memories necessarily have to start as short-term memories.
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Foraging For Froot Loops
To study a complex human behavior, such as remembering appropriate information at the right time, Eichenbaum had to train rats to memorize an important piece of information and then find a way for them to use it. So his team trained rats to find Froot Loops in flowerpots. Rats are absolutely nuts about Froot Loops, he says.
For example, the rats learned that in room A the cereal is hidden in a pot filled with purple plastic beads that smell sweet. But in room B, the goods are in the pot filled with black paper shreds that smell spicy. Rats are great with odors and textures, so were using textural and olfactory cues to direct them to express their memory, says Eichenbaum.
As the rats navigate from room to room, Eichenbaums team records their brain activity using electrodes inserted into the brain. They monitor both the hippocampus, known to be the seat of memory in the brain, and the prefrontal cortex, thought to be a coordinator.
For instance, when the rat enters room A, the ventral hippocampus transmits to the prefrontal cortex, setting the context to room A. The dorsal hippocampus begins firing as it recognizes flowerpots. The prefrontal cortex, which knows that the reward in room A is in the pot with purple beads, sends this information to the dorsal hippocampus, telling it which memory to act on. The two regions operate together as a system, kind of like handshaking, says Eichenbaum. Were seeing at the level of neurons what happens in cognitive life.
What Exactly Are Memories And How Does Storing Them Work
To understand how the brain stores a memory, you first have to understand how memories work. For example, no memory can exist solely on its own. Memories are broken down and sorted into windows of time. This is one of the reasons why it is harder to remember things that happened long ago, but when you start thinking about it, you begin to remember a lot of things that happened from that time period.
There are also several different types of memories. There are sensory memories, short-term memories, and long-term memories. Most people are familiar with the concept of short-term and long-term memories. Short-term memory typically involves remembering things such as your hotel room number when you are travelling and memorizing a shopping list. Long-term memories, on the other hand, are technically short-term memories that have been moved to a deeper part of the brain. Here, in the hippocampus, memories are condensed into more of an episodic-like format where you experience the whole event, rather than the specific sights, smells, and sounds. These are the two types of memories that you will need to know about if you want to understand how the brain stores memories.
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Specific Neurons That Map Memories Now Identified In The Human Brain
- Columbia University School of Engineering and Applied Science
- Neuroengineers have found the first evidence that individual neurons in the human brain target specific memories during recall. They studied recordings in neurosurgical patients who had electrodes implanted in their brains and examined how the patients’ brain signals corresponded to their behavior while performing a virtual-reality object-location memory task. The researchers identified ‘memory-trace cells’ whose activity was spatially tuned to the location where subjects remembered encountering specific objects.
An important aspect of human memory is our ability to conjure specific moments from the vast array of experiences that have occurred in any given setting. For example, if asked to recommend a tourist itinerary for a city you have visited many times, your brain somehow enables you to selectively recall and distinguish specific memories from your different trips to provide an answer.
The team measured the activity of neurons as the patients moved through the environment and marked their memory targets. Initially, they identified purely spatially tuned neurons similar to “place cells” that always activated when patients moved through specific locations, regardless of the subjects’ memory target. “These neurons seemed only to care about the person’s spatial location, like a pure GPS,” says Salman E. Qasim, Jacobs’ PhD student and lead author of the study.
Neurons That Fire Together Wire Together
Back in 1906, Camillo Golgi and Santiago Ramón y Cajal had been jointly awarded a Nobel Prize for advances in cell-staining techniques that demonstrated the anatomy of a neuron.
Thanks to their work, scientists knew there were millions of neurons in the brain that pass messages to each other in the form of electrical impulses. When an impulse reaches the end of one neuron, it causes the release of chemical messengers called neurotransmitters, which pass across the gap, or synapse, and latch onto a neighbouring neuron.
This makes the second neuron more or less likely to fire its own impulse. But how these neurons formed long-term memories was still a mystery
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That remained the case until 1949, when Donald Hebb published one of the most influential theories of neuroscience in the last century. He wrote that any two brain cells that are repeatedly active at the same time will tend to become associated.
Their anatomy and physiology will change so that they form new connections or strengthen existing ones. The activity in one, he said, will subsequently facilitate activity in the other. Youll often find this summarised as neurons that fire together, wire together.
Simply put, if two concepts, say the smell of a rose and its name, repeatedly stimulate their respecting neurons in the brain at the same time, those neurons will change shape and strengthen that connection.
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The Hippocampus And Long
A short-term memory can be consolidated into an enduring long-term memory. This involves a system of brain structures within the medial temporal lobe that are essential for forming declarative memories. The hippocampus is a key region in the medial temporal lobe, and processing information through the hippocampus is necessary for the short-term memory to be encoded into a long-term memory.
The long-term memory does not remain stored permanently in the hippocampus. These long-term memories are important and having them stored in only one brain location is risky damage to that area would result in the loss of all of our memories.
Instead, it is proposed that long-term memories become integrated into the cerebral cortex . This process is referred to as cortical integration; it protects the information stored in the brain.
However, damage to areas of the brain, particularly the hippocampus, results in loss of declarative memories, which is known as amnesia.
The famous case study of H.M. – Henry Molaison – demonstrated the hippocampus is vital to the formation of long-term memories. H.M. had his hippocampus removed as a 23-year-old in an attempt to treat epileptic seizures that originated in his medial temporal lobe.