Memory is an exciting and still mysterious function, and if we can easily find places of memory, do we know where in the brain our memory is stored?
We have seen it, it is plural. We cannot therefore associate it with a single place. There is, however, a place of "passage": the seahorse, involved through the circuit known as Papez. And cerebral imaging techniques make it possible to identify some structures particularly involved in memory processes, including the hippocampus, but also the amygdala and the prefrontal cortex. They operate in systems that are constantly interacting, and are linked to our emotions: there is no memory that is not linked to the emotional context in which it was recorded.
The fleeting memory of the sensory organs:
Our brain is the strategic center of memory circuits. Our five senses are constantly called upon to send their information to the hippocampus. These pass through the filter of the emotional brain, or limbic system, to be projected to the areas of the cerebral cortex that are dedicated to them. However, in the very short term, various sensory memories (visual, olfactory, tactile, taste, auditory) are managed by the sensory organs.
Thus, the back of the retina is involved in the phenomenon of retinal persistence, retaining information for a few hundred milliseconds, when the auditory nerve and the cells at the back of the ear retain over this same period of time the memory of a sound. . The same goes for the other three senses, always with this principle: withhold information for the time necessary to select the important elements. Knowing that this filtering takes place upstream of the short-term memory, to capture and encode a “good” message, it is therefore important to correct any sensory deficit. As for short and long term memories, they are fully taken care of by the brain.
There are several arguments in favor of an important role of a region located at the front of the brain - the prefrontal cortex - in short-term memory. In particular, when it comes to keeping the data necessary for reasoning available. As for the encoding of information in long-term memory, it involves the hippocampus, the temporal lobes and the structures of the limbic system which are connected to them.
The seahorse: an obligatory crossroads:
Made up of different structures buried deep in the center of the brain, the limbic system plays an important role in fear, pleasure, pain, aggression, and other emotional behaviors. But several of its components also participate in the memorization of certain memories. This is particularly the case with two structures: the hippocampus, and the amygdala.
The hippocampus is made up of several layers of neurons. It is greatly involved in the consolidation of long-term memory, in particular in a type of memory that can be brought out through language (it is said to be declarative), and more precisely that of facts and places (called episodic, because keeping traces of episodes of life). Its role is such that it is carefully examined on MRI scans in Alzheimer's disease. Because a radiological sign of atrophy of the hippocampi, linked to Alzheimer's disease, is visible on MRI and thus allows us to grade the course of the disease.
Memories of various and varied events are however not stored in the hippocampus: the information only passes through this structure, as in a crossroads where they are associated before reaching other areas of the cerebral cortex (occipital region for visual memories, temporal for auditory memories, etc.) where they will be kept. Conversely, our spatial memory is partly archived in the hippocampus.
At the heart of the emotional brain:
Another structure of the limbic system involved in memorization, the amygdala is shaped like a blackberry and is located in front of the hippocampus. Neuroimaging studies have shown that its activity increases when the individual visualizes an emotionally charged scene, and even more so in cases of post-traumatic stress, or fears. But like the hippocampus, it is not affected by any other type of long-term memory, called implicit or procedural.
Unconscious, that is to say automatic, this emotional memory is mobilized the longest. Moreover, this implicit memory is that of know-how - tying a shoe, tying a tie, driving a car, etc. And it is rather associated with changes in the cerebellum (cerebral structure closely related to motor control) or in the basal ganglia (also called basal ganglia) and the motor cortex.
The two hemispheres of the brain:
The cortex, also known as gray matter, is the outermost layer of the brain and includes all other structures. It is made up of two cerebral hemispheres. Separated by a groove, however, they are not isolated and communicate with each other through nerve fibers called the corpus callosum. We know that they are always helping each other and exchanging information. But since the work of the American Roger Sperry (Nobel Prize for physiology or medicine in 1981), it is classic to consider that they each have their specialties - each hemisphere also receives sensory information and controls the motor responses of half opposite of the body.
So when there is a stroke of the left hemisphere, the right side of the body is paralyzed (hemiplegia) and there may be aphasia, which is difficulty speaking or understanding language. The left hemisphere would in fact mainly take care of what is analytical: numbers, calculations and all the processes related to numbering. But he would also be responsible for most of the language processing, both oral and written. Knowledge allows us to stimulate and re-educate speech and memory through speech therapy, for example when a patient suffers from particular forms of Alzheimer's, or when he presents vascular signs in addition to neurodegeneration.
The right hemisphere, on the other hand, would be more involved in the management of novelty, learning, visuospatial skills, the perception of faces or the understanding of common expressions, metaphors or innuendo. This can make it difficult to organize your bedroom, write essays or analyze an abstract art painting in the event of an injury in the right hemisphere. Ultimately, if locating memory remains a matter of research, knowing the circuits involved helps us better understand and deal with its failures.
You are thinking anatomically and not physiologically in terms of connectivities and how that might play out with wave generation. The application of continuum theory might be a more effective approach in understanding possible mechanisms.
For example, in the hippocampus, physiological data show that, as predicted by Hebb, excitatory synapses between nearby excitatory cells are bound by simultaneous activation. In contrast with this local process, the preponderance of clinical and experimental evidence indicates that cortical recall of a “memory” involves uniting fragments stored in synaptically distant brain regions. A mathematical model of memory, and therefore of thought, must resolve this seeming contradiction as well as explain how many different memories and “thoughts” can be assembled within a given anatomical area. We have determined that growing and propagating waves of neural activity are the natural resonant modes of synaptic energy. In a layered geometry typifying the mammalian cortex, a time delay (T) in inter-layer signals effectively controls the waves temporal and spatial frequency. As a function of T, two very different types of wave can grow from pervasive noise. One is coherent, and its resonant spatial frequency increases with increasing T. However, further increase eventually leads to a discontinuous increase in both wavelength and temporal frequency. The result is a region of T values wherein two waves grow simultaneously and interfere in random fashion. This significant duality, whose origin lies in the phase relations of the amplified waves, leads us to propose that coherent waves are instrumental in the retrieval of memory and random waves embody original thought. Continuum theory, which treats an ensemble of “cell assemblies” or neural networks, we think, allows us to better understand networks of the brain. Deficits in the functioning of the system may also be evaluated potentially by means of “goodness-of-fit” of the clinical and spatially resolved data with the model.
Are memories stored in just one part of the brain, or are they stored in many different parts of the brain? The brain is key to our existence, but there’s a long way to go before neuroscience can truly capture its staggering capacity. For now though, our Brain Control series explores what we do know about the brain’s command of six central functions: language, mood, memory, vision, personality and motor skills – and what happens when things go wrong.
One of the critical functions of the brain is to encode and store information, which becomes our memories. Our memories provide us with insight into events and knowledge of the world around us and influence our actions and behaviors – forming important aspects of our personality.
There are multiple aspects and types of memories. What we usually think of as “memory” in daily usage is actually long-term memory. But there are also important short-term and sensory memory processes, which are required before a long-term memory can be established.
Memory is generally divided into two broad categories: explicit (declarative) and implicit (non-declarative) memory.
Implicit, or non-declarative, memories are behaviors that we have learned, but cannot verbalise. These memories typically operate without conscious awareness, encompassing skills, habits and behaviors.
These behaviors run on auto-pilot – for example, tying your shoelaces. It’s easy to do once learned, but it is very difficult to tell someone how you perform this task. Being able to tie your shoelaces is an implicit memory.
Multiple areas of the brain form implicit memories as they involve a variety of responses to be coordinated. A key region of the brain called the basal ganglia is involved in the formation of these “motor” programs. Additionally, the cerebellum at the back of the skull plays a vital role in the timing and execution of learned, skilled motor movement.
Explicit, or declarative, memories can verbally expressed. These include memories of facts and events, and spatial memories of locations. These memories can be consciously recalled and can be autobiographical – for instance, what you did for your last birthday – or conceptual, such as learning information for an exam.
These memories are easy to acquire. However, they are also easy to forget as they are susceptible to disruption during the process of forming and storing the information.
The prefrontal cortex and working memory. The prefrontal cortex is important in the formation of short-term or working memory. Although these short-term memories are lost due to interference with new incoming information, they are essential for planning behaviors and deciding what actions to perform based on the current situation.
A short-term memory can 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.
Are memories stored in just one part of the brain, or are they stored in many different parts of the brain? Karl Lashley began exploring this problem, about 100 years ago, by making lesions in the brains of animals such as rats and monkeys. He was searching for evidence of the engram: the group of neurons that serve as the “physical representation of memory” (Josselyn, 2010). First, Lashley (1950) trained rats to find their way through a maze. Then, he used the tools available at the time—in this case a soldering iron—to create lesions in the rats’ brains, specifically in the cerebral cortex. He did this because he was trying to erase the engram, or the original memory trace that the rats had of the maze.
Lashley did not find evidence of the engram, and the rats were still able to find their way through the maze, regardless of the size or location of the lesion. Based on his creation of lesions and the animals’ reaction, he formulated the equipotentiality hypothesis: if part of one area of the brain involved in memory is damaged, another part of the same area can take over that memory function (Lashley, 1950). Although Lashley’s early work did not confirm the existence of the engram, modern psychologists are making progress locating it. Eric Kandel, for example, spent decades working on the synapse, the basic structure of the brain, and its role in controlling the flow of information through neural circuits needed to store memories (Mayford, Siegelbaum, & Kandel, 2012).
Many scientists believe that the entire brain is involved with memory. However, since Lashley’s research, other scientists have been able to look more closely at the brain and memory. They have argued that memory is located in specific parts of the brain, and specific neurons can be recognized for their involvement in forming memories. The main parts of the brain involved with memory are the amygdala, the hippocampus, the cerebellum, and the prefrontal cortex.
The amygdala is involved in fear and fear memories. The hippocampus is associated with declarative and episodic memory as well as recognition memory. The cerebellum plays a role in processing procedural memories, such as how to play the piano. The prefrontal cortex appears to be involved in remembering semantic tasks.
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