H.M. developed severe amnesia (memory loss) at the age of 27 after undergoing brain surgery. His surgery involved the removal of large parts of the hippocampus on both sides of his brain. After which he was almost entirely unable to store any new information in his long-term memory. The extremity of his deficits was a surprise to experts since many of them believed that memory was distributed throughout the cerebral cortex.
The above case study was a revelation of its time. It hinted at the role of specific brain structures for memory formation and decay. It also paved the way for questions like what is memory? How does removing a brain region affect memory? What are the different types of memory? Though scientists are still trying to reach definite conclusions on various memory-related aspects, the current blog aims to make the readers aware of the basic concept of memory as per the current scientific understanding.
Memory is an integral part of human cognition. It allows individuals to recall past events to act in the present. It also gives them a framework to make sense of the present and future. Additionally, learning and memory operate together to increase our ability to perceive the environment around us, thus making learning a prerequisite to memory.
Memory is defined utilizing its process, storage, and capacity. According to Margaret W Matlin, “Memory is the process of maintaining information over time.” Various parts of the brain come into play in creating, storing, and recalling memories.
There exist different theories for explaining memory scientifically.
Tulving’s Model distinguishes between the two kinds of memory: Episodic and Semantic. Where episodic is like remembering details from your past, semantic is remembering general information.
Baddeley’s Model refers to working memory as having multiple components: a central executive acting as a supervisory system and controls flow of information between other parts that deal with verbal and visuo-spatial data.
Network processing model refers to memory as a result of the strength of neural networks.
The expansion in understanding of Memory has resulted in several classifications.
A common notion of classifying memory is based on its duration. Accordingly, it is divided into sensory memory, short term memory, and long-term memory.
Sensory memory refers to memory associated with senses for very short durations of time. The duration lasts for less than a second, but has high capacity to store information. Information received through sense organs is called Iconic, Echoic, and Haptic memory.
Iconic Memory refers to immediate visual memories. This is how the brain remembers an image you just saw moments ago.
Echoic Memory is also known as auditory sensory memory. The duration of echoic memories is slightly longer than iconic memories. Examples include remembering what one just said to you (voice playing in the head).
Haptic memory is the memory of touch. If you run your hand over any fruit, you’ll remember the exact sensation you felt for a few seconds. After that, the memory needs to be encoded into short-term memory for later recall.
Next, we have short-term memory (STM) with duration 20-30 seconds, sometimes also called working memory. The information in STM can be transferred to long term memory (LTM) through rehearsal. Working memory, which lasts between several seconds to many minutes, is often generalized as a lively and constant ‘refreshing’ of some experience.
Then we have long term memory, which is relatively permanent with unlimited capacity. It is slightly more sophisticated, as it involves a period of consolidation and a series of crucial neurochemical changes. There are many different forms of long-term memories. Sometimes they’re conscious, requiring us to recall a piece of information actively. Other times they’re unconscious, only appearing without an active attempt at the recollection, like remembering the route from home to work without actively thinking about it.
Let us now explore a little bit more of the Long term memory.
They are classified based on the type of information stored with examples.
Types of long term memory based on kind of information
Explicit Memory is also referred to as conscious memory. It oversees the voluntary recall of information. For example, every time Sam fills out a college application, he must consciously recall his full address, telephone number, and social security number. It is further divided into semantic and episodic memory.
Semantic memory is called the memory of facts. For example, semantic memory is responsible behind Sam remembering his mom’s birthday.
Episodic memory is the memory of specific context, including location or time. It is associated with emotions and has personal association of a particular place or time. With episodic memory we also have autobiographical memory- remembering incidents from one’s own life.
Procedural (Implicit) memory is the memory for skills like driving, tying shoelaces, etc. It is also referred to as “unconscious memory.” It also includes the retention of information from a moment in time that cannot specifically be recalled. For example, Sam’s parents took him on a family vacation to London when he was nine years old. Later, during his college vacation, Sam returned to London with her friends and remembered exactly how to get to London tower.
The next stage of long-term memory is consolidation where the networks are strengthened in the brain, particularly in the Hippocampus before being moved to their final storage destinations in the cortex.
n the case of complex memories, each portion of that memory experience is theorized to be stored within the cortical area related to that modality. Thus, when remembering some events such as riding the train, the sounds one remembers hearing on the train ride is stored in auditory areas, the sights in visual regions, etc.
Now let us explore how different brain regions are associated with memory.
The major regions which are involved with memory are Amygdala, hippocampus, cerebellum, and cortex.
Brain region involved with Memory
The amygdala is an essential structure for the creation and recall of memory. It helps in processing emotional information as well as in memory consolidation; i.e., it helps transfer new learning into long-term memory.
It is engaged in facilitating memories at a deeper level when the event is emotionally aroused, like when associated with joy or anger or sadness, etc. For example, the memory of a child securing first place at school in her 10th grade or memory of a child that faces abuse may leave long-lasting impressions.
The hippocampus is involved in creating new memories, precisely standard recognition memory, also known as spatial memory. It projects information to cortical regions that give memories their meaning and connects them to other bits of data.
Besides, it also plays a main role in memory consolidation. Suppose a person meets with an accident and is injured in the hippocampal. In that case, the power to make new memories will be affected but doesn’t significantly impair their ability to retrieve memories already stored.
THE CEREBELLUM AND PREFRONTAL CORTEX
The cerebellum plays a significant role in implicit memories (procedural memory, motor learning, and classical conditioning). It is responsible for the formation of implicit memories and conditioned responses. It maintains internal representations of the external world, which allow you to navigate through your living room to find your keys in complete darkness.
Prefrontal cortex helps in working memory—for example, holding a friend’s address in mind while listening to instructions about how to get there.
In summary, we explored the basic concepts about memory, its types, about various models that explain different types and processes of memory. At the end we also briefly explored how different brain regions are associated with certain types of memory. In our next blog, we will go through some of the critical process and mechanisms as to how memory works and its applications in the real world for us.
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Author: Jeevitha Ramesh, Research Associate