What are memories? Memory is the state or process of sequential encoding, storage, and retrieval ¹. The brain processes information learned (encoding), converts this information into a memory bank (warehouse) so that the individual can later pull information from this storage unit. Endel Tulving, a historical figure in describing memory, suggested similar and contrasting differences between two types of memory: episodic and semantic. Episodic memory stores the personal recollection of events at a specific time and place. Semantic memory stores factual information about the world and language without referencing when it was learned ². Figure 1 depicts the concept by Bird and Burgess (2008) ³ that the hippocampus is vital for long-term episodic memory. An example of episodic memory could be hitting a baseball for the first time.

In contrast, semantic memory could be that Major League Baseball (MLB) is the professional league for baseball players. Episodic memories can be subcategorized into flashbulb memories, memories of vivid moments that are emotionally linked ⁴. Memory can also be distinguished into limited immediate information, short-term memory, and long-lasting information, long-term memory, as evidenced by amnesia studies ⁵.

Short-term memory has a limited storage space that simultaneously holds 7 to 12 pieces of information ⁷. Items in the short-term memory bank disappear unless the information is repeated enough to make a more permanent form. One of the forms of short-term memory is working memory, which keeps track of information long enough to use in a task that occurs after the information has been acquired ⁸. In contrast, long-term memory is a storage area that can hold significantly more information. Long-term memory can be reflexive and declarative ⁹. Involuntary memory is automatic and does not require consciousness for recall or creation. It usually involves the amygdala and cerebellum ¹⁰. Declarative memory requires conscious attention for remembrance, as the memories can be reported verbally. When declarative memory turns into reflexive memory, this becomes muscle memory ¹¹.

But what happens if we are unable to form new memories? Memory loss is the inability to store or process new memories. Bilateral damage to either of the hippocampus regions of the brain makes it challenging to establish new memories (anterograde amnesia) and retrieve memories before the onset of amnesia (retrograde amnesia) ¹². The longer the period of amnesia, the higher the chance of permanent impairment of brain functioning ¹³. Memory loss tends to be associated with the older population but can also occur in younger people ¹⁴. In addition, various conditions and behaviors can lead to memory loss, including drugs and alcohol ¹⁵.

There are two types of memory loss: partial and complete. Partial memory loss is limited to events immediately before or after a traumatic event. In contrast, total memory loss, as described by amnesia, is the complete loss of memories from a certain period of time ¹⁶. Furthermore, memory loss can also be permanent or temporary (vacillate). Vacillate memory loss occurs when a person slips in and out of being able to remember events accurately ¹⁷. Finally, there are multiple forms of memory loss, such as benign senescent forgetfulness, dementia, multiple infarct dementia, depression, and head trauma.

Benign senescent forgetfulness is when memory is affected by most recent events, although this type of memory loss does not affect an individual’s social or professional life ¹⁸. One of the features of benign forgetfulness is its selectivity. It affects unimportant facts, such as forgetting one’s wallet at home. In addition, an individual with mild forgetfulness is aware of their deficit and often uses notes as a reminder ¹⁹.

Dementia is another form of memory impairment that includes memory loss and other problems like language deficits, lack of visual-spatial skills, and impaired judgment ²⁰. Dementia’s memory impairment affects the brain and interferes with an individual’s ability to have a social or professional life. One of the key facts about dementia is that there is only awareness of the disease in the earliest stage ²¹. As dementia progresses, disorientation about the time of day further affects a person’s environment. One of the critical differences between dementia and benign forgetfulness is that individuals with early dementia may write themselves notes. Still, they may misinterpret the information or forget to check the reminder ²². An example of dementia would be an individual who needs to pick up groceries and will write a note as a reminder to pick up groceries. However, this individual would continuously pick up groceries because they may have forgotten that this task has already been fulfilled. In addition, people with dementia are often disoriented by their time and environment, leading them not to recognize other people, especially people they should know. This can be difficult for families and caregivers ²³. In the latest stages of dementia, a person may not even recognize their own reflection.

One of the most common forms of dementia is Alzheimer’s disease. Laakso et al. (2000) suggest that in individuals with Alzheimer’s, there seems to be an accelerated rate of hippocampal volume loss ²⁴. However, when this study observed hippocampal loss after the three-year follow-up, there was no improvement in diagnostic accuracy for patients having Alzheimer’s ²⁵. As seen in Figure 2, a brain affected by Alzheimer’s, has a shriveled-up cortex.  This damages the neuronal connections typically, allowing neurons to travel and send signals throughout the brain.

Another form of dementia is multiple infarct dementia, caused by the destruction of brain cells from repeated strokes ²⁷. Repeated strokes lead to significant brain tissue damage and cause symptoms of dementia. In addition, these minor strokes can make the individual or their relatives unaware of when the disease strikes ²⁸. Multiple infarct dementia is also associated with paralysis or weakness in various body parts, like the arms, legs, and face. The progression of multiple infarcts dementia is unique because it occurs in steps ²⁹. Every time a stroke occurs, the individual’s condition increases memory loss ³⁰. The key takeaway about multiple infarct dementia is that it rarely affects the memory center, and if a patient’s memory is involved, it is due to amnesia ³¹. Multiple infarct dementia only occurs if repetitive strokes progress to brain damage. AD and multiple infarct dementia can happen to the same person, but these conditions require different treatments ³².

Depression can cause memory impairment due to problems that relate to attention and concentration ³³. Depression correlates with dementia, making differentiating the two memory impairments difficult. One of the primary differences between depression and dementia is that people with depression are almost always aware of their memory deficit and are moderately distressed ³⁴. People with dementia are unconscious of their memory deficit and not in distress, except in the early stages of dementia ³⁵. Characteristics of people with dementia include their ability to express stable emotions compared to people with depression who lack enthusiasm, interest, and concern. Individuals suffering from depression are often withdrawn and disturbed ³⁶. Anxiety can also lead to distorted memory. Distorted memory can confuse individuals to the point where they are unsure what is real and what their brain has conjured ³⁷.

Head trauma can also lead to amnesia if exposed to repeated head injuries ³⁸. In younger people, memory loss is often associated with repeated trauma to the brain (i.e., concussions) because of physical activities (i.e., sports). Honig et al. (2021) found that repetitive concussive and sub-concussive trauma can lead to long-term processes of neurodegeneration and memory decline ³⁹. The study also found that repeated concussive and sub-concussive head trauma can lead to a neuronal loss on the dentate gyrus and CA1—a subfield of the hippocampus. In addition, the repeatedly concussive mice in the study showed vascular abnormalities in their dorsal hippocampi. Each of the aforementioned forms of memory loss is associated with the hippocampus and other structures of memory, such as olfactory and gustatory memory.

The olfactory system (sense of smell) has a multitude of functions in physiological regulation, emotional responses (i.e., anxiety, fear, pleasure), reproductive functions (i.e., sexual and maternal behaviors), and social behaviors (i.e., recognition of conspecifics, family, clan, or outsiders) ⁴⁰. Interestingly, the olfactory system has its own distinct type of memory known as olfactory memory. An example of olfactory memory in action is when an individual smells something familiar, which reminds them of a specific memory. Semantic memory within the olfactory system refers to an individual’s knowledge or experience with a particular odorant ⁴¹. Odor identification and odor recognition describe odor memory ⁴². For instance, olfactory memory is subjected to familiarity (e.g., This smell is familiar) or hedonic qualities (e.g., This smell is fantastic) ⁴³. Episodic memory in the olfactory system occurs when an individual is exposed to various odors and asked to recognize the odors at a later time ⁴⁴. Classical conditioning can play a role in the olfactory system by pairing an unconditioned stimulus (a specific smell) with a conditioned stimulus (a particular sound) to produce a conditioned response. The conditioned response is that whenever the sound is heard, and the scent is smelled, this could trigger a reaction like hunger. Bahuleyan & Singh (2012) suggests that how smells are perceived, stored in memory, and recalled years later, has not been fully understood ⁴⁵. Long-term memory of the olfactory system requires the release of glutamate and norepinephrine—two neurotransmitters that send messages in the brain and can be linked to memory loss. Why there is, a loss of olfactory memory in the brain is unknown at this time. There is limited research on the correlation between the hippocampus and the olfactory system. Future studies should look into how these parts of the brain are connected regarding memory. It can be suggested that memory loss or any neurodegenerative diseases, like Alzheimer’s, cause neurons to diminish, become plaques, and decrease brain size—leading to a decline in neuronal connection. The decline in the neuronal connection may explain why the olfactory system is affected by memory loss since the hippocampus is not processing the information necessary for smell and memory.

For humans and other large land organisms, the gustatory system (sense of taste) records sensations that begin with stimulation from food or other substances ⁴⁶. Humans most commonly recognize four preferences through the tongue’s taste buds (Figure 3): salty, bitter, sweet, and sour ⁴⁷. Usually, the sweetness comes from sugars, saltiness from the table and mineral salts, bitterness from various factors, and sourness from acids ⁴⁸. As seen in Figure 1, the gustatory system has direct projections to the amygdala, one of the essential structures for memory ⁴⁹. Zald et al. (1998) found that exposure to an aversive gustatory stimulus activates the limbic system structures (amygdala, cingulate gyrus, hippocampus) ⁵⁰. Therefore, lesioning in any of the limbic structures could lead to the inability to recognize the taste of foods. The neural processing of regulating the detection of flavors and initiation of automatic feeding is understood better than the mechanisms of taste memory ⁵¹. Resembling olfactory memory, there is currently limited research on how the gustatory system is directly related to memory or memory loss.

Memories are a simple yet complex part of the human brain. Episodic and semantic memories provide insight into personal and environmental memories that can either be short- or long-term. Short-term memories are immediate and have limited storage area, while long-term memories can hold much more information and are long-lasting. However, memory loss can result in the inability to store or process new information in both short- and long-term memory systems. Memory loss has multiple forms and causes, such as benign forgetfulness, dementia, multiple infarct dementia, depression, and repeated head trauma. These forms of memory loss significantly impact individuals and the people closest to them. While memories in the hippocampus are associated the most with memory loss, other conditions of memory also exist. For example, Olfactory and gustatory memory have been explored limitedly. Whenever an individual taste or smell something familiar, the olfactory and gustatory systems are linked to memory. Overall, memories are an essential aspect of the human brain that requires exploration now and into the future.

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