There are multiple types of memory:
- Episodic: Episodic memories are what most people think of as memory and include information about recent or past events and experiences, such as where you parked your car this morning or the dinner you had with a friend last month. The recollection of experiences is contingent on three steps of memory processing: encoding, consolidation/storage, and retrieval. The hippocampus and surrounding structures in the temporal lobe are important in episodic memory and are part of an important network called the default mode network, which includes several brain areas, including frontal and parietal regions, and has been implicated in episodic memory functioning.
- Semantic: Semantic memory refers to your general knowledge including knowledge of facts. For example, your knowledge of what a car is and how an engine works are examples of semantic memory.
- Remote: The memory of events that occurred in the distant past is a type of episodic memory referred to as remote or long-term memory. The underlying anatomy of remote memory is poorly understood, in part because testing this type of memory must be personalized to a patient’s autobiographical past. What is known is that, like semantic memory, remote memory eventually becomes independent of the hippocampus and appears to be “stored” more broadly in the neocortex. Likely because of this unique neuroanatomy, remote episodic memories do not tend to be as severely disrupted as recent episodic memories in neurodegenerative diseases (e.g., Alzheimer’s disease).
- Working: Working memory is used to describe the process where one “holds on” and manipulates small bits of current information in mind, like a telephone number. Though commonly referred to as short-term memory, working memory is actually more closely related to attention and falls under the domain of executive function. The capacity of our working memory is limited, allowing us to keep only a few bits of information in mind at one time. Working memory involves the frontal cortex and parietal lobe.
Each type uses a different network in the brain, and therefore, one type can be affected by disease or injury while another type functions normally.
Creating a Memory
The initial step in forming an episodic memory is called encoding, which is the process of receiving and registering information. Encoding is necessary for creating memory representations of information or events that you experience. The process of encoding is dependent on you paying attention to an event or information. That is, if you are not paying attention to an event while it is happening because you are distracted, then you are less likely to remember the details of the event. Attention is a necessary component for effectively encoding events or information.
The encoding of episodic memories is also influenced by how you process the event. Encoding information can be strengthened by an elaboration process, which can involve making connections with the information at hand and/or relating the information to your personal experiences. For example, if you were asked to remember and buy ten items at the grocery store, you would likely remember more of the items if you used a strategy of making a mental connection between the items rather than if you were to simply repeat the items a couple of times. Using mnemonics or creating associations between the thing to be remembered and your personal experience can also enhance the encoding of memories. For example, if you were introduced to someone named Charlie, you might make a connection that this is the same name as your uncle as a strategy to help you remember the person’s name. Overall, effective encoding is the initial process necessary for the formation of a new memory.
Memory consolidation, the next step in forming an episodic memory, is the process by which memory traces of encoded information are strengthened, stabilized, and stored to facilitate later retrieval. Consolidation is also most effective when the information being stored can be linked to an existing network of information. It is also strengthened by repeated access to the information to be remembered. The neural pathways from the hippocampus to the cortex underlie the process of consolidation and storage. The number of neurons that are dedicated to a particular memory, as well as the frequency with which they fire together, help to strengthen the memory traces within the cortex. This process of consolidation occurs over the course of days to weeks and is subject to reorganization when new, relevant information is learned. This reorganization assists in the storage of the new information, but also continues to strengthen the previously assimilated information. When a memory trace has been consolidated, the memory trace can be stored for later retrieval indefinitely.
The last step in forming episodic memories is called retrieval, which is the conscious recollection of information that was encoded and stored. Retrieving information from episodic memory depends upon contextual information or cues and how effectively the information was encoded and stored in memory. Thus, if the information was not properly encoded because you were distracted, you may be less likely to retrieve details of the event or information. Emotional, semantic knowledge, olfactory, auditory, and visual factors can act as cues or contextual information to help in the retrieval of episodic memory. For example, when recalling where you parked your car, you may use the color of a sign you parked near and/or the floor of the parking structure as cues. Research also states that episodic retrieval can be associated with a sense of re-experiencing (i.e., “recollection”) of the event. In order to remember where you parked or did not park your car, you have to mentally travel back to the moment or time you parked.
Memory & Dementia
The hippocampus, its surrounding regions, and the default mode network are susceptible to many types of neurological insults. One particular type of insult commonly seen in older adults is Alzheimer’s disease. Alzheimer’s disease is caused by abnormal protein misfoldings (amyloid and tau) that most often originate in medial temporal structures, including the hippocampus, and is known to disrupt default mode network connectivity. Indeed, episodic memory impairment is a hallmark sign of Alzheimer’s disease. In addition to the hippocampus and default mode network, some other brain structures that play a role in memory are the thalamus, mammillary bodies, and the amygdala.
Many different neurologic diseases and conditions can affect episodic memory. These include but are not limited to, subarachnoid hemorrhage, trauma, hydrocephalus, tumors, metabolic conditions including vitamin B1 deficiency, infectious and inflammatory conditions such as Hashimoto’s encephalopathy, and neurodegenerative diseases such as Alzheimer’s disease. These diseases directly target memory structures (e.g., hippocampus) and/or parts of the memory networks (e.g., default mode network). As mentioned, episodic memory is also influenced by an individual’s ability to attend to the environment. Therefore, any conditions that disrupt attention can also impair the encoding of information. Attention is impacted by many conditions, such as head injury, Lewy body dementia, and delirium. Non-neurologic issues such as medications, anxiety, depression, or pain also adversely impact episodic memory.
Neuropsychological Testing
A common way to assess episodic memory abilities is by using neuropsychological tests, including pen-and-paper, verbal, and computer-based tasks. These measures give a clinician an objective method for evaluating how well a patient’s episodic memory is functioning compared to their peers. Neuropsychologists evaluate both verbal and visual episodic memory. Asking an examinee to remember a list of words or recall a story are common methods for assessing verbal episodic memory. Asking an examinee to copy a figure and then recall it at a later time is a common test of visual episodic memory. Neuropsychological evaluation complements other aspects of a comprehensive evaluation and is often able to detect deficits that are not captured using gross, neuroanatomical imaging.
