Episodic or implicit memory is the memory of an event or “episode”. This type of memory is declarative and is relevant both to recent and remote events. Examples of episodic memory would be the ability to recount what you did today and where you were when you heard shocking news. Another example would be remembering where you parked your car. Episodic memory can be thought of as a process with several different steps, each of which relies on a separate system of the brain.
The initial step in forming this type of memory is called encoding. Encoding is necessary for the acquisition of new factual knowledge. Encoding take effort, and more effort results in better encoding. For instance, if one were to hear a list of 10 words and asked to remember the list after a half an hour, one would likely remember more words if one were to think of a definition for each word than if one were to simply repeat the list a couple of times. The work done with the list in the encoding phase results in better memory later on. Another example of a frequently used encoding strategy is the use of mnemonics, or associations between something to be remembered and something one already knows. For instance, if one were introduced to someone named Charlie, one might make a note that this is the same name as one’s uncle.
The step by which the information is accessed and brought back into consciousness is called retrieval. This too takes effort. If someone were to ask a person where they were for dinner four nights ago, the answer might not readily come to mind. Rather, one might take a strategy to try to provide the answer, like thinking about what day was four days ago, say it was Tuesday, what one usually does for dinner on Tuesday, and whether this was a typical Tuesday or not.
Research and clinical experience suggest that information is not simply stored and retrieved, and that there may be an intermediate step in this process, which we call consolidation. Consolidation is the process by which recently learned information becomes more strongly represented in the brain, and more resistant to the effects of hippocampal injury (see below).
Anatomy The parts of the brain that are thought to be necessary for the effortful/variable aspects of encoding and retrieval are the frontal lobe and the lateral temporal lobes. When these regions are injured, memory encoding is disorganized, but can still occur. Memory retrieval will be disorganized as well, but can be aided with cues to organize retrieval. For instance, in someone who has impaired retrieval from frontal lobe damage, a cue such as where they met someone will help with the recall of the person’s name.
Studies show that the hippocampus and medial temporal lobes are critical for encoding. No matter how efficient the encoding strategy, if the hippocampus is severely injured, encoding will not occur. Because the encoding impairment is so profound in hippocampal damage, events will essentially disappear from memory once they are not attended to, and even hints or cues will not help to bring them back, because retrieval of these lost memories is impossible. Episodic memory is frequently impaired in Alzheimer’s disease because the hippocampus is very vulnerable to the changes occurring in this disorder.
The figure below shows a “slice” from an MRI scan of two patients, one from a normal older person and the other from a patient with Alzheimer’s disease. The brains are being viewed as if they are being cut about a third of the way back from their front tip (like cutting into a loaf of bread) and then looking at the cut surface. The hippocampi from these two individuals are circled in yellow, and one can see that the hippocampus is smaller in the Alzheimer’s disease patient.
The hippocampus is part of a network of regions in the brain important for memory. Research suggests that the mediodorsal nucleus of the thalamus, amygdala, and hippocampus may work in concert to regulate which information is consolidated in memory.
Anatomy of episodic memory
Because encoding and retrieval require effort, they can be affected by any problem that impairs the ability to make efforts for memory. This can occur from damage to several different parts of the brain, and can be associated with medication use, as well as psychological problems such as depression, or even physical problems such as pain. This is probably one reason why memory problems are the most common cognitive complaint.
There are a wide range of neurologic diseases and conditions that 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.