Explicit Memory | How Does Your Brain Consciously Recall Facts and Events?

Explicit memory, also known as declarative memory, is one of the two major subdivisions of long-term memory. It is responsible for the conscious, intentional recollection of factual information, previous experiences, and concepts. This type of memory is defined by the ability to "declare" or verbalize what is remembered. Explicit memory is further divided into two categories: episodic and semantic memory. Episodic memory pertains to the recollection of specific personal experiences, complete with contextual details such as the time and place the event occurred. For example, remembering your first day at college, including the sights, sounds, and emotions you felt, is an episodic memory. On the other hand, semantic memory refers to the storage of general worldly knowledge that is independent of personal experience. This includes facts, concepts, and vocabulary. Knowing that Paris is the capital of France is a semantic memory; you know the fact but likely do not remember the specific moment you learned it. Both systems work in concert. For instance, you can recall the episodic memory of learning about a historical event in a specific classroom, while the facts about the event itself are stored as semantic memory. Understanding this distinction is fundamental to comprehending how we build a coherent narrative of our lives and a stable base of knowledge about the world.

The formation and retrieval of explicit memory involve a network of specific brain regions, primarily within the medial temporal lobe. The most critical structure is the hippocampus, which is essential for the formation of new episodic memories. It acts as a temporary transit point, binding together the various sensory elements of an experience (what you saw, heard, felt) into a single, coherent memory trace. The adjacent entorhinal, perirhinal, and parahippocampal cortices also play crucial roles by processing "what" and "where" information related to the memory. Over time, a process called systems consolidation occurs, where these memories become less dependent on the hippocampus. They are gradually transferred and stored in a more distributed manner across the neocortex, the brain's outer layer responsible for higher-level thought. Therefore, while the hippocampus is the architect of new explicit memories, the vast neocortex serves as the long-term library for this knowledge and life history.

The creation of a lasting explicit memory is a three-stage process: encoding, consolidation, and retrieval. Encoding is the first step, where sensory information is transformed into a construct that can be stored. This process is heavily influenced by attention; you must actively attend to information to encode it effectively. The second stage, consolidation, involves stabilizing the memory trace after its initial acquisition. Synaptic consolidation happens rapidly at the level of neurons, while systems consolidation is the slower process, mentioned earlier, of reorganizing memory traces from the hippocampus to the neocortex. This strengthens the memory, making it less susceptible to interference. The final stage is retrieval, which is the conscious act of accessing stored information. A retrieval cue, which can be a thought or an external stimulus, triggers the reactivation of the neural pattern associated with the memory.

The primary distinction between explicit and implicit memory lies in the level of consciousness involved in recall. Explicit memory, as discussed, requires conscious thought—such as deliberately recalling a friend's name or a historical date. In contrast, implicit memory (or non-declarative memory) is recalled unconsciously. It influences our thoughts and behaviors without our awareness. The most common form of implicit memory is procedural memory, which governs skills and habits like riding a bicycle or typing on a keyboard. Once learned, these actions are performed automatically without the need to consciously recall the steps involved. Other types of implicit memory include priming, where exposure to one stimulus influences the response to a subsequent stimulus, and classical conditioning. Essentially, explicit memory is about "knowing that," while implicit memory is about "knowing how."

Explicit memory function, particularly episodic memory, is known to decline with normal aging. Older adults may find it more difficult to recall specific life events with the same level of detail as younger adults, although their semantic memory often remains relatively intact. This age-related decline is associated with natural changes in brain structure and function, including some volume loss in the hippocampus and frontal lobes. However, in neurodegenerative disorders like Alzheimer's disease, the impact on explicit memory is far more severe and devastating. Alzheimer's pathology begins in the medial temporal lobe, directly targeting the hippocampus and entorhinal cortex. As a result, one of the earliest and most prominent symptoms is a profound inability to form new episodic memories, a condition known as anterograde amnesia. As the disease progresses, it spreads to the neocortex, leading to the erosion of long-term semantic memories as well, causing individuals to forget fundamental facts about themselves and the world.