Types of memory

How does memory work?

Memory operates through a series of processes involving encoding, storage, and retrieval.

• Encoding involves the conversion of sensory information such as visual, auditory, tactile, olfactory, and gustatory, into a form that can be processed and stored in the brain. For example, reading a book converts visual input into meaningful information.
• Storage refers to the process of maintaining the encoded information over time, ranging from seconds to decades. Depending on the type of memory, storage occurs in different areas of the brain.
• Retrieval refers to the process of accessing stored information when needed such as remembering where you parked your car.

Memory is largely a function of the temporal lobe, along with the prefrontal cortex, involving cortical and subcortical structures such as the hippocampus and the amygdala. Memory can be observed in experiences recovered and brought into consciousness (e.g., remembering what you ate for lunch), or in behavioral changes (e.g., testing the improvement in your performance the next day after practice).

The physical embodiment of any memory in neuronal machinery is called engram; it is used to describe the biological “trace” of a memory within the brain, representing the network of neurons activated and modified upon memory formation, storage, and retrieval. Consolidation refers to the gradual stabilization of memories following the initial encoding of memory “traces”. Consolidation leads to persistence of memories at cellular level and, thus, entails changes in gene expression, protein synthesis, and synaptic plasticity mechanisms.

How is memory classified by duration?

Memory divides into three categories based on how long information is held: immediate memory (fractions of a second to a second), short-term memory (seconds to minutes), and long-term memory (days to a lifetime).

• Immediate memory refers to the ability of the brain to hold onto ongoing experience for fractions of a second to a second. Here is an example to understand the concept of immediate memory; typically every second we make two to three saccadic eye movements, i.e., rapid eye movements redirecting gaze, providing our visual system with new “snapshots” of our surroundings. Unless something striking captures our attention, these snapshots are forgotten and we perceive the visual scene as stable. However, if we close our eyes at some random moment, we are still able to recall a significant amount of detail from the image we last saw thanks to the immediate memory. Immediate memory involves mainly the brief storage of sensory input, and in such cases it is referred to as sensory memory. Iconic memory (visual information) fades almost instantly, while echoic memory (auditory information) lingers slightly longer.
• Short-term memory is the ability to temporarily hold information for seconds to minutes, while used to achieve a particular goal, e.g., remembering a phone number just long enough to dial it, or avoiding places already inspected when searching for a lost object. Due to its dynamic component, goal-driven short-term memory, is also referred to as working memory. An easy way to test the integrity of short-term memory bedside is the digit-span task, i.e., asking the patient to repeat a string of random numbers, with increasingly longer sequences being tested in each trial. The average correct digit span for normal adults is seven plus or minus two. Short-term memory is, also, closely related to attention, language, reasoning, and problem-solving.
• Long-term memory is where information is stored in a more permanent form, for extended periods ranging from days to even a lifetime. Immediate and short-term memories can enter long-term memory by conscious or unconscious rehearsal or practice. This is achieved through synaptic changes, neuronal circuitry growth and reorganization.

What are the main types of memory?

Based on whether stored information is consciously accessible, memory divides into two major categories: declarative (explicit) memory and non-declarative (implicit) memory.

Which brain regions are involved in memory?

Based on evidence from amnesic patients and neuroimaging knowledge about patterns of neural networks in the human brain, the sensory association cortices and the hippocampus as well as the medial temporal lobe and midline diencephalic structuresare all significantly involved in the acquisition and consolidation of declarative memories. However, the multimodal integration areas of the cerebral cortex are the major long-term repositories for many aspects of declarative memory. On the other hand, the integrity of the motor cortex, basal ganglia, amygdala, sensory association cortices, and cerebellum is critical for the acquisition and consolidation of non-declarative memories.

Learn the complex processes of learning and memory with the following interactive quiz!