Memory
Introduction
Memory is defined as the ability to retain information and recall it at a later time. It is a biologically fundamental function essential for survival. It shapes our identity by guiding our thoughts and decisions and influences our emotional reactions.
Classification Of Memory
Memory can be classified based on:
- Duration
- Behavioral manifestation
Based On Duration
Based on duration, memory can be classified into:
- Sensory storage
- Short-term memory
- Long-term memory
Sensory Storage
The stimulus is held for a fraction of a second. If the information has some meaning, it will be moved to short-term memory. Otherwise, it will be dumped.
Short-term Memory
It is the ability to hold and recall information for a short period, usually for a few seconds. The information can be "rehearsed" (repeated) silently or aloud, which will provide auditory cues – this will keep the information in short-term memory. The information can be lost. The information can be transferred into long-term memory.
Long-term Memory
This memory is consolidated from short-term to long-term memories. Here, information is stored for long-lasting periods, sometimes for an entire lifetime.
Based on Behavioral Manifestation
Based on behavioral manifestation, memory can be classified into:
- Implicit memories
- Explicit memories
They are divisions of long-term memory. Short-term memory is consolidated to long-term memories primarily in the hippocampus and stored throughout the cortex.
Explicit Memories
Explicit (or declarative) memories are memories that can be consciously recalled, such as facts, people, and events. Declarative memory could be divided into:
- Semantic memory – representing discrete facts such as dates, word definitions, and learned concepts.
- Episodic memory – representing explicit experiences one has lived through, such as a special birthday or one’s wedding day.
Implicit Memories
Implicit (or procedural) memories are those that store information about skills, for example, driving a car, riding a bike, or playing the piano.
Brain And Memory (Formation And Storage)
The portions of the brain associated with memory include:
- The association cortex of the frontal, parietal, occipital, and temporal lobes.
- Part of the limbic system, especially the hippocampus and amygdaloid nucleus.
- The diencephalon.
Research has also shown that some memory is stored throughout the body. This became evident with organ transplants. People who have received donor organs have reported experiencing cravings or emotional reactions to certain incidents that they never had before.
Phases of Memory Formation
- Encoding: Information for each memory is assembled from the different sensory systems and transplanted into whatever form necessary to be remembered. This is presumably the domain of the association cortices, hippocampus, and perhaps other areas.
- Storage: The actual deposition of the memories into their final resting places. This is believed to be in the association cortex.
- Consolidation: Converting the encoded information into a form that can be permanently stored. The hippocampal and surrounding areas apparently accomplish this.
- Retrieval: Memories are of little use if they cannot be read out for later use. Less is known about this process.
4 Sub-regions of the Hippocampus
- Dentate gyrus
- Hippocampus proper
- Subicular complex
- Entorhinal cortex
Encoding Of Memory
It is the act of classifying and placing memory items (signals or impulses) in their proper sequence in the brain to enable storage. Once we receive sensory information from the environment, our brains label or code it.
Brain Areas Concerned with Encoding of Memory
- Association cortices
- Hippocampus (major central role) – all bits of information go to it first
- Other areas:
- Amygdala – emotional memory
- Basal forebrain – nucleus basalis or Meynert nucleus
- Mammillary body of the hypothalamus
- Orbitofrontal cortex
Clinical Correlates
Amnesia
Loss of memory is known as amnesia. It is classified into two types:
- Anterograde amnesia: Inability to form new long-term memories. Occurs in hippocampal lesions.
- Retrograde Amnesia: Inability to recall memories from the past. Occurs in temporal syndrome. Damage in some thalamic areas may lead specifically to retrograde amnesia. A possible explanation for this is that the thalamus may play a role in helping the person "search" the memory storehouses and thus "read out" the memories.
Dementia
Dementia is the progressive deterioration of intellect, emotional control, social behavior, and motivation associated with loss of memory. It is an age-related disorder. Usually, it occurs above 65 years. When it occurs under 65, it is called presenile dementia.
Confabulation
Confabulation is an interesting though poorly understood condition that sometimes occurs in individuals with lesions of the ventromedial portions of the frontal lobes. These individuals perform poorly on memory tests but spontaneously describe events that never occurred. This has been called "honest lying."
Alzheimer’s disease
Alzheimer’s disease is the most common age-related neurodegenerative disorder. Memory decline initially manifests as a loss of episodic memory, which impedes the recollection of recent events. Loss of short-term memory is followed by a general loss of cognitive and other brain functions, the need for constant care, and, eventually, death.
