Decode the Mind: Memory Explained
The ability to acquire new information and retain it over time is a crucial skill for daily function, and a range of processes of memory facilitate this. Unlike a digital memory, human brains do not attempt or achieve exact storage, but instead prioritise information deemed salient for survival and everyday function. Excessively detailed memories would be cognitively inefficient and often a waste of processing, so this gives rise to an incomplete store. Memory activity is subdivided into: encoding, storage, and retrieval. Encoding refers to the processing of incoming information to be stored, which consists of acquisition and consolidation. Acquisition is when environmental input induces a sensory memory, and in consolidation, attention to this creates a stronger representation and facilitates committal to short-term memory. Information can then be recalled from short-term memory while it remains active, or rehearsed and thus transferred to a permanently maintained record in long-term storage. Retrieval is the utilisation of stored information to generate a conscious representation, or to execute a learned behaviour.
Types of memory:
Different forms of memory can be categorised into distinct forms of information, and may be stored and retrieved using separate neurological mechanisms and functionally specialised brain areas.
Explicit, or declarative memory is information that is consciously remembered. Learning a list of dates or useful formulae for an exam employs explicit memory. Types of explicit memory include episodic and semantic memory. Episodic memory is the long-term memory of specific autobiographical past events and experiences, often also including emotional charge and general context. Semantic memory refers to factual knowledge, which is usually independent of personal experience and the context in which it was acquired. This includes facts, meanings, concepts, and knowledge about the external world, such as recalling capital cities, functions of objects, or vocabulary.
Implicit memory is unconscious and unintentional. One type of implicit memory is procedural memory, such as how to perform a specific motor task, like brushing your teeth. Conscious recollection is not required to perform these tasks, but procedural memory still influences behaviour and total knowledge. The formation of implicit memories begins with learning and practising a new task, and this becomes automatic with repetition. To test the distinction between implicit and explicit memory, consider the activity of typing on a keyboard. Many people are able to touch type on their phone or computer keyboard, and yet most would struggle to name the order in which the letters appear. The procedural and implicit practice of touch typing requires a knowledge of where the letters are situated, and yet this is stored unconsciously, and is very difficult to access as explicit memory when consciously attempted.
Working memory is a part of short-term memory that acts as a temporary store for information being focused on currently. This is not only dependent on the hippocampus, but also on the prefrontal cortex, for goal-orientated behaviours based on working memory. Working memory holds multiple pieces of information in the mind temporarily. In order to measure its capacity, a subject can listen to a series of digits and letters, sort them into order in their mind, and then recall the ordered list out loud. The longest set that can be processed and recalled reflects the capacity of the working memory. This type of recollection is achieved through persistent firing of neurons in the prefrontal cortex for the duration of the period of remembering.
Patient HM:
Patient HM was perhaps the most famous and influential case study in the development of our understanding of memory. After suffering from severe epilepsy into his twenties, he received a major neurosurgical procedure called a temporal lobectomy – a surgical lesion of the hippocampus and adjacent cortex to remove the medial temporal lobe bilaterally. Though his epilepsy receded, patient HM suffered severe anterograde amnesia, meaning he was unable to form new long-term memories. Some childhood events could be recalled, but some retrograde amnesia (loss of recollection from before the operation) also resulted. Nevertheless, normal short-term and working memory were maintained. This was tested by asking the patient to recall a string of numbers; the length of numbers recalled is known as the ‘digit span’. Patient HM had a normal digit span (relying on short-term memory), but performed particularly poorly when long-term memory acquisition was introduced as a requirement. The distinction between long-term and short-term memory could be made, as the short-term memory seemed uncompromised. Additionally declarative and procedural memory could be separated, as the procedural memory demonstrated in learning motor skill tasks, such as mirror drawing, was also retained in patient HM. He could learn to play table tennis and develop his skills through practice, but he was unable to remember that he was skilled. Implicit memory used in motor and cognitive skills was undamaged, so dissociation of implicit and explicit memory could also be shown.
Brain areas involved in memory:
The human hippocampus is located in the medial temporal lobe, and is a crucial structure for consolidation of information from short-term to long-term memory. The hippocampus is particularly implicated in spatial (navigational) memory, and can be developed as spatial knowledge accumulates. For example, London taxi drivers must demonstrate extensive knowledge of routes and locations throughout London, requiring highly developed spatial memory. Upon MRI analysis, it was found that hippocampal volume (size) is correlated with the number of years spent working as a taxi driver. Conversely, in Alzheimer’s disease and other forms of dementia, the hippocampus often suffers damage, resulting in the short-term memory loss and disorientation that characterise the early symptoms.
The hippocampus was the first brain region where the process of long term potentiation was first observed. This mechanism is the basis for most types of memory and learning across the brain. Long term potentiation is the persistent strengthening of the connections between neurons (synapses) due to increased use. This activity-dependent synaptic plasticity (changes in synapse strength) is crucial in memory consolidation, as the replaying of memories allows connections to be strengthened, and the memory to become ‘stronger’, and more easily retrieved. This is a key cellular and molecular process that is fundamental to the understanding of memory formation and retrieval.
The encoding of semantic memory primarily activates the frontal and temporal cortexes, whereas episodic memory activity is processed in the hippocampus, then consolidated and stored. A single episodic memory comprises visual, olfactory, and auditory elements, and these components are distributed across the corresponding brain areas. The hippocampus ties the episode together when it is retrieved.
Memory as reconstructive:
Human memory is reconstructive, as opposed to reproductive. Using prior knowledge to supplement memory is an effective way to improve efficiency and capitalise on patterns in the world. However, this can result in implications having unwanted effects, such as misleading advertisement, inaccurate eyewitness testimony, or negative effects of stereotypes. Memory can be distorted and manipulated, and false memory can result.
Conversely, understanding memory processes can be applied to training memory and developing skills, such as in the case of ‘memorists’ such as Rajan Mahadevan. He earned a place in the Guinness Book of World Records in 1981 for reciting the first 31,811 digits of pi. This was achieved through employing spatial memory by pairing matrix locations with individual digits. Other memorists may employ visual images to reproduce number strings, mnemonics, ‘memory palaces’, or another unique method of encoding. The key to exceptional memory demonstration is using meaningful encoding and association. This translates what the brain would consider to be irrelevant data into a more familiar format that is deemed more important. These techniques can be learnt by almost any individual.
Improving understanding of how new memories are formed has exciting implications across a range of fields. Potential applications include understanding and refining learning, overcoming traumatic experiences, improving eyewitness testimony accuracy, and improving the lives of neurological patients with dementia or memory impairment.
