24th October 2023

An Overview of Memory and Forgetting

What is memory?

Aristotle once said, ‘Memory is, therefore, neither perception nor conception, but a state or affection of one of these, conditioned by lapse of time.’ By this, we can understand that memory describes past ideas and experiences, whereas perception and conception could also relate to the present (Ross, 1930). Memory is a broad term that describes multiple brain systems as an inter-relationship between our experiences, thoughts and impressions. Multiple brain structures of the temporal lobe and midbrain appear to be involved in memory, including the hippocampus, limbic system, basal ganglia and ventral tegmental area (Lisman and Grace, 2022, Squire et al., 2004). The relationship between these structures is documented in several studies. For example, studies into depression have suggested that chronic stress can sensitise the amygdala to negative information and suppress hippocampal neurogenesis (Dillon and Pizzagalli, 2018).

One way of dividing memory is into short-term and long-term, based on duration and capacity. Short-term memory typically holds a limited number of items for only a short period of time, whereas long-term memory is not bound by such limitations (Cowan, 2008). Working memory is sometimes described as a third division but will be described within short-term memory for this chapter. Memory can also be subcategorised based on the type of information that it pertains to. Semantic memory refers to acquired information about the world (Binder and Desai, 2011). For example, knowing the names of colours or that France is a country. It can also include more specific facts, such as historical names and dates. In comparison, episodic memory relates to our own autobiographical life events (Tulving, 2002). For example, remembering your first date or what you had for lunch. Both semantic and episodic memory are subdivisions of declarative memory. By contrast, non-declarative memory, otherwise known as implicit, consists of our procedural or motor memory (Krakauer and Shadmehr, 2006). For example, knowing how to swim or ride a bike.

Short-term memory

The working memory model

The term ‘working memory’ is sometimes used interchangeably with short-term memory. However, more specifically, it describes a way in which information is manipulated as well as stored. It is like a workspace for short-term memory (May et al, 1999). Working memory does not represent a single neural process but instead seems to be a set of interlinked abilities (Wynn and Coolidge, 2011).

Working memory is a highly researched area of cognitive neuroscience. The theory was conceived following an experiment where participants were asked to recall verbally spoken digits whilst being distracted with a visually presented grammatical reasoning task. The visual task had been intended as a distraction but, surprisingly, did little to influence the verbal information retained (Baddeley, 2012). This thereby inspired the original Baddeley and Hitch (1974) working memory model [see Figure 4]. The working memory model acknowledges that there can be multiple compartments to short-term memory, depending on the type of information that is being stored.

In the model, three components are described:

• The visuo-spatial sketch pad.

• The phonological loop.

• The overarching central executive.

Image Source: Wikimedia Commons (2011). Baddeley working memory, Licensed under a creative commons licence. Available for free at: https://commons.wikimedia.org/wiki/File:Baddeley_working_memory.jpg (Accessed on: 17 October 2023).

The phonological loop maintains and rehearses auditory information to maintain it within short-term memory, whilst the visuo-spatial sketchpad processes visual information (Wynn and Coolidge, 2011). Visuo-spatial and auditory memory are held in separate compartments and are therefore not limited by the usual capacity of short-term memory. (Baddeley, 2012). For example, you can listen to a conversation whilst driving with minimal interference. Later versions of the model also included a fourth component: a temporary memory store, or ‘episodic buffer’. The episodic buffer maintains data from the two subsystems in active attention so that it can be processed by the central executive. (Wynn and Coolidge, 2011).

Capacity and chunking

Miller (1956) proposed that short-term memory is limited to holding only seven +/- two items of information at any one time. We can observe that when a person is given more information than is their capacity to retain, new information displaces existing information in the short-term memory (Cowan, 2000). For example, when trying to remember a list of shopping items, remembering new items after a certain point will cause earlier ones to be forgotten. The primacy and recency effect suggests that the items in the middle of the list are most likely to be forgotten. A possible explanation for this is that the most recent items are recalled with the least delay, whilst earlier items have had a greater opportunity to be rehearsed. The recency effect seems to be less pronounced if there is a distraction or delay in recall, whereas the primacy effect is retained (Cowan, 2000).

‘Chunking’ is a technique which is used to overcome the limited capacity of short-term memory. For example, when learning a new phone number, we will tend to group numbers into ‘chunks’ which are easier to remember. McLean and Gregg (1967) highlighted that these chunks tend to be recited together quickly, for example, 123-456-789. If there are associations with items in long-term memory, chunking can be even more effective. Miller (1956) gave an example of the word ‘fbicbsibmirs’, which can be split into triads that many people already have in their long-term memory: FBI, CBS, IBM and IRS.

The levels of processing model 

Craik and Lockhart (1972) proposed an alternative theory for memory storage, which refers to both short and long-term memory. They also spoke of a third component, ‘sensory storage’, which includes all information received by the senses regardless of whether we have paid attention to it. They described sensory storage as having a duration of 0.5-2 seconds, short-term storage as having a duration of 5-20 seconds, and long-term storage as potentially unlimited (Craik and Lockhart, 1972).

Rather than discussing such defined stores, the levels of processing model describes shallow processing and deep processing depending on other factors. In shallow processing, the memory is lost when attention is diverted, as it is being repeated to be maintained in consciousness. In deep processing, there is a greater level of analysis, which allows the memory to be retained even when attention is shifted, for example, linking with pre-existing memories (Craik and Lockhart, 1972). This is another way of explaining the phenomena discussed under the working memory model because single words or chunks do not require phonemic analysis and thus remain in shallow processing. By contrast, full sentences and descriptive information are compatible with deeper learned structures and would, therefore, be moved into a deeper level of processing (Craik and Lockhart, 1972).

Long-term memory

Memory results from interactions between neurones and their activity in specific anatomical structures. The multiple memory systems model suggests that long-term memory is stored differently, depending on whether the information is declarative or non-declarative (Reber and Allen, 2022).

Declarative memory

The hippocampus is a structure in the temporal lobe of the brain which is necessary for encoding and recalling long-term episodic memories (Lisman and Grace, 2005). In the past two decades, there have been studies that suggest that the hippocampus forms a functional loop with the ventral tegmental area (VTA) using dopaminergic neurons. When the hippocampus receives new information, the loop is activated, and dopamine is released, and a signal is conveyed via several structures to the VTA (Lisman and Grace, 2005). Dopamine is just one of several neurotransmitters that seem to be involved in memory, in addition to glutamate, GABA and serotonin. The use of functional MRI has shown that, when learning new words, those that were better remembered demonstrated greater functional connectivity amongst the midbrain, hippocampus and ventral striatum (part of the basal ganglia). This has been tested in the absence of external reward to help clarify the mechanism of internally-guided learning (Ripollés et al, 2016).

Semantic memory overlaps with perception, as we use sensory information to identify objects and people as part of the process of memory. Neuroimaging studies have shown that there are modality-specific sensory, motor, and emotion systems concerned with understanding language. However, there are also significant regions of the brain that are involved in comprehension tasks but are not modality-specific (Binder and Desai, 2011).

Non-declarative memory

We have all heard the phrase ‘You’ll never forget how to ride a bike’ or heard of cases where a person with severe dementia is still able to play the piano. These phenomena relate to procedural or motor memory. Procedural learning has been explored experimentally to measure the progression of sequential movements into well-articulated behaviour (Doyon et al, 2009). Some parts of the brain that are thought to be important in procedural memory include the hippocampus, the prefrontal cortex and the basal ganglia (Reber and Allen, 2022). To learn a motor activity, we need to repeat the skill. However, there is also a period of post-activity learning known as motor consolidation. During this time, physiological and synaptic changes within the brain enable the activity to be consolidated as a memory (Doyon et al, 2009).

Retrieval of memories 

We all experience times when memory fails us. This could be due to a problem with the process of remembering or with retrieving the stored memory. The table below outlines some theories for forgetting.

One of the concepts introduced here is ‘Retrieval cues’. This means that a person is more likely to recall memories when they have similar cues to the initial memory input. Godden and Baddeley, mentioned in the table above, took a somewhat extreme example of this by drastically changing the external environment of the scuba divers. Dijkstra et al (2007) conducted another study on retrieval cues, which looked more selectively at body posture. They found that autobiographical events were recalled more quickly if body position was memory-congruent, i.e. the position during remembering was the same as during recall.

Clinical relevance 

One of the most cited cases in clinical neuroscience is Patient H.M., who became known for his severe memory impairment. The memory impairment was an unexpected outcome from undergoing temporal lobe neurosurgery for incapacitating seizures. He lived a long life and appeared intellectually unaffected but forgot events almost as soon as they occurred, which enables us to explore the link between short-term and long-term memory (Squire, 2009). H.M. could retain three-digit numbers for around 15 minutes with continuous rehearsal but would forget the entire event as soon as his attention shifted. This supports the idea of separate memory stores (Squire, 2009). Whilst the case of Patient H.M. is somewhat unique, the understanding that we have gained regarding temporal lobe structures can assist us in understanding the impact of other organic causes of memory loss, for example, malignancy.

More broadly, the group of patients that we are often most concerned with in terms of memory disorders are those with dementia. Previously, in this chapter, we discussed that neurotransmitters play a role in memory. This has translated, in part, to the emerging treatments of Alzheimer’s disease, such as NMDA receptor antagonists. Alzheimer’s disease is characterised by global memory loss and behaviour changes due to progressive damage to the hippocampus and neocortex (Liu et al, 2019). It is thought that over-activity of glutamate, leading to pathological levels of Ca2+, can cause neurotoxicity and synaptic dysfunction in Alzheimer’s disease. Hence, NMDA receptor antagonists are being used in the treatment of moderate to severe Alzheimer’s to reduce glutamate-induced excitotoxicity and slow progression of the condition (Liu et al, 2019).

Later in this chapter, we will discuss how other mental illnesses may interact with memory. For example, the role of attention in attention-deficit / hyperactivity disorder or perception in schizophrenia.

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