The term memory is commonly used to refer to the ability to remember information but it is important to understand that there are several different types of memory that subserve different functions. In the first instance, there is a distinction between motor and non-motor memories – the former is a form of implicit memory and typically involves the cerebellum, motor cortical areas and basal ganglia (see Chapters 38–42) and will not be discussed further in this chapter. The other forms of memory are more involved with the taking in, manipulating and storing of information for problem solving (working memory), events and factual knowledge (explicit memory).
In clinical practice it is not uncommon for patients and their families to complain about disorders of memory when they are referring to a range of different cognitive problems such as a deficit in language (see Chapter 28), attention or perception (see Chapter 34). In this chapter we discuss the different types of memory, their neurobiological basis, and disorders that affect these different systems and their clinical manifestations. In particular it is useful to distinguish between long-term and working memory (which is often erroneously referred to as short-term memory). While this distinction relates to the duration of a memory, it primarily refers to whether material is maintained in consciousness (working memory) or whether it is stored unconsciously and then retrieved into consciousness (long-term memory).
Working memory is the limited capacity (around seven items or chunks of information) to store information in consciousness that rapidly disappears when attention is diverted. A distinction is typically made between processes required for maintaining material and the control (‘executive’) processes required for manipulation of that material. Maintenance processes would typically be engaged by reciting a list of digits and requiring a subject to repeat them immediately (digit span). Executive control processes might be tested by requiring the subject to repeat the digits in reverse order.
Neurobiological basis and disorders of working memory
Studies in humans and monkeys have unequivocally demonstrated the importance of the lateral prefrontal cortex in working memory processes (see also Chapter 34). It has been suggested that different parts of the prefrontal cortex are important for the maintenance and control processes that constitute working memory. Other brain regions are clearly implicated in working memory processes in a modality-dependent way. Working memory for visuospatial material may rely on occipitotemporal regions (when remembering, for example, the visual properties of an object) or occipitoparietal regions (when remembering spatial properties). On the other hand, holding verbal or phonological material in working memory seems to require the lateral temporal cortex. Whatever the domain, it appears that the efficient flexible use of working memory processes depends upon coordinated interactivity of frontal control processes and modality-dependent ‘slave’ systems.
Abnormalities in this system typically occur with damage in the sites listed above, especially the prefrontal cortex, as well as in some disorders of the basal ganglia (e.g. Huntington’s and Parkin- son’s disease) where there is disruption of corticostriatal circuits (see Chapter 42). In these patients there is a difficulty in taking in information and as such the individuals have difficulty solving problems that require the ongoing manipulation of data.
Long-term memory is the store of practically unlimited capacity and the memories within this system may persist over a lifetime. Long-term memory is primarily divided in to explicit and implicit components.
· Explicit memory refers to memories that are accessible to consciousness. It is divided into episodic memory (memory for episodes or events; typically, memory with an autobiographical content) and semantic memory (knowledge of facts; memory that is not characterized by an autobiographical content). Thus, an episodic memory of Paris might comprise the memory of a visit there, while a semantic memory is that Paris is the capital of France, situated on the Seine, etc.
· Implicit memory refers to memory that is not accessible to consciousness and typically refers to motor memory; it encompasses the acquisition of motor skills, conditioning (e.g. Pavlov’s dogs salivating when hearing a bell), as well as priming. This latter process is defined as the subject’s ability to provide answers to general questions (e.g. the word ‘Paris’ when asked to name a city), even when they do not remember this prior exposure.
Neuroanatomical basis and disorders of long-term memory
The famous case of HM, in whom both medial temporal cortices were removed for intractable epilepsy, provided the first clear evidence that the episodic memory system depends on medial regions of the temporal lobe. In addition, his case also highlighted the difference between explicit and implicit memories and that different systems underlie episodic and semantic memory at the neuro- anatomical level. Subsequent to his operation, HM was unable to learn or recall new episodes or experiences in his life. However, his ability to learn new motor skills was preserved as was his factual knowledge. While there is a great deal of evidence underpinning the importance of medial temporal structures, especially the hippocampus, in episodic memory processes, it is clear that, as with working memory processes, distributed brain systems, frequently requiring prefrontally mediated control, are necessary for optimum autobiographical memory processes (see Chapters 11 and 34). In this respect, patients with certain forms of neurodegenerative dis- orders with relatively widespread pathology may have profound disorders of long-term memory, as for example in Alzheimer’s disease (see Chapter 60). In this condition there is pathology within the hippocampus and related structures (see Chapter 45) as well as temporal and parietal cortices, and patients develop problems of anterograde memory (i.e. the laying down of new memories) followed by progressive problems with retrograde memory (the retrieval of preformed established memories). This distinction in anterograde and retrograde memories is thought to have a basis in transferring information from hippocampal structures to the overlying cortex and thus as the pathology spreads out so the memory processes are affected in a similar fashion. While in Alzheimer’s disease the initial memory problem is more of an episodic nature, in some people there are problems within the semantic memory system. These cases of semantic dementia, wherein individuals begin to lose their knowledge of the meanings of words, depends on damage to the inferior and lateral temporal cortices and is seen in some patients with frontotemporal dementia (FTD).