Memory
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
Definition
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
Definition
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).
