Complaints of impaired memory are among the most common symptoms reported to neurologists. Moreover, impairment of memory is one of the most disabling aspects of many neurological disorders, including neurodegenerative diseases, strokes, tumours, head trauma, hypoxia, cardiac surgery, malnutrition, attention deficit disorder, depression, anxiety, medication adverse effects, and just normal aging. This memory loss often impairs the patient’s daily activities, profoundly affecting not just them but also their families.

Memory research that began with neuropsychological studies of patients with focal brain lesions now includes new methods such as positron emission tomography, functional MRI, and event-related potentials which have all provided a more refined and improved classification system. Instead of conceptualising memory as “short-term” versus “long-term”, we now think of memory as a collection of mental abilities that use different systems within the brain. A memory system is a way that the brain processes information in order to make it available for use at a later time. Some systems are associated with conscious awareness (explicit) and can be consciously recalled (declarative), whereas others are typically unconscious (non-declarative) and are instead expressed by a change in behaviour (implicit).

Here we will describe the four systems that are of clinical relevance: episodic memory, semantic memory, procedural memory, and working memory (table 1). Additional details may be found in Budson and Price¹ and the other references listed below.^2–6

EPISODIC MEMORY

Episodic memory is the explicit and declarative memory system that we all use to recall our personal experience, framed in our own context, such as remembering a short story or what we had for dinner last night. Episodic memory loss follows a predictable pattern known as Ribot’s law: recently acquired memories are more vulnerable to loss than longer established ones. This memory system depends on the medial temporal lobes …