Memory and Semantic Knowledge

Memory Basics

The creation of memories is a stream that includes sensory registration, short-term/working memory, and long-term storage. The memory stream starts with sensory input and finishes with stored, accessible memory traces or engrams ( Fig. 9.1 ). After an initial brief sensory registration in the sensory organ or point of entry (auditory or echoic 10 s; visual or iconic 500 ms), there follows a period of short-term (working) memory executed in the dorsolateral frontal lobes and related areas. Short-term/working memory involves holding information online for seconds to a minute (usually 20–30 s) if unrehearsed, but much longer if the patient verbally (phonological) or visually (visuospatial) rehearses the material. Short-term/working memory is clinically difficult to differentiate from sustained attention and is usually tested as part of mental control and attentional tests such as the digit span (see Chapter 7 ). If short-term/working memory is impaired, it is difficult to proceed along the memory stream to the creation of long-term memories.

Long-term memory is the actual encoding and storage of information in the brain and can be either declarative (explicit) or nondeclarative (implicit). Declarative memory is consciously stored and retrieved information that is itself further divided into episodic (context, time, and place dependent) experiences or semantic (context, time, and place independent) facts, which are processed in the limbic and semantic areas, respectively. In comparison, nondeclarative memory includes procedural learning (motor and cognitive skills), which is mediated by the basal ganglia and cerebellum; priming, or the positive influence of a prior exposure on subsequent learning; and classical conditioning, which is facilitated by prefrontal cortex and other structures.

When talking about memory disorders and testing memory, clinicians are primarily concerned with declarative memory loss (circled in Fig. 9.1 ), often from disease affecting hippocampi or related limbic areas ( Fig. 9.2 ). For testing purposes, these long-term memories may be divisible into those that occurred either in the recent past or the remote past. Recent memory includes the capacity to remember and learn current events and information and to be able to bring it forth after an interval ranging from minutes to days, as distinguished from the inability to retrieve remote information from years past. Episodic or time-tagged recent memory difficulty is the typical memory impairment in mild cognitive impairment, dementia, or amnestic disorders. Much rarer are disorders of semantic memory for time-independent facts/knowledge. The rest of this chapter is concerned with the testing of declarative memory, both episodic and semantic. Although this chapter does not focus on testing nondeclarative memory, when necessary, clinicians can test procedural memory by the ability to learn to trace an image in a mirror and priming by the speed of recognition from word fragments of previously seen words.

Declarative Episodic Memory