Memory function obeys certain neurologic laws. As memory fails, it first loses its hold on recent events. The extent in time of retrograde amnesia is generally proportionate to the magnitude of the underlying neurologic disorder. Early life memories are better preserved and often have been integrated into habitual responses; nevertheless, with natural aging, there is also a gradual loss of early life memories. In transitory amnesias (e.g., concussive head injury), memories are recovered in reverse order: first the remote and then the more recent. The enduring aspect of early life memories in contrast to more recently experienced and learned material, a restatement of the Ribot law, is apparent in both normal adults and in demented patients. As quoted by Kopelman, Ribot in 1882 stated: “The progressive destruction of memory follows a logical order—a law—it begins at the most recent recollections which, being rarely repeated and having no permanent associations, represent organization in its feeblest form.”
In the further analysis of the Korsakoff amnesic syndrome, it is necessary to consider the proposition that memory is not a unitary function, but takes several forms. One practical classification that adheres broadly to current ideas in the field is shown in Fig. 20-1 and Table 20-4. An initial distinction is made between the aforementioned immediate recall and the other types of memory. Short-term memory is exemplified by the common daily acts of hearing a phone number and retaining it in order to be able to walk across a room and dial the phone; or, performing a series of mental calculations that require holding an intermediate sum briefly in mind; all the numbers are soon forgotten. Long-term memory can be viewed from the perspective the individual’s awareness of the learning of new material (explicit memory), or not being conscious of the event of acquiring memory (implicit memory). Functions such the acquisition of physical skills (such as driving a car or playing tennis) are implicit memories that are termed procedural memory. Classic conditioning is considered another type of implicit memory.
Explicit memory subsumes what most persons consider to be memory and learning, that is, the ability to retain and recount events that were consciously experienced by the person, including the time and general circumstances of the acquisition (episodic, or autobiographical memory). Semantic memory, the learning of the nature of the environment and factual knowledge (such as the shape and color of a lion) is also a type of explicit memory but the event of acquiring the memory cannot be recalled.
A patient with virtually no capacity to learn any newly presented information can nonetheless still acquire some simple manual and pattern-analyzing skills. Moreover, having acquired these skills, the patient may have no memory of the circumstances in which they were acquired. The learning of simple mechanical skills has been referred to as procedural memory, in distinction to learning new data information. Cohen and Squire have described this dichotomy as “knowing how” as opposed to “knowing that.”
As confirmation of the separation of episodic from semantic memory functions, Gadian and colleagues have described young patients who showed severe impairments of episodic memory with relative preservation of semantic memory that was attributable to bilateral hippocampal damage from hypoxic-ischemic injury sustained early in life. Here, again, the subject matter most affected in this type of amnesia involved episodic, or autobiographical, memory. The same occurs in early Alzheimer disease in paraneoplastic and herpes simplex encephalitis.
A pervasive problem with these descriptive terms for various types of memory is the lack of uniformity in defining them. To Tulving, whose writings on this subject are recommended, the term episodic denotes a memory system for dating personal experiences and their temporal relationships; semantic memory is one’s repository of perceptual and factual knowledge, which makes it possible to comprehend language and make inferences. This hardly constitutes a novel concept; Korsakoff himself clearly recognized that certain aspects of mental function (among them those now being defined as semantic memory) are retained, despite profound impairment of episodic memory. Damasio has introduced yet another set of terms—generic in place of semantic and contextual for episodic. To Damasio, generic memory denotes the basic properties of acquired information, such as its class membership and function; he makes the point that in the amnesic syndrome, this component of declarative memory remains intact and only the contextual component is impaired.