Short-term and long-term memory. A few basic concepts of neuropsychology will be introduced here as important background knowledge for an understanding of the functioning of the limbic system with respect to memory.William James, a founding father of modern neuropsychology, divided memory into two types, which he called “primary memory” and “secondary memory.”The contents of primary memory are held in consciousness for a short time after the sensory impressions that produced them are no longer present(short-term memory). Secondary memory, on the other hand, enables the individual to call up earlier events or states that have “disappeared from consciousness” in the meantime (long-term memory). The distinction between short-termmemory (STM) and long-termmemory (LTM) is nowan empirically well-founded model in neuropsychology. Certain illnesses or lesions of the brain can impair these two memory systems to different extents. Both systems must function normally to enable normal cognitive performance. Dysfunction of either system can be revealed by standardized testing.
Neural substrates of short-term and long-term memory. Hebb, in the 1940s, postulated that the two forms of memory just described have different neural substrates. Hebb thought of STMas a circulating activation in a pool of neurons, and of LTMas the product of long-lasting structural changes at the level of synaptic connections. According to the hebbian model, a process of consolidation, takingminutes or hours, is required for this structural adaptation to take place. Later neuropsychological studies in patients with memory disorders revealed that the hippocampus indeed plays a crucial role in the consolidation of conscious memories.
Diagnostic tests of STMand LTM. A commonly used test of STMis performed as follows. The subject (or patient) is asked to listen to, and repeat, spoken sequences of numbers of increasing length. A normal individual can repeat seven, plus or minus two, numbers presented in this way. These memory traces are very rapidly lost and fail to enter LTM. LTM, on the other hand, can be tested by presenting certain stimuli (e. g., a list of terms or a set of objects) and asking the subject to take note of themover a defined interval of time, and then to recognize or reproduce them some time later. This is a test of voluntary recall of conscious memories.
Subtypes of LTM. There are two distinct subtypes (subsystems) of LTM, called episodic and semantic memory. Episodic memory deals with data that belong to a particular spatial and temporal setting, i.e., memories of personal experiences (a trip, concert, sporting event, etc.). Semanticmemory, on the otherhand, deals with facts belonging to general fields of knowledge (medicine, physics, etc.).
Part of LTM can influence behavior without the subject’s conscious knowledge. A basic distinction is drawn between explicit (declarative) and implicit (nondeclarative) memory. The former deals with conscious and verbally communicable memories, as already described, while the latter deals with nonverbal memory traces, such as those that must be learned and recalled during the performance of a motor task. Implicit memory is also responsible for classical conditioning (as demonstrated in Pavlov’s well-known experiment with the dog), as well as for perceptual and cognitive skills, and for the priming effect: information presented in one context can be processed latermore efficiently in another context, even if the subject does not consciously remember the earlier presentation. The type of memory involved in the priming effect is stored and recalled “unconsciously,” so to speak, and can only be recalled during the performance of the relevant tasks.
Complex patterns can also be stored in implicitmemory. Thus, chess playerscan remember a particular pattern of chessmen on a chessboard better than can nonplayers, but only if the pattern has been drawn from a real chess game; they perform no better than control subjects who do not play chess if the pattern to be remembered has been generated at random.
In summary, memory is not a single functional entity, but rather possesses multiple distinct components.
Squire’s taxonomy of memory. Squire (1987) proposed a classification scheme for the subtypes of memory. In addition to explicit and implicit memory structures, this scheme recognizes other subtypes of memory that are required to perform metacognitive tasks, such as evaluating one’s own memory performance or generating strategies to organize information storage and recall. Strategies of the latter type are called frontal-lobe-type memory functions, because they apparently depend on intact functioning of the frontal lobes. During the process of memory storage, there seems to be a transition from the concrete to the abstract: for example, onemight be able to remember the approximate appearance of the school one attended as a child, without being able to sketch it in detail. Yet, while memory storage suppresses some aspects of experience, it accentuates others. The “memories” laid down by the storage process thus bear less resemblance to a documentary film than to a subjectively colored reconstruction of events. In summary, LTM should be thought of as a dynamic process, which changes over the years and often becomes increasingly abstract, but nonetheless remains capable of storing vivid and detailed traces of certain experiences, particularly those that are of personal importance.
Diagnostic tests of STMand LTM. A commonly used test of STMis performed as follows. The subject (or patient) is asked to listen to, and repeat, spoken sequences of numbers of increasing length. A normal individual can repeat seven, plus or minus two, numbers presented in this way. These memory traces are very rapidly lost and fail to enter LTM. LTM, on the other hand, can be tested by presenting certain stimuli (e. g., a list of terms or a set of objects) and asking the subject to take note of themover a defined interval of time, and then to recognize or reproduce them some time later. This is a test of voluntary recall of conscious memories.
Subtypes of LTM. There are two distinct subtypes (subsystems) of LTM, called episodic and semantic memory. Episodic memory deals with data that belong to a particular spatial and temporal setting, i.e., memories of personal experiences (a trip, concert, sporting event, etc.). Semanticmemory, on the otherhand, deals with facts belonging to general fields of knowledge (medicine, physics, etc.).
Part of LTM can influence behavior without the subject’s conscious knowledge. A basic distinction is drawn between explicit (declarative) and implicit (nondeclarative) memory. The former deals with conscious and verbally communicable memories, as already described, while the latter deals with nonverbal memory traces, such as those that must be learned and recalled during the performance of a motor task. Implicit memory is also responsible for classical conditioning (as demonstrated in Pavlov’s well-known experiment with the dog), as well as for perceptual and cognitive skills, and for the priming effect: information presented in one context can be processed latermore efficiently in another context, even if the subject does not consciously remember the earlier presentation. The type of memory involved in the priming effect is stored and recalled “unconsciously,” so to speak, and can only be recalled during the performance of the relevant tasks.
Complex patterns can also be stored in implicitmemory. Thus, chess playerscan remember a particular pattern of chessmen on a chessboard better than can nonplayers, but only if the pattern has been drawn from a real chess game; they perform no better than control subjects who do not play chess if the pattern to be remembered has been generated at random.
In summary, memory is not a single functional entity, but rather possesses multiple distinct components.
Squire’s taxonomy of memory. Squire (1987) proposed a classification scheme for the subtypes of memory. In addition to explicit and implicit memory structures, this scheme recognizes other subtypes of memory that are required to perform metacognitive tasks, such as evaluating one’s own memory performance or generating strategies to organize information storage and recall. Strategies of the latter type are called frontal-lobe-type memory functions, because they apparently depend on intact functioning of the frontal lobes. During the process of memory storage, there seems to be a transition from the concrete to the abstract: for example, onemight be able to remember the approximate appearance of the school one attended as a child, without being able to sketch it in detail. Yet, while memory storage suppresses some aspects of experience, it accentuates others. The “memories” laid down by the storage process thus bear less resemblance to a documentary film than to a subjectively colored reconstruction of events. In summary, LTM should be thought of as a dynamic process, which changes over the years and often becomes increasingly abstract, but nonetheless remains capable of storing vivid and detailed traces of certain experiences, particularly those that are of personal importance.
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