Resumen

CARRILLO-MORA, Paul. Memory systems: historical background, classification, and current concepts. Part one: History, taxonomy of memory, long-term memory systems: Semantic memory. Salud Ment [online]. 2010, vol.33, n.1, pp.85-93. ISSN 0185-3325.

Memory is a fascinating brain function by means of which the nervous system can codify, store, organize, and recover a variety of information relevant to the subject. The formal study of memory started more than a century ago, providing in this time a considerable amount of scientific information on memory functioning. The actual knowledge of memory allow us to consider it far from being unique, isolated or static function, but more as a complex net of memory systems working in parallel for a common goal. Historic evolution of memory concepts and their classifications have progressed simultaneously to our knowledge on these systems. The first empiric approaches to memory description were found in ancient Greece by authors like Plato and Aristoteles, and were based on philosophy, thinking, and introspection and logic methods. However, the first real scientific approaches appeared in the XIX century, by authors such as Ebbinghaus and Lashley, who initiated the experimental study of memory in humans and animals, respectively. The contribution of the intellectual group known as <<behaviorist>> in the beginning of XX century, was also relevant during this period, and was represented by scientists like Pavlov, Watson, Skinner, and Thorndike, who detailed a variant of learning recognized in our days as associative learning. In turn, this variant can be divided into classic conditioning (associates a stimulus with a response) and instrumental or operant conditioning (associates a stimulus with a specific behavior). Behaviorists claimed that only on observational basis of behavior is possible to know processes linked to learning. However, these authors clearly made a mistake when they stated that knowledge on processes occurring in the brain will always be far of the understanding of the investigator. Later on, one of the hardest evidences for the study of memory processes came from clinic studies of patients with focal cerebral lesions. Penfield, Scoville, and Milner, during the 60's, documented the effects of surgical lesions of temporal lobe on declarative memory, finding selective alterations, such as severe anterograde amnesia and retrograde amnesia with temporal gradient. These findings were accompanied also by a description of memory subsystems remaining intact, despite of those temporal lobe lesions (procedural memory, long-term memory, etc.). Based on these findings, medial temporal lobe was described as a key structure for the acquisition of declarative information. In parallel to clinic studies, a first attempt to coordinate psycho-psychiatric research with the knowledge and scientific protocol of biology occurred in the 60's, thus allowing the emergence of disciplines known as cognitive neuroscience and cognitive psychology or psychophysiology, both created in an effort to explore the cellular and molecular mechanisms responsible for the storage of memory information. The list of significant findings provided by these two scientific disciplines is extensive, but among the most important are probably those derived from the study of the most elementary memory processes (i.e. habituation and sensitization) in invertebrate animal models, such as Aplysia spp. These researches established the basis for the basic cellular requirements for elementary learning, as well as the molecular basis for short-and long-term memory. The conjugation of these clinical descriptions together with experimental evidence led to the postulation of the first classifications of memory systems during the 80's. One of these classifications divided memory processes in two categories: declarative memory - the one containing information consciously acquired and easy to verbalize or transmit to other persons (this type of memory is also divided in semantic and episodic memories)-, and non-declarative -including that information not easy to verbalize or which acquisition is unconscious (this type of memory implies heterogeneous information in which the role of consciousness is complex and remains in discussion, and includes procedural memory, priming, classic, and operant conditioning, as well as the most elementary forms of learning, such as habituation and sensitization). To conceptualize these memory systems in an isolated form is a mistake since is results clear enough that all of them work together most of the time, and they work independently only very seldom, some other times cooperating, or even functioning in a competitive manner. Experimental studies have shown that the role of memory systems is different each time, even when two subjects perform exactly the same learning task, thus suggesting that codification, motivation and initial handling of information may determine whether this is processed as procedural, spatial or semantic information. The recent description of competitive relationships among the different memory systems (declarative vs. procedural) resulted to be an outstanding finding, although at the present there is hard clinical and experimental evidence indicating that a decrease of functions in procedural, spatial or declarative systems may induce the activation of another memory system. Initial studies in this field suggest that transient inactivation of striatal dorsal structures (implicated in performance of motor skills) or those from the hippocampus (implicated in the performance of spatial skills) facilitate learning in that system remaining active after pharmacological challenge. The real meaning of this competition among systems still remains unclear, although it has been proposed that both systems have evolved separately in response to distinct needs, which in turn might explain why eventually these systems can compete for the handling of information. Unfortunately, several studies have demonstrated that relationships among memory systems are complicated and poorly understood until now. Semantic memory Semantic memory mainly refers to information stored on characteristics defining concepts (facts lacking a well-defined spatial/ temporal frame), as well as processes allowing its efficient recovery for further utilization in language and thought. Knowledge about the anatomic location of semantic representations has gained attention with the use of new functional neuroimaging techniques. These studies have shown that the information about the features of specific objects needed to create concepts is stored in the same neuronal systems that remain active during the perception of different stimuli. The way in which this conceptual information is organized remains unclear so far, but there is evidence suggesting that its organization proceeds on the basis of grouped categories of concepts. Pathological studies in patients suffering semantic dementia have also demonstrated that some areas, such as temporal pole and perirhinal cortex, are relevant for semantic processing.

Palabras llave : Memory; semantic; episodic; procedural; working memory.

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