Sistemas de memoria: reseña histórica, clasificación y conceptos actuales. Segunda parte: Sistemas de memoria de largo plazo: Memoria episódica, sistemas de memoria no declarativa y memoria de trabajo / Memory systems: Historical background, classification and current concepts. Part two

ABSTRACT

According to recent evidence, memory can be conceptualized as a series of subsystems working together to reach the same final goal. In the present time, most authors coincide in proposing a main division or long-term memory systems in mammals. This division separates memory in two categories declarative and not declarative, often conceptualized also as explicit and implicit memories, respectively, although the latter terms are more appropriate to describe the role of consciousness during the development of codification and recovery tasks in regard to the information of each system. Declarative memory, as indicated by its name, includes a variety of memories implicating information that can be verbalized and effectively transmitted from person to person. This type of memory is conceptualized as conformed by two distinct memory sub-categories sometimes collaborating and coinciding in the use of memory. The first of these sub-categories is semantic memory, the one including the information about concepts and precise facts, and it is frequently defined as «general knowledge¼. It also makes reference to conceptual information lacking from the temporal-spatial frame (this issue is detailed in the first part of this paper). The second sub-category is episodic memory, which refers to the memory for personal experienced events, or the memory used for «what?, where?, and when?¼, in regard to the occurrence of a given personal event. Altogether, autonoesis (the conscious concept of a personally experienced event), subjective temporal consciousness (or subjective temporal frame about when a given event occurred), and self-consciousness are considered indispensable for the definition of episodic memory. In spite of these characteristics, there is yet controversy on whether this type of memory could be observed in animals. In this regard, some researchers have demonstrated that the processes needed to define episodic memory are effectively employed by non-human primates and rodents. In regard to the functional anatomy of episodic memory, an active role of the temporal lobe in the codification of some components of this memory is known from long time ago. Experimental studies employing hippocampal electro-physiological recordings have demonstrated that this region participates in two main components of episodic memory i) the strong links of the subject with the environment or the spatial context, and ii) the temporal organization of the stored information. Some studies have clearly demonstrated the important participation of specific hippocampal cell areas (CA1 and CA3) for the resolution of behavioral tasks requiring episodic memory. However, clinical studies employing functional imaging have shown a considerable diversity of cortical areas as involved in the several processes of codification or recovery of episodic information, going from prefrontal medial and left ventrolateral regions to medial and lateral temporal regions, retrosplenial cortex, posterior cingulus posterior, and even the cerebellum. Non-declarative memory systems Non-declarative memory contains different categories procedural memory, priming, associative learning (classic and operant conditioning) and non-associative learning (habituation and sensitization) a) Procedural memory refers to the storage and recovery of information on motor skills, or «know how to do¼ distinct tasks. Although this type of memory is considered as part of those memory systems often leading to unconscious learning, the relationship between procedural memory and consciousness is more complex. This is due in part to the fact that there is no evidence of a real association between the voluntary (conscious) desire of movement and the activation of motor brain areas. In addition, it has been demonstrated that movements consciously activated to start a given task may difficult the performance of the same task. Moreover, learning of motor skills exhibits a particular characteristic known as consolidation or off-line stabilization. This term refers to those motor skills exhibiting an improvement in performance during the interval of two practice sessions, meaning that the performance is improved if behavior is re-analyzed after an interval of rest typically occurring along the day or during sleep periods. This supports the theory that during sleep periods there is a recapitulation of events taking place during the day, thus favoring the strengthening of neuroplastic mechanisms involved in motor learning. b) Priming is a type of implicit memory not requiring any conscious recollection of previous experiences, and it shares some features with procedural and semantic memories. Similar to procedural memory, priming implies an increase of skills, but in this case, perceptual skills. It is also similar to semantic memory in terms of the involvement of cognitive representations of outdoor environment, and its representation is more cognitive than behavioral. The general characteristics of priming are i) it is related with the perceptual identification of objects in general terms, including words or concepts; ii) its neuronal substrate is not depending on those brain regions needed for episodic or semantic memories; iii) it is developed early in life and its capacity remains stable all the time; iv) its activity is not related with consciousness and its function is not sufficient to recall a previous experience; v) it is relatively immune to the effects of drugs affecting the other memory systems; vi) its information is distributed in multiple representations of specific words and objects; and vii) the access to this representations is highly specific to each category of objects. Finally, among short-term memory systems, the concept of working memory proposed by Baddeley and Hitch is still accepted in our days, and is characterized by the transitory storage and administration of all kind of information useful for the achievement of a specific task. This system is based on three main components i) a control system with limited attention capacity, also known as Executive Central Component; ii) the Phonologic Circuit, based on sound and language; and iii) the Visual-Spatial Scheme. The phonologic circuit is responsible for the transitory storage of language information based on sound and it is proponed to play an active role for learning of language. The Visual-Spatial Scheme is important for the visual-spatial representation of objects and their features, which in turn is relevant for the integration of concepts. The Executive Central Component is in charge of controlling the attention and the access to the other two subsidiary components. The knowledge on how the memory systems are functioning is and will remain in constant expansion, given the obvious contribution of recent studies on molecular biology and functional neuroimaging.

RESUMEN

La memoria episódica (ME) forma parte de los sistemas de memoria declarativa y es considerada como aquella de los sucesos experimentados de forma personal, o sea la memoria del «¿qué?¼, «¿dónde?¼ y «¿cuándo?¼ ocurrió cierto suceso de la experiencia personal. La triada de autonoesis (es decir, la conciencia de que un suceso le ocurrió personalmente), conciencia temporal subjetiva (o el marco temporal subjetivo de cuando ocurrió un suceso y en qué orden) y la conciencia de sí mismo, son consideradas como requisito indispensable para la definición de la memoria episódica. Hasta hace algunos años la memoria episódica era típicamente considerada como una habilidad exclusiva del ser humano, debido a los procesos cognitivos con los que se asocia, sin embargo, recientemente se ha comprobado que otras especies animales tienen y utilizan eficazmente un sistema análogo al de la ME para resolver diversas tareas de memoria en el laboratorio. Estos modelos experimentales de ME han impulsado la generación de conocimientos acerca de este sistema. La participación del lóbulo temporal, y en especial del hipocampo, en la ME es conocida desde hace tiempo, sin embargo se han identificado diversas áreas cerebrales implicadas en la recuperación y codificación de la información episódica. Una de las más estudiadas actualmente es la corteza parietal posterior, pero existen otras cuyo rol preciso en este tipo de memoria aún no se ha esclarecido. Por otro lado, la otra gran categoría central de la memoria la no declarativa, contiene diversas subcategorías heterogéneas en su contenido y función, como son la memoria procedimental, el priming, el aprendizaje asociativo (condicionamiento clásico y operante) y el aprendizaje no asociativo (habituación, deshabituación y sensibilización) mediado por vías reflejas. La memoria procedimental se refiere al almacenamiento y recuperación de información sobre habilidades motoras, es decir, el aprendizaje de «saber cómo hacer¼ distintas tareas. La memoria procedimental tiene relaciones complejas con la «conciencia¼, se ha demostrado que presenta un proceso de consolidación off-line, es decir que la información se consolida mejor cuando existe un periodo de reposo entre las sesiones de entrenamiento motor, sobre todo si durante este descanso ocurre el sueño. Existe una serie de estructuras tanto corticales como subcorticales que participan en diversas funciones de la memoria procedimental y cuya lesión selectiva produce distintos trastornos de las funciones motoras, como la parálisis, la apraxia, el temblor, los movimientos coreicos, las distonías, las dismetrías o la ataxia. El priming es un tipo de memoria implícita que no requiere de ninguna recolección consciente de experiencias previas, y que comparte algunas características con la memoria procedimental pero también con la semántica. Se trata de una variedad de memoria en la cual se propone que participan áreas neocorticales de asociación visual. Finalmente, dentro de los sistemas de memoria de corto plazo se encuentra el concepto de memoria de trabajo, propuesto por Baddeley y Hitch, que sigue teniendo aceptación en la actualidad y que es considerado como un sistema encargado de almacenar y administrar transitoriamente toda la información (de distintas modalidades) que se encuentra actualmente en uso para la realización de una tarea específica. Esta memoria de trabajo se conceptualiza como conformada por un componente ejecutivo central y dos sistemas subsidiarios el circuito fonológico (encargado entre otras cosas del reforzamiento verbal implicado en el aprendizaje del lenguaje) y el esquema visuo-espacial que estaría encargado del almacenamiento transitorio de la información espacial del ambiente externo. El conocimiento científico de la memoria continúa en expansión cotidiana, pero aún estamos lejos de conocer todos los detalles acerca de su funcionamiento desde el nivel molecular hasta el anatómico macroscópico. Sin duda los estudios moleculares y de imagen funcional ofrecerán en el futuro una panorámica más amplia al respecto.