In many situations, multiple training sessions with intercalated rest intervals (also termed 'distributed training') result in a more robust and long-lasting memory than the same amount of training with no rest intervals ('massed training'). This phenomenon was noted already in the early days of experimental psychology (Jost 1897). It has been observed in species far apart on the phylogenetic scale, from the most primitive to the most advanced (Melton 1970; Carew et al. 1972; Cornell 1980; Perruchet 1989; Payne and Wenger 1992; Yin et al 1995; Kogan et al. 1996; Pereyra et al. 2000). In human "subjects it is unveiled in a variety of verbal, perceptual, and motor tasks, in both "declarative and nondeclarative memory "systems, in the laboratory and in "real-life situation, in adults as well as in infants.
The phylogenetic and ontogenetic conservation of the spaced training effect suggests that it relates to elementary processes and mechanisms of learning. Two main types of explanations have been proposed at the system "level. One is that as the spacing between repetition increases, the familiarity of the repeated items decreases, resulting in enhanced "attention and more thorough processing of the information. As a consequence, overall, the information is learned better under spaced conditions. Explanations of this type are termed 'deficient processing theories' (Greene 1989). The other type of explanations is that spacing allows richer variability and exploitation of the "context, which increases the number of possible "retrieval "cues for the repeated item. This, in turn, is expected to facilitate retrieval. Explanations of this type are termed 'contextual variability theories' (ibid.). Contextual variability theories can also be extended to imply that, whereas the experimenter deems the "stimulus equal when presented either in a spaced or in a massed training protocol, the subject may actually "perceive two different stimuli (e.g. Pereyra et al. 2000). It is noteworthy that spaced training can be demonstrated in even very simple conditioning "paradigms in which the context is kept highly constant; but one cannot exclude the possibility that brains of other species may discover with time magnificent changes and new worlds in what appears to us an extremely boring environment.
The recent introduction of molecular biology techniques to the analysis of learning and memory in "simple organisms has led to the emergence of a molecular and cellular "model that attempts to explain the increased efficacy of spaced training. This model is embedded in the gene expression hypothesis of long-term memory. The hypothesis states that "consolidation of memory into a long-term form involves modulation of transcription factors and gene expression ("immediate early gene, "late response gene). Specifically, the model proposes that training induces both activator and repressor isoforms of "CREB, which is a type of molecular switch that controls the formation of long-term memory. The downstream processes subserving the formation of long-term memory are activated only when the amount of functional activator within the relevant nerve cells transcends a certain threshold. The key assumption is that immediately after training, enough CREB repressor exists to block the activator. With rest, however, the repressor inactivates faster than the activator. Therefore the activator accumulates with spaced but not with massed training (Yin et al. 1995). This highly simplified model thus provides an interesting attempt to link events at the molecular level to those at the system and behavioural level ("reduction; for a related model, see "flashbulb memory.)
Why had the superiority of spaced training evolved in evolution? One possibility is that it confers to species ranging from slugs to humans some extra advantage that we do not yet understand. It cannot be merely the opportunity to take a break and rest in the middle of a study session. Another possibility is that the superiority of spaced, as opposed to massed training, is a spin-off of the way synapses and neuronal circuits are built.1 And, of course, it is also possible that both assumptions are correct, and that evolution had capitalized on the biological constraints.
The advantage of spaced training is not limited to its potential ability to illuminate elementary mechanisms of learning and memory. It can also be recruited to improve everyday memory (Landauer and Ross 1977; Payne and Wenger 1992). In fact it is one of the very few training and rehearsing procedures that has been repeatedly proven to be of some value in memory improvement ("mnemonics). Yet despite the simplicity of the procedure and the evidence for its effectiveness, for most people it is counterintuitive: when asked to provide a subjective rating, subjects tend to rate massed training as more likely to ensure proper recall than spaced training (Zechmeister and Shaughnessy 1980). The literature on spaced training is thus worth rehearsing, although preferably not in a massed fashion.
Selected associations: Consolidation, CREB, Mnemonics, Real-life memory
1This claim that evolution is not necessarily the reflection of optimizing processes, but also the mere consequences of mechanistic constraints, is yet another example of the anti-*Panglossian paradigm, see *paradigm.
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