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as a homogeneous state. Despite much research, the function of sleep remains unknown. A considerable number of hypothetical functions of sleep have been proposed including homeostatic restoration, thermoregulation, tissue repair, immune control and memory processing. One of the most intriguing possibilities is that sleep contributes greatly to processes of memory and brain plasticity (Diekelmann & Born, 2010; Rasch & Born, 2007; Rauchs, Desgranges, Foret, & Eustache, 2005; Walker &
Stickgold, 2006). The term “memory,” like “sleep,” is not a single entity. Human memory is sorted into different classifications.
2. Categories of Memory
In recent years, a number of memory researchers have attempted to divide learning and memory into multiple memory systems. These include the distinctions between implicit and explicit, episodic and semantic memory, procedural and declarative memory, or data-driven and conceptually-driven processing. One of the most accepted categorizations was to categorize memory into declarative and
non-declarative memory. Declarative memory, also called explicit long-term memory, is that in which a person can call to mind, while non-declarative memory, also called implicit long-term memory, is that which is normally used without conscious
recollection (see figure 2). Declarative memory includes consciously accessible memories of fact-based information (i.e. knowing ‘what’), and contains several subcategories, including episodic memory that is memory for events in one’s past and semantic memory which is memory for general knowledge. By contrast,
non-declarative memory includes all non-conscious memories. It can be further divided into several subcategories, such as classical conditioning, non-associative learning (i.e. habituation and sensitization), implicit memory and procedural memory (i.e. knowing ‘how’) (Squire, 1986; Squire & Zola, 1996; Tulving, 1972; Zola, 1998).
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Neuroanatomically, declarative memory involves the hippocampus, along with adjacent entorhinal, parahippocampal, and perirhinal cortices (Eichenbaum, 2000;
Squire & Zola, 1996), which are thought to form a temporally ordered retrieval code for neocortically stored information. The task used to be involved in declarative memory is word paired-associates learning. Mary Whiton Calkins, who was president of the American Psychological Association in 1905, invented this experiment
procedure for studying memory. The paired associates method requires subjects to learn pairs of items by forming associations between them. There are two forms of paired associates tasks: one is “related” word pairs such as “doctor–nurse” which is semantically or associatively related (Backhaus & Junghanns, 2006; Gais et al., 2007;
Plihal & Born, 1997; Tucker et al., 2006). The other form of tasks are “unrelated”
word pairs such as “doctor–leaf” which are self evident (Barrett & Ekstrand, 1972;
Ekstrand, Sullivan, Parker, & West, 1971; Ficca, Lombardo, Rossi, & Salzarulo, 2000;
Gais, Molle, Helms, & Born, 2002; Grosvenor & Lack, 1984; Mazzoni et al., 1999;
Schmidt et al., 2006; Stickgold, Scott, Rittenhouse, & Hobson, 1999; Yaroush, Sullivan, & Ekstrand, 1971). Learning unrelated word pairs requires the forming of completely novel associations, which is hippocampus-dependent. Learning related word pairs, on the other hand, strengthens well-formed associations and is
hippocampus-independent (Stickgold, 2004).
In addition to word pair learning, there are various tasks that have been used to explore the relationship between sleep and declarative memory including (1) word list learning (Tamminen, Payne, Stickgold, Wamsley, & Gaskell, 2010), (2) face-name association (Backhaus & Junghanns, 2006; Clemens, Fabo, & Halasz, 2005), (3) foreign language or vocabulary learning (De Koninck, Lorrain, Christ, Proulx, &
Coulombe, 1989; Gais, Lucas, & Born, 2006; Meienberg, 1977), (4) story retention (Tilley & Empson, 1978), (5) visual-spatial learning (Rasch, Buchel, Gais, & Born,
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2007; Talamini, Nieuwenhuis, Takashima, & Jensen, 2008), (6) virtual maze- learning (Wamsley, Tucker, Payne, Benavides, & Stickgold, 2010), (7) semantic priming (Brualla, Romero, Serrano, & Valdizan, 1998; Plihal & Born, 1999) and (8) rule learning, in which participants learn a set of rules that they then apply to recombine sets of letters into logical sequences (Smith & Weeden, 1990).
On the other hand, non-declarative memory is composed of many different cognitive forms. These various processes are related to distinct neuroanatomical structures, involving both cortical and subcortical networks (Doyon, Penhune, &
Ungerleider, 2003). In the sleep-dependent non-declarative memory consolidation, three categories of learning were administrated in past studies: (1) motor learning (Fischer, Hallschmid, Elsner, & Born, 2002), (2) visual-perceptual learning (Gais, Plihal, Wagner, & Born, 2000; Karni, Tanne, Rubenstein, Askenasy, & Sagi, 1994), and (3) auditory learning (Atienza, Cantero, & Dominguez-Marin, 2002; Atienza, Cantero, & Stickgold, 2004). The tasks involved the components of motor skills broadly classified into two forms – “motor adaptation” (e.g. finger adaptation task) and “motor sequence” (e.g. sequential finger tapping task) (Doyon, et al., 2003).
Motor adaption tests the capacity to compensate for environmental changes, whereas motor sequence measures the incremental acquisition of movements into a
well-executed behavior. In daily life, learning how to use a computer mouse involves motor adaptation, whereas learning how to play a musical instrument like the piano is related to motor sequence. The finger adaptation task, for example, requires subjects to move a cursor with a computer mouse to reach a randomly moving target on a screen. Moreover, pursuit rotor task is also used in measuring motor adaptation. The pursuit rotor test includes a turntable with a metal dot on the platter. Subjects have to keep a flexible metal wand on the dot with motor coordination. The other task associated with motor function is the mirror-tracing task (Born, 1997; Tucker, nap
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study).
Additionally, a widely accepted mechanism of memory formation is brain plasticity. Brain plasticity refers to the ability of the brain to persistently change its structure and function according to the acquisition of new information. According to Hebb’s theory (1949), memory consolidation is functional changes at the synapses which increase the efficacy of synaptic connection and occurs when pre-synaptic neurons repeat and persistently take part in exciting the post-synaptic neurons. Hence, strengthening connections between neurons leads to synaptic plasticity to enhance memory. As a process, memory refers to the dynamic mechanisms associated with retaining and retrieving information as to past experience. Memory processing can be categorized into three stages: acquisition, consolidation, and retrieval. Acquisition is the time when the individual requires the information to be remembered, or
transforms data into a form of mental representation. Consolidation is the process by which recent memory is stabilized and enhanced into long-term memory. Retrieval is the process of recalling a memory.