Commenton//Llewellyn: Elaborative encoding (REM) sleep (as opposed to non-REM [NREM] sleep or slowwave sleep) in die formation of episodic memories, LleweUyn is partly in opposition to other frameworks (Diekelmann et al. 2009; Stiekgold & Walker 2007). Recently, for instance, Hahn et al. (2012) demonstrated diat entorhinal layer HI may be a mediator for memoiy consoUdation during slow-wave sleep. Aldiough Llewellyn reviews an impressive amount of data, she neidier elaborates on several estabUshed findings nor comments on die association between REM sleep, dreaming, and episodic memoiy in special populations, such as children. Young children seem to have at least die same amount of REM sleep as adults (Davis et al. 2004; Siegel 2011). However, developmental data show diat the episodic memory system emerges only later in Ufe (Fivush 2011; WiUoughby et al. 2012). Furthermore, motor-perceptual sküls-which probably are acquired very early in Ufe-also seem to bear relationships with bodi NREM and REM sleep (Rauchs et.al. 2005). One question diat subsequendy arises is why infants (and many mammalian species) have so much REM sleep but do not have episodic memory (Tulving 2005). Ghildren's dream recaU was found to increase significandy after age 7, correlating with mental imageiy abiUty and visuospatial sküls (Nir & Tononi 2010) radier dian odier inteUectual capacities.. These abiUties are subseived by neural substrates diat comprise extialiippocampal areas. LleweUyn puts special weight on the relationship between REM-sleep dreaming, hippocampus, and episodic memor)'. Her focus on the hippocampus is justified, mirroring that of odier researchers. However, during die first years of Ufe, die hippocampal formation-one of die crucial brain regions for the formation (or "indexing," as LleweUyn writes [sect. 4.2.5, para. 5]) of episodic memories-is stiU immature (Josselyn & Frankland 2012), whereas semantic memories can already be formed via paraliippocampal areas (e.g., Vargha-Khadem et al. 1997). Paraliippocampal cortex, which receives a diverse gamut of syndiesized sensory-specific in addition to multimodal cortical information (Nieuwenhuys et al. 2008), was also shown to be engaged in REM sleep (as Llewellyn also remarks). Activation of the parahippocampal region was furthermore found in relation to place perception (Kanwisher 2010) and processing of scenes and landmarks (Piefke et al. 2005). The amygdala is also activated during REM sleep and plays roles in different phases of episodic mnemonic memory processing (encoding, consoUdation, and retrieval) (AUy et al. 2012; Markowitsch & Staniloiu 2011a). Paraphrasing LleweUyn, we ought to say that the term episodic memory is rarely precisely defined before it is used. This is also die case in target aiticle. The various ways die term is currendy employed in the Uterature constitute a source of confusion for empirical data interpretation. Llewellyn herself points to data "difficult to reconcile" (sect 7, para. 2), inipl)'ing diat the relation between REM sleep and episodic memoiy may depend on tlie conceptuaUzation of episodic memory and testing paradigms employed. LleweUyn's definition of episodic memory-"enduring memories of personaUy significant past events witli conte.xtual place and time underpinnings (Squire 1987; Tulving 1983; 1991)" (sect. 1.1, para. 2)-is itself outdated (Markowitsch & StanUoiu 2011b; 2012; TuKdng 2005). As Szpunar and McDermott (2008) remark, episodic memory has been an "evolving concept." TuKang, who coined the constmct episodic memory, used to emphasize the refinements in its conceptuaUzation by giving a date for his most recent definition (e.g., Schacter & TuKang 1994). Whereas a few decades ago the term episodic could be applied to describe memory for laboratory stimuU widi a specific embedding in time and place, currendy die episodic memory system is viewed as equivalent to the episodic autoljiographical memory system. In 2005, Tulving's definition of episodic memory comprised 17 Unes, ending with diis sentence: "The essence of episodic memor)' Ues in die conjunction of three concepts-self, autonoetic awareness, and subjective time" (p. 9). This definition deviates considerably from previous ones

and from diose stiU in use in some Uterature - for example, "What-Where-When" tasks (sect. 7, para. 2), to which LleweUyn refers with respect to diefincUngsof Rauchs et al. (2004). It has been observed diat specific details, pertaining to "what, where, when," may be produced by some individuals without tme firstperson reexperiencing (Markowitsch et al. 1997) that is essential for strictly episodic memories (events) (Eustache & Desgranges 2008; Viard et al. 2012). On the odier hand, diere are individuals who judge dieir recalled material with a high confidence as representing tme reexperiencing wliile deUvering few episodic details (Lex'ine et al. 2009). This may reflect impoverished narrative abilities or that having several similar episodic events may reduce die memory strengdi for diem, leading to semanticization (Germak 1984). We concur with Llewellyn's reservation about using Verbal Paired Associates tests to uncover the relationship between REM sleep and episodic memory because diey do not match the current understanding of episodic autobiographical memory. Furdiermore, LePort and coUeagues (2012) found diat individuals widi higlily superior autobiograpliical memory did not perform significandy better on the Verbal Paired Associates test dian did controls. Since 2005, several audiors have increased die sophistication of their testing paradigms for episodic autobiographical memory (e.g., Levine et al. 2009). It seems dierefore plausible diat by employing testing paradigms diat tap into die more recent definition by Tulving (2005), the contribution of slowwave sleep to episodic memoiy wül be viewed from a different perspective. In conclusion, die Unk between REM sleep and strict episodic memory (as defined currendy) is far from being unequivocal. Also, die contribution of NREM sleep to strict episodic(-autobiographical) memory needs furdier exploration. How different stages of sleep support the Ukely interactive processing widiin different memory systems remains a topic for future research (Dew & Gabeza 2011).

The secret is at the crossways: Hodotopic organization and nonlinear dynamics of brain neural networks doi:10.1017/S0140525X13001386 Tobias A. Mattei Interdisciplinary Group for Research in Neuroseience, Epistemology and Cognition, Neurological Department, The Ohio State University, Columbus, OH

43210. tobias.mattei ©osumc.edu

Abstract: By integrating the classic psychological principles of ancient art of memor)' (AAOM) with the most recent paradigms in cognitive neuroseience (i.e., die concepts of hodotopic organization and nonlinear d)'namics of brain neural networks), Llewellyn prowdes an up-to-date model of the complex psychological relationships between memory, imagination, and dreams in accordance with current state-of-tbe-art principles in neuroseience.

In die target article. Sue LleweUyn proposes diat rapid eye movement (REM) dreaming is able to encode new episodic memories dirough several cognitive processes that enhance die UkeUhood of information retrieval by providing strong associations with odier remote and emotionaUy salient memories. After such presentation, one great question arises: Is diere any biological basis to support that the proposed connective pattems actually occur in a deeper physiological level at die neural networks involved in long-term memory encoding and dreaming (such as the mesial temporal stmctures and die Umbic networks), or is this just another purely speculative description (according to LleweUyn, based on the ancient art of memor)' [AAOM] principles) about some possible stmctural pattem in tlie relationships BEHAVIORAL AND BRAIN SCIENCES (2013) 36:6

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