The memory phases during which this stabilization takes place

It took the Muses, daughters of Memory, a single encounter with Hesiod on Mount Helicon to breathe into the poet divine voice and knowledge (Hesiod 8thC bc). Yet for most of us, who are not granted the privilege to mingle with the immortals, learning is often a much more lengthy, complicated, and frustrating process. Even if we do devote to training sufficient effort, memory may still betray us (false memory, "forgetting). And as if the burden of confusion and forgetfulness stemming from the continuing passage of time is not enough, the period immediately after learning also contributes its share to the fragility of our engrams. It has long been recognized that fresh memories need time to stabilize (Quintillian 1stC ad), and are particularly labile and prone to interference by agents ranging from distracting "stimuli to injury and drugs (Muller and Pilzecker 1900; McGaugh 1966, 2000; Dudai and Morris 2000). This brittle phase in the life of a memory is assumed to reflect the process of consolidation (consolidare, Latin for 'to make firm'). Consolidation, however, is not necessarily completed within a short time after learning; in some types of memory it may continue for weeks, months, even longer.

Consolidation is a term used to denote memory stabilization processes at different "levels of brain organization. Molecular and cellular neurobiologists use it to refer to 'local', or cellular consolidation (Dudai and Morris 2000). This is the time-dependent stabilization of information storage at local nodes, or "synapses and their cell body, in the neuronal circuit that encodes the memory. Cellular consolidation is accomplished within a few hours after training (e.g. Montarolo et al. 1986). In practice, it is commonly defined as that time window during which the formation of the long-term form of the newly acquired memory can be blocked by inhibitors of "protein or RNA synthesis. Certain other drugs, or electric shock, could also be used to block consolidation. The temporal parameters of the consolidation period will, however, depend on which treatment is used. This is because cellular consolidation involves multiple phases, some of which are sensitive to some treatments but not to others (Grecksch and Matthies 1980; Rosenzweig et al. 1993; DeZazzo and Tully 1995; Freeman et al. 1995; Ghirardi et al. 1995). The understanding of cellular consolidation has advanced impressively in recent years, as a consequence ofthe remarkable achievements in molecular and cellular neurobiology. We now know that in cellular consolidation, post-translational modifications, induced by activation of "intracellular signal transduction cascades, culminate in the modulation of neuronal gene expression and ultimately in the synthesis of new proteins ("CREB, "immediate early genes, "late response genes). These processes involve local synaptic mechanisms, cell-wide mechanisms, and cross-talk between the synapse, cell body, and nucleus (Frey and Morris 1997; Martin et al. 1997a; Casadio et al. 1999; Dudai and Morris 2000). The new proteins are believed to embody and subserve long-term modifications in the functional properties of the synapse. This probably involves morphological "plasticity as well (Bailey and Kandel 1993; "development). It is thought that once the use-dependent modulation of gene expression is triggered, the plasticity change becomes immune to molecular turnover and independent of certain signal transduction cascades, and a consequence, is no more sensitive to the consolidation blockers.

Cellular consolidation has been identified in the brain of every species that learns, and in every type of memory that lasts for more than a few hours or days. "Model neural networks can be shown to be capable of a consolidation-like process solely on the basis of modifications in local nodes (Rumelhart et al. 1986c); this may also be the case in simple neuronal circuits. However, local, cellular consolidation is not the whole story in some more complex circuits. In the mammalian brain, and possibly in other vertebrates, additional consolidating processes operate at the system level. These processes of system consolidation involve gradual recruitment and continuous reorganization of distributed brain circuits long after acquisition has occurred. System consolidation can be detected by noting the time-dependent sensitivity of certain types of memory to circumscribed brain lesions, or the change over time in the activation of specific brain circuits in a memory task, as revealed by "functional neuroimaging.1 System consolidation in some systems occurs within a few hours (Shadmehr and Holcomb 1997a,b), but in other systems it can take much more time: several days (Winocur 1990), weeks (Cho et al. 1993; Bontempi et al. 1999), even years (Schmidtke and Vollmer 1997; Reed and Squire 1998; Teng and Squire 1999;Haist et al. 2001; but see Nadel and Moscovitch 1997). It is believed to be promoted by endogenous brain activity, sensory input, and their interactions; the triggers and the circuit mechanisms are unknown, but the cellular mechanisms are likely to be similar to those mentioned above for cellular consolidation. The most notable example of system consolidation is provided by "declarative memories. These memories are assumed to be stored in the long run in circuits that involve the "cerebral cortex. But in the first period after learning, they depend on the "hippocampal formation. With time, this dependency disappears (ibid.; "amnesia).

An intriguing question, which has received much attention and stirred much debate in recent years, is whether for any memorized item, consolidation starts and ends just once. The data from cognitive psychology indicate that memory traces are reconstructed with use, and that "retrieving a memory item could involve mingling the "internal representations of the past with the "percepts of the present (e.g. Bartlett 1932; Tulving 1983; Schacter et al. 1998; "false memory). This mnemonic reconstruction process raises the possibility that engrams may be reconsolidated upon retrieval. There is evidence that a cellular consolidation phase may indeed ensue retrieval (Misanin et al. 1968; Spear and Mueller 1984; Nader et al. 2000; Sara 2000; but see, for example, Dawson and McGaugh 1969). It is not yet established, however, whether the entire reactivated trace2 could become sensitive to interference in this assumed reconsolidation, or, alternatively, whether a 'core memory' has privileged stability. What already becomes apparent is that the cellular mechanisms that consolidate novel and reactivated traces, respectively, differ from each other (Berman and Dudai 2001; Taubenfeld et al. 2001); and that the stability of the retrieved trace after post-retrieval interference is task and region dependent (Berman and Dudai 2001).

Why do memories consolidate in the first place? One could envisage a situation in which newly formed memories stabilize instantaneously, evading the risk of erasure by sensory and metabolic interference ("flashbulb memory). An appealing explanation is that instant acquisition of every percepts is bound to waste brain space on useless items ("capacity). Another possibility is that the post-stimulus time window of consolidation, during which the new information is particularly malleable and associates easily with other inputs, could assist the encoding and registration of selected, meaningful mental narratives (Frey and Morris 1997; Dudai and Morris 2000). Similarly, it has been suggested that system consolidation promotes better categorization and hence a more coherent, effective, and parsimonious construe of the world (McClelland et al. 1995). All these arguments share a teleological flavour. There is also the other, anti-zeitgeist type of possibility, that cellular consolidation, system consolidation, or both, are

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(b)

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System i i i

30 60 90 1 7 14 Min Days

30 60 90 1 7 14 Min Days

Fig. 19 Two types of consolidation windows in behaving animals. (a) The time course of cellular consolidation, determined by measuring the sensitivity of memory to the inhibition of *protein synthesis. Consolidated memory is defined as treatment-resistant long-term memory. The data are from experiments on shuttle box learning in the goldfish (Agranoff et al. 1966; *classic). The protein synthesis inhibitor was administered to the *subject at the indicated time point after training. The sensitivity of memory to protein synthesis inhibition is over by about 1 h. A consolidation process that depends on protein synthesis during and immediately after training is a universal property of the nervous system. (b) The time course of system consolidation, determined by measuring the sensitivity of long-term memory of *contextual *fear conditioning to a lesion in the rat *hippocampus. The lesions were inflicted at the indicated time points after training. The dependence on the hippocampus in this case is over by about 1 month. Data from Kim and Fanselow (1992). A system consolidation process that lasts weeks or even longer, during which the memory becomes independent of the hippocampus, is observed in *declarative memory tasks.

spin-offs of the constraints imposed on biological memory systems by the design and maintenance of the biological hardware, rather than functional properties selected in evolution.3

Selected associations: Acquisition, Immediate early genes, Phase, Protein synthesis, Retrieval

'Long-term memory may be subjected to continuous reorganization throughout life. Reorganization alone is therefore insufficient as a 'criterion for consolidation, because otherwise one will reach the conclusion (that some authors indeed reach) that system consolidation proceeds forever, which deprives the notion of consolidation phase of its usefulness. The criteria for system consolidation should therefore ultimately include a limited period of obligatory dependency on the activity of a specific brain region. This could be demonstrated by time-limited sensitivity to lesions or to inactivation of this region.

2On the distinction between active and inactive trace, see Lewis (1979); 'retrieval, 'taxonomy.

3This tension between adaptive selection and built-in biological constraints is a recurrent issue in this book; see the Panglossian paradigm, in 'paradigm.

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