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Memory relates to changes in the brain initiated by learning. These changes must represent the acquired information in forms that can be used to guide adapted perception and behavior. Since the memory content’s code is unknown, memory is accessible only via retrieval from a store, and thus memory always has two intimately entangled aspects: storage and retrieval. Mechanistic approaches to memory have focused on the storage processes, and retrieval has been mostly neglected because of the lack of experimental tools. Since Ebbinghaus (1885) it has been known that learning leads to memory, but memories are not made instantaneously by learning, rather, they develop and change over time. When Hermann Ebbinghaus began the scientiﬁc study of memory in humans, he discovered that memory formation is a time-consuming process and depends on the interval between sequential learning trials. Later, James (1890) developed the concept of primary and secondary memory, referring to the limited timespan and information capacity of primary memory, and the seemingly unlimited duration and content of secondary memory. Around the turn of the century, psychologists had established a framework of thinking about sequential memory stages, which was captured by the perserveration-consolidation hypothesis of Muller and Pilzecker (1900). Neural processes underlying newly formed memories initially perseverate in a labile form and then, over time, become consolidated into lasting neural traces.
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In this sense, memory dynamics is not restricted to humans and mammals, but is a general property in animals. Using invertebrate model systems one can ask basic questions of memory formation. Why should memory take hours, days, or weeks for ﬁnal adjustment of the circuit? Is the neural machinery so slow? The analysis of the memory trace’s molecular and neural properties has gained from studies of invertebrate species, because of their small number of large neurons, as in the marine slug Aplysia; their wellworked-out classical and molecular genetics, as in the fruit ﬂy Drosophila; or their potential to record memory stage correlates in alert and behaving animals even at the level of single neurons and circuits, as in an insect, the honeybee Apis mellifera. These studies prove that even in such rather simple systems memory is a highly dynamic process of multiple memory traces at multiple neural sites.
The combined temporal and spatial properties of the memory trace can be studied very well in the bee, because a bee will associate an odor with reward quickly, even under conditions when the brain is exposed to neural recordings (Menzel 1999). Natural learning behavior in bees is well deﬁned, because appetitive memory formation occurs during foraging on ﬂowers, which are rather unpredictable and widely distributed food sources. Perseveration and consolidation of memories appear to be adapted to the demands and constraints of the speciﬁc requirements to which the bee is exposed in its natural environment. Thus a bee, like other animals, behaves at any certain time with reference to information gathered over long periods of time, and at any particular moment information is evaluated according to genetically controlled internal conditions (thus the phylogenetic history of the species) and parameters of the experience gathering process such as reliability, context-dependence, and what new information means to the animal. These aspects are a function of time. New memories must be incorporated into existing ones based on their relevance.
The concepts evolving from these arguments can be studied in bees using a classical conditioning paradigm in which a harnessed bee learns to associate an odor with reward and expresses its retention by extending its tongue in response to the odor alone. The neural circuit (Fig. 1(A)) underlying this behavior is well described (Hammer and Menzel 1995, Hammer 1997), and learning-related changes can be monitored by intracellular el ectrodes or by imaging ﬂuorescence patterns of Ca + activity (Faber et al. 1999, Menzel and Muller 1996). Furthermore, molecular correlates can be measured at high temporal resolution by quickly blocking enzymatic activity and speciﬁc detection of second messenger-dependent reaction cascades in small compartments of the bee brain (Muller 1996).
The dynamics and distribution of the memory trace can be conceptualized as a stepwise, timeand eventdependent process (Fig. 1(B)) reﬂecting the temporalspatial characteristics of multiple memory traces. Such an analysis helps correlate the underlying mechanisms of behavioral data.
1. Associati e Induction and Early Short-term Memory (eSTM)
An associative learning trial leads to associative induction and an early form of short-term memory (eSTM) in the range of seconds. This memory is initially localized in the primary sensory neuropil, the antennal lobe, and is highly dominated by appetitive arousal and sensitization induced by the unconditioned stimulus (US), sucrose. Thus eSTM is restricted to the ﬁrst synaptic connections in the sensory pathway. It is rather unspeciﬁc and imprecise. The associative component, which connects speciﬁc stimuli with motor actions, already exists at a low level, and develops later over a period of several minutes (consolidation). eSTM covers the time window during which bees can expect to be exposed to the same stimuli, since ﬂowers most frequently occur in patches. No speciﬁc choices need to be performed at this time, and general arousal (depending on the strength of the US) will suﬃce to control whether the animal stays in the patch or postpones choices for a later time.
At the cellular level, stimulus association is reﬂected in the convergence of excitation of the pathways for the conditioned stimulus (CS) and the unconditioned, rewarding stimulus (US). There are three neuroanatomical convergence sites of these pathways (Fig. 1(A): antennal lobe, lip region of the mushroom bodies, lateral protocerebrum); two of these sites can independently form an associative memory trace for an odor stimulus. The dynamics of these two traces are diﬀerent: the antennal lobe (ﬁrst-order neuropil) establishes the trace quickly and gradually, the mushroom body stepwise by a consolidation process in the range of minutes during the transition to a late form of STM (lSTM). The molecular substrates for the formation of the associative links are unknown. Neither a glutamate receptor, as shown for the hippocampus, nor adenylylcyclase, as proposed for Aplysia, appear to play a role
in the bee. However, cAMP upregulation of PKA during STM is necessary for memory transition to mid-term memory (MTM) and long-term memory (LTM), and an increase in PKA activity is speciﬁcally connected with associative trials, indicating PKA’s role in consolidation to lasting memories established during consolidation (Fig. 1(B)).
2. Late Short-term Memory (lSTM)
The transition to the selective associative memory trace during lSTM is a rather slow process after a single learning trial lasting up to several minutes, and a quick one after multiple learning trials. Thus, consolidation is both time and event-dependent, where events must be associative experiences and not just CS or US repetitions. The mushroom bodies, a high-order, multiple sensory integration neuropil, appear to be selectively involved.
The behavioral relevance of these ﬁndings for foraging behavior under natural conditions may be related to the temporal separation between intraand interpatch visits. First, memory needs to be speciﬁc after leaving a patch, because distinctions need to be made between similar and diﬀerent ﬂowers. Second, such a speciﬁc memory trace should also be established after a single learning trial, because in some rare cases a single ﬂower may oﬀer a very high amount of reward. Third, discovering a rewarding ﬂower in a diﬀerent patch means that the local cues just learned are now presented in a diﬀerent context. lSTM is, therefore, a component of a working memory phase (together with eSTM), during which context dependencies are learned. Fourth, consolidation might be a highly beneﬁcial property for a foraging bee which must optimize its foraging eﬀorts with respect to reward distribution. In the ﬂower market bees can extract the proﬁtability of a ﬂower type only on the basis of its frequency within the foraging range and its probability of oﬀering food. Timeand eventdependence of memory consolidation in lSTM could be a simple means of extracting this information.
Consolidation into lasting memories depends not only on PKA activity but also on NO synthase activity, indicating that both cAMP and cGMP are important second messengers for consolidation. It is likely that the target of both second messengers is PKA, supporting the interpretation that high PKA activity is essential for the transition to long-lasting memories during consolidation (Fig. 1(B)).
3. Mid-term Memory (MTM)
At the beginning of MTM, behavior is controlled by consolidated, highly speciﬁc memory. At this stage memory is more resistant to extinction, conﬂicting information, and elapsing time, and some information about the context dependencies may have already been stored. Under natural conditions bees have usually returned to the hive and departed on a new foraging bout within the time window of MTM. Upon arrival at the feeding area, memory for ﬂower cues no longer resides in working memory (eSTM), but needs to be retrieved from a more permanent store. Therefore, MTM is a memory stage clearly disconnected from a continuous stream of working memory.
MTM is physiologically characterized by a wave of protease-dependent PKC activity in the antennal lobe (Fig. 1(B)). It might, be therefore, that the primary sensory neuropil is a substrate for MTM. Since the mushroom bodies appear to be the critical substrate for consolidation during lSTM, it has been speculated that the MTM trace in the antennal lobe is established under the guidance of the mushroom bodies. The mushroom bodies provide the information which relates the memory traces in the primary sensory neuropils to context stimuli across modalities. Output neurons of the mushroom bodies feed back to the antennal lobe. A particular output neuron, the Pe 1, projecting to a premotor area, shows associative plasticity only during lSTM (Mauelshagen 1993). It might thus be that the mushroom bodies are only involved during working memory (eSTM and lSTM), and provide the information necessary to couple the long-term stores in the sensory (and possibly also the motor) neuropils.
4. Long-term Memory (LTM)
LTM is divided into two forms, an early LTM (eLTM, 1–3 days) characterized by protein synthesis-dependent PKC activity, but not by protein synthesis-dependent retention, and late LTM (lLTM, 3 days) protein synthesis-dependent retention and no enhanced PKC activity. The transition from lSTM to both forms of LTM appears to be independent of MTM, because inhibiting the characteristic substrate of MTM (protease-dependent enhancement of PKC activity) does not prevent eLTM and lLTM being formed (Fig. 1(B)). LTM requires multiple learning trials, indicating that speciﬁc information which can be extracted only from multiple experiences (signal reliability, context dependence) controls transfer to LTM. This transfer can be related to a change in the proportion of the activator and repressor forms of the PKA-responsive transcription factor CREB in Drosophila (Yin et al. 1995a, 1995b), but the role of CREB in the bee is still unknown. The picture emerging from ﬁndings on Drosophila is that LTM formation can be actively suppressed, rather than it being automatically produced with associative events accumulating and time elapsing. The balance between the activator and repressor form of CREB should therefore depend on the information content gained by multiple learning trials, rather than their mere accumulation, a proposal which needs to be tested.
Structural changes in the connectivity between neurons have been proposed as the substrates for LTM in vertebrates and invertebrates (Bailey et al. 1996) and are believed to be at least one target of the interference eﬀects of protein synthesis inhibitors and memory blockers (for a review see Milner et al. 1998). In bees, direct evidence for LTM-related structural changes is lacking, but measurements of mushroom body subcompartment volume indicated that more experienced bees have bigger volumes and more elaborate dendrite branches (Durst et al. 1994, Fahrbach et al. 1995).
The biological circumstances of two forms of LTM may be related to the distinction between those forms of learning which usually lead to lifelong memories (e.g., visual and olfactory cues characterizing the home colony) and those which are stable but need updating on a regular basis (e.g., visual and olfactory cues of feeding places).
The bee, a small animal with a brain of merely 1 mm³ and a total of 950,000 neurons, establishes multiple and distributed memory traces not very diﬀerent from mammalian memory in the general temporal dynamics, characteristics of contents, and cellular substrates. It may thus serve as a suitable model for the study of memory structure and formation.
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