Hierarchical organization within memoryencoding neural clique assemblies

Through examining the overall organization of neural clique assembly involved in startle memory encoding, it is clear that the internal CA1 representations of any given startle episode involves a combinatorial set of neural cliques, invariantly consisting of the general startle clique, sub-general startle clique, startle identity-specific clique, and context-specific startle clique (Lin et al. 2005,2006). Thus, each clique assembly is organized in a categorical hierarchy manner and invariantly consists of a "feature-encoding pyramid" (Lin et al. 2006; Box 1): starting with the neural clique representing the most general and common features (to all categories) at the bottom layer, followed by neural cliques responding to less general features (covering multiple, but not all, common categories), and then moving gradually up towards more and more specific and discriminating features (responding to a specific category), with the most discriminating feature clique (corresponding to context-specificity) on the top of the feature-encoding pyramid.

According to this hierarchical structure of network-level memory encoding (Box 1), the general startle neural clique represents the neurons engaged in the extraction of the common features among various episodes (e.g., encoding abstract and generalized knowledge that "such events are scary and dangerous" by integrating neural inputs from the amygdala). The sub-general neural cliques are involved in identifying sub-common features across a subset of startling episodes (e.g., perhaps, the earthquake and drop-specific clique for encoding the semantic memory of the fact that "those events involve shaking and motion disturbances" by integrating inputs from the vestibular system), whereas the startle identity-specific cliques encode discriminative information about startle types (defining "what type" of event has happened) and the startle context-specific cliques provide an even more specific feature, such as contextual information about "where" a particular startling event has happened.

This invariant feature-encoding pyramid of neural clique assemblies reveals four basic principles for the organization of memory encoding in the brain (Box 1). First, the neural networks in the memory systems employ a categorical and hierarchical architecture in organizing memory coding units. Second, the internal representations of external events in the brain through such a feature-encoding pyramid is achieved not by recording exact details of the external event but rather by re-creating its own selective pictures based on the importance for survival and adaptation. Third, the feature-encoding pyramid structure provides a network mechanism, through a combinatorial and self-organizing process, for creating seemingly unlimited numbers of unique internal patterns, capable of dealing with potentially infinite numbers of behavioral episodes that an animal or human may encounter during its life. Fourth, in addition to its vast memory storage capacity, these neural clique-based, hierarchical-extraction and parallel-binding processes also enable the brain to achieve abstraction and generalization, cognitive functions essential for dealing with complex, ever-changing situations.

The finding that the memory-encoding neural clique assembly appears to invari-antly contain the coding units for processing the abstract and generalized information (Lin et al. 2005, 2006) is interesting. It fits well with the anatomical evidence that virtually all of the sensory input that the hippocampus receives arises from higher-order, multimodal cortical regions and the hippocampus has a high degree of sub-regional divergence and convergence at each loop. This unique anatomical layout supports the notion that whatever processing is achieved by the hippocampus in the service of long-term memory formation should have already engaged with fairly abstract, generalized representations of events, people, facts, and knowledge.

The observed feature-encoding pyramid structure of the neural clique assembly is likely to represent a general mechanism for memory encoding across different animal species. For example, single unit recordings in human hippocampus show that some hippocampal cells fire in response to faces or, more selectively, to a certain type of human facial emotions; others seem to exhibit highly selective firing to one specific person (e.g., the "actress Halle Berry cell", which fires selectively to her photo portraits, Cat-woman character, and even a string of her name (Quiroga et al. 2005). Although those cells were not recorded simultaneously, the findings are nonetheless consistent with the general-to-specific feature pyramid structure. In addition, it is also reported that, whereas some place cells in the rat hippocampus exhibit location-specific firing regardless of whether the animals engage in a random forage or goal-oriented food retrieval (or make aleftorright turn inaT-maze), others seem to fireselectively at their place fields only in association with a particular kind of experience (Markus et al. 1995; Wood et al. 2000). Thus, those studies also seem to support the existence of a hierarchical structure involved in space coding. Therefore, the hierarchical organization of the neural clique assembly, revealed through large-scale recordings of startling episodes and mathematical analyses, may represent a general feature for memory encoding in the brains. In addition, it further suggests that episodic memory is intimately linked with and simultaneously converted to semantic memory and generalized knowledge.

This form of hierarchical extraction and parallel binding along CNS pathways into the memory and other high cognition systems is fundamentally different from the strategies used in current computers, camcorders, or intelligent machines. These unique design principles allow the brain to extract the commonalities through one or multiple exposures and to generate more abstract knowledge and generalized experiences. Such generalization and abstract representation of behavioral experiences have enabled humans and other animals to avoid the burden of remembering and storing each mnemonic detail. More importantly, by extracting the essential elements and abstract knowledge, animals can apply past experiences to future encounters that share the same essential features but may vary greatly in physical detail. These higher cognitive functions are obviously crucial for the survival and reproduction of animal species.

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