Simple system

A *system, such as an organism or a preparation containing neural tissue, that is less complex than other systems in the same category, and is used to model the category and to facilitate research that involves this category.

Interest in the intelligence of simple organisms, and their use to investigate problems of learning and memory, draws mainly from two conceptual frameworks: Darwinism (Boakes 1984), and "reductionism. The notion that evolution applies to the mind as well, undoubtedly combined with a keen interest in zoology and behaviour, has directed some investigators already a century ago to study the mental powers of organisms such as protozoa and insects (Peckham and Peckham 1887; Jennings 1906; Day and Bentley 1911). Starting at the 1950s and 60s, the use of simple organisms in neurobiology (Benzer 1967), as well as that of simplified preparations from such organisms (Kandel and Spencer 1968), has been further reinforced by the remarkable success of utilizing simple organisms to decode the genetic code and to unravel metabolic pathways.

The selection of a 'simple' organism for the investigation of learning and memory depends on the objective and on the context of the research programme. If the objective is to study the neurobiology of a particular species per se, the selection is self-explanatory. If the objective is primarily to advance understanding of a general problem in the neurobiology of learning and memory, the selection is based on properties that offer particular experimental advantages. These could be, for example, simple reflex circuits, large accessible neurons (Kandel and Spencer 1968; T.E. Cohen et al. 1997), or amenability to genetic analysis (Benzer 1967; Dudai et al. 1976; Tully 1996). Furthermore, whether an organism is regarded 'simple' depends on what is it compared with. In the context of the analysis of human "amnesia, studying a "rat is switching to a simple system, although the rat, of course, is far from being simple.

Intact simple organisms could be employed to analyse distinct phenomena of learning and memory, such as non"associative learning, rudimentary associative learning, "cue revaluation, or elements of memory "consolidation. But the use of such organisms in the dissection of the physiological and molecular mechanisms that subserve the aforementioned phenomena requires a combination of "reductive and simplifying steps.1 This results in preparations in which fragments of the nervous system, or even individual neurons only, are analysed (Rayport and Schacher 1986; Krasne and Teshiba 1995; T.E. Cohen et al. 1997; Frysztak and Crow 1997). In the simplified preparations, certain molecular and cellular phenomena could be used to "model a molar behaviour, for example, "synaptic depression and facilitation to model "habituation and "sensiti-zation, respectively (T.E. Cohen et al. 1997), or activity dependent synaptic facilitation to model associative conditioning (Byrne and Kandel 1996).

The analysis of simple organisms and simplified neural preparations from such organisms has generated highly valuable insight into basic phenomena, processes, and mechanisms of learning and memory. Subsequent analysis in more complex systems has established that some basic findings "generalize from the simple systems to the more complex ones (Bailey et al. 1996; e.g. "CREB, "consolidation, "immediate early genes, "intracellular signal transduction cascades, "protein synthesis). The caveats should not, however, be overlooked. First, 'primitive' organisms are often not so primitive (e.g. Srinivasan and Chang 1998), and miniature brains do not necessarily imply simplicity ("subject). The so-called 'simple organisms' surely did not evolve to supply neuroscientists with convenient tools to approach complex problems; these organisms were shaped in evolution to survive in a circumscribed ecological niche, and are therefore endowed with certain specialized (and intriguing) properties, but not with others. This means that whatever is the physiology and behaviour of these species, it has been adapted to fulfil needs that might be different from those of other species. But it also means that individuals belonging to primitive species clearly cannot perform some feats that are within the repertoire of more advanced species: likening contemplation in a worm to the retrieval of explicit knowledge in humans, is carrying the analogy a bit too far indeed. Second, within their phylogenetic limitations, some of the specialized tasks carried out by so-called simple organisms are themselves rather elaborate (e.g. Zeil et al. 1996). Third, even if the task is simple, the neuronal "algorithms that subserve it may prove complex when the surface is scratched (e.g. Wolpaw 1997).

In recent years, some of the enthusiasm for using simple organisms to analyse the neurobiological bases of behaviour has dwindled, because state-of-the-art molecular biology could now be used to approach problems in higher organisms that previously were approachable only in lower ones. Powerful "neuro-genetics, for example, can already be practised in "mice, depriving Drosophila of its monopoly in the neurogenetic analysis of memory. It would be, however, a pity if simple organisms were to be abandoned. These species, in addition to being so interesting as such, do offer substantial advantages, for example, in deciphering the "representational code that are used in identified neural circuits that subserve discrete behaviours. In parallel with the aforementioned trend, the use of brain slices (e.g. Barkai and Hasselmo 1997) and of neuronal cell cultures (e.g. Tong et al. 1996) from complex nervous systems has gained much popularity, because such simplified preparations permit full exploitation of highly advanced cellular, molecular, and computational techniques (e.g. in the investigation of "long-term potentiation). It is likely that in the future, simple systems may come to include also bionic hybrids of neural tissue and printed circuits (Kuwana et al. 1995; Maher et al. 1999).

Selected associations: Aplysia, Drosophila, Honeybee, Model, Reduction

1For the definition of these two types of steps and the distinction between them, see *reduction.

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