A procedure or technique for the analysis of a phenomenon process or mechanism a test

Assays (from exagiere, Latin for 'to weigh out') are not merely research tools. They play a decisive part in the development and workings of scientific disciplines. They are also important in shaping the feasibility, progression, and outcome of particular research programmes. Sometimes they even play a decisive part in moulding the fate of individual academic careers.

Scientific assays are the nuts and bolts of scientific "methods and "paradigms. In experimental science, they are the end instrument used to embody the objectives of a 'method' and test the concepts of a 'paradigm'. They are thus more specific than 'methods'. The same method may be implemented by using a variety of assays. For example, one could employ a correlative method to probe the role of an "immediate early gene in memory in a given brain region, but use different assays to determine whether the expression of that gene is correlated with the behavioural change. A useful assay yields results that are then subjected to analysis and construed according to selected "criteria.

The spectrum of assays used in the neurosciences is rich and heterogeneous. Practically all these assays could also be incorporated into research programmes that target learning and memory. One useful classification of assays ("taxonomy) is by the "level of analysis involved. Other classifications are of course possible; for example, by the method that guides and utilizes the assay (i.e. correlation, intervention, etc.). Straightforward classification by level is into molecular, cellular, neuroanatomical, "system, and behavioural. In considering levels of analysis, one should note differences in the dialects of the scientific "culture. The term 'assay' is mostly popular in molecular and cellular studies. Neuroanatomists prefer to use 'technique' or 'method' (which, as noted above, is better reserved for a more comprehensive activity). Psychologists cling to 'test'. The latter term commonly carries the connotation of 'success' or 'failure' in "performance; 'assay' does not. 'Test' can also be used to denote particular instantiation of a type of assay in an experimental protocol.

Molecular assays, such as binding of drugs to "receptors or measuring enzyme activity, are shared by many branches of molecular and cellular biology (R. Martin 1997; e.g. "development). Cellular, neuroanatomical, and system procedures are shared by many subdisciplines of the neurosciences and are not unique to the study of plasticity and memory. One notable exception that comes to mind is "long-term potentiation, which under certain circumstances may be regarded as an assay to determine induction and maintenance of cellular "plasticity, although it is also a method, and moreover, a "paradigm. In contrast to molecular, cellular, and system assays, behavioural assays used in the field of learning and memory are unique to this field: they are specific 'memory assays' or 'tests'.

Some memory tests were groundbreaking at the time of their introduction. For a field of knowledge to become a scientific discipline, research techniques and assays are required that permit quantification of phenomena addressed in that field:'.. .the forces and actions of bodies are circumscribed and measured either by spatial intervals, or by moments of time, or by concentration of quantity, or by predominance of power; and unless these four are accurately and carefully weighed, the sciences concerned will be elegant speculations perhaps but of no practical use' (Bacon 1620). A handful of tests, by the mere fact that they had enabled for the first time the quantification of memory, had transformed the study of memory into a science. A prominent example is provided by tests involving "recall of series of so-called 'nonsense' syllables, introduced by Ebbinghaus to measure "forgetting (1885; see also Jacobs 1887). This type of experiment is considered to have opened the scientific era in research of human memory. Similarly, introduction of "classical and "instrumental conditioning has permitted the systematic experimental investigation of animal learning (Thorndike 1911; Pavlov 1927; for more on the history, see Boakes 1984).

Still another class of assays includes those that alter and reroute the course of a discipline. Here are some examples: Introduction of the "maze (Small 1901;

"classic) has paved the way to research on spatial learning, cognitive "maps, and other facets of memory. A popular descendent of those original mazes is the extensively used water maze (Morris 1981). Introduction of the "delay task (Hunter 1913) has permitted analysis of "recognition and "working memory, and development of "monkey models of "amnesia. Very useful versions are the trial-unique delay tasks, such as trial-unique delayed non-matching-to-sample (Gaffan 1974; Mishkin and Delacour 1975; "delay task). In some cases, adaptation of a well-known type of memory assay to a new organism could open a whole new field. An example is provided by olfactory conditioning in the fruit fly, "Drosophila. Sophisticated "neurogenetic analysis of memory became feasible only after classical conditioning had been adapted to the special needs of the fly (Quinn et al. 1974). And, of course, there are those many assays that are variations on a theme, introducing important improvements and modifications to already existing methods.

Lack of an appropriate assay may hinder the development of a field or the resolution of a major research problem. For example, some types of behavioural assays engage the "hippocampus and are sensitive to hippocampal damage. However, at the time of writing there is still no satisfying behavioural assay to tap exclusively into hippocampal function in primates. Such a task will be very useful in clarifying the role of the hippocampus in memory. The hippocampus can also be invoked to illustrate a potential problem in the use of assays. This is the problem of 'circular argumentation'. Thus, given that a hippocampal lesion impairs performance on task X under condition A, some investigators are quick to use task X under conditions other than A to determine whether the hippocampus is involved, as if task X is an established probe for hippocampal involvement. Failure or success on task X, however, may result from parameters specific to condition A that do not "generalize to other conditions of the "subject or the experiment. The problematics are further augmented when inference is made from one species to another. Here is an example that relates not only to the hippocampus but also to a profound issue in the evolution of mind: 'trace conditioning' of the eyelid reflex ("classical conditioning) is sensitive to hippocampal damage and involves "conscious awareness in normal human individuals (Clark and Squire 1998). However, this by itself is insufficient to propose trace conditioning as a cross-species assay for awareness, because other potential explanations ("Ockham's razor) must first be scrutinized, such as a failure to hold information off-line irrespective of awareness.

Another caveat that should be considered is that occasionally, an assay becomes a prison to imagination. This problem runs in two versions: individual and generic. Some individuals flirt with a single method, even a single assay, throughout their career, from their Ph.D. thesis on. Being inflicted with some unique version of separation anxiety, they refuse to give up a procedure that has worked for them, and entrust their future in the hands of the past. A more serious problem arises when an entire subdiscipline falls into the procedural drain. For example, in its first few years, the newly emerging discipline of mammalian neurogenetics has followed as a routine a very limited number of standard versions of the otherwise very useful water maze assay. This was also occasionally accompanied by the simplistic interpretation of performance in the maze, probably resulting in neglect of some intriguing effects of mutations on behaviour (on some of the complexities involved, see Bannerman et al. 1995; Wolfer et al. 1998).

It is likely that in due time, memory research will generate memory-specific assays based on direct observation of experience-dependent alterations in "internal representations of the nervous system ("map, "functional neuroimaging; example in "honeybee).

Selected associations: Delay task, Maze, Method, Paradigm

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