A figure of speech in which one entity is described in terms of another

Metaphora is 'a transfer' in Greek, the transference consisting 'in giving the thing a name that belongs to something else' (Aristotle, Poetics 1457b7-8). In a more formal notation, metaphors can be described as statements in figurative language, composed of two juxtaposed elements, the 'tenor' and the 'vehicle', which are presented as sharing common attributes or 'ground' (Richards 1936). The tenor is the subject of the metaphor, while the vehicle is the means by which the subject is referred to. (More recent discourse refers to the subject as the 'target domain' and to the vehicle as 'source domain'; Gibbs 1994.) For example, in 'memory is a storehouse' (after Locke 1690, and see below), 'memory' is the tenor (the target domain), 'storehouse' the vehicle (the source domain), and 'space'the ground. The interaction of the tenor with the vehicle, or the target with the source domain, produces an emergent meaning for the entire statement.

Classically, metaphors were regarded as artful, poetic decorations: 'Metaphor gives style clearness, charm, and distinction as nothing else can' (Aristotle, Rhetoric 1405a8-9). But in modern psychology and linguistics, metaphors are also considered essential in ordinary language, language development and change, and thought and conceptualization (Lakoff and Johnson 1980). Three major motives were proposed for using metaphors (Gibbs 1994). The first is similar to the classical poetic view, namely, metaphors capture effectively the vividness of experience. The second is to provide compactness in verbal communication. The third, to express what is otherwise inexpressible in literal language. It is this latter property that endows metaphors a central position in musing, hypothesizing and writing about learning and memory.

Ordinarily, there are two main reasons for inexpress-ibility in a normal person. The first is related to evolution of human brain and cognition. It is tempting to presume that "Homo sapiens has so far evolved to deal intuitively1 with only selected manifestations of the universe, characterized by circumscribed physical "dimensions (millimetres to kilometres, seconds to years), limited complexity (e.g. "attention, "capacity), and restricted access to inner mental processes. These segments of the universe comprise the 'mesoworld', to be contrasted with the 'micro' and 'macro' world, which are not accessible to the bare senses. It is also tempting to assume that ordinary language lags beyond the potential of the human brain to transcend the mesoworld into the micro, the macro, as well as the inner universe. Ultimately, the accumulated "insight of our "culture is formalized in formal, scientific terminology, some of which gravitates with time toward intuitiveness. But often, an interim stage is needed in which comprehension of the unfamiliar is mediated via the familiar. Here metaphors are useful. For a layperson, it is tough to comprehend intuitively what an electron is; depicting it by a process of analogical "transfer as a small planet encircling the nucleus-sun, makes life easier. A second reason for inexpressibility is simply the lack of appropriate knowledge.

Brain research uses metaphors for both aforementioned reasons. To describe the mind and memory, multiple classes of metaphors have been generated, most of them antedating the scientific era (Abrams 1953; Roediger 1980). The vehicle and ground in these metaphors involve either space, written records, vision, or technology. Especially common are spatial metaphors, depicting memory as a physical repository, and "retrieval as a search for the location of an item filed in storage (for classical examples see Plato, Theaetetus 197c-e; Augustine 400; Locke 1690; "engram, "mnemonics). Written record metaphors (which could be regarded as a subset of the spatial ones) advanced from etched wax tablets (Plato, Theaetetus 193a-a95a) to more fancy writing pads (Freud 1925). In an influential "model of "working memory, we encounter a subsystem termed the 'visuospatial scratch pad' or 'sketchpad' (Baddley 1986).2 Working memory as such was also referred to as 'the blackboard of the mind' (Goldman-Rakic 1996).

Visual metaphors of brain and memory often intermingle with spatial or written record metaphors. They refer to "perception and "acquisition in terms of passively perceiving the world or actively throwing light on it (Abrams 1953), and to "retrieval as illumination of items in mind (e.g. by an internal searchlight, Baars 1998). Visual metaphors are common when the discussion involves "attention and "conscious awareness (ibid.). Whereas the images in some of the above metaphors, e.g. the storehouse, remain rather similar over the ages, technological metaphors reflect the machines and gadgets of their period. They tend to be more dynamic than spatial metaphors and describe multiple phases in learning and memory. Examples of technological metaphors include hydraulic networks in the Renaissance (Descartes 1633), and electronics and computers nowadays (Churchland and Sejnowski 1992). Operational and system research by the Allies in the Second World War was influential in shaping metaphors of attention and working memory. In current discussions, working memory is occasionally referred to as the brain's 'desktop'; this is an interesting example of a second-order, or "palimpsestic metaphor: 'desktop' is borrowed from the computer world, which was imported from the pre-computer office environment.

Although metaphors of memory may help us in organizing our thoughts and even in proposing creative research ideas, their value is limited. A few caveats are appropriate here. First, metaphors are potential source for misunderstanding: 'Metaphor is the dreamwork of language and, like all dreamwork, its interpretation reflects as much on the interpreter as on the originator' (Davidson 1979). Second, in the absence of new data and theory, merely adding or exchanging metaphors could simply augment confusion. This is not unique to the scientific language: it was noted that 'mixed metaphors' (i.e. the application of two or more inconsistent metaphors to a given situation) 'always arouses derision' (Fowler and Burchfield 1996). Third, dominant metaphors ("zeitgeist) could hinder progress by fixating conceptual "paradigms (Watkins 1990; Koriat and Goldsmith 1996). For example, storage metaphors may lure us to think about memory as static, which is wrong. Only time will tell whether recent metaphors that drew from computer science and modern physics are distractive or not. In conclusion, in memory research, as in science in general, metaphors should better be regarded only as vehicles on the winding and bumpy road to formal understanding. But this is already another metaphor.

Selected associations: Engram, Flashbulb memory, Model, Palimpsest, Transfer

1For what is meant by 'intuition', see 'dimension. 2The 'central executive', used to describe the postulated supervisory controlling system in working memory, could also be considered a metaphor, but of a different ground. It belongs to the family of the *homunculus metaphors.

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