Complicaciones infecciosas

According to Lakoff (1987), in the classical view, “[categories] were assumed to be abstract containers, with things either inside or outside the category. Things were assumed to be in the same category if and only if they had certain properties in common. And the properties they had in common were taken as defining the category” (p.6). In other words, categories have clear all-or-none boundaries defined by necessary and sufficient conditions. This classical view of categories dominated psychology until the 1970s.

The first challenge to the classical view was presented by Wittgenstein (1974). Wittgenstein pointed out that certain categories, such as “game”, do not have defining features or common properties shared by all members, and do not have clear fixed boundaries. Instead, he argued that category members have “family resemblance,” meaning that, although they do not share the same set of properties, they are similar in various ways (perhaps sharing various combinations of features). A handful of other researchers also discussed specific problems with the classical view and provided evidence that the classical theory could not adequately explain the complex nature of categorization (for a comprehensive summary, see Lakoff, 1987).

previous studies, and developed empirical research methods to demonstrate that the classical view can not provide a full-scale theoretical account of categorization. Rosch (1978) suggested that category systems are structured along a vertical and a horizontal dimension, which are related to the basic-level categories and to the prototype effects, respectively. She also proposed that two basic principles of categorization underlie the formation of basic level categories and prototypes. The first principle states that the function of category systems is to provide maximum information with least cognitive effort. The second principle is based on the notion of a perceived structure of the world, which assumes that objects in the world are perceived to have correlated clusters of attributes that are highly probable to co-occur (e.g., wings and feathers). The perceived world’s structure, Rosch asserts, yields the structure of categories of the real world.

2.1.2.1.1 The vertical dimension - Basic level categories

The vertical dimension is concerned with the level of abstraction or the level of inclusiveness in a taxonomic arrangement of categories, where the most basic categories are placed in the middle of the hierarchical structure. In general, an object can be labeled in many different ways with varying degrees of abstraction (from general to specific), but among many alternatives a category at a particular level of abstraction has a primary status. This phenomenon was originally observed by Brown (1965), who also suggested that the basic level is constructed “at the level of distinctive action” (p.321). On the other hand, in the area of anthropology, ethnographic studies of folk taxonomies (Berlin, 1978; for a extensive review see Berlin, 1992) found certain regularities in the way people from different cultures categorize their environment, which also demonstrates the taxonomic category structure and

the primacy of categories in the middle of the taxonomy. For example, in his study of Tzeltal speakers, Berlin et al. (1974) found that Tzeltal natives tend to identify plants and animals at the level of ‘genus’ (oak, maple) which is in the middle of the folk classification, rather than one of the upper levels (plant, tree, leaf-bearing tree, etc.) or lower levels (sugar maple, live oak) of the classification structure. It is construed that “the genus was established as the level of biological discontinuity at which human beings could most easily perceive, agree on, learn, remember, and name the discontinuities” (Lakoff, 1987, p.34). Interestingly enough, Berlin also noted that folk taxonomies overlap with scientific taxonomies quite accurately at the basic level, but not at the other levels.

Drawing upon the findings from studies on folk classifications of natural (biological) objects, Rosch et al. (1978) proposed that “categories within taxonomies of concrete objects are structured such that there is generally one level of abstraction at which the most basic category cuts can be made… A taxonomy is a system by which categories are related to one another by means of class inclusion.” (p.30). Categories at a higher level of inclusiveness are called superordinate and those at a lower level are subordinate categories. The basic level is generally placed in the middle of the hierarchy. Rosch and her colleagues conducted a series of experiments using a variety of measures and found that the basic level is 1) the most inclusive level at which category members have similar appearances with a representative image, 2) the highest level at which category members are used in similar ways (in other words, people interact with them using similar motor movements), and 3) the level of abstraction at which category members have many common attributes distinguishing them from members of other categories, with few distinctive features among themselves1. For

example, members of the basic level category ‘chair’ have overall shapes with discernable common features such as legs and a seat, whereas members of a superordinate category ‘furniture’ do not have a similar appearance and share only a few attributes. In the case of subordinate categories such as kitchen chairs, comfort chairs, etc., although members of each category have common attributes, most of those attributes overlap with other categories at the same level (e.g. different kinds of chairs) because they all inherit attributes from the higher (basic) level. In terms of the second principle of categorization, basic categories best reflect the correlational structure of the environment. The results of a series of experiments show that objects are identified most rapidly at the basic level, and basic level categories (labels) are most frequently used when people name an object. In addition, as discovered in folk classification studies, children learn the basic level categories first and then learn other objects by generalizing (upward) or specializing (downward).

In summary, it has been found that categories of concrete objects (either natural or man-made) are organized in a hierarchy from general to specific and, more importantly, that there is a certain level in the hierarchy, usually in the middle of the hierarchy, at which cognitively basic and primary categories are situated. There is evidence that this basic level is by and large determined by the way people perceive and interact with objects, suggesting that categorization is not solely dependent on the attributes of the objects themselves, but also on the physical and mental capacities of humans.

concept defined as: “the validity of a given cue x as a predictor of a given category

y (the conditional probability of y|x) increases as the frequency with which cue x is associated with the category y increases and decreases as the frequency with which cue x is associated with categories other than y increases.” (p.30). Category resemblance is defined as “the weighted sum of the measures of all of the common features within a category minus the sum of the measures of all of the distinctive

2.1.2.1.2 The horizontal dimension – Prototype effects

The horizontal dimension has to do with the existence of prototypes within categories. In order to move beyond the classical view of categories dictating clear boundaries with necessary and sufficient conditions for category membership, Rosch (1978) argued that in many cases categories are conceived in terms of their prototypes rather than formal boundaries. Prototypes are regarded as ‘the clearest cases’ of a category, and operationally defined by ‘goodness-of-example’ ratings in her experiments. With the measure of goodness- of-example, it is shown that there exist asymmetries among category members. Certain category members are judged to be better examples or be more representative of a category, and there is a graded structure among category members as a function of how typical they are perceived as a member of the category. For example, ‘robin’ is judged to be more typical of ‘bird’ than falcon, which is more typical than ‘penguin.’ In a number of experiments conducted by Rosch and her colleagues as well as other researchers, it is repeatedly verified that subjects’ judgments of typicality are highly reliable and exhibit striking agreements (Rosch, 1975; Rosch & Mervis, 1975).

Given the empirical findings of degree of prototypicality, Rosch went on to test the effects of prototype structure on various psychological variables, including speed of processing, efficiency of learning, etc (Rosch & Mervis, 1975; Rosch et al., 1976). Rosch (1978) concluded that “the prototypicality of items within a category can be shown to affect virtually all of the major dependent variables used as measures in psychological research” (p.38).

Empirical verifications of prototype effects, indicating that categories have internal structures instead of being simple containers as the classical view assumes, entail the need

for a new model of category representation and structure. Since the 1970s a number of models of categorization have been proposed. According to Smith and Medin (1981), these models fall into one of two general views of categorization: the probabilistic view and the exemplar view.

The probabilistic view is directly influenced by Rosch’s research on prototypes. This view is based on two basic assumptions: (1) categories are represented in terms of some abstract summary (prototype), and (2) categories do not necessarily have a set of defining attributes (Smith & Medin, 1981). Instead, it is presumed that prototypes consist of clusters of attributes, which reflect the correlational structure of the world. Category membership of an object is then based on how similar it is to the prototype. In addition, the membership is graded depending on the level of similarity. In other words, category structure is established by the probability of objects matching the abstract summary.

The exemplar view posits that categories are represented by a set of exemplars or known instances, rather than by a single abstract summary. The underlying premise is that when people encounter a new object, exemplars stored in their memory from previous experiences play a critical role in processing the new object. Category membership of an object under this view is determined on the basis of the extent to which it is similar to the stored exemplars of the category.

For models based on the probabilistic view, the main problem is to explain the relative importance of features or combinations of features in membership judgments. For example, Rosch’s cue validity model formalizes individual features’ weight in terms of the frequency with which they appear in category members and nonmembers (Rosch & Mervis, 1975; Tversky, 1977). One the other hand, many models under the exemplar view are

concerned with what triggers particular exemplars being retrieved and used for categorization.