The vertical dimension

The vertical dimension derives from the discovery by Rosch and her colleagues (Rosch et al. 1976) that categories can be distinguished according to level of inclusiveness. Inclusiveness relates to what is subsumed within a particular category. As we have seen, the category FURNITURE is more inclusive than the category CHAIR because it includes entities like DESK and TABLE in addition to CHAIR. In turn, CHAIR is more inclusive than ROCKING CHAIR because it includes other types of chairs in addition to rocking chairs. The category ROCKING CHAIR

only includes rocking chairs, and therefore represents the least inclusive level of this category. Rosch and her colleagues found that there is a level of inclusive ness that is optimal for human beings in terms of providing optimum cognitive economy. This level of inclusiveness was found to be at the mid-level of detail, between the most inclusive and least inclusive levels: the level associated with categories like CAR, DOG and CHAIR. This level of inclusiveness is called the basic level, and categories at this level are called basic-level categories. Categories higher up the vertical axis, which provide less detail, are called superordinate categories. Those lower down the vertical axis, which provide more detail, are called subordinate categories. This is illustrated in Table 8.3.

 In a remarkable series of experiments, Rosch found that basic-level categories provided the most inclusive level of detail at which members of a particular category share features in common. In other words, while the superordinate level (e.g. MAMMAL) is the most inclusive level, members of categories at this level of inclusiveness share relatively little in common when compared to members of categories located at the basic level of inclusiveness (e.g. DOG).

Attributes Rosch et al. (1976) found that the basic level is the level at which humans are best able to list a cluster of common attributes for a category. To investigate this, Rosch and her colleagues gave subjects 90 seconds to list all the attributes they could think of for each of the individual items listed in a particular taxonomy. Six of the taxonomies used by Rosch et al. are presented in Table 8.4. (It is worth pointing out to British English readers that because Rosch’s experiments were carried out in the United States, some of the American English expressions may be unfamiliar.)

 Table 8.5 lists common attributes found for three of these taxonomies. In the table, lower levels are assumed to have all the attributes listed for higher levels and are therefore not repeated. Table 8.5 illustrates the fact that subjects were only able to provide a minimal number of shared attributes for superordinate categories. In contrast, a large number of attributes were listed as being shared by basic-level categories, while just one or two more specific attributes were added for subordinate categories. Hence, while subordinate categories have slightly more attributes, the basic level is the most inclusive level at which there is a cluster of shared attributes.

Motor movements

 In this experiment, Rosch et al. set out to establish the most inclusive level at which properties of human physical interaction with a category are found to cluster. This experiment also revealed that basic level categories were the most inclusive level at which members of categories share motor movements. To demonstrate this, subjects were asked to describe the nature of their physical interaction with the objects listed. It was found that while there are few motor movements common to members of a superordinate category, there are several specific motor movements listed for entities at the basic level, while entities at the subordinate level make use of essentially the same motor movements. This provides further evidence that the basic level is the most inclusive level, this time with respect to common interactional experiences. This is illustrated in Table 8.6.

 Similarity of shapes

 For this experiment, Rosch et al. sought to establish the most inclusive level of categorisation at which shapes of objects in a given category are most similar. In order to investigate this, the researchers collected around 100 images from sources like magazines and books representing each object at each level in the taxonomies listed in Table 8.4. The shapes were scaled to the same size and then superimposed upon one another. Areas of overlap ratios were then measured, which allowed the experimenters to determine the degree of similarity in shape. While objects at the superordinate level are not very similar in terms of shape (compare the outline shapes of car, bus and motorcycle, for example, as instances of the category VEHICLE), and while objects at the subordinate level are extremely similar, the basic level was shown to the most inclusive level at which object shapes are similar. In other words, the basic level includes a much greater number of instances of a category than the superordinate level (for example, DOG versus COLLIE) that can be identified on the basis of shape similarity.

Identification based on averaged shapes

In a fourth experiment, Rosch and her team devised averaged shapes of particular objects. They did this by overlapping outlines of entities belonging to a particular category. For all points where the two outlines did not coincide, the central point between the two lines was taken. Subjects were then shown the shapes and provided with superordinate, basic-level and subordinate terms to which they were asked to match the shapes. The success rate of matching shapes with superordinate terms was no better than chance, while subjects proved to be equally successful in matching averaged shapes with basic-level and subordinate terms. For example, the superordinate category VEHICLE consisted of overlapped shapes for car, bus and motorcycle, which are significantly different in shape and therefore less recognisable. On the other hand, the basic-level category CAR, rep resented by overlapping shapes of different types of cars, did not involve significant differences in shape, and was easily identifiable. Again, although there is a greater degree of similarity at the subordinate level, the basic level is more inclusive. The absence of shape similarity at the superordinate level compared to the evident shape similarity at the basic level goes some way towards explaining why the basic level is the optimum categorisation level for the human categorisation system, which is based, among other things, on perceptual similarity.

Cognitive economy versus level of detail

 The major finding to emerge from Rosch’s research on basic-level categorisation is that this level of categorisation is the most important level for human categorisation because it is the most inclusive and thus most informative level. It is worth emphasising why this should be the case. After all, Rosch et al.’s findings seem to show that the subordinate level is at least as informative as the basic level, if not more so, given that it provides more detailed information in addition to the information represented at the basic level. Recall that, when asked to list attributes of CAR and SPORTS CAR, subjects typically listed more attributes for SPORTS CAR than for CAR. This is because the subordinate cate gory SPORTS CAR is likely to be identified with the same attributes as CAR, plus some extra attributes specific to SPORTS CAR.

 The reason why the basic level is the most salient level of categorisation relates to the tension between similarity of members of a category and the principle of cognitive economy. While entities at the subordinate level are most alike (rocking chairs have most in common with other rocking chairs), different categories at the subordinate level are also very similar (rocking chairs are pretty similar to kitchen chairs). At the basic level, on the other hand, while there are also similarities within a particular category (all chairs are pretty similar to one another), there are far fewer between-category similarities (a chair is not that similar to a table). To illustrate this point, let’s compare and contrast the basic-level and sub ordinate level categories given in Table 8.7.

Crucially, for a category to achieve cognitive economy (to provide the greatest amount of information at the lowest processing cost), it must share as many common within-category attributes as possible, while maintaining the highest possible level of between-category difference. In intuitive terms, it is easier to spot the differences between a chair and a lamp than between a desk lamp and a floor lamp. This demonstrates why the basic level of categorisation is ‘special’: it is the level which best reconciles the conflicting demands of cognitive economy. Therefore the basic level is the most informative level of categorisation.

 This notion of cognitive economy has been described in terms of cue validity. According to Rosch (1977: 29) ‘cue validity is a probabilistic concept’ which predicts that a particular cue – or attribute – becomes more valid or rel evant to a given category the more frequently it is associated with members of that category. Conversely, a particular attribute becomes less valid or relevant to a category the more frequently it is associated with members of other categories. Thus ‘is used for sitting on’ has ‘high cue validity’ for the category CHAIR, but ‘is found in the home’ has low cue validity for the category CHAIR because many other different categories of object can be found in the home in addition to chairs.

Cue validity is maximised at the basic level, because basic level categories share the largest number of attributes possible while minimising the extent to which these features are shared by other categories. This means that basic-level categories simultaneously maximise their inclusiveness (the vertical dimension) and their distinctiveness (the horizontal dimension) which results in optimal cognitive economy by providing a maximally efficient way of representing information about frequently encountered objects.

Perceptual salience

 It is clear from Rosch’s findings that categorisation arises from perceptual stimuli. When we categorise objects, we do so according to various types of sensory perceptual input, including shape, size, colour and texture, as well as kinaesthetic input representing how we interact physically with objects. Another way of describing the importance of the basic level, then, is by relating it to perceptual salience. There are a number of additional lines of evidence that support the position that the basic level represents the most salient level of categorisation.

The basic level appears to be the most abstract (that is, the most inclusive and thus the least specific) level at which it is possible to form a mental image. After all, we are unable to form an image of the category FURNITURE without imagining a specific item like a chair or a table: a basic-level object. This is consistent with the finding that averaged shapes cannot be identified at the super ordinate level as there are insufficient similarities between entities at this very high level of inclusiveness. This is also consistent with the fact that Rosch’s subjects often struggled to list attributes for the superordinate level. You can try this experiment yourself: if you ask a friend to draw you a picture of ‘fruit’ or ‘furniture’ they will draw you apples and bananas or tables and chairs. These are all basic-level categories. There is no recognisable or meaningful shape that represents the superordinate level of categorisation.

 Based on a picture verification task, Rosch et al. (1976) also found that objects are perceived as members of basic-level categories more rapidly than as members of superordinate or subordinate categories. In this experiment, sub jects heard a word like chair. Immediately afterwards, they were presented with a visual image. If the word matched the image, subjects pressed a ‘match’ response key. If the word did not match the image, they pressed a different response key. This enabled experimenters to measure the reaction times of the subjects. It emerged that subjects were consistently faster at identifying whether an object matched or failed to match a basic level word than they were when verifying images against a superordinate or subordinate level word. This suggests that in terms of perceptual verification, objects are recognised more rapidly as members of basic-level categories than other sorts of categories.

Language acquisition

Rosch et al. (1976) found that basic-level terms are among the first concrete nouns to emerge in child language. This investigation was based on a case study of a single child, consisting of weekly two-hour recordings dating from the initial period of language production. All relevant utterances were independently rated by two assessors in order to determine whether they were superordinate, basic or subordinate level terms. The study revealed that the individual noun-like utterances were overwhelmingly situated at the basic level. Rosch et al. argued that this finding provided further support for the primacy of the basic level of categorisation.

Basic-level terms in language

The language system itself also reveals the primacy of the basic level in a number of ways. Firstly, basic-level terms are typically monolexemic: comprised of a single word-like unit. This contrasts with terms for subordinate level categories which are often comprised of two or more lexemes – compare chair (basic-level object) with rocking chair (subordinate-level object). Secondly, basic-level terms appear to occur more frequently in language use than super ordinate or subordinate level expressions. More speculatively, Rosch (1978) has even suggested basic-level terms may have emerged prior to superordinate- and subordinate-level terms in the process of language evolution. Of course, given that evidence for the primacy of the basic level is so overwhelming, we might wonder why we need the other levels of categorisation at all. In fact, the super ordinate and subordinate levels, while they may not be cognitively salient, have extremely useful functions. As Ungerer and Schmid (1996) explain, the super ordinate level (for example, VEHICLE) highlights the functional attributes of the category (vehicles are for moving people around), while also performing a collecting function (grouping together categories that are closely linked in our knowledge representation system). Subordinate categories, on the other hand, fulfil a specificity function.

Are basic-level categories universal?

Of course, if we can find evidence for basic-level categories among English speakers, two questions naturally arise. Firstly, do members of all cultures or speech communities categorise in this way? Given that all humans share the same cognitive apparatus, it would be surprising if the answer to this question were ‘no’. This being so, the second question that arises is whether the same basic-level categories are evident in all cultures or speech communities. Clearly, this question relates to ‘the extent to which structure is “given” by the world versus created by the perceiving organism’ (Rosch et al. 1976: 429). Put another way:

 [B]asic objects for an individual, subculture, or culture must result from interaction between potential structure provided by the world and the particular emphases and state of knowledge of the people who are categorizing. However, the environment places constraints on categorizations. (Rosch et al. 1976: 430)

It follows that while the environment partly delimits and thus determines the nature of the categories we create, these categories are also partly determined by the nature of the interaction between human experiencers and their environment. This finding, of course, is consonant with the thesis of embodied cognition.

This view of categorisation entails that while the organisation of conceptual categories into basic, superordinate and subordinate levels may be universal, the level at which particular categories appear may not be. This relates not only to cross-linguistic or cross-cultural variation in the broader sense, but is also reflected within a single speech community or culture where acquired specialist knowledge may influence an individual’s taxonomy of categories. For instance, Rosch et al. (1976) found that for most of their North American subjects the category AIRPLANE was situated at the basic level. However, for one of their subjects, a former aircraft mechanic, this category was situated at the superordinate level, with specific models of aircraft being situated at the basic level. This reveals how specialist knowledge in a particular field may influence an individual’s categorisation system. At the cross-cultural level, the cultural salience of certain objects may result in taxonomic differences. For example, the anthropologist Berlin and his colleagues (1974) investigated plant naming within the Mayan-speaking Tzeltal community in Southern Mexico. They found that in basic naming tasks members of this community most frequently named plants and trees at the (scientific) level of genus or kind (for example, pine versus willow) rather than at the (scientific) level of class (for example, tree versus grass). When Rosch et al. (1976) asked their North American students to list attributes for TREE, FISH and BIRD as well as subordinate instances of these categories, they found that, on average, the same number of attributes were listed for TREE, FISH and BIRD as for the subordinate examples, suggesting that for many speakers TREE, FISH and BIRD may be recognised as a basic-level category. The differences between the Tzeltal and North American speakers indicates that aspects of culture (for example, familiarity with the natural environment) can affect what ‘counts’ as the basic level of categorisation from one speech community to another. However, it does not follow from this kind of variation that any category can be located at any level. While our interaction with the world is one determinant of level of categorisation, the world itself provides structure that also partly determines categorisation, an issue to which we now turn.

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المزيد

مواضيع ذات صلة

Sources of typicality effects

The theory of idealised cognitive models

Problems with prototype theory

The horizontal dimension

The categorisation system

Principles of categorization

Prototype theory

The problem of prototypicality

The problem of conceptual fuzziness

The definitional problem

The classical theory

Categorisation and cognitive semantics