A Natural Bias For the Basic Level ?

It is well established that people can categorize the same objects at different levels of abstraction (i.e., superordinate, basic, and subordinate). Of these, the basic level is known to have a privileged status that is often attributed to the organization of categories in memory. Here, we argue that the bias could in part arise from the image formation process itself—i.e., the object properties for categorization that arise from the 2D retinal projections of distal 3D objects. In the real world, people do categorize objects from a variety of viewing distances and these modify the availability of object information on the retina. In two experiments, we tested the hypothesis that the information for basic categorizations is more resistant to changes in viewing distance than that of subordinate categorizations. Casual observers would experience little difficulty to categorize the animals in Figure 1 as exemplars of dog and those of Figure 2 as exemplars of whale. If they were “experts”, they could categorize these animals as SaintBernard dog, Doberman dog, Sperm whale, and Humpback whale. People can similarly apply different levels of category abstraction to the 3D distal objects that impinge on their retina. Rosch et al.’s (1976) seminal research isolated three “natural” levels of object categorization: the superordinate (animal, vehicle, furniture), the basic (dog, car, chair), and the subordinate (Saint-Bernard dog, Porsche, Chippendale chair). Of these, the basic and subordinate are thought to be closer to perception and we will focus on their main differences. The former level is superior to the later in a number of ways: (1) Categories at the basic-level are verified fastest (see also Hoffmann & Ziessler, 1983; Jolicoeur, Gluck & Kosslyn, 1984; Murphy, 1991; Murphy & Smith, 1982; Murphy & Brownell, 1985; Tanaka & Taylor, 1991). (2) Objects are named faster at the basic than at the subordinate level (Hoffmann & Ziessler, 1983; Jolicoeur, Gluck & Kosslyn, 1984; Murphy, 1991; Murphy & Smith, 1982; Murphy & Brownell, 1985; Rosch et al., 1976; Tanaka & Taylor, 1991; Johnson & Mervis, 1997). (3) Objects are preferentially designated with their basic-level names (Berlin, 1992; Brown, 1958; Rosch et al., 1976; Tanaka & Taylor, 1991; Wisniewski & Murphy, 1989). (4) Throughout development, basic names are learned before subordinate names (Anglin, 1977; Brown, 1958; Rosch et al., 1976; Horton & Markman, 1980; Markman, 1989; Markman and Hutchinson, 1984; Mervis and Crisafi, 1982). (5) Basic names tend to be shorter (Brown, 1956; Rosch et al., 1976). The origin of the bias to the basic level is still a matter of debate. In categorization, researchers have proposed that categories at the basic level are more differentiated that is, “. have the most attributes common to members of the category and the least attributes shared with members of other [contrasting] categories. ” (Rosch et al., 1976, p. 435) The first component of this differentiation definition has been called the specificity (Murphy & Brownell, 1985), or the informativeness (Murphy, 1991) of a category, and the second component the distinctiveness of a category (Murphy & Brownell, 1985; Murphy, 1991). The difference between basic and subordinate categorizations would thus stem from distinct differentiations at these two levels. But the origin of these remain unspecified. In recognition, researchers have sought to ground the basic level advantage on object properties (i.e., feature content). Rosch et al. (1976) found that basic-level categories are the most inclusive categories at which objects look alike. This suggests that shape is an important factor in the advantage of the basic over the subordinate level. One determinant of shape is part structure. Tversky and Hemenway (1984) found–for a broad range of natural categories including objects and organisms–a little increase in the number of listed parts from the basic to the subordinate level. Parts could therefore be a main determinant of basic-levelness. Jolicoeur, Gluck, & Kosslyn (1984) proposed that objects are initially recognized at the basic level on the basis of their parts, but also that these parts index the entry point to recognition. Entry point categories are usually at the basic-level but not always. To access categories below the entry point, such as Rosch’s subordinates, additional perceptual information is required (see also Biederman, 1987). This additional information was, however, left unspecified. Reflecting on the state of the art in object recognition, it is fair to say that the relationships between the basic level preference and its perceptual determinants are at a standstill. From this brief review of the literature, two main stances emerge regarding the advantage of basic level over the subordinate categorizations: (1) Categorization researchers have argued that the organization of categories in memory produces the faster access to the basic level (e.g. Murphy, 1991). (2) Recognition researchers have proposed that categorization is faster at the basic level because the visual system is geared to extract parts from the input, and parts represented categories at the basic level (e.g. Biederman, 1987). We will here present and test a third, and possibly simpler alternative: The bias for the basic level could arise from natural constraints on the image formation process that modifies the perceptual availability of object cues with changes of viewing distance. People who recognize common objects tend to do so over a wide range of viewing distances. For example, you need to recognize your car at a distance in a parking lot, but you also need to recognize it from a closer range, when you are about to unlock its door. Figure 1. Three-quarter right views of the Saint-Bernard and the Doberman Dogs used in experiments 1 and 2. T The figure respects the proportions of the stimuli, not their absolute sizes: the large animals occupied 12 deg of visual angle; the small ones (see dark spots at the bottomleft of each large animal) . 38 deg. However, a simple computational argument can be made that changing the size of the retinal projection also changes the information available in the image for identification. Simply put, reducing the retinal projection of an object by a factor of two reduces its sampling frequency by the same factor. Simplifying a little, if one starts with a 512x512 original image, the reduction samples one pixel every other pixel to produce a 256x256 image. Figure 2. Three-quarter right views of the Sperm and Humpback Whales used in experiments 1 and 2. The figure respects the proportions of the stimuli, not their absolute sizes: the large animals occupied 12 deg of visual angle; the small ones (see dark spots at the bottomleft of each large animal) . 38 deg. Any variation that was expressed between any two adjacent pixels of the original image (e.g. a black and white contrast) is lost in the reduced image. (Technically, shrinking an image eliminates its high spatial frequency information). This produces a marked loss in information for visual categorization. If we hypothesize that different basic and subordinate categorizations require visual information that resides at different scales of the stimulus, changing the scale of one stimulus could produce markedly different patterns of categorization performance. For example, removing large-scale information by reducing the size of objects could selectively impair the categorization level requiring the most specific details—i.e. the subordinate level. If this were the case, a bias to the basic level could arise from the statistics of categorization attributes over a wide range of viewing distances. Specifically, the attributes Saint-Bernard Dog Doberman Dog Sperm Whale